|
Model 7220
DSP Lock-in Amplifier
Instruction Manual
190171-A-MNL-C
Copyright © 1996 EG&G INSTRUMENTS CORPORATION
Table of Contents
Table of Contents
Chapter One, Introduction
1.1 How to Use This Manual. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1
1.2 What is a Lock-in Amplifier? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2
1.3 Key Specifications and Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3
Chapter Two, Installation & Initial Checks
2.1 Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1
2.1.01 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1
2.1.02 Rack Mounting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1
2.1.03 Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1
2.1.04 Line Cord Plug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1
2.1.05 Line Voltage Selection and Line Fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1
2.2 Initial Checks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3
2.2.01 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3
2.2.02 Procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3
Chapter Three, Technical Description
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
3.2 Principles of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
3.2.01 Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
3.2.02 Signal-Channel Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2
3.2.03 Line Frequency Rejection Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3
3.2.04 AC Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4
3.2.05 Anti-Aliasing Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4
3.2.06 Main Analog to Digital Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5
3.2.07 Reference Channel DSP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6
3.2.08 Internal Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7
3.2.09 Demodulator DSP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7
3.2.10 Output Processor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8
3.2.11 Main Microprocessor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8
3.3 Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8
3.3.01 Absolute Accuracy Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8
i
TABLE OF CONTENTS
3.3.02 Relative Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9
3.4 Full-Scale Sensitivity and AC Gain Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9
3.5 Dynamic Reserve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9
3.6 System Updates and Reference Frequency Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10
3.7 Reference Phase and Phase Shifter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10
3.8 Output Channel Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11
3.8.01 Slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11
3.8.02 Time Constants and Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12
3.8.03 Output Offset and Expand. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12
3.9 Use of Magnitude and Signal Phase Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12
3.10 Noise Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14
3.11 Power-up Defaults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14
3.12 Auto Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14
3.12.01 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14
3.12.02 Auto-Sensitivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15
3.12.03 Auto-Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15
3.12.04 Auto-Offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15
3.12.05 Auto-Measure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16
3.12.06 Default Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16
Chapter Four, Front and Rear Panels
4.1 Front Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1
4.1.01 A and B/I Signal Input Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1
4.1.02 OSC OUT Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1
4.1.03 REF IN Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2
4.1.04 Left-hand LCD Display Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2
4.1.05 MENU Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-4
4.1.06 90º Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5
4.1.07 SET Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5
4.1.08 Right-hand LCD Display Panel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5
4.2 Rear Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6
4.2.01 Line Power Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6
4.2.02 Line Power Input Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6
4.2.03 RS232 Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7
4.2.04 AUX RS232 Connector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7
4.2.05 GPIB Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7
4.2.06 DIGITAL OUTPUTS Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7
i i
TABLE OF CONTENTS
4.2.07 PREAMP POWER Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7
4.2.08 REF MON Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7
4.2.09 REF TTL Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7
4.2.10 SIG MON Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7
4.2.11 CH1, CH2 Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8
4.2.12 TRIG Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8
4.2.13 ADC1, ADC2 Connectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8
4.2.14 DAC1, DAC2 Connectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8
4.2.15 FAST X, FAST Y Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8
Chapter Five, Front Panel Operation
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1
5.2 Setup Menu Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1
5.2.01 Input Setup Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2
5.2.02 Reference Setup Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4
5.2.03 Output Setup Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5
5.2.04 Control Options Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-8
5.2.05 Miscellaneous Options Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10
5.2.06 RS232 Setup 1 Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11
5.2.07 RS232 Setup 2 Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12
5.2.08 RS232 Setup 3 Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-13
5.2.09 GPIB Setup 1 Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-14
5.2.10 GPIB Setup 2 Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-15
5.2.11 Digital Outputs Setup Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-16
5.2.12 Control Setup Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-17
5.3 Auto Functions Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-18
5.4 Main Display Mode - Left-hand LCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-20
5.5 Main Display Mode - Right-hand LCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-26
5.6 Typical Lock-in Amplifier Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-35
Chapter Six, Computer Operation
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
6.2 Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
6.2.01 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
6.2.02 Curve Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
6.2.03 Burst Mode Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
6.2.04 Internal Oscillator Frequency Sweep Generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2
iii
TABLE OF CONTENTS
6.3 RS232 and GPIB Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2
6.3.01 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2
6.3.02 RS232 Interface - General Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2
6.3.03 Choice of Baud Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3
6.3.04 Choice of Number of Data Bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3
6.3.05 Choice of Parity Check Option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3
6.3.06 Auxiliary RS232 Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4
6.3.07 GPIB Interface - General Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4
6.3.08 Handshaking and Echoes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5
6.3.09 Terminators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6
6.3.10 Command Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6
6.3.11 Delimiters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7
6.3.12 Compound Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7
6.3.13 Status Byte, Prompts and Overload Byte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7
6.3.14 Service Requests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9
6.4 Command Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10
6.4.01 Signal Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10
6.4.02 Reference Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13
6.4.03 Signal Channel Output Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14
6.4.04 Signal Channel Output Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-15
6.4.05 Instrument Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16
6.4.06 Internal Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-19
6.4.07 Auxiliary Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-20
6.4.08 Auxiliary Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-21
6.4.09 Output Data Curve Buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-23
6.4.10 Computer Interfaces (RS232 and GPIB). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-27
6.4.11 Instrument Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-29
6.4.12 Front Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-30
6.4.13 Default Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-30
6.5 Programming Examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-30
6.5.01 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-30
6.5.02 Basic Signal Recovery. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-30
6.5.03 Frequency Response Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-31
6.5.04 X and Y Output Curve Storage Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-32
6.5.05 Transient Recorder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-32
6.5.06 Frequency Response Measurement using Curve Storage and Frequency Sweep . . . . . . . . . . . . . 6-33
Appendix A, Specifications
i v
TABLE OF CONTENTS
Appendix B,Pinouts
B.1 RS232 Connector Pinout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1
B.2 Preamplifier Power Connector Pinout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1
B.3 Digital Output Port Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-2
Appendix C, DemonstrationPrograms
C.1 Simple Terminal Emulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1
C.2 RS232 Control Program with Handshakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1
C.3 GPIB User Interface Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-3
Appendix D, Cable Diagrams
D.1 RS232 Cable Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-1
Appendix E, Alphabetical Listing ofCommands
Appendix F, Default Settings
Default Setting Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F-1
Index
WARRANTY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . End of Manual
v
TABLE OF CONTENTS
v i
Introduction
Chapter 1
1.1 How to Use This Manual
This manual gives detailed instructions for setting up and operating the EG&G
Instruments Model 7220 Digital Signal Processing (DSP) dual phase lock-in
amplifier. It is split into the following chapters:-
Chapter 1 - Introduction
Provides an introduction to the manual, briefly describes what a lock-in amplifier is
and the types of measurements it may be used for, and lists the major specifications
of the model 7220.
Chapter 2 - Installation and Initial Checks
Describes how to install the instrument and gives a simple test procedure which you
may perform to check that the unit has arrived in full working order.
Chapter 3 - Technical Description
Provides an outline description of the design of the instrument and discusses the effect
of the various controls. A good understanding of the design will enable you to get the
best possible performance from the unit.
Chapter 4 - Front and Rear Panels
Describes the connectors, controls and indicators which are to be found on the unit
and which are referred to in the subsequent chapters.
Chapter 5 - Front Panel Operation
Describes the capabilities of the instrument when used as a manually operated unit,
and shows how to operate it using the front panel controls.
Chapter 6 - Remote Operation
This chapter provides detailed information on operating the instrument from a
computer over either the GPIB (IEEE-488) or RS232 interfaces. It includes
information on how to establish communications, the functions available, the
command syntax and a detailed command listing.
Appendix A
Gives the detailed specifications of the unit.
Appendix B
Details the pinouts of the multi-way connectors on the rear panel.
Appendix C
Lists three simple terminal programs which may be used as the basis for more
complex user-written programs.
1-1
Chapter 1, INTRODUCTION
Appendix D
Shows the connection diagrams for suitable RS232 null-modem cables to couple the
unit to an IBM-PC or 100 % compatible computer.
Appendix E
Gives an alphabetical listing of the computer commands for easy reference.
Appendix F
Provides a listing of the instrument settings produced by using the default setting
function.
If you are a new user, it is suggested that you unpack the instrument and carry out the
procedure in chapter 2 to check that it is working satisfactorily. You should then make
yourself familiar with the information in chapters 3, 4 and 5, even if you intend that
the unit will eventually be used under computer control. Only when you are fully
conversant with operation from the front panel should you then turn to chapter 6 for
information on how to use the instrument remotely. Once you are familiar with the
structure of the computer commands, appendix E will prove to be convenient as it
provides a complete alphabetical listing of these commands in a single easy-to-use
section.
1.2 What is a Lock-in Amplifier?
In its most basic form the lock-in amplifier is an instrument with dual capability. On
the one hand it can recover signals in the presence of an overwhelming noise
background or alternatively it can provide high resolution measurements of relatively
clean signals over several orders of magnitude and frequency.
Modern instruments, such as the model 7220, offer far more than these two basic
characteristics and it is this increased capability which has led to their acceptance in
many fields of scientific research, such as optics, electrochemistry, materials science,
fundamental physics and electrical engineering, as units which can provide the
optimum solution to a large range of measurement problems.
The model 7220 lock-in amplifier can function as a:-
n AC Signal Recovery Instrument n Transient Recorder
n Vector Voltmeter
n Phase Meter
n Precision Oscillator
n Frequency Meter
n Spectrum Analyzer
n Noise Measurement Unit
These characteristics, all available in a single compact unit, make it an invaluable
addition to any laboratory.
1-2
Chapter 1, INTRODUCTION
1.3 Key Specifications and Benefits
The EG&G Instruments Model 7220 represents the latest in DSP Lock-in Amplifier
technology at an affordable price, and offers:-
n Frequency range:
0.001 Hz to 120 kHz
20 nV to 1 V full-scale
n Voltage sensitivity:
n Current input mode sensitivities:
20 fA to 1 µA full-scale
20 fA to 10 nA full-scale
n Line frequency rejection filter
n Dual phase demodulator with X-Y and R-θ outputs
n Very low phase noise of < 0.0001o rms
n 5-digit output readings
n Direct Digital Synthesizer (DDS) oscillator with variable output amplitude
and frequency
n Oscillator frequency sweep generator
n Output time constant: 10 µs to 5 ks
n 8-bit programmable digital output port for system control
n Two external ADCs, two external DACs
n Full range of auto-modes
n Standard IEEE-488 and RS232 interfaces with RS232 daisy-chain
capability
n Dual back-lit liquid crystal display (LCD) with variable contrast control
n 32768 point curve storage buffer
1-3
Chapter 1, INTRODUCTION
1-4
Installation &
Initial Checks
Chapter 2
2.1 Installation
2.1.01 Introduction
Installation of the model 7220 in the laboratory or on the production line is very
simple. Because of its low power consumption, the model 7220 does not incorporate
forced-air ventilation. It can be operated on almost any laboratory bench or be rack
mounted, using the optional accessory kit, at the user’s convenience. With an ambient
operating temperature range of 0 oC to 35 oC, it is highly tolerant to environmental
variables, needing only to be protected from exposure to corrosive agents and liquids.
2.1.02 Rack Mounting
An optional accessory kit, part number K02002, is available from EG&G
Instruments to allow the model 7220 to be mounted in a standard 19-inch rack.
2.1.03 Inspection
Upon receipt the model 7220 Lock-in Amplifier should be inspected for shipping
damage. If any is noted, EG&G INSTRUMENTS should be notified immediately
and a claim filed with the carrier. The shipping container should be saved for
inspection by the carrier.
2.1.04 Line Cord Plug
A standard IEC 320 socket is mounted on the rear panel of the instrument and a
suitable line cord is supplied.
2.1.05 Line Voltage Selection and Line Fuses
Before plugging in the line cord, ensure that the model 7220 is set to the voltage of
the AC power supply to be used.
A detailed discussion of how to check and, if necessary, change the line voltage
setting follows.
CAUTION! The model 7220 may be damaged if the line voltage is set for 110 V
AC operation and it is turned on with 220 V AC applied to the power input
connector.
The model 7220 can operate from any one of four different line voltage ranges,
90-110 V, 110-130 V, 200-240 V, and 220-260 V, at 50-60 Hz. The change from one
range to another is made by repositioning a plug-in barrel selector internal to the Line
Input Assembly on the rear panel of the unit.
2-1
Chapter 2, INSTALLATION AND INITIAL CHECKS
Instruments are normally shipped from the factory with the line voltage selector set to
110-130 V AC, unless they are destined for an area known to use a line voltage in the
220-260 V range, in which case, they are shipped configured for operation from the
higher range.
The line voltage setting can be seen through a small rectangular window in the line
input assembly on the rear panel of the instrument (figure 2-1). If the number
showing is incorrect for the prevailing line voltage (refer to table 2-1), the barrel
selector will need to be repositioned as follows.
Observing the instrument from the rear, note the plastic door immediately adjacent to
the line cord connector (figure 2-1) on the left-hand side of the instrument. When the
line cord is removed from the rear panel connector, the plastic door can be opened
outwards by placing a small, flat-bladed screwdriver in the slot on the right-hand side
and levering gently. This gives access to the fuse and to the voltage barrel selector,
which is located at the right-hand edge of the fuse compartment. Remove the barrel
selector with the aid of a small screwdriver or similar tool. With the barrel selector
removed, four numbers become visible on it: 100, 120, 220, and 240, only one of
which is visible when the door is closed. Table 2-1 indicates the actual line voltage
range represented by each number. Position the barrel selector such that the required
number (see table 2-1) will be visible when the barrel selector is inserted and the door
closed.
Figure 2-1, Line Input Assembly
VISIBLE #
VOLTAGE RANGE
100
120
220
240
90 - 110 V
110 - 130 V
200 - 240 V
220 - 260 V
Table 2-I, Range vs Barrel Position
Next check the fuse rating. For operation from a nominal line voltage of 100 V or
120 V, use a 20 mm slow-blow fuse rated at 1.0 A, 250 V. For operation from a
nominal line voltage of 220 V or 240 V, use a 20 mm slow-blow fuse rated at 0.5 A,
250 V.
To change the fuse, first remove the fuse holder by pulling the plastic tab marked with
an arrow. Remove the fuse and replace with a slow-blow fuse of the correct voltage
and current rating. Install the fuse holder by sliding it into place, making sure the
arrow on the plastic tab is pointing downwards. When the proper fuse has been
2-2
Chapter 2, INSTALLATION AND INITIAL CHECKS
installed, close the plastic door firmly. The correct selected voltage setting should now
be showing through the rectangular window. Ensure that only fuses with the required
current rating and of the specified type are used for replacement. The use of
makeshift fuses and the short-circuiting of fuse holders is prohibited and potentially
dangerous.
2.2 Initial Checks
2.2.01 Introduction
The following procedure checks the performance of the model 7220. In general, this
procedure should be carried out after inspecting the instrument for obvious shipping
damage (NOTE: any damage must be reported to the carrier and to EG&G
INSTRUMENTS immediately; take care to save the shipping container for
inspection by the carrier).
Note that this procedure is intended to demonstrate that the instrument has arrived in
good working order, not that it meets specifications. Each instrument receives a
careful and thorough checkout before leaving the factory, and normally, if no shipping
damage has occurred, will perform within the limits of the quoted specifications. If
any problems are encountered in carrying out these checks, contact EG&G
INSTRUMENTS or the nearest authorized representative for assistance.
2.2.02 Procedure
1) Ensure that the model 7220 is set to the line voltage of the power source to be
used, as described in section 2.1.05
2) With the rear panel mounted power switch (located at the extreme left-hand side
of the instrument when viewed from the rear) set to 0 (off), plug in the line cord
to an appropriate line source.
3) Turn the model 7220 power switch to the I (on) position.
4) The instrument’s front panel displays will now briefly display the following
message:-
Figure 2-2, Opening Display
5) Press the key marked MENU twice to enter the setup menu screens. (N.B. on
early units this key was marked AUTO).
6) Press one of the keys on the left-hand side of the left-hand display repeatedly until
2-3
Chapter 2, INSTALLATION AND INITIAL CHECKS
the CONTROL SETUP menu is displayed, which will look similar to the
following:-
Figure 2-3, Control Setup Menu
7) Press one of the keys on the right-hand side of the left-hand display once. This
will set all the instrument’s controls and displays to a known state. The displays
will revert to the normal mode, with the left-hand panel showing the AC Gain and
Full Scale Sensitivity controls and the right-hand one the instrument’s outputs in
the form of magnitude as a percentage of full-scale and phase in degrees.
8) The right-hand display should now look as follows:-
Figure 2-4, Right-hand LCD - Main Display
9) Connect a BNC cable between the OSC OUT and A input connectors on the
front panel.
10) The right-hand display should now indicate a magnitude close to 100 % of full-
scale (i.e. the sinusoidal oscillator output, which was set to 1 kHz and a signal
level of 0.5 V rms by the Default Setting key is being measured with a full-scale
sensitivity of 500 mV rms) and a phase of near zero degrees, if a short cable is
used.
This completes the initial checks. Even though the procedure leaves many functions
untested, if the indicated results were obtained the user can be reasonably sure that
the unit incurred no hidden damage in shipment and is in good working order.
2-4
Technical Description
Chapter 3
3.1 Introduction
The model 7220 lock-in amplifier is capable of outstandingly good signal recovery
performance, provided that it is operated correctly. This chapter describes the design
of the instrument, enabling the best use to be made of its facilities. Of particular
importance is the correct adjustment of the AC Gain parameter, described in section
3.2.04.
3.2 Principles of Operation
3.2.01 Block Diagram
The model 7220 utilizes two digital signal processors, a microprocessor and a
dedicated digital waveform synthesizer, together with very low-noise analog
circuitry to achieve its specifications. A block diagram of the instrument is shown in
figure 3-1, and the sections that follow describe how each functional block operates
and the effect it has on the instrument’s performance.
Figure 3-1, Model 7220 - Block Diagram
3-1
Chapter 3, TECHNICAL DESCRIPTION
3.2.02 Signal-Channel Inputs
The signal input amplifier may be configured for either single-ended or differential
voltage mode operation, or single-ended current mode operation. In voltage mode a
choice of AC or DC coupling is available and the input may be switched between
FET and bipolar devices. In current mode two conversion gains are selectable to
allow for optimum matching to the signal input. In both modes the input connector
“shells” may be either floated or grounded to the instrument’s chassis ground. These
various features are discussed in the following paragraphs.
Input Connector Selection, A / (A - B)
When set to the A mode, the lock-in amplifier measures the voltage between the
centre and the outer of the A input BNC connector, whereas when set to (A-B) mode
it measures the difference in voltage between the centre pins of theA and B input
connectors.
The latter, differential, mode is often used to eliminate ground loops, although it is
worth noting that at very low signal levels it may be possible to make a substantial
reduction in unwanted offsets by using this mode, with a short-circuit terminator on
the B connector, rather than by simply using the A input mode.
The specification defined as the Common Mode Rejection Ratio, CMRR, defines how
well the instrument rejects common mode signals applied to the A and B inputs when
operating in differential input mode. It is usually given in decibels. Hence a
specification of > 100 dB implies that a common mode signal (i.e. a signal
simultaneously applied to both A and B inputs) of 1 V will give rise to less than
10 µV of signal out of the input amplifier.
Input Connector Shell Ground / Float
The input connector shells may be connected either directly to the instrument’s
chassis ground or they can be “floated” by being connected via a 1 kΩ resistor. When
in the float mode, the presence of this resistor substantially reduces the problems
which often occur in low-level lock-in amplifier measurements due to ground loops.
Input Device Selection, FET / Bipolar
The voltage preamplifier may be switched between bipolar and FET input devices.
The bipolar device, which has an input impedance of 10 kΩ, shows a relatively high
level of added current noise (2 pA/√Hz), but less than 50 percent of the voltage noise
of the FET device. As such, it is intended for use where the source impedance is
resistive or inductive with a resistance of 100 Ω or less, and there is no input voltage
offset.
WARNING: Signal channel overload may occur if the bipolar device is selected
and no DC bias path is provided.
The FET device provides an input impedance of 10 MΩ.
AC / DC Coupling
In normal operation, with reference frequencies above a few hertz, AC coupled
operation is always used.
3-2
Chapter 3, TECHNICAL DESCRIPTION
The primary purpose of the DC coupling facility is to enable the use of the instrument
at reference frequencies below 0.5 Hz. It may also be used to reduce the effect of
phase and magnitude errors introduced by the AC coupling circuitry below a few
hertz.
However, the use of DC coupling introduces serious problems where the source has a
DC offset or is of such high impedance that bias currents cause significant offsets. In
these cases it may be necessary to include some form of signal conditioning between
the signal source and the lock-in amplifier.
The instrument always reverts to the AC coupled mode on power-up, to protect the
input circuitry.
Input Signal Selection, V / I
Although the voltage mode input is most commonly used, a current to voltage
converter may be switched into use to provide current mode input capability, in
which case the signal is connected to theB/I connector. High impedance sources
(> 100 kΩ) are inherently current sources and need to be measured with a low
impedance current mode input. Even when dealing with a voltage source in series
with a high impedance, the use of the current mode input may provide advantages in
terms of improved bandwidth and immunity from the effects of cable capacitance.
The converter may be set to low-noise or wide bandwidth conversion settings, but it
should be noted that even at the wide bandwidth setting the -3 dB point is at 50 kHz.
Better performance may be achieved using a separate current preamplifier, such as
the EG&G Instruments Model 5182.
3.2.03 Line Frequency Rejection Filter
Following the signal input amplifier, there is an option to pass the signal through a
line frequency rejection filter, which is designed to give greater than 40 dB of
attenuation at the power line frequencies of 50 Hz or 60 Hz and their second
harmonics at 100 Hz and 120 Hz.
Early instruments use a simple single stage band-rejection filter, which has a
relatively broad bandwidth. This introduces significant gain and phase errors, at least
in the range 5 to 500 Hz, and this should be taken into account if it is used in
conjunction with reference frequencies in or near to this range. The filter control
settings for these units are simply “ON” or “OFF”.
Instruments manufactured after June 1996 use a more sophisticated type of filter,
which uses two cascaded rejection stages with “notch” characteristics. This allows
the filter to be set to reject signals at frequencies equal to either of, or both of, the
fundamental and second harmonic of the line frequency. Hence the filter control
settings for these instruments are “OFF”, “F”, “2F” or “F & 2F”.
Although instruments are supplied with the line frequency filter set to match the line
frequency of the country for which they are destined, it should be appreciated that if
a unit is moved from a 50 Hz area to a 60 Hz area then the filter will need to be
adjusted. The later instruments therefore respond to a computer command, LINE50,
which allows this to be done (see section 6.4).
3-3
Chapter 3, TECHNICAL DESCRIPTION
3.2.04 AC Gain
The signal channel contains a number of analog filters and amplifiers, the gain of
which are defined by the “AC Gain” parameter, which is specified in terms of
decibels (dB). For each value of AC Gain there is a corresponding value of the
INPUT LIMIT parameter, which is the maximum instantaneous (peak) voltage or
current that can be applied to the input without input overload, as shown in table 3-1
below.
It is a basic property of the DSP lock-in amplifier that the best demodulator
performance is obtained by presenting as large a signal as possible to the main
analog to digital converter. Therefore, in principle, the AC Gain value should be made
as large as possible without causing amplifier or converter overload. This constraint
is not too critical however and the use of a value 10 or 20 dB below the optimum
value makes little difference.
Note that when signal overload occurs, the only action required is to reduce the
AC Gain value.
AC Gain (dB)
INPUT LIMIT (mV)
0
10
20
30
40
50
60
70
80
90
3000
1000
300
10
30
10
3
1
0.3
0.1
Table 3-1, Input Limit vs AC Gain
Further information on the control of AC Gain is given in section 3.4
3.2.05 Anti-Aliasing Filter
Prior to the main analog to digital converter (ADC) the signal passes through an anti-
aliasing filter to remove unwanted frequencies which would cause a spurious output
from the ADC due to the nature of the sampling process.
Consider the situation when the lock-in amplifier is measuring a sinusoidal signal of
frequency fsignal Hz, which is sampled by the main ADC at a sampling frequency
fsampling Hz. In order to ensure correct operation of the instrument the output values
representing the fsignal frequency must have been uniquely generated by the signal to
be measured, and not by any other process.
However, if the input to the ADC has, in addition, an unwanted analog sinusoid with
frequency f1 Hz, where f1 is greater than half the sampling frequency, then this will
appear in the output as a sampled-data sinusoid with frequency less than half the
sampling frequency, falias = |f1 - nfsampling|, where n is an integer.
3-4
Chapter 3, TECHNICAL DESCRIPTION
This alias signal is indistinguishable from the output generated when a genuine signal
at frequency falias is sampled. Hence if the frequency of the unwanted signal were
such that the alias signal frequency produced from it was close to, or equal to, that of
the wanted signal then it is clear that a spurious output would result.
For example, if the sampling frequency were 160 kHz then half the sampling
frequency would be 80 kHz. Let the instrument be measuring a signal of 55 kHz
accompanied by an interfering signal of 100 kHz. The output of the ADC would
therefore include a sampled-data sinusoid of 55 kHz (the required signal) and,
applying the above formula, an alias signal of 60 kHz (i.e. |100 kHz - 160 kHz|). If
the signal frequency were now increased towards 60 kHz then the output of the lock-
in amplifier would increasingly be affected by the presence of the alias signal and the
accuracy of the measurement would deteriorate.
To overcome this problem the signal is fed through the anti-aliasing filter, which
restricts the signal bandwidth. When operating at reference frequencies below
60 kHz, the reference frequency is less than half the sampling frequency and a
conventional elliptic-type, low-pass anti-alias filter is used. This enables the system
to provide the lowest possible noise bandwidth. At frequencies above 60 kHz an
adaptive bandpass anti-alias filter is used. The noise bandwidth of this filter is
dependent on the reference frequency and is higher than that of the conventional
type of filter, but typically the noise penalty is negligible.
It should be noted that the dynamic range of a lock-in amplifier is normally so high
that practical anti-alias filters are not capable of completely removing the effect of a
full-scale alias. For instance, even if the filter gives 100 dB attenuation, an alias at
the input limit and at the reference frequency will give a one percent output error
when the dynamic reserve is set to 60 dB, or a full-scale error when the dynamic
reserve is set to 100 dB.
In a typical low-level signal recovery situation, many unwanted inputs need to be
dealt with and it is normal practice to make small adjustments to the reference
frequency until a clear point on the frequency spectrum is reached. In this context an
unwanted alias is treated as just another interfering signal and its frequency is
avoided when setting the reference frequency.
A buffered version of the analog signal just prior to the main ADC is available at the
rear panel signal monitor (SIG MON) connector; it may be viewed on an
oscilloscope to monitor the effect of the signal channel filters and amplifiers.
3.2.06 Main Analog to Digital Converter
Following the anti-alias filter the signal passes to the main 18-bit analog to digital
converter running at a sampling rate of 166 kHz. This rate is not fixed but is adjusted
automatically by up to ±1 %, as a function of the reference frequency, to ensure that
the sampling process does not generate a “beat” frequency close to zero hertz. For
example, if the reference frequency were 82.95 kHz and the sampling frequency were
not adjusted, a beat frequency of 50 Hz (|82.95 kHz - (166 kHz/2)|) would be
generated and would appear at the output if the time constant were not set to a large
enough value.
3-5
Chapter 3, TECHNICAL DESCRIPTION
There is one situation where this automatic correction might not be sufficient to give
good performance. Consider the case where the signal being measured is at 73 kHz,
which is 10 kHz away from half the sampling frequency. If there were also a strong
interfering signal at 93 kHz (i.e. 166 kHz/2 + 10 kHz), then an alias of this would
give rise to a spurious output. Note that under these circumstances, the reference
frequency is not sufficiently close to half the sampling frequency to cause the latter to
be automatically adjusted. The problem is overcome by providing the Sample Rate
control which allows the user to adjust the main ADC sampling rate in steps of about
1 %. A 1 % change moves the alias by about 1 kHz, which is normally sufficient to
ensure rejection by the output low-pass filters and thereby remove any error.
The output from the converter feeds the first of the digital signal processors, which
implements the digital multiplier and the first stage of the output low-pass filtering
for each of the X and Y channels.
3.2.07 Reference Channel DSP
The second DSP in the instrument is responsible for implementing the reference
trigger/phase-locked loop, digital phase shifter and internal oscillator look-up table
functional blocks on the block diagram. The processor generates two main outputs,
the first being a series of phase values which are used to drive the other DSP’s
reference channel input and the second being a sinusoidal signal which may be used
as the instrument’s internal oscillator output.
The normal operating mode of the instrument incorporates two reference frequency
ranges, namely the baseband from 1 mHz to 60 kHz and the highband from 60 kHz to
120 kHz. Different hardware configurations are used in the two bands, transitions
between which are made automatically according to the value of the reference
frequency. These transitions are generally transparent to the user.
External Reference Mode
In external reference mode at frequencies above 300 mHz, the reference source may
be applied to either a general purpose input, designed to accept virtually any periodic
waveform with a 50:50 mark-space ratio and of suitable amplitude, or to a TTL-logic
level input. At frequencies below 300 mHz the TTL-logic level input must be used.
Following the trigger buffering circuitry the reference signal is passed to a digital
phase-locked loop (PLL) implemented in the reference DSP. This measures the
period of the applied reference waveform and from this generates the phase values.
Internal Reference Mode
With internal reference operation in the baseband mode (i.e. at reference frequencies
< 60 kHz), the reference processor is free-running at the selected reference frequency
and is not dependent on a phase-locked loop (PLL), as is the case in most other lock-
in amplifiers. Consequently, the phase noise is extremely low, and because no time is
required for a PLL to acquire lock, reference acquisition is immediate. See appendix
A for numerical values of phase noise.
In the internal reference highband mode (i.e. reference frequencies > 60 kHz), the
instrument essentially operates as if in external mode, except that the reference
trigger input is now provided by an internal link from the output of the direct digital
synthesizer.
3-6
Chapter 3, TECHNICAL DESCRIPTION
3.2.08 Internal Oscillator
The model 7220, in common with many other lock-in amplifiers, incorporates an
internal oscillator which may be used to drive an experiment. However, unlike most
other instruments, the oscillator in the model 7220 is digitally synthesized with the
result that the output frequency is extremely accurate and stable. The oscillator
operates over the same frequency range as the lock-in amplifier, 1 mHz to 120 kHz.
The source of the oscillator depends on whether the instrument is operating on
internal or external reference mode and on the selected frequency.
In internal reference baseband mode (< 60 kHz) the oscillator is derived from the
reference channel DSP. This outputs a series of digital values, corresponding to a
sinusoid at the required frequency, to a 16-bit DAC which in turn feeds a variable
attenuator. The output of the attenuator is the internal oscillator output.
In internal reference highband mode (> 60 kHz) and external reference mode, the
oscillator is derived from a dedicated direct digital synthesizer (DDS).
A further choice of output at the OSC OUT connector is offered when the unit is
operating in external reference mode. In this situation, if the synchronous oscillator
(also called the demodulator monitor) control is turned on, then the OSC OUT signal
becomes a direct analog representation of the sinusoidal signal at the reference input
to the X channel phase sensitive detector. Consequently it is affected by both the
reference phase shifter and harmonic controls of the reference channel.
For example, if an external reference at 1 kHz were applied, the unit were set to
operate in the 2F mode and the synchronous oscillator were turned on, then the signal
at the OSC OUT connector would be a 2 kHz sinusoid whose phase could be
adjusted using the reference phase shifter.
When used in the synchronous oscillator (demodulator monitor) mode, OSC OUT is
updated at the rate at which the reference channel generates new values for the
demodulators. Since this occurs approximately once every 6 µs, this should be taken
into account when viewing the waveform on an oscilloscope.
3.2.09 Demodulator DSP
The essential operation of the demodulator DSP is to multiply the digitized output of
the signal channel by data sequences called the X and Y demodulation functions and
to operate on the results with digital low-pass filters (the output filters). The
demodulation functions, which are derived by use of a look-up table from the phase
values supplied by the reference channel DSP, are sinusoids with frequency equal to
an integer multiple, nfref, of the reference frequency fref. The Y demodulation
function is the X demodulation function delayed by a quarter of a period. The integer
n is called the reference harmonic number and in normal lock-in amplifier operation is
set to unity. Throughout the remainder of this text, the reference harmonic number
will be assumed to be unity unless specifically stated to have a non-unity value.
The outputs from the X channel and Y channel multipliers feed the first stage of the X
3-7
Chapter 3, TECHNICAL DESCRIPTION
and Y channel output filters. The outputs of these in turn drive two 16-bit digital to
analog converters (DACs) which generate the instrument’s FAST X and FAST Y
analog outputs. In addition, the signals are fed to further low-pass filters before
subsequent processing by the instrument’s host microprocessor.
3.2.10 Output Processor
Although shown on the block diagram as a separate entity, the output processor is in
fact part of the instrument’s main microprocessor. It provides more digital filtering of
the X and Y channel signals if required, calculates the vector magnitude, R, where
R = √(X2 + Y2) and phase angle, θ, where θ = tan-1(Y/X), and routes any of these
signals via two further 16-bit DACs to the unit’s CH1 and CH2 output connectors. It
also allows one of the two auxiliary analog inputs, ADC1 and ADC2, which are
digitized by a 16-bit analog to digital converter, to be used in ratio calculations.
3.2.11 Main Microprocessor
All functions of the instrument are under the control of a microprocessor which in
addition drives the front panel displays, processes front panel key operations and
supports the RS232 and GPIB (IEEE-488) computer interfaces. This processor also
drives the instrument’s 8-bit digital programmable output port, which may be used
for controlling auxiliary apparatus.
The microprocessor has access to memory which may be used for storage of the
instrument’s outputs as curves prior to transferring them to a computer via the
computer interfaces. In addition to using this function for the normal outputs, such as
the X and Y output signals, it may also be used with the auxiliary ADC inputs to
allow the instrument to operate as a transient recorder. The internal oscillator
frequency sweep function is also controlled by the microprocessor.
A particularly useful feature of the design is that only part of the controlling
firmware program code, which the microprocessor runs, is permanently resident in
the instrument. The remainder is held in flash EEPROM and can be updated via the
RS232 computer interface. It is therefore possible to change the functionality of the
instrument, perhaps to include a new feature or update the computer command set,
simply by connecting it to a computer and running an Update program.
3.3 Accuracy
3.3.01 Absolute Accuracy Specifications
When the demodulator is operating under correct conditions, the absolute gain
accuracy of the instrument is limited by the analog components in the signal channel,
and the absolute phase accuracy is limited by the analog components in both the
signal channel and the reference channel. The resulting typical accuracy is ±0.5
percent of the full-scale sensitivity and ±0.5 degree. When the higher values of AC
Gain are in use, the errors tend to increase in the upper part of the frequency range
(above 25 kHz).
3-8
Chapter 3, TECHNICAL DESCRIPTION
3.3.02 Relative Accuracy
The majority of lock-in amplifier measurements are concerned with the variation of
the input signal with time, temperature, etc. or with the comparison of two different
specimens. In these cases the absolute accuracy is of less importance than the
accuracy with which readings can be transferred from range to range.
A new feature of the model 7220 is the introduction of a separate control function
(“AC Gain”) for the gain of the signal channel. Where appropriate, this can be set to
accommodate the existing noise level and subsequent changes in the instrument’s full-
scale sensitivity do not cause any of the errors which might arise from a change in the
analog gain.
3.4 Full-Scale Sensitivity and AC Gain Control
The full-scale sensitivity is indicated as SEN on the left-hand LCD and is adjusted by
the use of the adjacent keys. The analog outputs and analog meter limit at a level a
few percent above the full-scale sensitivity value, but the digital displays do not limit
until a level of ±300 percent full-scale has been reached.
As stated in section 3.2.04, the best performance is obtained by making the AC Gain
value as large as possible without causing amplifier overload.
Note that the demodulator gain is adjusted automatically when the AC Gain value is
changed, in order to maintain the SEN value. However, the user is prevented from
setting an illegal AC Gain value, i.e. one that would result in overload on a full-scale
input signal. Similarly, if the user selects a SEN value which causes the present AC
Gain value to be illegal, the AC Gain will change to the nearest legal value.
In practice, this system is very easy to operate. However, the user may prefer to
make use of the AUTOMATIC AC Gain facility which gives very good results in
most circumstances.
Note that when signal channel overload occurs, the only action required is to reduce
the AC Gain.
At reference frequencies above 1 Hz, the Auto-Sensitivity and Auto-Measure
functions can be used to adjust the full-scale sensitivity.
3.5 Dynamic Reserve
At any given setting, the ratio
DR = 0.7 (INPUT LIMIT) / (FULL-SCALE SENSITIVITY)
represents the factor by which the largest acceptable sinusoidal interference input
exceeds the full-scale sensitivity and is called the Dynamic Reserve of the lock-in
amplifier at that setting. (The factor 0.7 is a peak to rms conversion). The dynamic
3-9
Chapter 3, TECHNICAL DESCRIPTION
reserve is often expressed in decibels, for which
DR( in dB) = 20 log(DR( as a ratio))
Applying this formula to the model 7220 we may put in the maximum value of
INPUT LIMIT (3 V) and the smallest available value of FULL-SCALE
SENSITIVITY (20 nV) to reach a value of about 1E8 or 160 dB for the maximum
available dynamic reserve. Figures of this magnitude are available from any DSP
lock-in amplifier but are based only on arithmetical identities and do not give any
indication of how the instrument actually performs. In fact, all current DSP lock-in
amplifiers become too noisy and inaccurate for most purposes at reserves of greater
than about 100 dB.
For the benefit of users who prefer to have the AC Gain value expressed in this form,
the model 7220 displays the current value of Dynamic Reserve in decibels on the
input full-scale sensitivity control.
3.6 System Updates and Reference Frequency Changes
Both the signal channel and the reference channel contain calibration parameters
which are dependent on the reference frequency. These include corrections to the
anti-alias filter and to the analog circuits in the reference channel.
In external reference operation the processor uses the reference frequency meter to
monitor the reference frequency and updates these parameters when a change of
about 2 percent has been detected.
All the parameters are also updated when the SET key is pressed or the LOCK
command is executed. Therefore if the most accurate and reproducible settings are
required, the SET key should be pressed or the LOCK command executed after
every intentional change in reference frequency, when in the external reference
modes. Note that sufficient time must be allowed for the frequency meter to give a
fully accurate value.
With internal reference operation, regardless of the frequency mode, the frequency-
dependent parameters are updated on any change of reference frequency, without the
need to press the front panel SET key or to issue the LOCK command.
3.7 Reference Phase and Phase Shifter
If the reference input is a sinusoid applied to the REF IN socket, the reference phase
is defined as the phase of the X demodulation function with respect to the reference
input.
This means that when the reference phase is zero and the signal input to the
demodulator is a full-scale sinusoid in phase with the reference input sinusoid, the X
output of the demodulator is a full-scale positive value and the Y output is zero.
3-10
Chapter 3, TECHNICAL DESCRIPTION
The circuits connected to the REF IN socket actually detect a positive-going crossing
of the mean value of the applied reference voltage. Therefore when the reference input
is not sinusoidal, its effective phase is the phase of a sinusoid with positive-going zero
crossing at the same point in time, and accordingly the reference phase is defined with
respect to this waveform. Similarly, the effective phase of a reference input to the
TTL REF IN socket is that of a sinusoid with positive-going zero crossing at the
same point in time.
The reference phase is adjusted to its required value by the use of a digital phase
shifter, which is accessed from the front panel, by the REFP computer command or
with the use of the Auto-Phase function.
In basic lock-in amplifier applications the purpose of the experiment is to measure
the amplitude of a signal which is of fixed frequency and whose phase with respect
to the reference input does not vary. This is the scalar measurement, often
implemented with a chopped optical beam. Many other lock-in amplifier
applications are of the signed scalar type, in which the purpose of the experiment is
to measure the amplitude and sign of a signal which is of fixed frequency and whose
phase with respect to the reference input does not vary apart from reversals of phase
corresponding to changes in the sign of the signal. A well known example of this
situation is the case of a resistive bridge, one arm of which contains the sample to be
measured. Other examples occur in derivative spectroscopy, where a small
modulation is applied to the angle of the grating (in optical spectroscopy) or to the
applied magnetic field (in magnetic resonance spectroscopy). Double beam
spectroscopy is a further common example.
In this signed scalar measurement the phase shifter must be set, after removal of any
zero errors, to maximize the X or the Y output of the demodulator. This is the only
method that will give correct operation as the output signal passes through zero, and
is also the best method to be used in an unsigned scalar measurement where any
significant amount of noise is present.
3.8 Output Channel Filters
3.8.01 Slope
As with most lock-in amplifiers, the output filter configuration in the model 7220 is
controlled by the SLOPE variable. This may seem somewhat strange, and a few
words of explanation may be helpful.
In traditional audio terminology, a first-order low-pass filter is described as having
“a slope of 6 dB per octave” because in the high frequency limit its gain is inversely
proportional to frequency (6 dB is approximately a factor of 2 in amplitude and an
octave is a factor of 2 in frequency); similarly a second-order low-pass filter is
described as having “a slope of 12 dB per octave”. These terms have become part of
the accepted terminology relating to lock-in amplifier output filters and are used in the
model 7220 to apply to the envelope of the frequency response function of the digital
FIR (finite impulse response) output filters. Accordingly the front panel display
control which selects the configuration of the output filters is labelled SLOPE and the
options are labelled 6, 12, 18, 24 dB/octave.
3-11
Chapter 3, TECHNICAL DESCRIPTION
The 6 dB/octave filters are not satisfactory for most purposes because they do not
give good rejection of periodic components in the demodulator output, including the
inevitable component at double the reference frequency. However, the 6 dB/octave
filter finds use where the lock-in amplifier is incorporated in a feedback control loop,
and in some situations where the form of the time-domain response is critical. The
user is recommended to use 12 dB/octave unless there is some definite reason for not
doing so.
Note that the filter slope for the rear panel FAST X and FAST Y outputs is fixed at
6 dB/octave.
3.8.02 Time Constants and Synchronization
The output time constant can be varied between 10 µs and 5 ks. Values from 10 µs to
640 µs are available at the rear panel FAST X and FAST Y outputs, while values
from 5 ms to 5 ks apply to all other outputs, including CH1, CH2 and the digital
displays.
The filters are of the Finite Impulse Response (FIR) type with the averaging time of
each section being equal to double the nominal time constant.
These filters offer a substantial advantage in response time compared with analog
filters or digital Infinite Impulse Response (IIR) filters.
When the reference frequency is below 10 Hz the synchronous filter option is
available. This means that the actual time constant of the filter is not generally the
selected value T but a value which is equal to an integer number of reference cycles.
If T is greater than 1 reference cycle, the time constant is between T/2 and T.
Where random noise is relatively small, synchronous filter operation gives a major
advantage in low-frequency measurements by enabling the system to give a constant
output even when the output time constant is equal to only 1 reference cycle.
3.8.03 Output Offset and Expand
The output offset facility enables ±300 % full-scale offset to be applied to the X, Y or
both displays. Note however that the rear panel analog outputs limit at ±120 %
full-scale.
The output expand facility allows a ×10 expansion to be applied to the X, Y, both or
neither outputs, and hence to the analog meter indication and the CH1 and CH2
analog outputs, if these are set to output X or Y values.
3.9 Use of Magnitude and Signal Phase Outputs
If the input signal Vs(t) is a reference frequency sinusoid of constant amplitude, and
the output filters are set to a sufficiently long time constant, the demodulator outputs
2
2
are constant levels Vx and Vy. The function √(Vx + Vy ) is dependent only on the
amplitude of the required signal Vs(t) (i.e. it is not dependent on the phase of Vs(t)
3-12
Chapter 3, TECHNICAL DESCRIPTION
with respect to the reference input) and is computed by the output processor in the
lock-in amplifier and made available as the “magnitude” output. The phase angle
between Vs(t) and the X demodulation function is called the “signal phase”: this is
equal to the angle of the complex quantity (Vx + jVy) (where j is the square root of -1)
and is also computed by the processor by means of a fast arctan algorithm.
The magnitude and signal phase outputs are used in cases where phase is to be
measured, or alternatively where the magnitude is to be measured under conditions
of uncertain or varying phase.
One case of varying phase is that in which the reference input is not derived from the
same source as that which generates the signal, and is therefore not exactly at the
same frequency. In this case, if the input signal is a sinusoid of constant amplitude,
the X and Y demodulator outputs show slow sinusoidal variations at the difference
frequency, and the magnitude output remains steady.
However, the magnitude output has disadvantages where significant noise is present
at the outputs of the demodulator. When the required signal outputs (i.e. the mean
values of the demodulator outputs) are less than the noise, the outputs take both
positive and negative values but the magnitude algorithm gives only positive values:
this effect, sometimes called noise rectification, gives rise to a zero error which in
the case of a Gaussian process has a mean value equal to 0.798 times the combined
root-mean-square (rms) value of the X and Y demodulator noise. Note that unlike
other forms of zero error this is not a constant quantity which can be subtracted from
all readings, because when the square root of the sum of the squares of the required
outputs becomes greater than the total rms noise the error due to this mechanism
disappears.
A second type of signal-dependent error in the mean of the magnitude output occurs
as a result of the inherent non-linearity of the magnitude formula: this error is always
positive and its value, expressed as a fraction of the signal level, is half the ratio of
the mean-square value of the noise to the square of the signal.
These considerations lead to the conclusion that when the magnitude output is being
used, the time constants of the demodulator should be set to give the required signal/
noise ratio at the X and Y demodulator outputs; improving the signal/noise ratio by
averaging the magnitude output itself is not to be recommended.
For analogous reasons, the magnitude function also shows signal-dependent errors
when zero offsets are present in the demodulator. For this reason, it is essential to
reduce zero offsets to an insignificant level (usually by the use of the Auto-Offset
function) when the magnitude output is to be used.
Note that the majority of signal recovery applications are scalar measurements, where
the phase between the required signal and the reference voltage is constant apart from
possible phase reversals corresponding to changes in the sign of the quantity being
measured. In this situation the lock-in amplifier is used in the normal X-Y mode, with
the phase shifter adjusted to maximize the X output and to bring the mean Y output to
zero. (Refer to section 3.12.03 for further information on the correct use of the Auto-
Phase function for this purpose.)
3-13
Chapter 3, TECHNICAL DESCRIPTION
3.10 Noise Measurements
The noise measurement facility is available only in the baseband mode (i.e. at
reference frequencies less than 60 kHz) and uses the output processor to perform a
noise computation on the Y output where it is assumed that the waveform is
Gaussian with zero mean. The zero mean is usually obtained by using the reference
phase control or the Auto-Phase function with a comparatively long time constant
(say 1 s) and the time constant is then reduced (to say 10 ms) for the noise
measurement.
The user is strongly advised to use an oscilloscope attached to the rear panel SIG
MON (signal monitor) output when making noise measurements as this is the best
way of ensuring that one is measuring a random process rather than line pick-up.
The indicated value of the noise (in V/√Hz or A/√Hz) is the square root of the mean
spectral density over the bandwidth defined by the setting of the output filter time
constant.
3.11 Power-up Defaults
All instrument settings are retained when the unit is switched off. When the
instrument is switched on again the settings are restored but with the following
exceptions:-
a) The signal channel reverts to AC coupling.
b) The GPIB mask byte is set to zero.
c) The REMOTE parameter is set to zero (front panel control enabled).
d) The curve buffer is cleared.
e) Any sweep that was in progress at switch-off is terminated.
f) Synchronous time constants are enabled.
g) Display backlights are turned on
3.12 Auto Functions
3.12.01 Introduction
The auto functions are groups of control operations which can be executed by means
of a single command or two key-presses. The auto functions allow easier, faster
operation in most applications, however, direct manual operation or special purpose
control programs may give better results in certain circumstances. During application
of several of the auto functions, decisions are made on the basis of output readings
made at a particular moment. Where this is the case, it is important for the output
time constant set by the user to be long enough to reduce the output noise to a
sufficiently low level so that valid decisions can be made and sufficient time is
allowed for the output to settle.
The following sections contain brief descriptions of the auto functions.
3-14
Chapter 3, TECHNICAL DESCRIPTION
3.12.02 Auto-Sensitivity
This function only operates when the reference frequency is above 1 Hz. A single
Auto-Sensitivity operation consists of increasing the full-scale sensitivity range if the
magnitude output is greater than 90 % of full-scale, or reducing the range if the
magnitude output is less than 30 % of full-scale. After the Auto-Sensitivity function
is called, Auto-Sensitivity operations continue to be made until the required criterion
is met.
In the presence of noise, or a time-varying input signal, it may be a long time before
the Auto-Sensitivity sequence comes to an end, and the resulting setting may not be
what is really required.
3.12.03 Auto-Phase
In an Auto-Phase operation the value of the signal phase is computed and an
appropriate phase shift is then introduced into the reference channel so as to bring
the value of the signal phase to zero. The intended result is to null the output of the
Y channel while maximizing the output of the X channel.
Any small residual phase can normally be removed by calling Auto-Phase for a
second time after a suitable delay to allow the outputs to settle.
The Auto-Phase facility is normally used with a clean signal which is known to be of
the required phase. It usually gives very good results provided that the X and Y
channel outputs are steady when the procedure is called.
If a zero error is present, it must be nulled with an Auto-Offset operation before the
signal is applied and Auto-Phase executed. If the error is due to unwanted interaction
between signal and reference (crosstalk) the Auto-Phase, while actually setting the
correct phase, will not give a zero value of signal phase. The final step is to remove
the signal and execute another Auto-Offset operation. This is a powerful method of
removing the effect of crosstalk which is not generally in the same phase as the
required signal.
3.12.04 Auto-Offset
In an Auto-Offset operation the X offset and Y offset functions are turned on and are
automatically set to the values required to give zero values at both the X and the Y
outputs. Any small residual values can normally be removed by calling Auto-Offset
for a second time after a suitable delay to allow the outputs to settle.
The primary use of the Auto-Offset is to cancel out zero errors which are usually
caused by unwanted coupling or crosstalk between the signal channel and the
reference channel, either in the external connections or possibly under some
conditions in the instrument itself.
Note that if a zero error is present, the Auto-Offset function should be executed
before any execution of Auto-Phase.
3-15
Chapter 3, TECHNICAL DESCRIPTION
3.12.05 Auto-Measure
This function only operates when the reference frequency is greater than 1 Hz. It
performs the following operations:
The line filter is disabled, AC coupling is established, the voltage measurement
mode is entered, with the single-ended A input mode, the FET input device is selected
and the FLOAT mode is set. If the reference frequency is more than 10 Hz the output
time constant is set to 100 ms, otherwise it is set to the lowest synchronous value, the
filter slope is set to 12 dB/octave, output expand is switched off, the reference
harmonic mode is set to 1, the X offset and Y offset functions are switched off and the
Auto-Phase and Auto-Sensitivity functions are called. The Auto-Sensitivity function
also adjusts the AC Gain if required.
The Auto-Measure function is intended to give a quick setting of the instrument
which will be approximately correct in typical simple single-ended measurement
situations. For optimum results in any given situation, it may be convenient to start
with Auto-Measure and to make subsequent modifications to individual controls.
3.12.06 Default Setting
With an instrument of the design of the model 7220, where there are many controls
of which only a few are regularly adjusted, it is very easy to overlook the setting of
one of them. Consequently a Default Setting function is provided, which sets all the
controls to a defined state. This is most often used as a rescue operation to bring the
instrument into a known condition when it is giving unexpected results. A listing of
the settings which are invoked by the use of this function can be found in appendix F.
3-16
Front and Rear Panels
Chapter 4
4.1 Front Panel
Figure 4-1, Model 7220 Front Panel Layout
As shown in figure 4-1 there are four BNC connectors with associated LED
indicators, two LCD display panels, an edge-indicating analog meter, eight double
and three single keys mounted on the model 7220’s front panel. The following
sections describe the function and location of these items.
4.1.01 A and B/I Signal Input Connectors
The A connector is the signal input connector for use in single-ended and differential
voltage mode. The B/I connector is the signal input connector for use in differential
voltage mode (A-B) and is also the signal input connector when current input mode is
selected. LEDs adjacent to the connectors light to indicate which of them is active
when a particular input mode is selected (see figure 4-2). One or both of these LEDs
will flash to indicate that the input is in overload.
Figure 4-2, Signal Inputs
4.1.02 OSC OUT Connector
This is the output connector for the internal oscillator. When internal reference mode
is selected the LED adjacent to the connector will be lit (see figure 4-3).
4-1
Chapter 4, FRONT AND REAR PANELS
4.1.03 REF IN Connector
This is the input connector for a general purpose external reference signal. When
external reference mode is selected the LED adjacent to the connector will be lit (see
figure 4-3). Under unlock conditions the LED will flash.
Figure 4-3, OSC OUT and REF IN Connectors
4.1.04 Left-hand LCD Display Panel
This panel and the two pairs of keys on each side of it are normally used to select and
adjust the instrument’s controls, such as the full-scale sensitivity, time constant, filter
slope, oscillator frequency and voltage, etc. In this mode the display shows two of the
possible range of controls and their present settings, one on the upper menu line and
one on the lower (see figure 4-4).
Figure 4-4, Main Display - Left-hand LCD
To select a given control, press the left-hand up or down SELECT keys repeatedly
until it is displayed on the corresponding menu line. The current setting of the control
is then shown adjacent to the description and may be adjusted using the corresponding
left-hand up or down ADJUST keys.
Some controls, such as time constant and full-scale sensitivity, have only a limited
range of settings, and so the use of the ADJUST keys allows the required value to be
chosen with only a few keypresses. Other controls, such as the internal oscillator
amplitude and frequency, may be set over a wide range of values and to a high
4-2
Chapter 4, FRONT AND REAR PANELS
precision. In these cases a significant number of keypresses are required to make
adjustments.
Adjustment of the latter type of control is made easier by the use of one or other of
the two methods described below.
Auto Repeat
If an up or down ADJUST key is pressed and held, then its action is automatically
repeated such that the displayed control setting will increment or decrement at a rate
approximately ten times faster than that which could be achieved by repeated manual
keypresses.
Active Cursor
The ADJUST keys can be used initially to place a cursor over a given digit in the
displayed control setting, prior to changing that digit. This is done by using the
procedure discussed below.
Step 1 Press both the up and down ADJUST keys simultaneously. A cursor
appears over one of the displayed digits (see figures 4-5 and 4-6).
Step 2 With the cursor visible, repeating step 1 causes the cursor to move to the
left. When the cursor reaches the most significant digit available (left end of
control setting) the next keypress returns the cursor to the least significant
digit (right end of control setting). Continue this action until the cursor
covers the required digit.
Step 3 Press the up or down ADJUST key to change the digit to the required value.
Figure 4-5, Active Cursor Activation
As an example of this operation, suppose that the oscillator frequency is 50 kHz and
it is required to change it to 51 kHz. Simultaneously press both the up and down
ADJUST keys adjacent to the oscillator frequency display. Move the cursor, by
repeated double keypresses, until it is over the required digit, in this case the zero to
the right of the leading 5. Then press the up ADJUST key to increment the frequency
by 1 kHz. The cursor will disappear as soon as the frequency is adjusted but its
position remains active until changed (see figure 4-6).
4-3
Chapter 4, FRONT AND REAR PANELS
Figure 4-6, Active Cursor Operation
The double keypress action can also be performed with one finger by firmly pressing
the center of the up and down ADJUST key rocker which will deform to press both
keys. The active cursor can be used to set any particular digit. For example, if you
only want to adjust the reference phase in 1 degree steps leave the cursor over the first
digit to the left of the decimal point of the reference phase value.
4.1.05 MENU Key
The left-hand LCD is also used to access the four auto functions that are built into the
instrument. To do this, from the power-up display default, press the key marked
MENU once (Note: On early units this key was labelled AUTO).
The left-hand LCD now changes to that shown in figure 4-7.
Figure 4-7, Auto Functions Menu - Left-hand Display
To activate one of the auto functions press the appropriate adjacent key, as shown in
figure 4-7. The display will immediately change to a message indicating that the
selected function is in progress, and will revert to the Main Display mode when the
function is completed.
When in the AUTO menu, a second press of the MENU key affects both the left and
right-hand displays and sets these to the setup menu mode. It is from this mode that
the instrument controls, which rarely need adjusting once set for a given experiment,
are activated (see figure 4-8 which shows a typical setup menu).
4-4
Chapter 4, FRONT AND REAR PANELS
Figure 4-8, Setup Menu Mode - Left and Right-hand LCD Displays
In the setup menu mode, the left-hand SELECT keys adjacent to the left-hand
display cycle through a series of twelve setup menus. In general each menu allows
three controls to be adjusted, one via the right-hand side of the left-hand display and
the other two via the right-hand display. The setup menu description is shown on the
left-hand side of the left-hand display. Figure 4-8 makes this clear and the various
menus are discussed more fully in chapter 5.
When in the setup menu mode a further single press of the MENU key returns the
instrument to the normal Main Display mode.
4.1.06 90º Key
This key increments the reference phase shifter by 90º each time the key is pressed.
4.1.07 SET Key
This key updates all frequency dependent parameters within the lock-in amplifier.
Press this key after any change to the External Reference frequency.
4.1.08 Right-hand LCD Display Panel
This panel is normally used to display two out of the set of fourteen possible
instrument outputs, with the keys on each side of it being used to make the selection.
In this mode it is also used to allow adjustment of the level of the output offsets
applied to the X and Y outputs. In addition, the panel is used in the setup menu mode
as described in section 4.1.05 above.
In the normal mode, the panel is divided vertically into two sections, with the output
descriptions shown on the upper line and the output values on the lower line. The
output type selection is made using the SELECT keys on each side of the upper line;
simply press the key repeatedly until the required output type is shown. Note that it is
not possible to set both sides of the display to the same output type. Figure 4-9 makes
this clear.
The SELECT keys associated with the right-hand display also allow easy switching
of the display between an output expressed as a percentage of full-scale and the
corresponding output expressed in volts or amps using the “simultaneous double
4-5
Chapter 4, FRONT AND REAR PANELS
keypress” feature. To perform such a switch, simply press both sides of the SELECT
keys simultaneously. This feature avoids the need to cycle through a number of
outputs, thereby reducing the number of keypresses needed.
The edge-indicating, analog panel meter is linked to the display on the left-hand side
of the right-hand display, with full-scale corresponding to a digital reading of 100 %.
However, the panel meter limits at a few percent above full-scale whereas the digital
displays limit at ±300 % full-scale.
Figure 4-9, Main Display - Right-hand LCD
4.2 Rear Panel
Figure 4-10, Model 7220 Rear Panel Layout
As shown in figure 4-10, the line power switch, line power voltage selector, two
RS232 connectors, a GPIB (IEEE-488) connector, digital output port, preamplifier
power connector and twelve BNC signal connectors are mounted on the rear panel of
the instrument. Brief descriptions of these are given in the following text.
4.2.01 Line Power Switch
Press the end of the switch marked I to turn on the instrument’s power, and the other
end, marked O, to turn it off.
4.2.02 Line Power Input Assembly
This houses the line voltage selector and line input fuse. To check, and if necessary
change, the fuse or line voltage see the procedure in section 2.1.05.
4-6
Chapter 4, FRONT AND REAR PANELS
4.2.03 RS232 Connector
This 9-pin D type RS232 interface connector implements pins 1, 2, 3 and 7 (Earth
Ground, Transmit Data, Receive Data, Logic Ground) of a standard DTE interface.
To make a connection to a PC-compatible computer, it is normally sufficient to use a
three-wire cable connecting Transmit Data to Receive Data, Receive Data to
Transmit Data, and Logic Ground to Logic Ground. Appendix D shows the
connection diagrams of cables suitable for computers with 9-pin and 25-pin serial
connectors. Pinouts for this connector are given in appendix B.
4.2.04 AUX RS232 Connector
This connector is used to link other compatible EG&G equipment together in a
“daisy-chain” configuration. Up to an additional 15 units can be connected in this
way. Each unit must be set to a unique address (see section 5.2.08). Pinouts for this
connector are given in appendix B.
4.2.05 GPIB Connector
The GPIB interface connector conforms to the IEEE-488 1978 Instrument Bus
Standard. The standard defines all voltage and current levels, connector
specifications, timing and handshake requirements.
4.2.06 DIGITAL OUTPUTS Connector
This connector provides eight TTL output lines, each of which can be set high or low
by the use of a front panel setup menu or via the computer interfaces. It is most
commonly used for controlling auxiliary apparatus, such as lamps, shutters and
heaters. Pinouts for this connector are given in appendix B.
4.2.07 PREAMP POWER Connector
This connector supplies ±15 V at up to 100 mA and can be used for powering any of
several optional remote preamplifiers available from EG&G Instruments. Pinouts for
this connector are given in appendix B.
4.2.08 REF MON Connector
The signal at this connector is a TTL-compatible waveform synchronous with the
reference. This output monitors correct reference channel operation but its polarity is
not uniquely defined so that it does not necessarily show the correct phase
relationship with the SIG MON output.
4.2.09 REF TTL Connector
This connector is provided to allow TTL compatible pulses to be used as the
reference input.
4.2.10 SIG MON Connector
The signal at this connector is that immediately prior to the main analog to digital
converter and after the preamplifier, line filter and anti-alias filters.
4-7
Chapter 4, FRONT AND REAR PANELS
4.2.11 CH1, CH2 Connectors
The signal at these connectors is an analog voltage corresponding to a selected
output, such as X, Y, R, θ, etc., as specified in the Output Setup menu. The minimum
time constant that can be used is 5 ms. The full-scale output voltage range is ±10.0 V
although the outputs remain valid to ±12.0 V to provide some overload capability.
4.2.12 TRIG Connector
This connector accepts a TTL-compatible input and can be used for triggering the
auxiliary Analog to Digital Converters (ADCs). The input operates on the positive
edge only.
4.2.13 ADC1, ADC2 Connectors
The input voltages at these connectors may be digitized using the auxiliary ADCs and
read either from the front panel or by the use of a computer interface command. The
input voltages are sampled and held when the ADC is triggered, and several different
trigger modes are available. These modes can be set either from the front panel or by
using a remote computer command. The input voltage range is ±10.0 V and the
resolution is 1 mV.
4.2.14 DAC1, DAC2 Connectors
There are two DAC (Digital to Analog Converter) output connectors. The output
voltages at these connectors can be set either from the front panel or by the use of
remote computer commands. The output range is ±10.0 V and the resolution is 1 mV.
4.2.15 FAST X, FAST Y Connectors
The signals at these two connectors are the X channel and Y channel output signals
derived from a point after the first stage of output low-pass filtering. The maximum
time constant that can be used is 640 µs, with a fixed slope of 6 dB/octave. Visual
interpretation of the waveforms at these connectors, as displayed on an oscilloscope,
when the instrument is operating in the highband mode (i.e. above 60 kHz) is
difficult.
4-8
Front Panel Operation
Chapter 5
5.1 Introduction
This chapter describes how to operate the model 7220 using the front panel controls,
and describes its capabilities when used in this way. Chapter 6 provides similar
information in the situation where the unit is operated remotely using one of the
computer interfaces.
Readers should refer to chapter 4 for a detailed description of the use of the
SELECT and ADJUST keys, and the functions of the left and right-hand display
panels. However, for ease of use, some of this information is repeated here.
The model 7220 uses a flexible, menu based, control structure which allows many
instrument controls to be adjusted from the front panel with only a few keys.
Furthermore this design makes it very easy to introduce new features or improve
existing ones without the limitation resulting from a fixed front panel layout.
The following sections describe the instrument control menus in a logical sequence,
from the setup menus, which typically only need changing occasionally, through to
the output display selection.
5.2 Setup Menu Mode
When in the normal Main Display mode, two presses of the MENU key set the left
and right-hand displays to the setup menu mode, (see figure 5-1 which shows a
typical setup menu).
Figure 5-1, Setup Menu Mode - Left and Right-hand LCD Displays
In the setup menu mode, the left-hand SELECT keys of the left-hand display cycle
through a series of twelve setup menus. In general each menu allows three controls to
be adjusted, one via the right-hand side of the left-hand display and the other two via
5-1
Chapter 5, FRONT PANEL OPERATION
the right-hand display. The setup menu description is shown on the left-hand side of
the left-hand display. Figure 5-1 makes this clear.
One further press of the MENU key causes the instrument to leave the setup menu
mode and return to the main display mode. On leaving the setup menu mode the last
menu displayed is held in memory and will be displayed on re-entry. The following
sections describe each menu in sequence.
5.2.01 Input Setup Menu
Figure 5-2, Input Setup Menu
In this menu, shown in figure 5-2, three controls affecting the lock-in amplifier’s
signal channel input are displayed. Changes to the setting of these controls can be
made by using the ADJUST keys adjacent to the appropriate LCD.
Input Mode
This control has four settings:-
A Volts
In this setting the signal channel input is a single-ended voltage input connected to
the front panel BNC connector marked “A”.
A-B Volts
In this setting the signal channel input is a differential voltage input connected to
the front panel BNC connectors marked “A” and “B/I”.
B Amps (HB)
In this setting the signal channel input is a single-ended current input connected to
the front panel BNC connector marked “B/I”, and uses a high bandwidth (HB)
current to voltage converter.
B Amps (LN)
In this setting the signal channel input is a single-ended current input connected to
5-2
Chapter 5, FRONT PANEL OPERATION
the front panel BNC connector marked “B/I”, and uses a low-noise (LN) current
to voltage converter.
Input
This control has four settings:-
Flt/DC
The shells of the “A” and “B/I” connectors are connected to chassis ground via a
1 kΩ resistor and the signal channel input is DC coupled.
Flt/AC
The shells of the “A” and “B/I” connectors are connected to chassis ground via a
1 kΩ resistor and the signal channel input is AC coupled. Note that DC coupling
should be used at frequencies of < 1 Hz.
Gnd/DC
The shells of the “A” and “B/I” connectors are connected to chassis ground and
the signal channel input is DC coupled.
Gnd/AC
The shells of the “A” and “B/I” connectors are connected to chassis ground and
the signal channel input is AC coupled. Note that DC coupling should be used at
frequencies of < 1 Hz.
Device
This control allows the selection of the input device when operating in voltage input
mode and has two settings:-
FET
Uses a FET as the input device, for which case the input impedance is 10 MΩ.
This is the usual setting.
Bipolar
Uses a bipolar device in the input stage, for the lowest possible voltage input
noise. In this case the input impedance is 10 kΩ. Note that this selection is not
possible when using the DC coupled input modes.
5-3
Chapter 5, FRONT PANEL OPERATION
5.2.02 Reference Setup Menu
Figure 5-3, Reference Setup Menu
In this menu, shown in figure 5-3, there are three controls affecting the reference
channel of the instrument. They are:-
Ref Source
This control allows selection of the source of reference signal used to drive the
reference circuitry, and has three settings:-
INTERNAL
The lock-in amplifier’s reference is taken from the instrument’s internal oscillator.
Note that this setting gives the best phase and gain measurement accuracy under
all operating conditions, and is always to be preferred if it is possible to design
the experiment so that the lock-in amplifier acts as a source of reference signal.
EXT, FRONT
In this setting, suitable for use with reference frequencies above 300 mHz, the
lock-in amplifier’s reference should be applied to the front panel REF IN
connector. A wide variety of signal waveforms may be employed but at
frequencies lower than 1 Hz, square waveforms should be used.
EXT, REAR
In this setting, the lock-in amplifier’s reference should be applied to the rear panel
TTL compatible REF TTL connector. The use of this input is preferable to the
front panel input when a TTL logic reference signal is available.
Harmonic
This control allows selection of the harmonic at which the lock-in amplifier will
detect. It has three settings, but most commonly is set to “1st”. Note that the “2F”
setting found on other lock-in amplifiers corresponds to setting this control to “2nd”.
Demod Mon,
The Demodulator Monitor control has two settings, ON and OFF, which are only
meaningful when the lock-in amplifier is operated in external reference mode:-
5-4
Chapter 5, FRONT PANEL OPERATION
ON
When the Demodulator Monitor is switched ON and the instrument is operating
in External Reference mode, the signal at the OSC OUT connector changes from
that of the internal oscillator to an analog representation of the drive from the
reference channel to the X output demodulator. The amplitude of this signal may
be controlled by the internal oscillator amplitude controls, but the internal
oscillator frequency control is inactive since the frequency is related to the
external reference.
If the harmonic mode is set to 1st, the signal at the OSC OUT connector will be
at the same frequency as the applied reference, but if set to 2nd or 3rd then the
output will be at two or three times the reference frequency respectively.
OFF
When the Demodulator Monitor is switched OFF the OSC OUT connector
functions as the output from the internal oscillator. The signal provided at it may
be adjusted both in amplitude and frequency using the instrument’s controls. This
is the most common setting.
5.2.03 Output Setup Menu
Figure 5-4, Output Setup Menu
This menu, shown in figure 5-4, has three controls affecting the output channels of
the instrument. They are:-
Expand
This control allows a ×10 output expansion to be applied to the X, Y or both output
channels, or to be switched off:-
5-5
Chapter 5, FRONT PANEL OPERATION
OFF
Output expansion is turned off.
X ONLY
A ×10 output expansion is applied to the X output only.
Y ONLY
A ×10 output expansion is applied to the Y output only.
X & Y
A ×10 output expansion is applied to both the X and Y outputs.
CH1 OUTPUTS CH2
This control, shown on the right-hand LCD, allows the two rear panel analog outputs
CH1and CH2 to be connected to the required instrument outputs. The left-hand
ADJUST keys are used to select the output provided at the CH1 connector and the
right-hand ones select that at the CH2 connector. Each output may be set to one of
the following settings:-
X %
When set to X % the corresponding rear panel CH1/CH2 connector will output a
voltage related to the X %fs front panel display as follows:-
X %fs
+120
+100
0
CH1/2 Voltage
12.0 V
10.0 V
0.0 V
-100
-120
-10.0 V
-12.0 V
Y %
When set to Y % the corresponding rear panel CH1/CH2 connector will output a
voltage related to the Y %fs front panel display as follows:-
Y %fs
+120
+100
0
CH1/2 Voltage
12.0 V
10.0 V
0.0 V
-100
-120
-10.0 V
-12.0 V
MAG %
When set to MAG % the corresponding rear panel CH1/CH2 connector will
output a voltage related to the MAG %fs front panel display as follows:-
5-6
Chapter 5, FRONT PANEL OPERATION
MAG %fs CH1/2 Voltage
+120
+100
0
12.0 V
10.0 V
0.0 V
-100
-120
-10.0 V
-12.0 V
PHASE1
When set to PHASE1 the corresponding rear panel CH1/CH2 connector will
output a voltage related to the PHA deg front panel display as follows:-
PHA deg CH1/2 Voltage
+180
+90
0
-90
-180
9.0 V
4.5 V
0.0 V
-4.5 V
-9.0 V
PHASE2
When set to PHASE2 the corresponding rear panel CH1/CH2 connector will
output a voltage related to the PHA deg front panel display as follows:-
PHA deg CH1/2 Voltage
+360
+180
0
+9.0 V
0.0 V
-9.0 V
NOISE
When set to NOISE the corresponding rear panel CH1/CH2 connector will
output a voltage related to the N %fs front panel display as follows:-
N %fs
+120
+100
0
CH1/2 Voltage
12.0 V
10.0 V
0.0 V
-100
-120
-10.0 V
-12.0 V
RATIO
When set to RATIO the corresponding rear panel CH1/CH2 connector will
output a voltage related to the RATIO front panel display as follows:-
RATIO
+12
+10
0
CH1/2 Voltage
12.0 V
10.0 V
0.0 V
-10
-12
-10.0 V
-12.0 V
5-7
Chapter 5, FRONT PANEL OPERATION
5.2.04 Control Options Menu
Figure 5-5, Control Options Setup Menu
This menu, shown in figure 5-5, has three controls affecting the line frequency
rejection filter, AC Gain control and output time constants, as follows:-
Linefilt
This control sets the mode of operation of the internal line frequency rejection filter.
Early instruments have two possible settings for this control, ON or OFF. Note that in
these units the filter introduces significant gain and phase errors when measuring
signals in the frequency range from 5 Hz to 500 Hz.
Instruments manufactured after June 1996 which are fitted with the updated filter
hardware offer four possible settings for the control, as defined by the following
table:-
Legend
OFF
F
2F
F&2F
Function
Line filter inactive
Enable 50 or 60 Hz notch filter
Enable 100 or 120 Hz notch filter
Enable both filters
AC Gain
As discussed in section 3.2.04, the correct adjustment of the AC Gain in a DSP lock-
in amplifier is necessary to achieve the best results. This control allows the user to
select whether this adjustment is carried out automatically or remains under manual
control.
5-8
Chapter 5, FRONT PANEL OPERATION
MANUAL
In this setting the AC Gain may be manually adjusted from the main display.
AUTOMATIC
In this setting the AC Gain value is automatically selected by the instrument,
depending on the full-scale sensitivity.
TC’s
This control affects the output time constants, and has two settings:-
SYNC
When set to SYNC, the actual time constant used is chosen to be some multiple
of the reference frequency period. In this mode the output will be much more
stable at low frequencies than it would otherwise be. Note: when set to this mode,
output time constants shorter than 100 ms cannot be used.
ASYNC
In this setting, the normal mode, time constants are not related to the reference
frequency period.
5-9
Chapter 5, FRONT PANEL OPERATION
5.2.05 Miscellaneous Options Menu
Figure 5-6, Miscellaneous Options Setup Menu
This menu, shown in figure 5-6, has three controls affecting the auxiliary ADC
trigger rate and the front panel display as follows:-
Trigger
This control selects the trigger which is used to initiate the conversion of voltages
applied to the rear panel ADC1 and ADC2 connectors, as follows:-
200Hz
In this setting, ADC conversions occur at a 200 Hz rate.
EXTERNAL
In this setting, ADC conversions are started by an external trigger signal applied
to the rear panel TRIG connector.
The other eight settings of this control, BURST #2 to BURST #9 are only
meaningful when using the instrument under computer control, since they cause
data to be stored to the internal curve buffer. They will not therefore be discussed
further here, but their function is described in chapter 6.
Lights
This control switches the back lighting of the two LCD displays and the LEDs
adjacent to the front panel BNC connectors ON or OFF.
Contrast
This control adjusts the contrast of the LCD displays and may be set to a value
between 0 and 50.
5-10
Chapter 5, FRONT PANEL OPERATION
5.2.06 RS232 Setup 1 Menu
Figure 5-7, RS232 Setup 1 Menu
This menu, shown in figure 5-7, has three controls affecting the RS232 computer
interface, as follows:-
BaudRate
Thirteen values of Baud Rate are available in the range 75 to 19200 bits per second.
Format
There are four data formats available:
7D + 1P
Sets up 7 Data Bits + 1 Parity bit (1 = Parity ON)
8D + 1P
Sets up 8 Data Bits + 1 Parity bit (1 = Parity ON)
8D + 0P
Sets up 8 Data Bits + 0 Parity bit (0 = Parity OFF)
9D + 0P
Sets up 9 Data Bits + 0 Parity bit (0 = Parity OFF)
Parity
If the Format control is set to a mode with the Parity bit enabled, the Parity control
offers two modes, ODD and EVEN. If however, the Parity bit is not enabled then the
Format control displays NONE.
5-11
Chapter 5, FRONT PANEL OPERATION
5.2.07 RS232 Setup 2 Menu
Figure 5-8, RS232 Setup 2 Menu
This menu, shown in figure 5-8, has three controls affecting the RS232 computer
interface, as follows:-
Prompt
This function can be switched ON or OFF:-
ON
The prompt character is sent out by the lock-in amplifier after each command
response to indicate that the response is finished and the instrument is ready for a
new command. It can also be used to signal overload conditions (see section
6.3.13 for further information).
OFF
No prompt character is sent.
Echo
This function can be switched ON or OFF (see section 6.3.08 for further
information):-
ON
The lock-in amplifier echoes each character received over the RS232 interface
back to the controlling computer, to indicate it is ready to receive the next one. In
order for this to work, the controlling computer should ensure that it does not
send the next character until it has read the echo.
OFF
No character echo occurs.
Delimiter
Some instrument control commands generate more than one output value, such as the
5-12
Chapter 5, FRONT PANEL OPERATION
MP command (report Magnitude and Phase). Hence it is necessary for the controlling
program to be able to determine when all of the first value has been sent. The
delimiter is a separator character sent between each response which may be used for
this purpose. The control allows any ASCII character with decimal value between 32
and 125, or 13, to be used.
5.2.08 RS232 Setup 3 Menu
Figure 5-9, RS232 Setup 3 Menu
The third setup menu with controls affecting the RS232 interface is shown in
figure 5-9.
Address
This control sets the RS232 address which is used when daisy-chaining other
compatible instruments. Each instrument used in the chain must be set to a unique
address in the range 0 to 15. All instruments receive commands, but only the
currently addressed instrument will implement or respond to the commands, except,
of course, the command to change the instrument to be addressed.
Information only
The right-hand LCD displays the version level of the instrument’s internal firmware.
Please be prepared to give this number when contacting EG&G Instruments with a
technical query.
5-13
Chapter 5, FRONT PANEL OPERATION
5.2.09 GPIB Setup 1 Menu
Figure 5-10, GPIB Setup 1 Menu
This menu, shown in figure 5-10, has two controls affecting the GPIB computer
interface, as follows:-
Address
Each instrument used on the GPIB interface must have a unique address, in the range
0 to 31, and this control is used to set this address. The default setting is 12.
Terminator
This control selects the method by which the instrument signals to the controlling
computer the completion of transmission of a response to a command. Three choices
are available:-
CR,LF
A carriage return followed by a line feed are transmitted at the end of a response
string, and in addition the GPIB interface line EOI (end of instruction) is asserted
with the line feed character.
EOI
The GPIB interface line EOI (end of instruction) is asserted at the end of a
response string. This gives the fastest possible operation since other termination
characters are not needed.
CR
A carriage return is transmitted at the end of a response string and in addition the
GPIB interface line EOI (end of instruction) is asserted.
5-14
Chapter 5, FRONT PANEL OPERATION
5.2.10 GPIB Setup 2 Menu
Figure 5-11, GPIB Setup 2 Menu
This menu, shown in figure 5-11, has two controls affecting the GPIB computer
interface, as follows:-
SRQ Mask
The instrument includes the ability to generate a Service Request on the GPIB
interface, to signal to the controlling computer that urgent attention is required. The
request is generated when the result of a logical bit-wise AND operation between the
Service Request Mask byte, set by this control, and the instrument’s Status Byte, is
non-zero. The bit assignments of the Status Byte are as follows:-
Bit
0
1
Decimal value
Function
command complete
invalid command
1
2
2
3
4
8
command parameter error
reference unlock
4
5
16
32
overload
auto-mode active or new ADC values available after
external trigger
6
7
64
128
asserted SRQ
data available
Hence, for example, if the SRQ mask byte is set to decimal 16 (i.e. bit 4 is asserted),
a service request would be generated as soon as an overload occurred; if the SRQ
mask byte were set to 0 (i.e. no bits asserted), then service requests would never be
generated.
Test echo
When this control is enabled, all transmissions to and from the instrument via the
GPIB interface are echoed to the RS232 interface. Hence if a terminal is connected to
the latter port, it will display any commands sent to the instrument and any responses
generated, which can be useful during program development. When disabled, echoing
does not occur. The control should always be disabled when not using this feature,
since it slows down communications.
5-15
Chapter 5, FRONT PANEL OPERATION
5.2.11 Digital Outputs Setup Menu
Figure 5-12, Digital Outputs Setup Menu
This menu, shown in figure 5-12, is used to control the 8 TTL lines of the rear panel
digital output port, used for controlling external equipment.
Decimal
The bit pattern appearing at the digital output port is set by an unsigned eight-bit
binary number, the decimal equivalent of which can range from 0 to 255. This
decimal value may be set using the left-hand display, so that for example if it were 0
the eight output lines would all be low, whereas if set to 255, then they would all be
high.
The centre of the right-hand display shows the number in binary format, with the
value of each bit shown below its bit number. Each of the keys around this display
will toggle one bit, as indicated by the numbers adjacent to them, providing a second
way of controlling the port.
Any changes made to the digital output in decimal format will be reflected in the
binary display and vice-versa.
5-16
Chapter 5, FRONT PANEL OPERATION
5.2.12 Control Setup Menu
Figure 5-13, Control Setup Menu
The final setup menu, shown in figure 5-13, is used to set the instrument to a known
state and to adjust the sampling rate of the main analog to digital converter.
Default Setting
Pressing a key adjacent to this label will set all of the instrument’s controls to a
known state. This can be useful when performing the initial checks procedure, or after
taking the instrument over from another user. Note that on completion, the instrument
will leave the setup menu mode and revert to the main display mode. A listing of the
settings invoked by the use of this function can be found in appendix F.
Caution
The default setting includes controls affecting the RS232 and GPIB interfaces, so
these will need to be readjusted following use of this function unless you have used
their default settings.
Sample rate
This control adjusts the sampling rate of the instrument’s main ADC, offering four
possible settings. It should only be necessary to use it if you suspect that an
interfering signal is being aliased into the instrument’s output.
5-17
Chapter 5, FRONT PANEL OPERATION
5.3 Auto Functions Menu
When in the Main Display mode, one press of the MENU key accesses the AUTO
MENU showing four auto functions that are built into the instrument. The left-hand
LCD changes to that shown in figure 5-14.
Figure 5-14, Auto Functions Menu - Left-hand Display
To activate one of the auto functions press one of the keys adjacent to it, as shown in
figure 5-14. The display will immediately change to a message indicating that the
selected function is in progress, and will revert to the Main Display mode when the
function is completed.
The four functions operate as follows:-
AUTO SEN
This function only operates when the reference frequency is greater than 1 Hz. A
single Auto-Sensitivity operation consists of increasing the full-scale sensitivity range
if the magnitude output is greater than 90 % of full-scale, or reducing the range if the
magnitude output is less than 30 % of full-scale. After the Auto-Sensitivity function
is called, Auto-Sensitivity operations continue to be made until the required criterion
is met.
In the presence of noise, or a time-varying input signal, it may be a long time before
the Auto-Sensitivity sequence comes to an end, and the resulting setting may not be
what is really required.
AUTO PHASE
In an Auto-Phase operation the value of the signal phase is computed and an
appropriate phase shift is introduced into the reference channel so as to bring the
value of the signal phase to zero. The intended result is to null the output of the Y
channel while maximizing the output of the X channel.
Any small residual phase error can normally be removed by calling Auto-Phase for a
second time after a suitable delay to allow the outputs to settle.
The Auto-phase facility usually gives good results when the X and Y channel outputs
5-18
Chapter 5, FRONT PANEL OPERATION
are steady, implying the signal phase is stable, when the procedure is called.
If a zero error is present on the outputs, such as may be caused by unwanted coupling
between the reference and signal channel inputs, then the following procedure should
be adopted:-
1) Remove the source of input signal, without disturbing any of the connections to
the signal input which might be picking up interfering signals from the reference
signal. In an optical experiment, for example, this could be done by shielding the
detector from the source of chopped light.
2) Execute an Auto-Offset operation, which will reduce the X and Y outputs to zero.
3) Re-establish the source of input signal. The X and Y channel outputs will now
indicate the true level of the input signal, at the present reference phase setting.
4) Execute an Auto-Phase operation. This will set the reference phase shifter to the
phase angle of the input signal. However, because the offset levels which were
applied in step 2 were calculated at the original reference phase setting, they will
not now be correct and the instrument will in general display a non-zero Y output
value.
5) Remove the source of input signal again.
6) Execute a second Auto-Offset operation, which will reduce the X and Y outputs
to zero at the new reference phase setting.
7) Re-establish the source of input signal.
This technique, although apparently complex, is the only way of effectively removing
crosstalk which is not generally in the same phase as the required signal.
AUTO OFFSET
In an Auto-Offset operation the X offset and Y offset functions are turned on and are
automatically set to the levels required to give zero values at both the X and the Y
outputs. Any small residual values remaining after the initial Auto-Offset operation
can normally be removed by calling the function for a second time after a suitable
delay to allow the outputs to settle.
The primary use of the Auto-Offset is to cancel out zero errors which are usually
caused by unwanted coupling or crosstalk between the signal channel and the
reference channel, either in the external connections or possibly, under some
conditions, in the instrument itself.
Note that if a zero error is present, the Auto-Offset function should be executed
before any execution of Auto-Phase.
AUTO MEASURE
This function only operates when the reference frequency is greater than 1 Hz. It
performs the following operations:
5-19
Chapter 5, FRONT PANEL OPERATION
The line filter is disabled; AC coupling is established; the voltage measurement mode
is entered, using the single-ended, A, input; the FET input devices are enabled; the
FLOAT mode is set. If the reference frequency is more than 10 Hz the output time
constant is set to 100 ms, otherwise it is set to the lowest synchronous value; the filter
slope is set to 12 dB/octave; output expand is switched off; the reference harmonic
mode is set to 1; the X offset and Y offset functions are switched off; Auto-Phase and
Auto-Sensitivity functions are called. The Auto-Sensitivity function also adjusts the
AC Gain if required.
The Auto-Measure function is intended to provide a means of setting the instrument’s
controls quickly to conditions which will be approximately correct for typical simple,
single-ended measurement situations. For optimum results in any given situation it
may be convenient to start with an Auto-Measure operation and subsequently fine
tune the setup conditions manually.
The Auto-Measure function is most often used as a rescue operation to bring the
instrument into a well-defined state when it is giving unexpected results. The length of
the above list demonstrates that one or more items can easily be overlooked if
performed manually.
5.4 Main Display Mode - Left-hand LCD
The Main Display Mode is the default mode on instrument power-up. Two controls,
chosen from a possible ten, may be adjusted using the left-hand display, although it is
not possible to show the same control on both lines of the display simultaneously. The
choice of control is made using the SELECT keys adjacent to the left-hand side of
the left-hand display. Once selected, the control is adjusted using the ADJUST keys
adjacent to the right-hand side of the left-hand display.
The ten controls are now discussed in turn. Note that in the following figures the
vertical lines below the heading “Left-hand LCD” depict the horizontal limits of the
display to indicate how control information is shown. It should be remembered that
there are two lines available on the display and the user may choose to show a control
on either the upper or the lower line (see figure 4-4).
Sensitivity
When set to voltage input mode, the instrument’s full-scale voltage sensitivity may be
set to any value between 20 nV and 1 V in a 1-2-5 sequence.
5-20
Chapter 5, FRONT PANEL OPERATION
Figure 5-15, Sensitivity Control - Voltage Input Mode
When set to current input mode, the instrument’s full-scale current sensitivity may be
set to any value between 20 fA and 1 µA (wide bandwidth mode) or 20 fA and 10 nA
(low-noise mode), in a 1-2-5 sequence.
Figure 5-16, Sensitivity Control - Wide Band Current Input Mode
Figure 5-17, Sensitivity Control - Low-noise Current Input Mode
5-21
Chapter 5, FRONT PANEL OPERATION
AC Gain
If the AC Gain control is set to Manual (using the Input Setup menu), then this
control allows it to be adjusted from 0 dB to 90 dB in 10 dB steps, although not all
settings are available at all full-scale sensitivity settings.
Figure 5-18, AC Gain Control
Time Constant
The time constant of the output filters is set using this control. The settings between
10 µs and 640 µs are in a binary sequence and apply only to outputs at the rear panel
FAST X and FAST Y connectors. Settings between 5 ms and 5 ks are in a 1-2-5
sequence and apply only to all the other instrument outputs.
Figure 5-19, Time Constant Control
5-22
Chapter 5, FRONT PANEL OPERATION
Slope
The roll-off of the output filters is set, using this control, to any value from 6 dB to
24 dB/octave, in 6 dB steps. Note this control does not affect the roll-off of outputs at
the FAST X and FAST Y connectors which are fixed at 6 dB/octave.
Figure 5-20, Output Filter Slope Control
Oscillator Frequency
The frequency of the instrument’s internal oscillator may be set, using this control, to
any value between 1 mHz and 120 kHz with a 1 mHz resolution. Adjustment is faster
if use is made of the Active Cursor control - see section 4.1.04
Figure 5-21, Internal Oscillator Frequency Control
5-23
Chapter 5, FRONT PANEL OPERATION
Oscillator Amplitude
The amplitude of the instrument’s internal oscillator may be set, using this control, to
any value between 1 mV and 5 V rms with a 1 mV resolution. Adjustment is faster if
use is made of the Active Cursor control - see section 4.1.04.
Figure 5-22, Internal Oscillator Amplitude Control
DAC 1
This control sets the voltage appearing at the rear panel DAC1 output connector to
any value between +10 V and -10 V with a resolution of 1 mV. Adjustment is faster if
use is made of the Active Cursor control - see section 4.1.04.
Figure 5-23, DAC 1 Output Control
5-24
Chapter 5, FRONT PANEL OPERATION
DAC 2
This control sets the voltage appearing at the rear panel DAC2 output connector to
any value between +10 V and -10 V with a resolution of 1 mV. Adjustment is faster if
use is made of the Active Cursor control - see section 4.1.04.
Figure 5-24, DAC 2 Output Control
Offset
This control allows an output offset to be added to the X output, Y output, neither or
both outputs. The actual value of offset applied, which may range from -300 % to
+300 % of the selected full-scale sensitivity setting, is set using the right-hand LCD
panel, or automatically by the Auto-Offset function. Note that the Auto-Offset
function automatically switches both X and Y output offsets on.
Figure 5-25, Output Offset Status Control
5-25
Chapter 5, FRONT PANEL OPERATION
Reference Phase
This control allows the reference phase to be adjusted over the range -360° to + 360°
in 10m° steps, although readers will appreciate that a setting of -180° is equivalent to
+180°, and that ±360° is equivalent to 0°.
The Auto-Phase function also affects the value displayed here.
Figure 5-26, Reference Phase Control
5.5 Main Display Mode - Right-hand LCD
The right-hand display, in Main Display Mode, is used to display two out of the
possible fourteen instrument outputs. In addition, it is used to set the levels of the X
and Y output offsets.
The panel is divided vertically into two halves, with the output description on the
upper line and the output value on the lower line. Selection of the required output type
is made using the upper SELECT keys on each side of the right-hand display. Each
press of the key advances the display to the next output choice, with the display
“wrapping” so that repeated presses cycle through those available. Note that it is not
possible to set both displayed outputs to the same type, i.e. both sides of the display
cannot, for example, be set to “X%”.
The fourteen possible outputs and two controls are now discussed in sequence. Note
that in the following figures the horizontal lines below the heading “Right-hand LCD”
depict the vertical limits of the display to indicate how information is shown. The user
may choose to display this information in either the left or the right half of the display
so no horizontal limits are depicted (see figure 4-9).
5-26
Chapter 5, FRONT PANEL OPERATION
X %fs
Figure 5-27, X Output as % Full-Scale
Shows the X output as a percentage of the selected full-scale sensitivity setting.
Hence if the sensitivity setting were 100 mV and a 50 mV signal were applied, with
the instrument’s reference phase adjusted for maximum X output, the display would
read 50.00 %
Y %fs
Figure 5-28, Y Output as % Full-Scale
Shows the Y output as a percentage of the selected full-scale sensitivity setting.
Hence if the sensitivity setting were 100 mV and a 50 mV signal were applied, with
the instrument’s reference phase adjusted for maximum Y output, the display would
read 50.00 %
5-27
Chapter 5, FRONT PANEL OPERATION
MAG %fs
Figure 5-29, Magnitude Output as % Full-Scale
Shows the signal magnitude, where magnitude = √((X output)2 + Y output)2), as a
percentage of the selected full-scale sensitivity setting. Hence if the sensitivity setting
were 100 mV and a 50 mV signal were applied, regardless of the setting of the
instrument’s reference phase, the display would read 50.00 %
Noise %fs
Figure 5-30, Noise Output as % Full-Scale
Shows the noise accompanying the signal, in a bandwidth defined by the setting of the
output filter time constant, where it is assumed that the noise is Gaussian. The value
is given as a percentage of the instrument’s selected full-scale sensitivity setting. Note
that the displayed value will change as the time constant control is adjusted. The
instrument offers a second Noise output (discussed later in this section), expressed
directly in terms of volts or amps per root Hertz, which takes this effect into account.
5-28
Chapter 5, FRONT PANEL OPERATION
Phase in Degrees
Figure 5-31, Phase Output in Degrees
Shows the relative phase, where phase = tan-1 (Y output/X output), in degrees.
Reference Frequency
Figure 5-32, Reference Frequency Display
Shows the reference frequency at which the lock-in amplifier is operating. Note that
the display shows values in kHz only when the frequency is greater than 3 kHz. At all
other values the units are Hz.
5-29
Chapter 5, FRONT PANEL OPERATION
X Volts (or Amps)
Figure 5-33, X Output in Volts or Amps
Shows the X output directly in terms of volts or amps (depending on whether voltage
or current input mode is selected). Hence if the sensitivity setting were 100 mV and a
50 mV signal were applied, with the instrument’s reference phase adjusted for
maximum X output, the display would read 50.00 mV
Y Volts (or Amps)
Figure 5-34, Y Output in Volts or Amps
Shows the Y output directly in terms of volts or amps (depending on whether voltage
or current input mode is selected). Hence if the sensitivity setting were 100 mV and a
50 mV signal were applied, with the instrument’s reference phase adjusted for
maximum Y output, the display would read 50.00 mV
5-30
Chapter 5, FRONT PANEL OPERATION
MAG Volts (or Amps)
Figure 5-35, Magnitude Output in Volts or Amps
Shows the signal magnitude, where magnitude = √((X output)2 + (Y output)2),
directly in terms of volts or amps (depending on whether voltage or current input
mode is selected). Hence if the sensitivity setting were 100 mV and a 50 mV signal
were applied, regardless of the setting of the instrument’s reference phase, the display
would read 50.00 mV
Noise in Volts or Amps per Root Hertz
Figure 5-36, Noise Output in Volts or Amps per Root Hertz
Shows the noise accompanying the signal, in a noise bandwidth defined by the setting
of the output filter time constant, where it is assumed that the noise is Gaussian. Note
that the noise floor of the instrument is 2 nV and so the output value will always be
greater than this.
5-31
Chapter 5, FRONT PANEL OPERATION
Ratio
Figure 5-37, Ratio Output
Shows the ratio, where ratio = (X output / ADC1 Input), usually used to compensate
for source intensity fluctuations in optical experiments.
Log Ratio
Figure 5-38, Log Ratio Output
Shows the logarithm to base 10 of the ratio, where ratio = (X output / ADC1 Input),
usually used to compensate for source intensity fluctuations in optical experiments.
5-32
Chapter 5, FRONT PANEL OPERATION
ADC1 Volts
Figure 5-39, ADC 1 Input
Shows the voltage applied to the rear panel ADC1 auxiliary input.
ADC2 Volts
Figure 5-40, ADC 2 Input
Shows the voltage applied to the rear panel ADC2 auxiliary input.
5-33
Chapter 5, FRONT PANEL OPERATION
X Offset
Figure 5-41, X Output Offset Control
This control allows the X output offset to be adjusted, using the lower ADJUST keys
adjacent to the displayed value. Note that although this control adjusts the level of the
offset, it is only applied if the OFFSET control on the left-hand LCD is set to X or
BOTH. If the X offset is ON then the description on the LCD display of all outputs
which would be affected, such as X% fs, or MAG % fs, is alternated with the
warning message OFFSET!
Y Offset
Figure 5-42, Y Output Offset Control
This control allows the Y output offset to be adjusted, using the lower ADJUST keys
adjacent to the displayed value. Note that although this control adjusts the level of the
offset, it is only applied if the OFFSET control on the left-hand LCD is set to Y or
BOTH. If the Y offset is ON then the description on the LCD display of all outputs
which would be affected, such as Y% fs, or MAG % fs, is alternated with the
5-34
Chapter 5, FRONT PANEL OPERATION
warning message OFFSET!
The instrument provides a quick way to switch between the following pairs of
outputs, by simply pressing simultaneously both ends of the SELECT keys adjacent
to their description:-
X %fs
Y %fs
Mag %fs
Noise %fs
↔
↔
↔
↔
X volts or amps
Y volts or amps
Mag volts or amps
Noise volts/√Hz or amps/√Hz
5.6 Typical Lock-in Amplifier Experiment
Having discussed how the instrument’s controls may be adjusted and outputs
displayed, readers may find the following basic checklist helpful in setting up the
instrument for manual operation.
Auto-Default
Use the Auto-Default function on the Control Setup menu to set the instrument to a
known state.
Selection of Signal Input
Use the Input Setup menu to select voltage (single-ended or differential) or current
input mode, and connect your signal source to the relevant A and/or B/I input
connector(s).
Selection of Reference Mode
The default setting function will have set the reference mode to internal, which
assumes that the internal oscillator will be used as a source of excitation to your
experiment. Use the Internal Oscillator amplitude and frequency controls to set the
required oscillator output, and connect the output signal from the OSC OUT
connector to your experiment.
If using external reference mode, use the Reference Setup menu to select one of the
two External modes, and connect your reference signal to the specified connector.
Auto-Measure
Use the Auto-Measure function to set the instrument so that it is correctly displaying
your signal.
Other Adjustments
You may now adjust the other controls as required, and choose the outputs you wish
to display. In particular, if you want to use the analog outputs of the instrument, use
the Output Setup menu to specify what the output signals at the CH1 and CH2
connectors should represent.
5-35
Chapter 5, FRONT PANEL OPERATION
5-36
Computer Operation
Chapter 6
6.1 Introduction
The model 7220 includes both RS232 and IEEE-488 (also known as GPIB for
General Purpose Interface Bus) interface ports, designed to allow the lock-in
amplifier to be completely controlled from a remote computer. All the instrument’s
controls may be operated, and all the outputs read, via these interfaces. In addition,
there are some functions, such as curve storage and oscillator frequency sweeps
which may only be accessed remotely.
This chapter describes the capabilities of the instrument when operated remotely and
discusses how this is done.
6.2 Capabilities
6.2.01 General
All instrument controls which may be set using the front panel may also be set
remotely. In addition, the current setting of each control may be determined by the
computer. All instrument outputs which may be displayed on the front panel may also
be read remotely.
When operated via the interfaces, the following features are also available:-
6.2.02 Curve Storage
A 32768 point memory is included in the instrument. This may be used as a single
curve or split into a number of curves, each of which will record chosen instrument
outputs when an acquisition is initiated. The memory is useful for overcoming speed
limitations in the interfaces, allowing outputs to be recorded faster than would be
possible by transferring them to the computer. It also finds use in experiments where
data is recorded over a long period of time, since it frees the computer from the need
to measure time intervals and send requests for output to the instrument. On
completion of the acquisition, the stored curves are transferred to the computer for
processing.
6.2.03 Burst Mode Acquisition
A special use of the curve storage memory is as a transient recorder, when the voltage
at the ADC1 or ADC2 input is sampled and stored at rates up to 40 kHz. Again,
stored curves are transferred to the computer for processing.
6-1
Chapter 6, COMPUTER OPERATION
6.2.04 Internal Oscillator Frequency Sweep Generator
The instrument’s internal oscillator may be swept in frequency both linearly and
logarithmically over a specified range. This facility allows the instrument to function
as a simple swept-frequency oscillator, or, in conjunction with the curve storage
capability, allows frequency response curves to be recorded.
6.3 RS232 and GPIB Operation
6.3.01 Introduction
Control of the lock-in amplifier from a computer is accomplished by means of
communications over the RS232 or GPIB interfaces. The communication activity
consists of the computer sending commands to the lock-in amplifier, and the lock-in
amplifier responding, either by sending back some data or by changing the setting of
one of its controls. The commands and responses are encoded in standard 7-bit ASCII
format, with one or more additional bits as required by the interface (see below).
The two ports cannot be used simultaneously, but when a command has been
completed, the lock-in amplifier will accept a command at either port. Also when the
test echo facility has been activated all output from the computer to the GPIB can be
monitored by a terminal attached to the RS232 connector.
Although the interface is primarily intended to enable the lock-in amplifier to be
operated by a computer program specially written for an application, it can also be
used in the direct, or terminal, mode. In this mode the user enters commands on a
keyboard and reads the results on a video screen.
The simplest way to establish the terminal mode is to connect a standard terminal, or
a terminal emulator, to the RS232 port. A terminal emulator is a computer running
special-purpose software that makes it act as a terminal. In the default (power-up)
state of the port, the lock-in amplifier sends a convenient prompt character when it is
ready to receive a command, and echoes each character that is received.
Microsoft Windows versions 3.1 and 3.11 include a program called Terminal, usually
in the Accessories group, which may be used as a terminal emulator. On the other
hand, a simple terminal program with minimal facilities can be written in a few lines
of BASIC code (see appendix C.1).
6.3.02 RS232 Interface - General Features
The RS232 interface in the model 7220 is implemented with three wires; one carries
digital transmissions from the computer to the lock-in amplifier, the second carries
digital transmissions from the lock-in amplifier to the computer and the third is the
Logic Ground to which both signals are referred. The logic levels are ±12 V referred
to Logic Ground, and the connection may be a standard RS232 cable in conjunction
with a null modem or alternatively may be made up from low-cost general purpose
cable. The pinout of the RS232 connectors are shown in appendix B and cable
diagrams suitable for coupling the instrument to a computer are shown in
appendix D.
6-2
Chapter 6, COMPUTER OPERATION
The main advantages of the RS232 interface are:
1. It communicates via a serial port which is present as standard equipment on
nearly all computers, using leads and connectors which are available from
suppliers of computer accessories or can be constructed at minimal cost in the
user’s workshop.
2. It requires no more software support than is normally supplied with the computer,
for example Microsoft’s GWBASIC, QBASIC or Windows Terminal mode.
A single RS232 transmission consists of a start bit followed by 7 or 8 data bits, an
optional parity bit, and 1 stop bit. The rate of data transfer depends on the number of
bits per second sent over the interface, usually called the baud rate. In the model 7220
the baud rate can be set to a range of different values up to 19,200, corresponding to
a minimum time of less than 0.5 ms for a single character.
Mostly for historical reasons, there are a very large number of different ways in
which RS232 communications can be implemented. Apart from the baud rate options,
there are choices of data word length (7 or 8 bits), parity check operation (even, odd
or none), and number of stop bits (1 or 2). With the exception of the number of stop
bits, which is fixed at 1, these settings may be adjusted using the three front-panel
RS232 SETUP menus, discussed in chapter 4. They may also be adjusted by means
of the RS command.
In order to achieve satisfactory operation, the RS232 settings must be set to
exactly the same values in the terminal or computer as in the lock-in amplifier.
6.3.03 Choice of Baud Rate
Where the lock-in amplifier is connected to a terminal or to a computer implementing
an echo handshake, the highest available baud rate of 19,200 is normally used if, as is
usually the case, this rate is supported by the terminal or computer. Lower baud rates
may be used in order to achieve compatibility with older equipment or where there is
some special reason for reducing the communication rate.
6.3.04 Choice of Number of Data Bits
The model 7220 lock-in amplifier uses the standard ASCII character set, containing
127 characters represented by 7-bit binary words. If an 8-bit data word is selected,
the most significant bit is set to zero on output from the lock-in amplifier and ignored
on input. The result is that either the 8-bit or the 7-bit option may be used, but the
7-bit option can result in slightly faster communication.
6.3.05 Choice of Parity Check Option
Parity checks are not required at the baud rates available in the model 7220, that is up
to 19,200 baud, with typical cable lengths of up to a few meters. Therefore no
software is provided in the model 7220 for dealing with parity errors. Where long
cables are in use, it may be advisable to make use of a lower baud rate. The result is
that any of the parity check options may be used, but the no-parity option will result
in slightly faster communication.
6-3
Chapter 6, COMPUTER OPERATION
Where the RS232 parameters of the terminal or computer are capable of being set to
any desired value, an arbitrary choice must be made. In the model 7220 the
combination set at the factory is even parity check, 7 data bits, and one stop bit
(fixed) because these are the MS-DOS default.
6.3.06 Auxiliary RS232 Interface
The auxiliary RS232 interface allows up to sixteen model 7220s or a mixture of
compatible instruments to be connected to one serial port on the computer. The first
lock-in amplifier is connected to the computer in the usual way. Additional lock-in
amplifiers are connected in a daisy-chain fashion using null-modem cables, the
AUX RS232 port of the first to the RS232 port of the second, the AUX RS232 port
of the second to the RS232 port of the third, etc. The address of the lock-in amplifier
must be set up from the front panel before any communication takes place. At power-
up the RS232 port of each lock-in amplifier is fully active irrespective of its address.
Since this means that all lock-in amplifiers in the daisy-chain are active on power-up,
the first command must be \N n where n is the address of one of the lock-in
amplifiers. This will deselect all but the addressed lock-in amplifier. When it is
required to communicate with another lock-in amplifier, send a new \N n command
using the relevant address.
Note: When programming in C remember that in order to send the character \ in a
string it is necessary to type in \\.
6.3.07 GPIB Interface - General Features
The GPIB is a parallel digital interface with 8 bidirectional data lines, and 8 further
lines which implement additional control and communication functions.
Communication is through 24-wire cables (including 8 ground connections) with
special-purpose connectors which are constructed in such a way that they can be
stacked on top of one another to enable numerous instruments to be connected in
parallel. By means of internal hardware or software switches, each instrument is set
to a different address on the bus, usually a number in the range 0 to 31. In the model
7220 the address is set using the GPIB SETUP 1 setup menu or by means of the GP
command.
A most important aspect of the GPIB is that its operation is defined in minute detail
by the IEEE-488 standard, usually implemented by highly complicated special-
purpose semiconductor devices that are present in each instrument and communicate
with the instrument’s microprocessor. The existence of this standard greatly simplifies
the problem of programming the bus controller, i.e. the computer, to implement
complex measurement and test systems involving the interaction of numerous
instruments. There are fewer interface parameters to be set than with RS232
communications.
The operation of the GPIB requires the computer to be equipped with special-purpose
hardware, usually in the form of a plug-in card, and associated software which enable
it to act as a bus controller. The control program is written in a high-level language,
usually BASIC or C, containing additional subroutines implemented by software
supplied by the manufacturer of the interface card.
6-4
Chapter 6, COMPUTER OPERATION
Because of the parallel nature of the GPIB and its very effective use of the control
lines including the implementation of a three-wire handshake (see below),
comparatively high data rates are possible, up to a few hundred thousand bytes per
second. In typical setups the data rate of the GPIB itself is not the factor that limits
the rate of operation of the control program.
6.3.08 Handshaking and Echoes
A handshake is a method of ensuring that the transmitter does not send a byte until
the receiver is ready to receive it, and, in the case of a parallel interface, that the
receiver reads the data lines only when they contain a valid byte.
GPIB Handshaking
The GPIB interface includes three lines (*DAV, *NRFD, *NDAC) which are used to
implement a three-wire handshake. The operation of this is completely defined by the
IEEE-488 standard and is fully automatic, so that the user does not need to know
anything about the handshake when writing programs for the GPIB. Note that each
command must be correctly terminated.
RS232 Handshaking
In the RS232 standard there are several control lines called handshake lines (RTS,
DTR outputs and CTS, DSR, DCD inputs) in addition to the data lines (TD output
and RD input). However, these lines are not capable of implementing the handshaking
function required by the model 7220 on a byte-by-byte basis and are not connected in
the model 7220 apart from the RTS and DTR outputs which are constantly asserted.
Note that some computer applications require one or more of the computer’s RS232
handshake lines to be asserted. If this is the case, and if the requirement cannot be
changed by the use of a software switch, the cable may be used in conjunction with a
null modem. A null modem is an adaptor which connects TD on each side through to
RD on the other side, and asserts CTS, DSR, and DCD on each side when RTS and
DTR are asserted on the other side.
With most modern software there is no need to assert any RS232 handshake lines and
a simple three-wire connection can be used. The actual handshake function is
performed by means of bytes transmitted over the interface.
The more critical handshake is the one controlling the transfer of a command from the
computer to the lock-in amplifier, because the computer typically operates much
faster than the lock-in amplifier and bytes can easily be lost if the command is sent
from a program. (Note that because of the limited speed of human typing, there is no
problem in the terminal mode.) Therefore an echo handshake is used, which works in
the following way: after receiving each byte, the lock-in amplifier sends back an echo,
that is a byte which is a copy of the one that it has just received, to indicate that it is
ready to receive the next byte. Correspondingly, the computer does not send the next
byte until it has read the echo of the previous one. Usually the computer makes a
comparison of each byte with its echo, and this constitutes a useful check on the
validity of the communications.
Where the echo is not required, it can be suppressed by negating bit 3 in the RS232
6-5
Chapter 6, COMPUTER OPERATION
parameter byte. The default (power-up) state of this bit is for it to be asserted.
The program RSCOM2.BAS in section C.2 illustrates the use of the echo handshake.
6.3.09 Terminators
In order for communications to be successfully established between the lock-in
amplifier and the computer, it is essential that each transmission, i.e. command or
command response, is terminated in a way which is recognizable by the computer and
the lock-in amplifier as signifying the end of that transmission.
In the model 7220 there are three input termination options for GPIB
communications, selected from the front panel under the GPIB SETUP 1 menu or by
means of the GP command. The lock-in amplifier may be set to expect the <CR> byte
(ASCII 13) or the <CR,LF> sequence (ASCII 13 followed by ASCII 10) to be
appended by the controller as a terminator to the end of each command, or
alternatively instead of a terminator it may expect the EOI signal line (pin 5 on the
GPIB connector) to be asserted during the transmission of the last character of the
command. The third option is normally to be preferred with modern interface cards
which can easily be set to a wide variety of configurations.
The selected GPIB termination option applies also to the output termination of any
responses sent back by the lock-in amplifier to the controller, i.e. the lock-in amplifier
will send <CR> or <CR,LF> or no byte as appropriate. In all cases the lock-in
amplifier asserts the EOI signal line during the transmission of the last byte of a
response.
In RS232 communications, the lock-in amplifier automatically accepts either <CR>
or <CR,LF> as an input command terminator, and sends out <CR,LF> as an output
response terminator except when the noprompt bit (bit 4 in the RS232 parameter
byte) is set in which case the terminator is <CR>. The default (power-up) state of this
bit is zero.
6.3.10 Command Format
The simple commands listed in section 6.4 have one of five forms:
CMDNAME terminator
CMDNAME n terminator
CMDNAME [n] terminator
CMDNAME [n1 [n2]] terminator
CMDNAME n1 [n2] terminator
where CMDNAME is an alphanumeric string that defines the command, and n, n1, n2
are parameters separated by spaces. When n is not enclosed in square brackets it must
be supplied. [n] means that n is optional. [n1 [n2]] means that n1 is optional and if
present may optionally be followed by n2. Upper-case and lower-case characters are
equivalent. Terminator bytes are defined in section 6.3.09.
Where the command syntax includes optional parameters and the command is
6-6
Chapter 6, COMPUTER OPERATION
sent without the optional parameters, the response consists of a transmission of
the present values of the parameter(s).
Any response transmission consists of one or more numbers followed by a response
terminator. Where the response consists of two or more numbers in succession, they
are separated by a delimiter (section 6.3.11).
Some commands have an optional floating point mode which is invoked by appending
a . (full stop) character to the end of the command and before the parameters. This
allows some parameters to be entered or read in floating point format. The floating
point output format is given below.
±1.234E±01
The number of digits between the decimal point and the exponent varies depending on
the number but is a minimum of one and a maximum of eight. The input format is not
as strict but if a decimal point is used there must be a digit before it. An exponent is
optional. The following are all legal commands for setting the oscillator frequency to
100.1 Hz:
OF. 100.1
OF. 1.001E2
OF. +1.001E+02
OF. 1001E-1
6.3.11 Delimiters
Any response transmissions consist of one or two numbers followed by a response
terminator. Where the response of the lock-in amplifier consists of two numbers in
succession, they are separated by a byte called a delimiter. This delimiter can be any
printing ASCII character and is selected via the RS232 SETUP 2 setup screen or by
the use of the DD command.
6.3.12 Compound Commands
A compound command consists of two or more simple commands separated by
semicolons (ASCII 59) and terminated by a single command terminator. If any of the
responses involve data transmissions, each one is followed by an output terminator.
6.3.13 Status Byte, Prompts and Overload Byte
An important feature of the IEEE-488 standard is the serial poll operation by which a
special byte, the status byte, may be read at any time from any instrument on the bus.
This contains information which must be urgently conveyed from the instrument to
the controller.
The function of the individual bits in the status byte is instrument dependent, apart
from bit 6 (the request service bit) whose functions are defined by the standard.
In the model 7220, bits 0 and 7 signify ‘command complete’ and ‘data available’
6-7
Chapter 6, COMPUTER OPERATION
respectively. In GPIB communications, the use of these bits can lead to a useful
simplification of the control program by providing a response subroutine which is the
same for all commands, whether or not they send a response over the bus. The
principle is that after any command is sent, serial poll operations are repeatedly
executed. After each operation bit 0 is tested; if this is found to be negated then bit 7
is tested, and if this is asserted then a read operation is performed. Serial poll
operations continue until the lock-in amplifier asserts bit 0 to indicate that the
command-response sequence is complete. This method deals successfully with
compound commands.
In RS232 communications, comparatively rapid access to the status byte is provided
by the prompt character which is sent by the lock-in amplifier at the same time as
bit 0 becomes asserted in the status byte. This character is sent out by the lock-in
amplifier after each command response (whether or not the response includes a
transmission over the interface) to indicate that the response is finished and the
instrument is ready for a new command. The prompt takes one of two forms. If the
command contained an error, either in syntax or by a command parameter being out
of range, or alternatively if an overload or reference unlock is currently being
reported by the front-panel indicators, the prompt is ? (ASCII 63). Otherwise the
prompt is * (ASCII 42).
These error conditions correspond to the assertion of bits 1, 2, 3 or 4 in the status
byte. When the ? prompt is received by the computer, the ST command may be issued
in order to discover which type of fault exists and to take appropriate action.
The prompts are a rapid way of checking on the instrument status and enable a
convenient keyboard control system to be set up simply by attaching a standard
terminal, or a simple computer-based terminal emulator, to the RS232 port. Where
the prompt is not required it can be suppressed by setting the noprompt bit, bit 4 in
the RS232 parameter byte. The default (power-up) state of this bit is zero.
Because of the limited number of bits in the status byte, it can indicate that an
overload exists but cannot give more detail. An auxiliary byte, the overload byte
returned by the N command, gives details of the location of the overload.
A summary of the bit assignments in the status byte and the overload byte is given
below.
Status Byte
Overload Byte
bit 0
bit 1
bit 2
bit 3
bit 4
bit 5
command complete
invalid command
command parameter error
reference unlock
overload
new ADC values available
after external trigger
asserted SRQ
not used
CH1 output overload
CH2 output overload
Y output overload
X output overload
not used
input overload
reference unlock
bit 6
bit 7
data available
6-8
Chapter 6, COMPUTER OPERATION
6.3.14 Service Requests
The interface defined by the IEEE-488 standard includes a line (pin 10 on the
connector) called the SRQ (service request) line which is used by the instrument to
signal to the controller that urgent attention is required. At the same time that the
instrument asserts the SRQ line, it also asserts bit 6 in the status byte. The controller
responds by executing a serial poll of all the instruments on the bus in turn and testing
bit 6 of the status byte in order to discover which instrument was responsible for
asserting the SRQ line. The status byte of that instrument is then further tested in
order to discover the reason for the service request and to take appropriate action.
In the model 7220 the assertion of the SRQ line is under the control of a byte called
the SRQ mask byte which can be set by the user with the MSK command. If any bit
in the status byte becomes asserted, and the corresponding bit in the mask byte has a
non-zero value, the SRQ line is automatically asserted. If the value of the mask byte
is zero, the SRQ line is never asserted.
Hence, for example, if the SRQ mask byte is set to 16, a service request would be
generated as soon as an overload occurred; if the SRQ mask byte were set to 0, then
service requests would never be generated.
6-9
Chapter 6, COMPUTER OPERATION
6.4 Command Descriptions
This section lists the commands in logical groups, so that, for example, all commands
associated with setting controls affecting the signal channel are shown together.
Appendix E gives the same list of commands but in alphabetical order.
6.4.01 Signal Channel
IMODE [n] Controls whether the instrument input is connected to a current or a voltage
preamplifier
The value of n sets the input mode according to the following table:
n
0
1
Input mode
Current mode off - voltage mode input enabled
High bandwidth (HB) current mode enabled - connect signal to B input
connector
2
Low noise (LN) current mode enabled - connect signal to B inputconnector
If n = 0 then the input configuration is determined by the VMODE command.
If n > 0 then current mode is enabled irrespective of the VMODE setting.
VMODE [n]
Voltage input configuration
The value of n sets up the input configuration according to the following table:
n
0
1
3
Input configuration
Both inputs grounded (test mode)
A input only
A-B differential mode
Note that the IMODE command takes precedence over the VMODE command.
FET [n]
Voltage mode input device control
The value of n selects the input device according to the following table:
n
0
1
Selection
Bipolar device, 10 kΩ input impedance, 2 nV/√Hz voltage noise
FET, 10 MΩ input impedance, 5 nV/√Hz voltage noise
FLOAT [n] Input connector shield float / ground control
The value of n sets the input shield switch according to the following table:
n
0
1
Selection
Ground
Float (connected to ground via a 1 kΩ resistor)
6-10
Chapter 6, COMPUTER OPERATION
CP [n]
Input coupling control
The value of n sets the input coupling mode according to the following table:
n
0
1
Coupling mode
AC
DC
SEN[.] [n] Full-scale sensitivity control
The value of n sets the full-scale sensitivity according to the following table,
depending on the setting of the IMODE control:
n
full-scale sensitivity
IMODE=1
20 fA
IMODE=0
20 nV
50 nV
100 nV
200 nV
500 nV
1 µV
IMODE=2
n/a
4
5
6
7
8
50 fA
n/a
n/a
n/a
n/a
100 fA
200 fA
500 fA
1 pA
9
n/a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
2 µV
5 µV
2 pA
5 pA
10 pA
20 pA
20 fA
50 fA
100 fA
200 fA
500 fA
1 pA
10 µV
20 µV
50 µV
100 µV
200 µV
500 µV
1 mV
50 pA
100 pA
200 pA
500 pA
1 nA
2 nA
5 nA
10 nA
20 nA
2 pA
5 pA
10 pA
20 pA
50 pA
100 pA
200 pA
500 pA
1 nA
2 nA
5 nA
10 nA
2 mV
5 mV
10 mV
20 mV
50 mV
100 mV
200 mV
500 mV
1 V
50 nA
100 nA
200 nA
500 nA
1 µA
Floating point mode can only be used for reading the sensitivity, which is reported in
volts or amps. For example, if IMODE = 0 and the sensitivity is 1 mV the command
SEN would report 18 and the command SEN. would report +1.0E-03. If IMODE was
changed to 1, SEN would still report 18 but SEN. would report +1.0E-09.
AS
Perform an Auto-Sensitivity operation
The instrument adjusts its full-scale sensitivity so that the magnitude output lies
between 30 % and 90 % of full-scale.
6-11
Chapter 6, COMPUTER OPERATION
ASM Perform an Auto-Measure operation
The instrument adjusts its full-scale sensitivity so that the magnitude output lies
between 30 % and 90 % of full-scale, and then performs an auto-phase operation to
maximize the X output and minimize the Y output.
ACGAIN [n] AC Gain control
Sets the gain of the signal channel amplifier. Values of n from 0 to 9 can be entered,
corresponding to the range 0 dB to 90 dB in 10 dB steps.
AUTOMATIC [n]
AC Gain automatic control
The value of n sets the status of the AC Gain control according to the following table:
n
0
Status
AC Gain is under manual control, either using the front panel or the ACGAIN
command
1
Automatic AC Gain control is activated, with the gain being adjusted
according to the full-scale sensitivity setting
LF [n]
Signal channel line frequency rejection filter control
In instruments manufactured prior to June 1996, the value of n sets the mode of the
line frequency notch filter according to the following table:
n
0
1
Selection
Off
On (i.e. reject 50/60 Hz and 100/120 Hz)
In instruments manufactured after June 1996, the value of n sets the mode of the line
frequency notch filter according to the following table:
n
0
1
2
3
Selection
Off
Enable 50 or 60 Hz notch filter
Enable 100 or 120 Hz notch filter
Enable both filters
Users may identify which version of the instrument they have by sending the
command LF 3; if this is accepted by the instrument, it was made after June 1996,
but if it generates a command error, it was made prior to this date.
Units made after June 1996 respond in addition to a new command, LINE50, which
sets the notch filter centre frequency.
LINE50 [n]
Signal channel line frequency rejection filter centre frequency control
The value of n sets the line frequency notch filter centre frequency according to the
following table:
6-12
Chapter 6, COMPUTER OPERATION
n
0
1
Notch filter mode
60 Hz (and/or 120 Hz)
50 Hz (and/or 100 Hz)
Units made prior to June 1996 generate an Invalid Command (bit 1 of the serial poll
status byte is asserted) to the LINE50 command.
SAMPLE [n] Main analog to digital converter sample rate control
The sampling rate of the main analog to digital converter, which is nominally
166 kHz, may be adjusted from this value to avoid problems caused by the aliasing
of interfering signals into the output passband.
n may be set to 0, 1, 2 or 3, corresponding to four different sampling rates (not
specified) near 166 kHz.
6.4.02 Reference Channel
IE [n]
Reference channel source control (Internal/External)
The value of n sets the reference input mode according to the following table:
n
0
1
2
Selection
INT (internal)
EXT LOGIC (external rear panel TTL input)
EXT (external front panel analog input)
FNF [n]
Reference harmonic mode control
The value of n sets the reference channel to one of the NF modes, or restores it to the
default F mode according to the following table:
n
1
2
3
Mode selected
The lock-in amplifier measures signals at the reference frequency F
The lock-in amplifier measures at 2F
The lock-in amplifier measures at 3F
REFP[.] [n] Reference phase control
In fixed point mode n sets the phase in millidegrees in the range ±360000.
In floating point mode n sets the phase in degrees.
AQN
Auto-Phase (auto quadrature null)
The instrument adjusts the reference phase to maximize the X output and minimize
the Y output signals.
6-13
Chapter 6, COMPUTER OPERATION
FRQ[.]
Reference frequency meter
If the lock-in amplifier is in the EXT or EXT LOGIC reference source modes, the
FRQ command causes the lock-in amplifier to respond with 0 if the reference channel
is unlocked, or with the reference input frequency if it is locked.
If the lock-in amplifier is in the INT reference source mode, it responds with the
frequency of the internal oscillator.
In fixed point mode the frequency is in mHz.
In floating point mode the frequency is in Hz.
LOCK
System lock control
Updates all frequency dependent gain and phase correction parameters.
6.4.03 Signal Channel Output Filters
SLOPE [n] Output low-pass filter slope (roll-off) control
The value of n sets the slope of the output filters according to the following table:
n
0
1
2
3
Slope
6 dB/octave
12 dB/octave
18 dB/octave
24 dB/octave
TC [n]
TC.
Filter time constant control
The value of n sets the time constant of the output according to the following table:
n
0
1
2
3
4
5
6
7
time constant
10 µs
20 µs
40 µs
80 µs
160 µs
320 µs
640 µs
5 ms
8
9
10 ms
20 ms
50 ms
100 ms
200 ms
500 ms
1 s
10
11
12
13
14
15
16
2 s
5 s
6-14
Chapter 6, COMPUTER OPERATION
17
18
19
20
21
22
23
24
25
10 s
20 s
50 s
100 s
200 s
500 s
1 ks
2 ks
5 ks
The TC. command is only used for reading the time constant, and reports the current
setting in seconds. Hence if a TC 11 command were sent, TC would report 11 and
TC. would report 1.0E-01, i.e. 0.1 s or 100 ms.
SYNC [n] Synchronous time constant control
At reference frequencies below 10 Hz, if the synchronous time constant is enabled,
then the actual time constant of the output filters is not generally the selected value T
but rather a value equal to an integer number of reference cycles. If T is greater than
1 cycle, the time constant is between T/2 and T. The parameter n has the following
significance:
n
0
1
Effect
Synchronous time constant disabled
Synchronous time constant enabled
6.4.04 Signal Channel Output Amplifiers
XOF [n1 [n2]] X output offset control
The value of n1 sets the status of the X offset facility according to the following table:
n1
0
1
Selection
Disables offset facility
Enables offset facility
The range of n2 is ±30000 corresponding to ±300 % full-scale.
YOF [n1 [n2]] Y output offset control
The value of n1 sets the status of the Y offset facility according to the following table:
n1
0
1
Selection
Disables offset facility
Enables offset facility
The range of n2 is ±30000 corresponding to ±300 % full-scale.
6-15
Chapter 6, COMPUTER OPERATION
AXO
Auto-Offset
The X and Y output offsets are turned on and set to levels giving zero X and Y
outputs. Any changes in the input signal then appear as changes about zero in the
outputs.
EX [n]
Output expansion control
Expands X and/or Y outputs by a factor of 10. Changes meter, CH1 and CH2
outputs full-scale to ±10 % if X or Y selected. The value of n has the following
significance:
n
0
1
2
3
Expand mode
Off
Expand X
Expand Y
Expand X and Y
CH n1 [n2] Analog output control
Defines what outputs appear on the rear panel CH1 and CH2 connectors according
to the following table:
n2 Signal
0
1
2
3
4
5
6
X %FS
Y %FS
Magnitude %FS
Phase 1:- +9 V = +180°, -9 V = -180°
Phase 2:- +9 V = 360°, - 9 V = 0°
Noise %FS
Ratio:- (1000 × X)/ADC 1
n1 is compulsory and is either 1 for CH1 or 2 for CH2
6.4.05 Instrument Outputs
X[.]
X output
In fixed point mode causes the lock-in amplifier to respond with the X demodulator
output in the range ±30000, full-scale being ±10000.
In floating point mode causes the lock-in amplifier to respond with the X demodulator
output in volts or amps.
Y[.]
Y output
In fixed point mode causes the lock-in amplifier to respond with the Y demodulator
output in the range ±30000, full-scale being ±10000.
In floating point mode causes the lock-in amplifier to respond with the Y demodulator
output in volts or amps.
6-16
Chapter 6, COMPUTER OPERATION
XY[.]
X, Y outputs
Equivalent to the compound command X[.];Y[.]
MAG[.]
Magnitude
In fixed point mode causes the lock-in amplifier to respond with the magnitude value
in the range 0 to 30000, full-scale being 10000.
In floating point mode causes the lock-in amplifier to respond with the magnitude
value in the range +3.000E0 to +0.001E-9 volts or +3.000E-6 to +0.001E-15 amps.
PHA[.]
Signal phase
In fixed point mode causes the lock-in amplifier to respond with the signal phase in
centidegrees, in the range ±18000.
In floating point mode causes the lock-in amplifier to respond with the signal phase in
degrees.
MP[.]
RT[.]
Magnitude, phase
Equivalent to the compound command MAG[.];PHA[.]
Ratio output
In integer mode the RT command reports a number equivalent to 1000 × X / ADC1
where X is the value that would be returned by the X command and ADC1 is the
value that would be returned by the ADC1 command.
In floating point mode the RT. command reports a number equivalent to X / ADC 1.
Log Ratio output
LR[.]
In integer mode, the LR command reports a number equivalent to
1000 log (X / ADC1) where X is the value that would be returned by the X command
and ADC1 is the value that would be returned by the ADC1 command. The response
range is -3000 to +2079.
In floating point mode, the LR. command reports a number equivalent to
log (X / ADC 1). The response range is -3.000 to +2.079.
NHZ.
Causes the lock-in amplifier to respond with the square root of the noise spectral
density measured at the Y output, expressed in volt/√Hz or amp/√Hz referred to the
input. This measurement assumes that the Y output is Gaussian with zero mean.
(Section 3.10). The command is only available in floating point mode.
This command is not available when the reference frequency exceeds 60 kHz.
6-17
Chapter 6, COMPUTER OPERATION
ENBW[.]
Equivalent noise bandwidth
In fixed point mode, reports the equivalent noise bandwidth of the output low-pass
filters at the current time constant setting in microhertz.
In floating point mode, reports the equivalent noise bandwidth of the output low-pass
filters at the current time constant setting in hertz.
This command is not available when the reference frequency exceeds 60 kHz.
NN[.]
Noise output
In fixed point mode causes the lock-in amplifier to respond with the mean absolute
value of the Y output in the range 0 to 12000, full-scale being 10000. If the mean
value of the Y output is zero, this is a measure of the output noise.
In floating point mode causes the lock-in amplifier to respond in volts.
STAR [n]
Star mode setup command
The star mode allows faster access to instrument outputs than is possible using the
conventional commands listed above. The mode is set up using the STAR command
to specify the output(s) required and invoked by sending an asterisk (ASCII 42) to
request the data. The data returned is specified by the value of n, as follows:
n
0
1
2
3
4
5
6
7
Data returned by * command
X
Y
MAG
PHA
ADC1
XY
MP
ADC1;ADC2
*
Transfer command
This command establishes the high-speed transfer mode. Use the STAR command to
set up the desired response to the * command, and then send an * (ASCII 42), without
terminator, to the instrument. The instrument will reply with the selected output as
quickly as possible, and then wait for another *. If the computer processes the reply
quickly and responds immediately with another *, then very rapid controlled data
transfer is possible.
The first transfer takes a little longer than subsequent ones because some overhead
time is required for the model 7220 to get into the high speed transfer mode. When in
this mode, the front panel controls are inactive and display is frozen.
The mode is terminated by sending any command other than an *, when the
instrument will exit the mode and process the new command, or after a period of 10
seconds following the last * command.
6-18
Chapter 6, COMPUTER OPERATION
Caution: Check that the computer program does not automatically add a carriage
return or carriage return-line feed terminator to the * command, since these
characters will slow down communications.
6.4.06 Internal Oscillator
OA[.] [n] Oscillator amplitude control
In fixed point mode n sets the oscillator amplitude in mV. The range of n is 0 to 5000
representing 0 to 5 V rms.
In floating point mode n sets the amplitude in volts
OF[.] [n]
Oscillator frequency control
In fixed point mode n sets the oscillator frequency in mHz. The range of n is 0 to
120,000,000 representing 0 to 120 kHz.
In floating point mode n sets the oscillator frequency in Hz. The range of n is 0 to
1.2E5.
SYNCOSC [n]
Synchronous oscillator (demodulator monitor) control
This control operates only in external reference mode. The parameter n has the
following significance:
n
0
1
Effect
Synchronous Oscillator (Demodulator Monitor) disabled
Synchronous Oscillator (Demodulator Monitor) enabled
When enabled and in external reference mode, the instrument’s OSC OUT connector
functions as a demodulator monitor of the X channel demodulation function.
FSTART[.] [n] Oscillator frequency sweep start frequency
Sets the start frequency for a subsequent sweep of the internal oscillator frequency.
In fixed point mode, n is in millihertz.
In floating point mode n is in hertz.
FSTOP[.] [n] Oscillator frequency sweep stop frequency
Sets the stop frequency for a subsequent sweep of the internal oscillator frequency.
In fixed point mode, n is in millihertz.
In floating point mode n is in hertz.
6-19
Chapter 6, COMPUTER OPERATION
FSTEP[.] [n1 n2]
Oscillator frequency sweep step size and type
The frequency may be swept either linearly or logarithmically, as specified by
parameter n2. The step size is specified by parameter n1.
Log sweep n2 = 0
In fixed point mode, n1 is the step size in thousandths of a percent.
In floating point mode n1 is in percent. The range of n1 is 0 to 100.00 %
Linear sweep n2 = 1
In fixed point mode, n1 is the step size in millihertz.
In floating point mode n1 is in hertz. The range of n1 is 0 to 10 kHz
SRATE[.] [n] Oscillator frequency sweep step rate
Sets the sweep rate in time per step in the range 50 ms to 1000 s, in 5 ms increments.
SWEEP [n] Oscillator frequency sweep start/stop
Starts/stops the internal oscillator frequency sweep depending on the value of n
according to the following table:
n
0
1
Frequency sweep status
Stop/Pause
Run
When a frequency sweep has been defined, applying SWEEP 1 will start it. The
sweep will continue until the stop frequency is reached. If, during the sweep,
SWEEP 0 is applied, the sweep will stop at the current frequency. If SWEEP 1 is
then applied, the sweep will restart from this point. Once the sweep reaches the stop
frequency and stops, the next SWEEP 1 command will reset the frequency to the start
frequency and restart the sweep.
6.4.07 Auxiliary Outputs
DAC[.] n1 [n2] Auxiliary DAC output controls
Sets the voltage appearing at the rear panel DAC1 and DAC2 outputs.
The first parameter n1, which specifies the DAC, is compulsory and is either 1 or 2.
The value of n2 specifies the voltage to be output.
In fixed point mode it is an integer in the range -12000 to +12000, corresponding to
voltages from -12.000 V to +12.000 V.
In floating point mode it is in volts.
6-20
Chapter 6, COMPUTER OPERATION
BYTE [n] Digital output port control
The value of n, in the range 0 to 255, determines the bits to be output on the rear
panel digital output port. When n = 0, all outputs are low, and when n = 255, all are
high.
6.4.08 Auxiliary Inputs
ADC[.] n Read auxiliary analog to digital inputs
Reads the voltage appearing at the rear panel ADC1 (n = 1) and ADC2 (n = 2)
inputs.
In fixed point mode the response is an integer in the range -12000 to +12000,
corresponding to voltages from -12.000 V to +12.000 V.
In floating point mode it is in volts.
TADC [n] Auxiliary ADC trigger mode control
The value of n sets the trigger mode of the auxiliary ADC inputs according to the
following table:
n
0
1
2
3
4
5
6
7
Trigger mode
Asynchronous (5 ms intervals)
External (rear panel TRIG input)
Burst mode, fixed rate, triggered by command (ADC1 only)
Burst mode, fixed rate, triggered by command (ADC1 and ADC2)
Burst mode, variable rate, triggered by command (ADC1 only)
Burst mode, variable rate, triggered by command (ADC1 and ADC2)
Burst mode, fixed rate, External trigger (rear panel TRIG input) (ADC1 only)
Burst mode, fixed rate, External trigger (rear panel TRIG input) (ADC1 and
ADC2)
8
9
Burst mode, variable rate, External trigger (rear panel TRIG input) (ADC1
only)
Burst mode, variable rate, External trigger (rear panel TRIG input) (ADC1
and ADC2)
In the burst modes, data is stored in the curve buffer. Use the LEN command to set
the number of points required. Note that it may be necessary to enter CBD 32 before
setting the length, if the curve buffer has previously been used for more than one data
type. The data is read out from the buffer using DC[.] 5 for ADC1 and DC[.] 6 for
ADC2. If the length is set to more than 16384 and a burst mode which stores both
ADC1 and ADC2 is specified then the curve length will automatically be reduced to
16384 points. Note also that setting the TADC parameter to any value other than 0 or
1 may affect the CBD parameter, as follows:
6-21
Chapter 6, COMPUTER OPERATION
TADC parameter
Effect on CBD parameter
none
none
0
1
2
3
4
5
6
7
8
9
automatically set to 32
automatically set to 96
automatically set to 32
automatically set to 96
automatically set to 32
automatically set to 96
automatically set to 32
automatically set to 96
The maximum sampling rate depends on the number of ADC inputs used and whether
the sampling is timed or simply runs as fast as possible. In the modes above described
as Fixed Rate, sampling runs at the maximum possible rate, nominally 20 kHz when
sampling both ADC1 and ADC2 or 40 kHz when sampling ADC1 only. In the
Variable Rate modes, the sampling speed is set by the BURSTRATE command.
BURSTRATE [n]
Sets the burst mode sampling rate for ADC1 and ADC2
n sets the sample rate for the Variable Rate burst modes according to the following
equations:
When storing only to ADC1:
(i.e. TADC 2, TADC 4, TADC 6 and TADC 8)
16,000,000
Sample Rate = ————— Hz
((25 × n) + 157)
When storing to ADC1 and ADC 2:
(i.e. TADC 3, TADC 5, TADC 7 and TADC 9)
16,000,000
Sample Rate = ————— Hz
((25 × n) + 1031)
Note that these equations apply only to units manufactured after December 1995.
Earlier instruments used a 16.384 MHz instead of a 16.0 MHz crystal, so the above
equations should be modified accordingly by replacing the 16,000,000 figure with
16,384,000.
For example when n = 20, the sample rate will be 24,353 Hz for ADC1 for an
instrument with a 16.0 MHz crystal, and 24,937 Hz for a unit with a 16.384 MHz
crystal.
6-22
Chapter 6, COMPUTER OPERATION
6.4.09 Output Data Curve Buffer
CBD [n]
Curve buffer define
Defines which data outputs are stored in the curve buffer when subsequent TD (take
data) or TDC (take data continuously) commands are issued. Up to 16 curves, or
outputs, may be acquired, as specified by the CBD parameter.
The CBD is an integer between 0 and 65,535, being the decimal equivalent of a 16-bit
binary word. When a given bit in the word is asserted, the corresponding output is
selected for storage. When a bit is negated, the output is not stored. The bit function
and range for each output are shown in the table below:
Bit Decimal value
Output and range
0
1
1
2
X Output (±10000 FS)
Y Output (±10000 FS)
2
3
4
8
Magnitude Output (±10000 FS)
Phase (±18000 = ±180°)
4
5
6
7
8
9
10
11
12
13
14
15
16
32
64
Sensitivity setting (4 to 27) + IMODE (0, 1, 2 = 0, 32, 64)
ADC1 (±10000 = ±10.0 V)
ADC2 (±10000 = 10.0 V)
Unassigned
DAC1 (±10000 = 10.0 V)
DAC2 (±10000 = 10.0 V)
Noise (±10000 FS)
128
256
512
1024
2048
4096
8192
16384
32768
Ratio (±10000 FS)
Log ratio (-3000 to +2000)
Last value given to EVENT command
Reference frequency bits 0 to 15 (mHz)
Reference frequency bits 16 to 32 (mHz)
32768 points are available for data storage, shared equally between the specified
curves. For example, if all 16 outputs are stored then the maximum number of
storage points would be 2048 (i.e. 32768/16). The LEN command sets the actual
curve length, which cannot therefore be longer than 32768 divided by the number of
curves selected. If more curves are requested than can be stored with the current
buffer length, then the buffer length will be automatically reduced. Its actual length
can of course be determined by sending the LEN command without a parameter.
The reason why bit 4 is needed, which stores both the sensitivity and the IMODE
setting, is to allow the instrument to transfer the acquired curves to the computer in
floating point mode. Without this information, the unit would not be able to determine
the correct calibration to apply.
Curves 14 and 15 store the reference frequency in millihertz. The calculation needed
to translate these two 16-bit values to one 32-bit value is:
Reference Frequency = (65536 × value in Curve 15) + (value in Curve 14)
Note that the CBD command directly determines the allowable parameters for the DC
6-23
Chapter 6, COMPUTER OPERATION
and HC commands. It also interacts with the LEN command and affects the values
reported by the M command.
LEN [n]
Curve length control
The value of n sets the curve buffer length in effect for data acquisition. The
maximum allowed value depends on the number of curves requested using the CBD
command, and a parameter error results if the value given is too large. For this
reason, if the number of points is to be increased and the number of curves to be
stored is to be reduced using the CBD command, then the CBD command should be
issued first.
NC
New curve
Initializes the curve storage memory and status variables. All record of previously
taken curves is removed.
STR [n]
Storage interval control
Sets the time interval between successive points being acquired under the TD or TDC
commands. n specifies the time interval in ms with a resolution of 5 ms, input values
being rounded up to a multiple of 5. The longest interval that can be specified is
1000000 s corresponding to one point in about 12 days.
In addition, n may be set to 0, which sets the rate of data storage to the curve buffer
to 800 Hz. However this only allows storage of the X and Y outputs. There is no need
to issue a CBD 3 command to set this up since it happens automatically when
acquisition starts.
If the time constant is set to 5 ms or longer, then the actual time constant applied to
the stored X and Y output values will be 640 µs, but if it is set to a shorter value then
this will be the time constant actually used.
TD
Take data
Initiates data acquisition. Acquisition starts at the current position in the curve buffer
and continues at the rate set by the STR command until the buffer is full.
TDC
Take data continuously
Initiates data acquisition. Acquisition starts at the current position in the curve buffer
and continues at the rate set by the STR command until halted by an HC command.
The buffer is circular in the sense that when it has been filled, new data overwrites
earlier points.
EVENT [n] Event marker control
During a curve acquisition, if bit 13 in the CBD command has been asserted, the
lock-in amplifier stores the value of the Event variable at each sample point. This can
be used as a marker indicating the point at which an experimental parameter was
changed. The EVENT command is used to set this variable to any value between 0
and 32767.
6-24
Chapter 6, COMPUTER OPERATION
HC
Halt curve acquisition
Halts curve acquisition in progress. It is effective during both single (data acquisition
initiated by TD command) and continuous (data acquisition initiated by TDC
command) curve acquisitions. The curve may be restarted by means of the TD or
TDC command, as appropriate.
M
Curve acquisition status monitor
Causes the lock-in amplifier to respond with four values that provide information
concerning data acquisition, as follows.
First value, Curve Acquisition Status:a number with five possible values, defined
by the following table:
First Value Significance
0
1
2
5
6
No curve activity in progress.
Acquisition via TD command in progress and running.
Acquisition via TDC command in progress and running.
Acquisition via TD command in progress but halted by HC command
Acquisition via TDC command in progress but halted by HC command
Second value, Number of Sweeps Acquired: This number is incremented each time
a TD is completed and each time a full cycle is completed on a TDC acquisition. It is
zeroed by the NC command and also whenever a CBD or LEN command is applied
without parameters.
Third value, Status Byte: The same as the response to the ST command. The
number returned is the decimal equivalent of the status byte and refers to the
previously applied command.
Fourth value, Number of Points Acquired: This number is incremented each time a
point is taken. It is zeroed by the NC command and whenever CBD or LEN is applied
without parameters.
DC[.] n
Dump acquired curve(s) to computer
In fixed point mode, causes a stored curve to be dumped via the computer interface in
decimal format.
In floating point mode the SEN curve (bit 4 in CBD) must have been stored if one or
more of the following outputs are required in order that the lock-in amplifier can
perform the necessary conversion from %FS to volts or amps: X, Y, Magnitude,
Noise.
One curve at a time is transferred. The value of n is the bit number of the required
curve, which must have been stored by the most recent CBD command. Hence n can
range from 0 to 15. If for example CBD 5 had been sent, equivalent to asserting bits
0 and 2, then the X and Magnitude outputs would be stored. The permitted values of
n would therefore be 0 and 2, so that DC 0 would transfer the X output curve and
DC 2 the Magnitude curve.
6-25
Chapter 6, COMPUTER OPERATION
The computer program’s subroutine which reads the responses to the DC command
needs to run a FOR...NEXT loop of length equal to the value set by the LEN (curve
length) command.
Note that when using this command with the GPIB interface the serial poll must be
used. After sending the DC command, perform repeated serial polls until bit 7 is set,
indicating that the instrument has an output waiting to be read. Then perform
repeated reads in a loop, waiting each time until bit 7 is set indicating that a new
value is available. The loop should continue until bit 1 is set, indicating that the
transfer is completed.
DCT n
Dump acquired curves to computer in table format
This command is similar to the DC command described above, but allows transfer of
several curves at a time and only operates in fixed point mode. Stored curve(s) are
transferred via the computer interface in decimal format.
The DCT parameter is an integer between 1 and 65,535, being the decimal equivalent
of a 16-bit binary number. When a given bit in the number is asserted, the
corresponding curve is selected for transfer. When a bit is negated, the curve is not
transferred. The bit corresponding to each curve is shown in the table below:
Bit Decimal value
Curve and output range
0
1
1
2
X Output (±10000 FS)
Y Output (±10000 FS)
2
3
4
8
Magnitude Output (±10000 FS)
Phase (±18000 = ±180°)
4
5
6
7
8
9
10
11
12
13
14
15
16
32
64
Sensitivity setting (4 to 27) + IMODE (0, 1, 2 = 0, 32, 64)
ADC1 (±10000 = ±10.0 V)
ADC2 (±10000 = ±10.0 V)
Not used
DAC1 (±10000 = ±10.0 V)
DAC2 (±10000 = ±10.0 V)
Noise (±10000 FS)
128
256
512
1024
2048
4096
8192
16384
32768
Ratio (±10000 FS)
Log ratio (-3000 to +2000)
EVENT variable (0 to 32767)
Reference frequency bits 0 to 15 (mHz)
Reference frequency bits 16 to 32 (mHz)
The values of the selected curves at the same sample point are transferred as a group
in the order of the above table, separated by the chosen delimiter character and
terminated with the selected terminator. This continues until all the points have been
transferred.
As an example, suppose CBD 5 had been sent, equivalent to asserting bits 0 and 2,
then the X and Magnitude outputs would be stored. The permitted values of n would
therefore be 1, 4 and 5. DCT 1 and DCT 4 would only transfer one curve at a time,
but DCT 5 would transfer the X output curve and the Magnitude curve
simultaneously. A typical output data sequence would be:
6-26
Chapter 6, COMPUTER OPERATION
<X output value1><delim><Magnitude value1><term>
<X output value2><delim><Magnitude value2><term>
<X output value3><delim><Magnitude value3><term>
<X output value4><delim><Magnitude value4><term>
<X output value5><delim><Magnitude value5><term>
etc, where <delim> and <term> are the delimiter and terminator characters
respectively.
The computer program’s subroutine which reads the responses to the DCT command
needs to run a FOR...NEXT loop of length equal to the value set by the LEN (curve
length) command, and must be able to separate the responses on each line for storage
or processing.
Note that when using this command with the GPIB interface the serial poll must be
used. After sending the DCT command, perform repeated serial polls until bit 7 is set,
indicating that the instrument has an output waiting to be read. Then perform
repeated reads in a loop, waiting each time until bit 7 is set indicating that a new
value is available. The loop should continue until bit 1 is set, indicating that the
transfer is completed.
6.4.10 Computer Interfaces (RS232 and GPIB)
RS [n1 [n2]] Set/read RS232 interface parameters
The values of n1 set the baud rate of the RS232 interface according to the following
table:
n1 Baud rate (bits per second)
0
1
75
110
2
3
134.5
150
4
300
5
600
6
7
8
9
10
11
12
1200
1800
2000
2400
4800
9600
19200
The lowest five bits in n2 control the other RS232 parameters according to the
following table:
6-27
Chapter 6, COMPUTER OPERATION
bit number
bit negated
bit asserted
0
1
2
3
4
data + parity = 8 bits
no parity bit
even parity
echo disabled
prompt disabled
data + parity = 9 bits
1 parity bit
odd parity
echo enabled
prompt enabled
GP [n1 [n2]] Set/read GPIB parameters
n1 sets the GPIB address in the range 0 to 31
n2 sets the GPIB terminator and the test echo function according to the following
table:
n
0
1
2
3
4
5
Terminator
[CR], test echo disabled
[CR], test echo enabled
[CR,LF], test echo disabled
[CR,LF], test echo enabled
no terminator, test echo disabled
no terminator, test echo enabled
When the test echo is on, every character transmitted or received via the GPIB port is
echoed to the RS232 port. This is provided solely as an aid to program development
and should not be enabled during normal operation of the instrument.
\N n
Address command
When the model 7220 is daisy-chained with other compatible instruments this
command will change which instrument is addressed. All daisy-chained instruments
receive commands but only the currently addressed instrument will implement or
respond to the commands. The exception is the \N n command. If n matches the
address set from the front panel the instrument will switch into addressed mode. If n
does not match the address set from the front panel the instrument will switch into
unaddressed mode.
Note: The \N n command does not change the address of an instrument but which
instrument is addressed.
Warning: All instruments must have a unique address.
DD [n]
Define delimiter control
The value of n, which can be set to 13 or 32 to 125, determines the ASCII value of
the character sent by the lock-in amplifier to separate two numeric values in a two-
value response, such as that generated by the MP (magnitude and phase) command.
ST
Report status byte
Causes the lock-in amplifier to respond with the status byte.
Note: this command is not normally used in GPIB communications, where the status
6-28
Chapter 6, COMPUTER OPERATION
byte is accessed by performing a serial poll.
Bit 0
Bit 1
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
Bit 7
Command complete
Invalid command
Command parameter error
Reference unlock
Overload
New ADC values available after external trigger
Asserted SRQ
Data available
N
Report overload byte
Causes the lock-in amplifier to respond with the overload byte.
Bit 0
Bit 1
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
Bit 7
not used
CH1 output overload (> ±120 %FS)
CH2 output overload (> ±120 %FS)
Y output overload (> ±300 %FS)
X output overload (> ±300 %FS)
not used
input overload
reference unlock
MSK [n]
Set/read service request mask byte
The value of n sets the SRQ mask byte in the range 0 to 255
REMOTE [n] Remote only (front panel lock-out) control
Allowed values of n are 0 and 1. When n is equal to 1, the lock-in amplifier enters
remote only mode in which the front panel control functions are inoperative and the
instrument can only be controlled with the RS232 or the GPIB interfaces. When n is
equal to 0, the front panel control functions normally.
6.4.11 Instrument Identification
ID
Identification
Causes the lock-in amplifier to respond with the number 7220.
REV
Report firmware revision
Causes the lock-in amplifier to respond with the firmware revision number. This gives
a four line response which the controlling program must be able to accept.
VER
Report firmware version
Causes the lock-in amplifier to respond with the firmware version number. The
firmware version number is the number displayed on the front-panel RS232 SETUP 3
screen.
6-29
Chapter 6, COMPUTER OPERATION
6.4.12 Front Panel
LTS [n]
Lights on/off control
The value of n controls the front panel LEDs and LCD backlights according to the
following table:
n
0
1
Selection
All lights off
Normal operation
6.4.13 Default Setting
ADF
Default Setting command
This command will automatically set all the instrument controls and displays to the
factory set default values. However, if the command is used when the interface
parameters are at values other than their default settings, then communication will be
lost.
6.5 Programming Examples
6.5.01 Introduction
This section gives some examples of the commands that need to be sent to the lock-in
amplifier for typical experimental situations.
6.5.02 Basic Signal Recovery
In a typical simple experiment, the computer is used to set the instrument controls and
then to record the chosen outputs, perhaps as a function of time. At sampling rates of
up to a few points per second, there is no need to use the internal curve buffer. The
commands to achieve this would therefore be similar to the following sequence:
IE 2
VMODE 1
FET 1
AUTOMATIC 1
FLOAT 1
LF 0
Set reference to external front panel input
Single-ended voltage input mode
10 MΩ input impedance using FET stage
AC Gain control automatic
Float input connector shell using 1 kΩ to ground
Turn off line frequency rejection filter
ASM
TC 12
Auto-Measure (assumes reference frequency > 1 Hz)
Set time constant to 200 ms, since previous ASM changed it
Then the outputs could be read as follows:
X.
Reads X output in volts
Y.
Reads Y output in volts
MAG.
PHA.
FRQ.
Reads Magnitude in volts
Reads Phase in degrees
Reads reference frequency in hertz
6-30
Chapter 6, COMPUTER OPERATION
The controlling program would send a new output command each time a new reading
were required. Note that a good “rule of thumb” is to wait for a period of five time-
constants after the input signal has changed before recording a new value. Hence in a
scanning type experiment, the program should issue the commands to whatever
equipment causes the input signal to the lock-in amplifier to change, wait for five
time-constants, and then record the required output.
6.5.03 Frequency Response Measurement
In this example, the lock-in amplifier’s internal oscillator output signal is fed via the
filter stage under test back to the instrument’s signal input. The oscillator frequency is
stepped between a lower and an upper frequency and the signal magnitude and phase
recorded. At sampling rates of up to a few points per second, there is no need to use
the internal curve buffer or oscillator frequency sweep generator. The commands to
achieve this would therefore be similar to the following sequence:
IE 0
Set reference mode to internal
VMODE 1
FET 1
AUTOMATIC 1
FLOAT 1
LF 0
OA. 1.0
OF. 100.0
SEN 27
TC 10
Single-ended voltage input mode
10 MΩ input impedance using FET stage
AC Gain control automatic
Float input connector shell using 1 kΩ to ground
Turn off line frequency rejection filter
Set oscillator amplitude to 1.0 V rms
Set oscillator frequency to 100 Hz (starting frequency)
Set sensitivity to 1 V full-scale
Set time constant to 50 ms
AQN
Auto-Phase
The frequency sweep would be performed and the outputs recorded by sending the
following commands from a FOR...NEXT program loop:
OF. XX
Set oscillator frequency to new value XX hertz
software delay of 250 ms (5 × 50 ms) allowing output to stabilize
MAG.
PHA.
FRQ.
Read Magnitude in volts
Read Phase in degrees
Read reference frequency in hertz. This would be same as the
oscillator frequency since the unit is operating in internal
reference mode.
until the stop frequency is reached.
6-31
Chapter 6, COMPUTER OPERATION
6.5.04 X and Y Output Curve Storage Measurement
In this example, the lock-in amplifier is measuring a current input signal applied to
the B input connector and the measured X output and Y output are recorded for 10
seconds at a 100 Hz sampling rate. The acquired curves as read back to the computer
are required in floating point mode.
The sequence of commands is therefore as follows:
IE 2
Set reference mode to external front panel input
High bandwidth current input mode
AC Gain control automatic
Float input connector shell using 1 kΩ to ground
Turn off line frequency rejection filter
Set sensitivity to 1 nA full-scale
Set time constant to 50 ms
IMODE 1
AUTOMATIC 1
FLOAT 1
LF 0
SEN 18
TC 10
AQN
Auto-Phase
Now the curve storage needs to be set up:
NC
Clear and reset curve buffer
CBD 19
LEN 1000
STR 10
Stores X output, Y output and sensitivity (i.e. bits 0, 1 and 4)
Number of points = 100 Hz × 10 seconds
Store a point every 10 ms (1/100 Hz)
The data is acquired by issuing:
TD
Acquires data
As the acquisition is running, the M command reports the status of the curve
acquisition. Once this indicates the acquisition is complete (i.e. parameter 1 = 0,
parameter 2 = 1), the acquired data may be transferred to the computer using:
DC. 0
DC. 1
Transfers X output values in floating point mode.
Transfers Y output values in floating point mode.
The input routine of the program must be prepared to read and store 1000 responses
to each of these commands.
6.5.05 Transient Recorder
In this example, the signal recovery capabilities of the lock-in amplifier are not used,
but the auxiliary inputs are. The voltage applied to the rear panel ADC1 input is
sampled and digitized at a rate of approximately 40 kHz, with the values being stored
to the curve buffer. Sampling is required to start on receipt of a trigger at the rear
panel TRIG IN connector and must last for 500 ms.
The sequence of commands is therefore as follows:
6-32
Chapter 6, COMPUTER OPERATION
Clear and reset curve buffer
NC
LEN 20000
TADC 6
500 ms recording time at 40 kHz = 20,000 points
Set ADC1 sampling to burst mode, fixed rate (≈ 40 kHz),
external trigger, and arm trigger
As soon as a trigger occurs, the acquisition starts. Once it completes the acquired
data may be transferred to the computer using:-
DC. 5
Transfers ADC1 values in floating point mode
The input routine of the program must be prepared to read and store 20,000
responses to this command.
6.5.06 Frequency Response Measurement using Curve
Storage and Frequency Sweep
In this example, a more sophisticated version of that given in section 6.5.03, the
internal oscillator frequency sweep generator is used in conjunction with curve
storage, allowing the acquisition of a frequency response without the need for the
computer to perform the frequency setting function for each point.
As before, the lock-in amplifier’s internal oscillator output signal is fed via the filter
stage under test to the signal input. The oscillator frequency is stepped between a
lower and an upper frequency and the signal magnitude and phase are recorded.
The required sequence of commands is therefore as follows:-
IE 0
VMODE 1
FET 1
AUTOMATIC 1
FLOAT 1
LF 0
OA. 1.0
OF. 100.0
Set reference mode to internal
Single-ended voltage input mode
10 MΩ input impedance using FET stage
AC Gain control automatic
Float input connector shell using 1 kΩ to ground
Turn off line frequency rejection filter
Set oscillator amplitude to 1.0 V rms
Set initial oscillator frequency to 100 Hz so that AQN
runs correctly
SEN 27
TC 8
Set sensitivity to 1 V full-scale
Set time constant to 10 ms
AQN
Auto-Phase
The next group of commands set up the frequency sweep:
FSTART. 100.0
FSTOP. 1000.0
FSTEP. 10 1
SRATE. 0.1
Set initial oscillator frequency to 100 Hz
Set final oscillator frequency to 1000 Hz
Step size = 10 Hz, linear law
100 ms per step
There will therefore be 100 steps (100 Hz to 1000 Hz inclusive in 10 Hz steps). Now
we need to specify the curve storage:
6-33
Chapter 6, COMPUTER OPERATION
NC
Clear and reset curve buffer
CBD 49180
Stores Magnitude, Phase, Sensitivity and Frequency
(i.e. bits 2, 3, 4, 14 and 15)
LEN 100
STR 100
Number of points = 100
Store a point every 100 ms - must match SRATE parameter
The data may now be acquired by issuing the compound command:
TD; SWEEP 1
Starts sweep and curve acquisition
Note that the order of these two commands is important. If used as shown then the
data will be acquired and then the oscillator frequency will be changed at each data
point, prior to waiting the time set by the SRATE and STR commands. This gives
sufficient time for the instrument output to stabilize after each change of frequency.
If the commands were used in the reverse order (i.e. SWEEP 1; TD) then the
output(s) would never have time to settle by the time at which they were recorded.
The frequency sweep starts and the magnitude and phase outputs are recorded to the
curve buffer. As it runs the M command reports the status of the acquisition, and
once this indicates it is complete (i.e. parameter 1 = 0, parameter 2 = 1), the acquired
data may be transferred to the computer using:
DC. 2
DC. 3
DC 14
DC 15
Transfers Magnitude curve
Transfers Phase curve
Transfers Reference frequency - lower 16 bits
Transfers Reference frequency - upper 16 bits
6-34
Specifications
AppendixA
Measurement Modes
X In-phase
Y Quadrature
The unit can simultaneously present any
R
θ
Magnitude
Phase Angle
Noise
two of these as outputs
Harmonic
Noise
2F or 3F
Measures noise in a given bandwidth centered
on frequency F
Displays
Two LED backlit, two-line, 16-character alphanumeric dot-matrix LCDs giving
digital indication of current instrument set-up and output readings. Edge indicating
analog panel meter. Menu system with dynamic key function allocation.
Signal Channel
Voltage Inputs
Modes
A only or Differential (A-B)
20 nV to 1 V in a 1-2-5 sequence
> 100 dB
Full-scale Sensitivity
Dynamic Reserve
Impedance
FET Device
Bipolar Device
Voltage Noise
FET Device
Bipolar Device
CMRR
10 MΩ // 30 pF
10 kΩ // 30 pF
5 nV/√Hz at 1 kHz
2 nV/√Hz at 1 kHz
> 100 dB at 1 kHz degrading by 6 dB/octave
Frequency Response
Gain Accuracy
Distortion
0.001 Hz to 120 kHz
0.5 % typ (full bandwidth)
-90 dB THD (60 dB AC Gain, 1 kHz)
Line Filter
Grounding
attenuates 50, 60, 100, 120 Hz
BNC shields can be grounded or floated via
1 kΩ to ground
A-1
Appendix A, SPECIFICATIONS
Current Input
Mode
Low Noise or Wide Bandwidth
Full-scale Sensitivity
Low Noise
Wide Bandwidth
Dynamic Reserve
Frequency Response
Low Noise
20 fA to 10 nA in a 1-2-5 sequence
20 fA to 1 µA in a 1-2-5 sequence
> 100 dB (with no signal filters)
-3 dB at 500 Hz
-3 dB at 50 kHz
Wide Bandwidth
Impedance
Low Noise
< 2.5 kΩ at 100 Hz
Wide Bandwidth
Noise
< 250 Ω at 1 kHz
Low Noise
13 fA/√Hz at 500 Hz
Wide Bandwidth
Gain Accuracy (midband)
Low Noise
130 fA/√Hz at 1 kHz
≤ 0.6 % typ
Wide Bandwidth
Line Filter
Grounding
≤ 0.6 % typ
attenuates 50, 60, 100, 120 Hz
BNC shields can be grounded or floated via
1 kΩ to ground
Reference Channel
TTL Input (rear panel)
Frequency Range
1 mHz to 120 kHz
Analog Input (front panel)
Impedance
1 MΩ // 30 pF
Sinusoidal Input
Level
1.0 V rms**
Frequency Range
Squarewave Input
Level
1 Hz to 120 kHz
100 mV rms**
Frequency Range
300 mHz to 120 kHz
**Note: Lower levels can be used with the
analog input at the expense of increased phase
errors.
Phase
Set Resolution
0.01º increments
0.5º typ
Accuracy
Noise at 100 ms TC, 12 dB/octave
Internal Reference
External Reference
< 0.0001º rms
< 0.01º rms at 1 kHz
A-2
Appendix A, SPECIFICATIONS
Orthogonality
Drift
90º ±0.0001º
< 0.01º/ºC below 10 kHz
< 0.1º/ºC above 10 kHz
Acquisition Time
Internal Reference
External Reference
instantaneous acquisition
2 cycles + 50 ms
Reference Frequency Meter Accuracy
120 kHz > F > 40 kHz
40 kHz > F > 400 Hz
±4 Hz
±0.8 Hz at F = 40 kHz improving to
±0.008 Hz at F = 400 Hz
400 Hz > F > 1 mHz
±0.040 Hz at F = 400 Hz improving to
better than ±0.0001 Hz at F = 1 mHz
Demodulator and Output Processing
Description
2 × 18-bit ADCs driving two DSP elements
managed by a powerful 68000-series host
processor
Output Zero Stability
Digital Outputs
Displays
No zero drift on all settings
No zero drift on all settings
< 5 ppm/ºC
Analog Outputs
Harmonic Rejection
-90 dB
Time Constants
Digital Outputs
Fast Outputs
Roll-off
5 ms to 100 ks in a 1-2-5 sequence
10 µs to 640 µs in a binary sequence
6, 12, 18 and 24 dB/octave
Synchronous Filter Operation
Offset
Available for F < 20 Hz
Auto and Manual on X and Y: ±300 % FS
Oscillator
Frequency
Range
0.001 Hz to 120 kHz
0.001 Hz
25 ppm + 30 µHz
Setting Resolution
Absolute Accuracy
Distortion (THD)
-80 dB at 1 kHz
A-3
Appendix A, SPECIFICATIONS
Amplitude
Range
1 mV to 5 V
Setting Resolution
1 mV to 500 mV
501 mV to 2 V
2.001 V to 5 V
1 mV
4 mV
10 mV
Accuracy
0.001 Hz to 60 kHz
60 kHz to 120 kHz
Stability
±0.3 %
±0.5 %
50 ppm/ºC
Output
Impedance
50 Ω
Auxiliary Inputs
ADC 1 and 2
Maximum Input
±10 V
Resolution
1 mV
Accuracy
±0.2 %
Input Impedance
Sample Rate
ADC 1 only
ADC 1 and 2
Trigger Mode
Trigger input
1 MΩ // 30 pF
40 kHz max
13 kHz max
Int, ext or burst
TTL compatible
Outputs
CH1 CH2 Outputs
Function
X, Y, R, θ, Noise and aux functions
Amplitude
±10 V
Impedance
1 kΩ
Fast X Output
Time Constant
Amplitude
≤ 640 µs
±10 V
Update Rate
170 kHz
1 kΩ
Output Impedance
Fast Y Output
Time Constant
Amplitude
≤ 640 µs
±10 V
Update Rate
170 kHz
1 kΩ
Output Impedance
A-4
Appendix A, SPECIFICATIONS
Signal Monitor
Amplitude
±10 V FS
Impedance
1 kΩ
Aux D/A Output 1, 2
Maximum Output
Resolution
±10 V
1 mV
Accuracy
±0.1 %
Output Impedance
1 kΩ
8-bit Digital Output
8 TTL compatible lines that can be
independently set high or low to activate
external equipment
Reference Output
Waveform
0 to 5 V square wave
TTL compatible
Impedance
Power - Low Voltage
±15 V at 100 mA rear panel DIN connector for
powering EG&G preamplifiers
Data Storage
Data Buffer
Size
32k 16-bit data points, may be organized as
1×32k, 2×16k, 3×10.6k, 4×8k, etc.
Max Storage Rate
From LIA
up to 800 16-bit values per second
up to 40,000 16-bit values per second
From ADC
Interfaces
RS232, IEEE-488. A auxiliary RS232 port is
provided to allow "daisy-chain" connection and
control of multiple units from a single RS232
computer port.
Power Requirements
Voltage
Frequency
Power
110/120/220/240 VAC
50/60 Hz
< 40 VA
A-5
Appendix A, SPECIFICATIONS
General
Dimensions
Width
Depth
Height
432 mm (17 ")
415 mm (16.4 ")
With feet
Without feet
74 mm (2.9 ")
60 mm (2.4 ")
Weight
7.4 kg (16.3 lb)
A-6
Pinouts
AppendixB
B.1 RS232 Connector Pinout
5
4
3
2
1
9
8
7
6
Figure B-1, RS232 and AUX RS232 Connector (Female)
PIN
FUNCTION
COMMENT
2
3
5
7
RXD
TXD
GND
RTS
Data In
Data Out
Signal Ground
(Always at +12 V)
All other pins are not connected.
B.2 Preamplifier Power Connector Pinout
Figure B-2, Preamplifier Power Connector
PIN
FUNCTION
1
2
3
-15 V
GROUND
+15 V
All other pins are unused.
Shell is shield ground.
B-1
Appendix B, PINOUTS
B.3 Digital Output Port Connector
Figure B-3, Digital Output Port Connector
8-bit TTL-compatible output set from the front panel or via the computer interfaces;
each line can drive 3 LSTTL loads. This connector mates with a 20-pin IDC Header
Plug. The pinout is as follows.
PIN #
FUNCTION
1
2
3
GROUND
GROUND
D0
4
5
GROUND
D1
6
7
GROUND
D2
8
9
GROUND
D3
10
11
12
13
14
15
16
17
18
19
20
GROUND
D4
GROUND
D5
GROUND
D6
GROUND
D7
GROUND
+5 V
+5 V
D0 = Least Significant Bit
D7 = Most Significant Bit
B-2
Demonstration
Programs
AppendixC
C.1 Simple Terminal Emulator
This is a short terminal emulator with minimal facilities, which will run on a PC-compatible computer
in a Microsoft GW-BASIC or QuickBASIC environment, or can be compiled with a suitable compiler.
10 'MINITERM 9-Feb-96
20 CLS: PRINT "Lockin RS232 parameters must be set to 9600 baud, 7 data bits, 1 stop bit and
even parity"
30 PRINT "Hit <ESC> key to exit"
40 OPEN "COM1:9600,E,7,1,CS,DS" FOR RANDOM AS #1
50 '..............................
60 ON ERROR GOTO 180
70 '..............................
100 WHILE (1)
110
120
130
140
150
B$ = INKEY$
IF B$=CHR$(27) THEN CLOSE #1: ON ERROR GOTO 0: END
IF B$<>"" THEN PRINT#1, B$;
LL% = LOC(1)
IF LL%>0 THEN A$ = INPUT$(LL%,#1): PRINT A$;
160 WEND
170 '..............................
180 PRINT "ERROR NO.";ERR: RESUME
C.2 RS232 Control Program with Handshakes
RSCOM2.BAS is a user interface program which illustrates the principles of the echo handshake.
The program will run on a PC-compatible computer either in a Microsoft GW-BASIC or QuickBASIC
environment, or in compiled form.
The subroutines in RSCOM2 are recommended for incorporation in the user’s own programs.
10 'RSCOM2 9-Feb-96
20 CLS: PRINT "Lockin RS232 parameters must be set to 9600 baud, 7 data bits, 1 stop bit and
even parity"
30 OPEN "COM1:9600,E,7,1,CS,DS" FOR RANDOM AS #1
40 CR$=CHR$(13)
50 '
' carriage return
60 '...main loop..................
70 WHILE 1
' infinite loop
80
90
INPUT "command (00 to exit) ";B$
IF B$="00" THEN END
' no commas are allowed in B$
C-1
Appendix C, DEMONSTRATION PROGRAMS
100
110
120
130
140
B$=B$+CR$
GOSUB 180
GOSUB 310: PRINT Z$;
IF A$="?" THEN GOSUB 410: GOSUB 470
' append a carriage return
' output the command B$
' read and display response
' if "?" prompt fetch STATUS%
' and display message
150 WEND
' return to start of loop
160 '
170 '
180 '...output the string B$..............
190 ON ERROR GOTO 510
200 IF LOC(1)>0 THEN A$=INPUT$(LOC(1),#1)
210 ON ERROR GOTO 0
220 FOR J1%=1 TO LEN(B$)
' enable error trapping
' clear input buffer
' disable error trapping
' LEN(B$) is number of bytes
' send byte
' wait for byte in input buffer
' read input buffer
' input byte should be echo
' next byte to be sent or
' return if no more bytes
230
240
250
260
C$=MID$(B$,J1%,1): PRINT#1,C$;
WHILE LOC(1)=0: WEND
A$=INPUT$(1,#1)
IF A$<>C$ THEN PRINT”handshake error”
270 NEXT J1%
280 RETURN
290 '
300 '
310 '....read response..................
320 A$="": Z$=""
330 WHILE (A$<>"*" AND A$<>"?")
' read until prompt received
' append next byte to string
' wait for byte in input buffer
' read byte from buffer
340
350
360
Z$=Z$+A$
WHILE LOC(1)=0: WEND
A$=INPUT$(1,#1)
370 WEND
' next byte to be read
380 RETURN
' return if it is a prompt
390 '
400 '
410 '....fetch status byte..............
420 B$="ST"+CR$
430 GOSUB 180
440 GOSUB 310
450 STATUS%=VAL(Z$)
460 RETURN
' "ST" is the status command
' output the command
' read response into Z$
' convert to integer
470 '....instrument error message.......
480 PRINT"Error prompt, status byte = ";STATUS%
490 PRINT
' bits are defined in manual
500 RETURN
510 '....I/O error routine..............
520 RESUME
C-2
Appendix C, DEMONSTRATION PROGRAMS
C.3 GPIB User Interface Program
GPCOM.BAS is a user interface program which illustrates the principles of the use of the serial poll
status byte to coordinate the command and data transfer.
The program runs under Microsoft GW-BASIC or QuickBASIC on a PC-compatible computer fitted
with a National Instruments IEEE-488 interface card and the GPIB.COM software installed in the
CONFIG.SYS file. The program BIB.M, and the first three lines of GPCOM, are supplied by the card
manufacturer and must be the correct version for the particular version of the interface card in use.
The interface card may be set up, using the program IBCONF.EXE, to set EOI with the last byte of
Write in which case no terminator is required. (Read operations are automatically terminated on EOI
which is always sent by the lock-in). Normally, the options called ‘high-speed timing’, ‘interrupt jumper
setting’, and ‘DMA channel’ should all be disabled.
The principles of using the Serial Poll Status Byte to control data transfer, as implemented in the
main loop of GPCOM, are recommended for incorporation in the user’s own programs.
10 'GPCOM 9-Feb-96
20 '....the following three lines and BIB.M are supplied by the.......
30 '....manufacturer of the GPIB card, must be correct version........
40 CLEAR,60000!:IBINIT1=60000!:IBINIT2=IBINIT1+3:BLOAD"BIB.M",IBINIT1
50 CALL
IBINIT1(IBFIND,IBTRG,IBCLR,IBPCT,IBSIC,IBLOC,IBPPC,IBBNA,IBONL,IBRSC,IBSRE,IBRSV,IBPAD,
IBSAD,IBIST,IBDMA,IBEOS,IBTMO,IBEOT,IBRDF,IBWRTF,IBTRAP)
60 CALL
IBINIT2(IBGTS,IBCAC,IBWAIT,IBPOKE,IBWRT,IBWRTA,IBCMD,IBCMDA,IBRD,IBRDA,IBSTOP,
IBRPP,IBRSP,IBDIAG,IBXTRC,IBRDI,IBWRTI,IBRDIA,IBWRTIA,IBSTA%,IBERR%,IBCNT%)
70 '.................................................
80 CLS: PRINT"DEVICE MUST BE SET TO CR TERMINATOR"
90 '....assign access code to interface board........
100 BDNAME$="GPIB0"
110 CALL IBFIND(BDNAME$,GPIB0%)
120 IF GPIB0%<0 THEN PRINT "board assignment error":END
130 '....send INTERFACE CLEAR.........................
140 CALL IBSIC(GPIB0%)
150 '....set bus address, assign access code to device..........
160 SUCCESS% = 0
170 WHILE SUCCESS% = 0
180
190
200
210
220
230
240
INPUT "BUS ADDRESS ";A%
DEVNAME$ = "DEV"+RIGHT$(STR$(A%),LEN(STR$(A%))-1)
CALL IBFIND(DEVNAME$,DEV%)
IF DEV%<0 THEN PRINT "device assignment error": END
A$ = CHR$(13): GOSUB 480
' assign access code
' test: write <CR> to bus
IF IBSTA%>0 THEN SUCCESS% = 1
IF (IBSTA%<0 AND IBERR%=2) THEN BEEP: PRINT "NO DEVICE AT THAT ADDRESS"
250 WEND
260 '....send SELECTED DEVICE CLEAR...................
270 CALL IBCLR(DEV%)
280 '....set timeout to 1 second......................
C-3
Appendix C, DEMONSTRATION PROGRAMS
290 V%=11: CALL IBTMO(DEV%,V%)
300 '....set status print flag........................
310 INPUT "Display status byte y/n "; R$
320 IF R$="Y" OR R$="y" THEN DS% = 1 ELSE DS% = 0
330 '....main loop....................................
340 WHILE 1
' infinite loop
350
360
370
380
390
400
410
420
430
440
445
450
INPUT "command (00 to exit) ";A$
IF A$="00" THEN END
A$ = A$ + CHR$(13)
GOSUB 480
S% = 0
' terminator is <CR>
' write A$ to bus
' initialize S%
' while command not complete
' serial poll, returns S%
WHILE (S% AND 1)=0
GOSUB 530
IF DS% THEN PRINT "S%= ";S%
IF (S% AND 128) THEN GOSUB 500: PRINT B$
WEND
IF (S% AND 4) THEN PRINT "parameter error"
IF (S% AND 2) THEN PRINT "invalid command"
' read bus into B$ and print
460 WEND
470 '....end of main loop.............................
480 '....write string to bus..........................
490 CALL IBWRT(DEV%,A$): RETURN
500 '....read string from bus.........................
510 B$ = SPACE$(32)
' B$ is buffer
520 CALL IBRD(DEV%,B$): RETURN
530 '......serial poll................................
540 CALL IBRSP(DEV%,S%): RETURN
C-4
Cable Diagrams
AppendixD
D.1 RS232 Cable Diagrams
Users who choose to use the RS232 interface to connect the model 7220 lock-in
amplifier to a standard serial port on a computer, will need to use one of two types of
cable. The only difference between them is the number of pins used on the connector
which goes to the computer. One has 9 pins and the other 25; both are null modem
(also called modem eliminator) cables in that some of the pins are cross-connected.
Users with reasonable practical skills can easily assemble the required cables from
parts which are widely available through computer stores and electronics components
suppliers. The required interconnections are given in figures D-1 and D-2.
Figure D-1, Interconnecting RS232 Cable Wiring Diagram
D-1
Appendix D, CABLE DIAGRAMS
Figure D-2, Interconnecting RS232 Cable Wiring Diagram
D-2
Alphabetical Listing of
Commands
AppendixE
ACGAIN [n] AC Gain control
Sets the gain of the signal channel amplifier. Values of n from 0 to 9 can be entered
corresponding to the range 0 dB to 90 dB in 10 dB steps.
ADC[.] n Read auxiliary analog to digital inputs
Reads the voltage appearing at the rear panel ADC1 (n = 1) and ADC2 (n = 2)
inputs.
In fixed point mode the response is an integer in the range -12000 to +12000,
corresponding to voltages from -12.000 V to +12.000 V.
In floating point mode it is in volts.
ADF
Default setting command
This command will automatically set all the instrument controls and displays to the
factory set default values. However, if the command is used when the interface
parameters are at values other than their default settings, then communication will be
lost.
AQN
AS
Auto-Phase (auto-quadrature null)
The instrument adjusts the reference phase to maximize the X output and minimize
the Y output signals.
Perform an Auto-Sensitivity operation
The instrument adjusts its full-scale sensitivity so that the X output lies between 30 %
and 90 % of full-scale.
ASM
Perform an Auto-Measure operation
The instrument adjusts its full-scale sensitivity so that the magnitude output lies
between 30 % and 90 % of full-scale, and then performs an Auto-Phase operation to
maximize the X output and minimize the Y output
AUTOMATIC [n]
AC Gain automatic control
The value of n sets the status of the AC Gain control according to the following table:
n
0
Status
AC Gain is under manual control, either using the front panel or the ACGAIN
command
1
Automatic AC Gain control is activated, with the gain being adjusted
according to the full-scale sensitivity setting
E-1
Appendix E, ALPHABETICAL LISTING OF COMMANDS
AXO
Auto-Offset
The X and Y output offsets are turned on and set to levels giving zero X and Y
outputs. Any changes in the input signal then appear as changes about zero in the
outputs.
BURSTRATE [n]
Sets the burst mode sampling rate for ADC1 and ADC2
n sets the sample rate for the Variable Rate burst modes according to the following
equations:
When storing only to ADC1:
(i.e. TADC 2, TADC 4, TADC 6 and TADC 8)
16,000,000
Sample Rate = ————— Hz
((25 × n) + 157)
When storing to ADC1 and ADC 2:
(i.e. TADC 3, TADC 5, TADC 7 and TADC 9)
16,000,000
Sample Rate = ————— Hz
((25 × n) + 1031)
Note that these equations apply only to units manufactured after December 1995.
Earlier instruments used a 16.384 MHz instead of a 16.0 MHz crystal, so the above
equations should be modified accordingly by replacing the 16,000,000 figure with
16,384,000.
For example when n = 20, the sample rate will be 24,353 Hz for ADC1 for an
instrument with a 16.0 MHz crystal, and 24,937 Hz for a unit with a 16.384 MHz
crystal.
BYTE [n] Digital output port control
The value of n, in the range 0 to 255, determines the bits to be output on the rear
panel digital output port. When n = 0, all outputs are low, and when n = 255, all are
high.
CBD [n]
Curve buffer define
Defines which data outputs are stored in the curve buffer when subsequent TD (take
data) or TDC (take data continuously) commands are issued. Up to 16 curves, or
outputs, may be acquired, as specified by the CBD parameter.
The CBD is an integer between 0 and 65,535, being the decimal equivalent of a 16-bit
binary word. When a given bit in the word is asserted, the corresponding output is
selected for storage. When a bit is negated, the output is not stored. The bit function
and range for each output are shown in the table below:
E-2
Appendix E, ALPHABETICAL LISTING OF COMMANDS
Bit Decimal value
Output and range
0
1
1
2
X Output (±10000 FS)
Y Output (±10000 FS)
2
3
4
8
Magnitude Output (±10000 FS)
Phase (±18000 = ±180°)
4
5
6
7
8
9
10
11
12
13
14
15
16
32
64
Sensitivity setting (4 to 27) + IMODE (0, 1, 2 = 0, 32, 64)
ADC1 (±10000 = ±10.0 V)
ADC2 (±10000 = 10.0 V)
Unassigned
DAC1 (±10000 = 10.0 V)
DAC2 (±10000 = 10.0 V)
Noise (±10000 FS)
128
256
512
1024
2048
4096
8192
16384
32768
Ratio (±10000 FS)
Log ratio (-3000 to +2000)
Last value given to EVENT command
Reference frequency bits 0 to 15 (mHz)
Reference frequency bits 16 to 32 (mHz)
32768 points are available for data storage, shared equally between the specified
curves. For example, if all 16 outputs are stored then the maximum number of
storage points would be 2048 (i.e. 32768/16). The LEN command sets the actual
curve length, which cannot therefore be longer than 32768 divided by the number of
curves selected. If more curves are requested than can be stored with the current
buffer length, then the buffer length will be automatically reduced. Its actual length
can of course be determined by sending the LEN command without a parameter.
The reason why bit 4 is needed, which stores both the sensitivity and the IMODE
setting, is to allow the instrument to transfer the acquired curves to the computer in
floating point mode. Without this information, the unit would not be able to determine
the correct calibration to apply.
Curves 14 and 15 store the reference frequency in millihertz. The calculation needed
to translate these two 16-bit values to one 32-bit value is:
Reference Frequency = (65536 × value in Curve 15) + (value in Curve 14)
Note that the CBD command directly determines the allowable parameters for the DC
and HC commands. It also interacts with the LEN command and affects the values
reported by the M command.
E-3
Appendix E, ALPHABETICAL LISTING OF COMMANDS
CH n1 [n2] Analog output control
Defines what outputs appear on the rear panel CH1 and CH2 connectors according
to the following table:
n2 Signal
0
1
2
3
4
5
6
X %FS
Y %FS
Magnitude %FS
Phase 1: +9 V = +180°, -9 V = -180°
Phase 2: +9 V = 360°, -9 V = 0°
Noise %FS
Ratio: (1000 × X)/ADC 1
n1 is compulsory and is either 1 for CH1 or 2 for CH2
CP [n]
Input coupling control
The value of n sets the input coupling mode according to the following table:
n
0
1
Coupling mode
AC
DC
DAC[.] n1 [n2]
Auxiliary DAC output controls
Sets the voltage appearing at the rear panel DAC1 and DAC2 outputs.
The first parameter n1, which specifies the DAC, is compulsory and is either 1 or 2.
The value of n2 specifies the voltage to be output.
In fixed point mode it is an integer in the range -12000 to +12000, corresponding to
voltages from -12.000 V to +12.000 V.
In floating point mode it is in volts.
DC[.] n
Dump acquired curve(s) to computer
In fixed point mode, causes a stored curve to be dumped via the computer interface in
decimal format.
In floating point mode the SEN curve (bit 4 in CBD) must have been stored if one or
more of the following outputs are required in order that the lock-in amplifier can
perform the necessary conversion from %FS to volts or amps: X, Y, Magnitude,
Noise.
One curve at a time is transferred. The value of n is the bit number of the required
curve, which must have been stored by the most recent CBD command. Hence n can
range from 0 to 15. If for example CBD 5 had been sent, equivalent to asserting bits
0 and 2, then the X and Magnitude outputs would be stored. The permitted values of
n would therefore be 0 and 2, so that DC 0 would transfer the X output curve and
DC 2 the Magnitude curve.
E-4
Appendix E, ALPHABETICAL LISTING OF COMMANDS
The computer program’s subroutine which reads the responses to the DC command
needs to run a FOR...NEXT loop of length equal to the value set by the LEN (curve
length) command.
Note that when using this command with the GPIB interface the serial poll must be
used. After sending the DC command, perform repeated serial polls until bit 7 is set,
indicating that the instrument has an output waiting to be read. Then perform
repeated reads in a loop, waiting each time until bit 7 is set indicating that a new
value is available. The loop should continue until bit 1 is set, indicating that the
transfer is completed.
DCT n
Dump acquired curves to computer in table format
This command is similar to the DC command described above, but allows transfer of
several curves at a time and only operates in fixed point mode. Stored curve(s) are
transferred via the computer interface in decimal format.
The DCT parameter is an integer between 1 and 65,535, being the decimal equivalent
of a 16-bit binary number. When a given bit in the number is asserted, the
corresponding curve is selected for transfer. When a bit is negated, the curve is not
transferred. The bit corresponding to each curve is shown in the table below:
Bit Decimal value
Curve and output range
0
1
1
2
X Output (±10000 FS)
Y Output (±10000 FS)
2
3
4
8
Magnitude Output (±10000 FS)
Phase (±18000 = ±180°)
4
5
6
7
8
9
10
11
12
13
14
15
16
32
64
Sensitivity setting (4 to 27) + IMODE (0, 1, 2 = 0, 32, 64)
ADC1 (±10000 = ±10.0 V)
ADC2 (±10000 = ±10.0 V)
Not used
DAC1 (±10000 = ±10.0 V)
DAC2 (±10000 = ±10.0 V)
Noise (±10000 FS)
128
256
512
1024
2048
4096
8192
16384
32768
Ratio (±10000 FS)
Log ratio (-3000 to +2000)
EVENT variable (0 to 32767)
Reference frequency bits 0 to 15 (mHz)
Reference frequency bits 16 to 32 (mHz)
The values of the selected curves at the same sample point are transferred as a group
in the order of the above table, separated by the chosen delimiter character and
terminated with the selected terminator. This continues until all the points have been
transferred.
As an example, suppose CBD 5 had been sent, equivalent to asserting bits 0 and 2,
then the X and Magnitude outputs would be stored. The permitted values of n would
therefore be 1, 4 and 5. DCT 1 and DCT 4 would only transfer one curve at a time,
but DCT 5 would transfer the X output curve and the Magnitude curve
simultaneously. A typical output data sequence would be:
E-5
Appendix E, ALPHABETICAL LISTING OF COMMANDS
<X output value1><delim><Magnitude value1><term>
<X output value2><delim><Magnitude value2><term>
<X output value3><delim><Magnitude value3><term>
<X output value4><delim><Magnitude value4><term>
<X output value5><delim><Magnitude value5><term>
etc, where <delim> and <term> are the delimiter and terminator characters
respectively.
The computer program’s subroutine which reads the responses to the DCT command
needs to run a FOR...NEXT loop of length equal to the value set by the LEN (curve
length) command, and must be able to separate the responses on each line for storage
or processing.
Note that when using this command with the GPIB interface the serial poll must be
used. After sending the DCT command, perform repeated serial polls until bit 7 is set,
indicating that the instrument has an output waiting to be read. Then perform
repeated reads in a loop, waiting each time until bit 7 is set indicating that a new
value is available. The loop should continue until bit 1 is set, indicating that the
transfer is completed.
DD [n]
Define delimiter control
The value of n, which can be set to 13 or 32 to 125, determines the ASCII value of
the character sent by the lock-in amplifier to separate two numeric values in a two-
value response, such as that generated by the MP (magnitude and phase) command.
ENBW[.] Equivalent noise bandwidth
In fixed point mode, reports the equivalent noise bandwidth of the output low-pass
filters at the current time constant setting in microhertz.
In floating point mode, reports the equivalent noise bandwidth of the output low-pass
filters at the current time constant setting in hertz.
This command is not available when the reference frequency exceeds 60 kHz.
EVENT [n]
Event marker control
During a curve acquisition, if bit 13 in the CBD command has been asserted, the
lock-in amplifier stores the value of the Event variable at each sample point. This can
be used as a marker indicating the point at which an experimental parameter was
changed. The EVENT command is used to set this variable to any value between 0
and 32767.
E-6
Appendix E, ALPHABETICAL LISTING OF COMMANDS
Output expansion control
EX [n]
Expands X and/or Y outputs by a factor of 10. Changes meter, CH1 and CH2
outputs full-scale to ±10 % if X or Y selected. The value of n has the following
significance:
n
0
1
2
3
Expand mode
Off
Expand X
Expand Y
Expand X and Y
FET [n]
Voltage mode input device control
The value of n selects the input device according to the following table:
n
0
1
Selection
Bipolar device, 10 kΩ input impedance, 2 nV/√Hz voltage noise
FET, 10 MΩ input impedance, 5 nV/√Hz voltage noise
FLOAT [n]
Input connector shield float / ground control
The value of n sets the input shield switch according to the following table:
n
0
1
Selection
Ground
Float (connected to ground via a 1 kΩ resistor)
FNF [n]
Reference harmonic mode control
The value of n sets the reference channel to one of the NF modes, or restores it to the
default F mode according to the following table:
n
1
2
3
Mode selected
The lock-in amplifier measures signals at the reference frequency F
The lock-in amplifier measures at 2F
The lock-in amplifier measures at 3F
FRQ[.]
Reference frequency meter
If the lock-in amplifier is in the EXT or EXT LOGIC reference source modes, the
FRQ command causes the lock-in amplifier to respond with 0 if the reference channel
is unlocked, or with the reference input frequency if it is locked.
If the lock-in amplifier is in the INT reference source mode, it responds with the
frequency of the internal oscillator.
In fixed point mode the frequency is in mHz.
In floating point mode the frequency is in Hz.
E-7
Appendix E, ALPHABETICAL LISTING OF COMMANDS
FSTART[.] [n]
Oscillator frequency sweep start frequency
Sets the start frequency for a subsequent sweep of the internal oscillator frequency.
In fixed point mode, n is in millihertz.
In floating point mode n is in hertz.
FSTEP[.] [n1 n2]
Oscillator frequency sweep step size and type
The frequency may be swept either linearly or logarithmically, as specified by
parameter n2. The step size is specified by parameter n1.
Log sweep n2 = 0
In fixed point mode, n1 is the step size in thousandths of a percent.
In floating point mode n1 is in percent. The range of n1 is 0 to 100.00 %
Linear sweep n2 = 1
In fixed point mode, n1 is the step size in millihertz.
In floating point mode n1 is in hertz. The range of n1 is 0 to 10 kHz
FSTOP[.] [n] Oscillator frequency sweep stop frequency
Sets the stop frequency for a subsequent sweep of the internal oscillator frequency.
In fixed point mode, n is in millihertz.
In floating point mode n is in hertz.
GP [n1 [n2]]
Set/read GPIB parameters
n1 sets the GPIB address in the range 0 to 31.
n2 sets the GPIB terminator and the test echo function according to the following
table:
n
0
1
2
3
4
5
Terminator
[CR], test echo disabled
[CR], test echo enabled
[CR,LF], test echo disabled
[CR,LF], test echo enabled
no terminator, test echo disabled
no terminator, test echo enabled
When the test echo is on, every character transmitted or received via the GPIB port is
echoed to the RS232 port. This is provided solely as an aid to program development
and should not be enabled during normal operation of the instrument.
E-8
Appendix E, ALPHABETICAL LISTING OF COMMANDS
Halt curve acquisition
HC
Halts curve acquisition in progress. It is effective during both single (data acquisition
initiated by TD command) and continuous (data acquisition initiated by TDC
command) curve acquisitions. The curve may be restarted by means of the TD or
TDC command, as appropriate.
ID
Identification
Causes the lock-in amplifier to respond with the number 7220.
IE [n]
Reference channel source control (Internal/External)
The value of n sets the reference input mode according to the following table:
n
0
1
2
Selection
INT (internal)
EXT LOGIC (external rear panel TTL input)
EXT (external front panel analog input)
IMODE [n] Controls whether the instrument input is connected to a current or a voltage
preamplifier
The value of n sets the input mode according to the following table:
n
0
1
Input mode
Current mode off - voltage mode input enabled
High bandwidth (HB) current mode enabled - connect signal to B input
connector
2
Low noise (LN) current mode enabled - connect signal to B inputconnector
If n = 0 then the input configuration is determined by the VMODE command.
If n > 0 then current mode is enabled irrespective of the VMODE setting.
LEN [n]
Curve length control
The value of n sets the curve buffer length in effect for data acquisition. The
maximum allowed value depends on the number of curves requested using the CBD
command, and a parameter error results if the value given is too large. For this
reason, if the number of points is to be increased and the number of curves to be
stored is to be reduced using the CBD command, then the CBD command should be
issued first.
E-9
Appendix E, ALPHABETICAL LISTING OF COMMANDS
LF [n] Signal channel line frequency rejection filter control
In instruments manufactured prior to June 1996, the value of n sets the mode of the
line frequency notch filter according to the following table:
n
0
1
Selection
Off
On (i.e. reject 50/60 Hz and 100/120 Hz)
In instruments manufactured after June 1996, the value of n sets the mode of the line
frequency notch filter according to the following table:
n
0
1
2
3
Selection
Off
Enable 50 or 60 Hz notch filter
Enable 100 or 120 Hz notch filter
Enable both filters
Users may identify which version of the instrument they have by sending the
command LF 3; if this is accepted by the instrument, it was made after June 1996,
but if it generates a command error, it was made prior to this date.
Units made after June 1996 respond in addition to the command, LINE50, which sets
the notch filter centre frequency.
LINE50 [n]
Signal channel line frequency rejection filter centre frequency control
The value of n sets the line frequency notch filter centre frequency according to the
following table:
n
0
1
Notch filter mode
60 Hz (and/or 120 Hz)
50 Hz (and/or 100 Hz)
Units made prior to June 1996 generate an Invalid Command (bit 1 of the serial poll
status byte is asserted) to the LINE50 command.
LOCK
System lock control
Updates all frequency dependent gain and phase correction parameters.
LR[.]
Log ratio output
In integer mode, the LR command reports a number equivalent to
1000 log (X / ADC1) where X is the value that would be returned by the X command
and ADC1 is the value that would be returned by the ADC1 command. The response
range is - 3000 to +2079.
In floating point mode, the LR. command reports a number equivalent to
log (X / ADC 1). The response range is -3.000 to +2.079.
E-10
Appendix E, ALPHABETICAL LISTING OF COMMANDS
Lights on/off control
LTS [n]
The value of n controls the front panel LEDs and LCD backlights according to the
following table:
n
0
1
Selection
All lights off
Normal operation
M
Curve acquisition status monitor
Causes the lock-in amplifier to respond with four values that provide information
concerning data acquisition, as follows.
First value, Curve Acquisition Status: a number with five possible values, defined
by the following table:
First Value Significance
0
1
2
5
6
No curve activity in progress.
Acquisition via TD command in progress and running.
Acquisition via TDC command in progress and running.
Acquisition via TD command in progress but halted by HC command
Acquisition via TDC command in progress but halted by HC command
Second value, Number of Sweeps Acquired: This number is incremented each time
a TD is completed and each time a full cycle is completed on a TDC acquisition. It is
zeroed by the NC command and also whenever a CBD or LEN command is applied
without parameters.
Third value, Status Byte: The same as the response to the ST command. The
number returned is the decimal equivalent of the status byte and refers to the
previously applied command.
Fourth value, Number of Points Acquired: This number is incremented each time a
point is taken. It is zeroed by the NC command and whenever CBD or LEN is applied
without parameters.
MAG[.]
Magnitude
In fixed point mode causes the lock-in amplifier to respond with the magnitude value
in the range 0 to 30000, full-scale being 10000.
In floating point mode causes the lock-in amplifier to respond with the magnitude
value in the range +3.000E0 to +0.001E-9 volts or +3.000E-6 to +0.001E-15 amps.
MP[.]
Magnitude, phase
Equivalent to the compound command MAG[.];PHA[.]
MSK [n]
Set/read service request mask byte
The value of n sets the SRQ mask byte in the range 0 to 255.
E-11
Appendix E, ALPHABETICAL LISTING OF COMMANDS
\N n
Address command
When the 7220 is daisy-chained with other compatible instruments this command will
change which instrument is addressed. All daisy-chained instruments receive
commands but only the currently addressed instrument will implement or respond to
the commands. The exception is the \N n command. If n matches the address set from
the front panel the instrument will switch into addressed mode. If n does not match
the address set from the front panel the instrument will switch into unaddressed mode.
Note: The \N n command does not change the address of an instrument but which
instrument is addressed.
Warning: All instruments must have a unique address.
N
Report overload byte
Causes the lock-in amplifier to respond with the overload byte.
Bit 0
Bit 1
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
Bit 7
not used
CH1 output overload (> ±120 %FS)
CH2 output overload (> ±120 %FS)
Y output overload (> ±300 %FS)
X output overload (> ±300 %FS)
not used
input overload
reference unlock
NC
New curve
Initializes the curve storage memory and status variables. All record of previously
taken curves is removed.
NHZ.
Causes the lock-in amplifier to respond with the square root of the noise spectral
density measured at the Y output, expressed in volt/√Hz or amp/√Hz referred to the
input. This measurement assumes that the Y output is Gaussian with zero mean.
(Section 3.10). The command is only available in floating point mode.
This command is not available when the reference frequency exceeds 60 kHz.
NN[.]
Noise output
In fixed point mode causes the lock-in amplifier to respond with the mean absolute
value of the Y output in the range 0 to 12000, full-scale being 10000. If the mean
value of the Y output is zero, this is a measure of the output noise.
In floating point mode causes the lock-in amplifier to respond in volts.
E-12
Appendix E, ALPHABETICAL LISTING OF COMMANDS
OA[.] [n] Oscillator amplitude control
In fixed point mode n sets the oscillator amplitude in mV. The range of n is 0 to 5000
representing 0 to 5 V rms.
In floating point mode n sets the amplitude in volts.
OF[.] [n]
Oscillator frequency control
In fixed point mode n sets the oscillator frequency in mHz. The range of n is 0 to
120,000,000 representing 0 to 120 kHz.
In floating point mode n sets the oscillator frequency in Hz. The range of n is 0 to
1.2E5.
PHA[.]
Signal phase
In fixed point mode causes the lock-in amplifier to respond with the signal phase in
centidegrees, in the range ±18000.
In floating point mode causes the lock-in amplifier to respond with the signal phase in
degrees.
REFP[.] [n]
Reference phase control
In fixed point mode n sets the phase in millidegrees in the range ±360000.
In floating point mode n sets the phase in degrees.
REMOTE [n]
Remote only (front panel lock-out) control
Allowed values of n are 0 and 1. When n is equal to 1, the lock-in amplifier enters
remote only mode in which the front panel control functions are inoperative and the
instrument can only be controlled with the RS232 or the GPIB interfaces. When n is
equal to 0, the front panel control functions normally.
REV
Report firmware revision
Causes the lock-in amplifier to respond with the firmware revision number. This gives
a four line response which the controlling program must be able to accept.
E-13
Appendix E, ALPHABETICAL LISTING OF COMMANDS
RS [n1 [n2]]
Set/read RS232 interface parameters
The values of n1 set the baud rate of the RS232 interface according to the following
table:
n1 Baud rate (bits per second)
0
1
75
110
2
3
134.5
150
4
300
5
600
6
7
8
9
10
11
12
1200
1800
2000
2400
4800
9600
19200
The lowest five bits in n2 control the other RS232 parameters according to the
following table:
bit number
bit negated
bit asserted
0
1
2
3
4
data + parity = 8 bits
no parity bit
even parity
echo disabled
prompt disabled
data + parity = 9 bits
1 parity bit
odd parity
echo enabled
prompt enabled
RT[.]
Ratio output
In integer mode the RT command reports a number equivalent to 1000 × X / ADC1
where X is the value that would be returned by the X command and ADC1 is the
value that would be returned by the ADC1 command.
In floating point mode the RT. command reports a number equivalent to X / ADC 1.
SAMPLE [n] Main analog to digital converter sample rate control
The sampling rate of the main analog to digital converter, which is nominally
166 kHz, may be adjusted from this value to avoid problems caused by the aliasing
of interfering signals into the output passband.
n may be set to 0, 1, 2 or 3, corresponding to four different sampling rates (not
specified) near 166 kHz.
E-14
Appendix E, ALPHABETICAL LISTING OF COMMANDS
Full-scale sensitivity control
SEN[.] [n]
The value of n sets the full-scale sensitivity according to the following table,
depending on the setting of the IMODE control:
n
full-scale sensitivity
IMODE=1
20 fA
IMODE=0
20 nV
50 nV
100 nV
200 nV
500 nV
1 µV
IMODE=2
n/a
4
5
6
7
8
50 fA
n/a
n/a
n/a
n/a
100 fA
200 fA
500 fA
1 pA
9
n/a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
2 µV
5 µV
2 pA
5 pA
10 pA
20 pA
20 fA
50 fA
100 fA
200 fA
500 fA
1 pA
10 µV
20 µV
50 µV
100 µV
200 µV
500 µV
1 mV
50 pA
100 pA
200 pA
500 pA
1 nA
2 nA
5 nA
10 nA
20 nA
2 pA
5 pA
10 pA
20 pA
50 pA
100 pA
200 pA
500 pA
1 nA
2 nA
5 nA
10 nA
2 mV
5 mV
10 mV
20 mV
50 mV
100 mV
200 mV
500 mV
1 V
50 nA
100 nA
200 nA
500 nA
1 µA
Floating point mode can only be used for reading the sensitivity, which is reported in
volts or amps. For example, if IMODE = 0 and the sensitivity is 1 mV the command
SEN would report 18 and the command SEN. would report +1.0E-03. If IMODE was
changed to 1, SEN would still report 18 but SEN. would report +1.0E-09.
SLOPE [n]
Output low-pass filter slope (roll-off) control
The value of n sets the slope of the output filters according to the following table:
n
0
1
2
3
slope
6 dB/octave
12 dB/octave
18 dB/octave
24 dB/octave
SRATE[.] [n]
Oscillator frequency sweep step rate
Sets the sweep rate in time per step in the range 50 ms to 1000 s, in 5 ms increments.
E-15
Appendix E, ALPHABETICAL LISTING OF COMMANDS
ST
Report status byte
Causes the lock-in amplifier to respond with the status byte.
Note: this command is not normally used in GPIB communications, where the status
byte is accessed by performing a serial poll.
Bit 0
Bit 1
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
Bit 7
Command complete
Invalid command
Command parameter error
Reference unlock
Overload
New ADC values available after external trigger
Asserted SRQ
Data available
STAR [n] Star mode setup command
The star mode allows faster access to instrument outputs than is possible using the
conventional commands listed above. The mode is set up using the STAR command
to specify the output(s) required and invoked by sending an asterisk (ASCII 42) to
request the data. The data returned is specified by the value of n, as follows:
n
0
1
2
3
4
5
6
7
Data returned by * command
X
Y
MAG
PHA
ADC1
XY
MP
ADC1;ADC2
*
Transfer command
This command establishes the high-speed transfer mode. Use the STAR command to
set up the desired response to the * command, and then send an * (ASCII 42), without
terminator, to the instrument. The instrument will reply with the selected output as
quickly as possible, and then wait for another *. If the computer processes the reply
quickly and responds immediately with another *, then very rapid controlled data
transfer is possible.
The first transfer takes a little longer than subsequent ones because some overhead
time is required for the model 7220 to get into the high speed transfer mode. When in
this mode, the front panel controls are inactive and display is frozen.
The mode is terminated by sending any command other than an *, when the
instrument will exit the mode and process the new command, or after a period of 10
seconds following the last * command.
Caution: Check that the computer program does not automatically add a carriage
E-16
Appendix E, ALPHABETICAL LISTING OF COMMANDS
return or carriage return-line feed terminator to the * command, since these
characters will slow down communications.
STR [n]
Storage interval control
Sets the time interval between successive points being acquired under the TD or TDC
commands. n specifies the time interval in ms with a resolution of 5 ms, input values
being rounded up to a multiple of 5. The longest interval that can be specified is
1000000 s corresponding to about one point in 12 days.
In addition, n may be set to 0, which sets the rate of data storage to the curve buffer
to 800 Hz. However this only allows storage of the X and Y outputs. There is no need
to issue a CBD 3 command to set this up since it happens automatically when
acquisition starts.
If the time constant is set to 5 ms or longer, then the actual time constant applied to
the stored X and Y output values will be 640 µs, but if it is set to a shorter value then
this will be the time constant actually used.
SWEEP [n]
Oscillator frequency sweep start/Stop
Starts/stops the internal oscillator frequency sweep depending on the value of n
according to the following table:
n
0
1
Frequency sweep status
Stop/Pause
Run
When a frequency sweep has been defined, applying SWEEP 1 will start it. The
sweep will continue until the stop frequency is reached. If, during the sweep,
SWEEP 0 is applied, the sweep will stop at the current frequency. If SWEEP 1 is
then applied, the sweep will restart from this point. Once the sweep reaches the stop
frequency and stops, the next SWEEP 1 command will reset the frequency to the start
frequency and restart the sweep.
SYNC [n]
Synchronous time constant control
At reference frequencies below 10 Hz, if the synchronous time constant is enabled,
then the actual time constant of the output filters is not generally the selected value T
but rather a value equal to an integer number of reference cycles. If T is greater than
1 cycle, the time constant is between T/2 and T. The parameter n has the following
significance:
n
0
1
Effect
Synchronous time constant disabled
Synchronous time constant enabled
E-17
Appendix E, ALPHABETICAL LISTING OF COMMANDS
SYNCOSC [n]
Synchronous oscillator (demodulator monitor) control
This control operates only in external reference mode. The parameter n has the
following significance:
n
0
1
Effect
Synchronous Oscillator (Demodulator Monitor) disabled
Synchronous Oscillator (Demodulator Monitor) enabled
When enabled and in external reference mode, the instrument’s OSC OUT connector
functions as a demodulator monitor of the X channel demodulation function.
TADC [n]
Auxiliary ADC trigger mode control
The value of n sets the trigger mode of the auxiliary ADC inputs according to the
following table:
n
0
1
2
3
4
5
6
7
Trigger mode
Asynchronous (5 ms intervals)
External (rear panel TRIG input)
Burst mode, fixed rate, triggered by command (ADC1 only)
Burst mode, fixed rate, triggered by command (ADC1 and ADC2)
Burst mode, variable rate, triggered by command (ADC1 only)
Burst mode, variable rate, triggered by command (ADC1 and ADC2)
Burst mode, fixed rate, External trigger (rear panel TRIG input) (ADC1 only)
Burst mode, fixed rate, External trigger (rear panel TRIG input) (ADC1 and
ADC2)
8
9
Burst mode, variable rate, External trigger (rear panel TRIG input) (ADC1
only)
Burst mode, variable rate, External trigger (rear panel TRIG input) (ADC1
and ADC2)
In the burst modes, data is stored in the curve buffer. Use the LEN command to set
the number of points required. Note that it may be necessary to enter CBD 32 before
setting the length, if the curve buffer has previously been used for more than one data
type. The data is read out from the buffer using DC[.] 5 for ADC1 and DC[.] 6 for
ADC2. If the length is set to more than 16384 and a burst mode which stores both
ADC1 and ADC2 is specified then the curve length will automatically be reduced to
16384 points. Note also that setting the TADC parameter to any value other than 0 or
1 may affect the CBD parameter, as follows:
TADC parameter
Effect on CBD parameter
none
none
0
1
2
3
4
5
6
7
8
9
automatically set to 32
automatically set to 96
automatically set to 32
automatically set to 96
automatically set to 32
automatically set to 96
automatically set to 32
automatically set to 96
E-18
Appendix E, ALPHABETICAL LISTING OF COMMANDS
The maximum sampling rate depends on the number of ADC inputs used and whether
the sampling is timed or simply runs as fast as possible. In the modes above described
as Fixed Rate, sampling runs at the maximum possible rate, nominally 20 kHz when
sampling both ADC1 and ADC2 or 40 kHz when sampling ADC1 only. In the
Variable Rate modes, the sampling speed is set by the BURSTRATE command.
TC [n]
TC.
Filter time constant control
The value of n sets the time constant of the output according to the following table:
n
0
1
2
3
4
5
6
7
time constant
10 µs
20 µs
40 µs
80 µs
160 µs
320 µs
640 µs
5 ms
8
9
10 ms
20 ms
50 ms
100 ms
200 ms
500 ms
1 s
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
2 s
5 s
10 s
20 s
50 s
100 s
200 s
500 s
1 ks
2 ks
5 ks
The TC. command is only used for reading the time constant, and reports the current
setting in seconds. Hence if a TC 11 command were sent, TC would report 11 and
TC. 1.0E-01, i.e. 0.1 s or 100 ms
TD
Take data
Initiates data acquisition. Acquisition starts at the current position in the curve buffer
and continues at the rate set by the STR command until the buffer is full.
E-19
Appendix E, ALPHABETICAL LISTING OF COMMANDS
TDC
Take data continuously
Initiates data acquisition. Acquisition starts at the current position in the curve buffer
and continues at the rate set by the STR command until halted by an HC command.
The buffer is circular in the sense that when it has been filled, current data overwrites
earlier points.
VER
Report firmware version
Causes the lock-in amplifier to respond with the firmware version number. The
firmware version number is the number displayed on the front panel RS232 SETUP 3
setup screen.
VMODE [n]
Voltage input configuration
The value of n sets up the input configuration according to the following table:
n
0
1
3
Input configuration
Both inputs grounded (test mode)
A input only
A-B differential mode
Note that the IMODE command takes precedence over the VMODE command.
X output
X[.]
In fixed point mode causes the lock-in amplifier to respond with the X demodulator
output in the range ±30000, full-scale being ±10000.
In floating point mode causes the lock-in amplifier to respond with the X demodulator
output in volts or amps.
XOF [n1 [n2]]
X output offset control
The value of n1 sets the status of the X offset facility according to the following table:
n1
0
1
Selection
Disables offset facility
Enables offset facility
The range of n2 is ±30000 corresponding to ±300 % full-scale.
XY[.]
Y[.]
X, Y outputs
Equivalent to the compound command X[.];Y[.]
Y output
In fixed point mode causes the lock-in amplifier to respond with the Y demodulator
output in the range ±30000, full-scale being ±10000.
In floating point mode causes the lock-in amplifier to respond with the Y demodulator
output in volts or amps.
E-20
Appendix E, ALPHABETICAL LISTING OF COMMANDS
YOF [n1 [n2]]
Y output offset control
The value of n1 sets the status of the Y offset facility according to the following table:
n1
0
1
Selection
Disables offset facility
Enables offset facility
The range of n2 is ±30000 corresponding to ±300 % full-scale.
E-21
Appendix E, ALPHABETICAL LISTING OF COMMANDS
E-22
Default Settings
AppendixF
Default Setting Function
The default setting function sets the model 7220’s controls and output displays as
follows:-
Left-hand LCD
Displays the AC Gain control on the upper line and the full-scale sensitivity control
on the lower line.
Right-hand LCD
Displays the magnitude as a percentage of full-scale output on the left-hand side and
the phase angle in degrees output on the right-hand side.
The ten controls accessed via the left-hand LCD are set to the following values:-
Full-scale sensitivity
AC Gain
500 mV
0 dB
Time constant
Slope
100 ms
12 dB/octave
1000.000 Hz
0.500 mV rms
0.000 V
0.000 V
OFF
Oscillator frequency
Oscillator amplitude
DAC1 output
DAC2 output
Output offsets
Phase
0.00°
The two output offset controls accessed via the right-hand LCD are set as follows:-
X channel output offset
Y channel output offset
0.00 %
0.00 %
The remaining instrument controls, accessed via the setup menus, are set as follows:-
Input mode
Single-ended voltage mode, A input connector
Coupling
AC
Input connector shell
Input device
Floating
FET
Internal
1st
Off
Off
Reference mode
Reference harmonic
Demodulator monitor
Output expansion
CH1 analog output
CH2 analog output
X %
Y %
F-1
Appendix F, DEFAULT SETTINGS
Line frequency rejection filter Off
AC Gain control
Time constant mode
ADC trigger rate
Front panel lights
Display contrast
Manual
Sync
200 Hz
On
0
RS232 interface settings
Baud rate
Data bits
9600
7
Stop bits
1
Parity
Even
On
On
, (044)
1
Prompt character
Character echo
Delimiter
Address
GPIB interface settings
Address
12
Terminator
SRQ mask byte
Test Echo
CR,LF
0
Disabled
Digital outputs
Sample rate adjustment
0 (i.e. D0 - D7 are at logic zero)
0
F-2
Index
Index
* command 6-18, E-16
\N n command 6-28, E-12
2F reference mode 3-7, 5-4
8-bit programmable output port 3-8
90° Key 4-5
AUTO menu 4-4
Auto repeat 4-3
AUTOMATIC [n] command 6-12, E-1
AUX RS232 connector 4-7
Auxiliary ADCs 3-8
Auxiliary RS232 interface 6-4
AXO command 6-16, E-2
Absolute accuracy 3-8
AC Gain
and full-scale sensitivity 3-9
and input overload 3-4
control 5-8, 5-22
Baseband reference mode 3-6
Baud rate control 5-11
Bipolar input device 3-2, 5-3
Burst mode acquisition 6-1
BURSTRATE [n] command 6-22, E-2
BYTE [n] command 6-21, E-2
description of 3-4
AC GAIN [n] command 6-12, E-1
AC/DC input coupling 3-2
Accuracy 3-8
Active cursor 4-3
ADC
sample rate control 5-17
sampling frequency 3-4
trigger control 5-10
ADC[.] n command 6-21, E-1
ADC1
CBD [n] command 6-23, E-2
CH n1 [n2] command 6-16, E-4
CH1
connector 4-8
output 3-8
output connector 5-6
output control 5-6
auxiliary input 3-8
connector 4-8
display 5-33
CH2
connector 4-8
output 3-8
ADC2
output connector 5-6
output control 5-6
Commands
auxiliary input 3-8
connector 4-8
display 5-33
ADF command 6-30, E-1
ADJUST keys 4-2
alphabetical listing of E-1 – E-21
compound 6-7
descriptions of 6-10
for auxiliary inputs 6-21
for auxiliary outputs 6-20
for computer interfaces 6-27
for default setting 6-30
for front panel 6-30
for instrument identification 6-29
for instrument outputs 6-16
for internal oscillator 6-19
for output data curve buffer 6-23
for reference channel 6-13
for signal channel 6-10
for signal channel output amplifiers 6-15
for signal channel output filters 6-14
format 6-6
Analog meter 3-9, 4-1, 4-6
Analog to digital converter (ADC) 3-4
Anti-aliasing filter 3-4
AQN command 6-13, E-1
AS command 6-11, E-1
ASM command 6-12, E-1
Auto functions
Auto-Measure 3-9, 3-16, 5-19
Auto-Offset 3-15, 5-19
Auto-Phase 3-11, 3-15, 5-18
Auto-Sensitivity 3-9, 3-15, 5-18
introduction 3-14
menu 5-18
AUTO key 4-4
Common mode rejection ratio (CMRR) 3-2
Index-1
INDEX
Computer control, sample programs 6-30
Contrast control 5-10
Control options menu 5-8
FAST X
connector 4-8
output 3-7
FAST Y
connector 4-8
output 3-7
FET [n] command 6-10, E-7
FET input device 3-2, 5-3
FIR filters 3-12
Control setup menu 5-17
CP [n] command 6-11, E-4
Current input mode 3-2, 5-2
Current to voltage converter 5-2
Current/voltage input mode selection 3-3
Curve storage 6-1
Firmware version display 5-13
FLOAT [n] command 6-10, E-7
Floating point command format 6-7
FNF [n] command 6-13, E-7
Front panel
DAC[.] n1 [n2] command 6-20, E-4
DAC1
auxiliary output 3-8
connector 4-8
control 5-24
layout 4-1
DAC2
operation 5-1
auxiliary output 3-8
connector 4-8
control 5-25
FRQ[.] command 6-14, E-7
FSTART[.] [n] command 6-19, E-8
FSTEP[.] [n1 n2] command 6-20, E-8
FSTOP[.] [n] command 6-19, E-8
Full-scale sensitivity 3-9
Fuses, line 2-1, 2-2
DC[.] n command 6-25, E-4
DCT n command 6-26, E-5
DD [n] command 6-7, 6-28, E-6
Default setting 3-16
Default setting control 5-17
Default settings, list of F-1 – F-2
Delimiters 6-7
General purpose reference input 3-6
GP [n1 [n2]] command 6-4, 6-6, 6-28, E-8
GPIB interface
address 6-4
Demodulator
address control 5-14
connector 4-7
general features 6-4
handshaking 6-5
microprocessor support of 3-8
service request 5-15, 6-9
service request mask byte 5-15
setup 1 menu 5-14
setup 2 menu 5-15
status byte 5-15
DSP 3-7
gain 3-9
monitor 3-7, 5-4
Differential voltage input mode 3-2, 4-1, 5-2
Digital displays 3-9
Digital low-pass filter 3-7
Digital output port 3-8
DIGITAL OUTPUTS connector 4-7
Digital outputs setup menu 5-16
Digital phase locked loop 3-6
Digital phase shifter 3-6
Direct digital synthesizer (DDS) 3-6
Direct mode 6-2
terminator control 5-14
test echo control 5-15
Handshaking and echos 6-5
Harmonic measurement selection 5-4
Harmonic measurements 3-7
HC command 6-25, E-9
Display contrast control 5-10
Dynamic range 3-4
Dynamic reserve 3-9
Highband reference mode 3-6
ENBW[.] command 6-18, E-6
Equivalent noise bandwidth 3-14
EVENT [n] command 6-24, E-6
EX [n] command 6-16, E-7
External reference 5-4
ID command 6-29, E-9
IE [n] command 6-13, E-9
IMODE [n] command 6-10, E-9
Initial checks 2-3
External reference mode 3-6
Index-2
INDEX
Input
connector ground/float 5-3
Miscellaneous options menu 5-10
MP[.] command 6-17, E-11
connector selection 5-2
connector shell ground/float 3-2
coupling 3-2, 5-3
device selection 3-2, 5-3
float/ground control 3-2
impedance 3-2, 5-3
mode 5-2
MSK [n] command 6-9, 6-29, E-11
N command 6-8, 6-29, E-12
NC command 6-24, E-12
nF reference mode 3-7, 5-4
NHZ. command 6-17, E-12
NN[.] command 6-18, E-12
Noise %fs display 5-28
mode selection 3-3
overload 3-4
Noise in volts/amps per root hertz display 5-31
Noise measurements 3-14
NOISE output 5-7
overload indicators 4-1
selection 3-2
setup menu 5-2
Inspection 2-1
Null modem 6-2, 6-5
Installation 2-1
OA[.] [n] command 6-19, E-13
OF[.] [n] command 6-19, E-13
Offset status control 5-25
Operating environment 2-1
OSC OUT connector 3-7, 4-1
Oscillator
Internal oscillator 3-7
Internal oscillator frequency sweep 3-8
Internal reference 5-4
Internal reference mode 3-6
Key specifications 1-3
amplitude control 5-24
frequency control 5-23
Output
LCD contrast control 5-10
LED indicators 4-1
Left-hand LCD display panel 4-2
LEN [n] command 6-24, E-9
LF [n] command 6-12, E-10
Lights control 5-10
channel filters 3-7, 3-11
expand 3-12
expand control 5-5
offset 3-12
offset indicators 5-34
processor 3-8
setup menu 5-5
Line cord 2-1
Line frequency rejection filter 3-3, 5-8
Line fuses 2-1, 2-2
Line power input assembly 4-6
Line power switch 4-6
Line voltage selection 2-1
LINE50 [n] command 3-3, 6-12, E-10
LOCK command 3-10, 6-14, E-10
Log ratio display 5-32
Overload byte 6-7
PHA[.] command 6-17, E-13
Phase 3-8
Phase in degrees display 5-29
Phase noise 3-6
Phase sensitive detector 3-7
Phase shifter 3-10
Phase values 3-6
LR[.] command 6-17, E-10
LTS [n] command 6-30, E-11
PHASE1 output 5-7
PHASE2 output 5-7
M command 6-25, E-11
MAG % output 5-6
Power consumption 2-1
Power input assembly 2-1
Power-up defaults 3-14
PREAMP POWER connector 4-7
Programming examples 6-30
Prompt character 6-7
MAG %fs display 5-28
MAG in volts/amps display 5-31
MAG[.] command 6-17, E-11
Main Display mode 5-1, 5-20
Main display mode 4-4
Main microprocessor 3-8
MENU key 4-4
Rack mounting 2-1
Microsoft Windows Terminal program 6-2
Ratio calculations 3-8
Index-3
INDEX
Ratio display 5-32
SET key 4-5
RATIO output 5-7
Rear panel layout 4-6
Setup menu mode 4-4, 5-1
SIG MON connector 3-4, 4-7
Signal channel inputs 3-2
Signal channel overload 3-9
Signal input connectors 4-1
Single-ended voltage input 5-2
Single-ended voltage input mode 4-1
Slope 3-11
REF IN connector 3-10, 4-2, 5-4
REF MON connector 4-7
REF TTL connector 4-7, 5-4
Reference channel DSP 3-6
Reference frequency changes 3-10
Reference frequency display 5-29
Reference harmonic 5-4
Reference harmonic number 3-7
Reference mode
SLOPE [n] command 6-14, E-15
Slope control 5-23
Specifications
external 3-6
internal 3-6
Reference phase 3-10
detailed listing of A-1 – A-6
for Auxiliary Inputs A-4
for Data Storage A-5
for Demodulator and Output Processing A-3
for Measurement Modes A-1
for Oscillator A-3
for Outputs A-4
for Reference Channel A-2
for Signal Channel A-1
Reference phase control 5-26
Reference setup menu 5-4
Reference source control 5-4
Reference unlock indicator 4-2
REFP[.] [n] command 3-11, 6-13, E-13
Relative accuracy 3-8
REMOTE [n] command 6-29, E-13
REV command 6-29, E-13
Right-hand LCD display panel 4-5
RS [n1 [n2]] command 6-27, E-14
RS232 interface
SRATE[.] [n] command 6-20, E-15
ST command 6-8, 6-28, E-16
STAR [n] command 6-18, E-16
Status byte 6-7
STR [n] command 6-24, E-17
SWEEP [n] command 6-20, E-17
SYNC [n] command 6-15, E-17
Synchronous filters 3-12
Synchronous oscillator 3-7
SYNCOSC [n] command 6-19, E-18
System updates 3-10
address 6-4
address control 5-13
character echo control 5-12
choice of baud rate 6-3
choice of number of data bits 6-3
choice of parity check option 6-3
connector 4-7
TADC [n] command 6-21, E-18
TC [n] command 6-14, E-19
TC. command 6-14, E-19
TD command 6-24, E-19
TDC command 6-24, E-20
Technical description 3-1
Terminal emulator 6-2
Terminal mode 6-2
Terminators 6-6
Time constants 3-12
data delimiter control 5-12
data format control 5-11
data prompt character control 5-12
general features 6-2
handshaking 6-5
input terminator 6-6
microprocessor support of 3-8
output terminator 6-6
setup 1 menu 5-11
setup 2 menu 5-12
setup 3 menu 5-13
Time Constants control 5-22
Time constants control 5-9
Transient recorder 3-8
TRIG connector 4-8
TTL logic reference input 3-6
Typical experiment description 5-35
RT[.] command 6-17, E-14
SAMPLE [n] command 6-13, E-14
Sample rate control 5-17
SELECT keys 4-2, 5-1
SEN[.] [n] command 6-11, E-15
Sensitivity control 5-20
Index-4
INDEX
Update program 3-8
Vector magnitude 3-8
Ventilation 2-1
VER command 6-29, E-20
VMODE [n] command 6-10, E-20
Voltage input mode 3-2
What is a lock-in amplifier? 1-2
X % output 5-6
X %fs display 5-27
X & Y demodulation functions 3-7
X in volts/amps display 5-30
X output offset level control 5-34
X[.] command 6-16, E-20
XOF [n1 [n2]] command 6-15, E-20
XY[.] command 6-17, E-20
Y % output 5-6
Y %fs display 5-27
Y in volts/amps display 5-30
Y output offset level control 5-34
Y[.] command 6-16, E-20
YOF [n1 [n2]] command 6-15, E-21
Zero error 3-15
Index-5
INDEX
Index-6
WARRANTY
EG&G Instruments Corporation warrants each instrument of its own manufacture to be free of defects in material and
workmanship for a period of ONE year from the date of delivery to the original purchaser. Obligations under this Warranty
shall be limited to replacing, repairing or giving credit for the purchase, at our option, of any instruments returned, shipment
prepaid, to our Service Department for that purpose, provided prior authorization for such return has been given by an
authorized representative of EG&G Instruments Corporation.
This Warranty shall not apply to any instrument, which our inspection shall disclose to our satisfaction, to have become
defective or unusable due to abuse, mishandling, misuse, accident, alteration, negligence, improper installation, or other
causes beyond our control. This Warranty shall not apply to any instrument or component not manufactured by EG&G
Instruments Corporation. When products manufactured by others are included in EG&G Instruments Corporation equipment,
the original manufacturers Warranty is extended to EG&G Instruments customers.
EG&G Instruments Corporation reserves the right to make changes in design at any time without incurring any obligation to
install same on units previously purchased.
THERE ARE NO WARRANTIES WHICH EXTEND BEYOND THE DESCRIPTION ON THE FACE HEREOF. THIS WARRANTY IS
IN LIEU OF, AND EXCLUDES ANY AND ALL OTHER WARRANTIES OR REPRESENTATIONS, EXPRESSED, IMPLIED OR
STATUTORY, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, AS WELL AS ANY AND ALL
OTHER OBLIGATIONS OR LIABILITIES OF EG&G INSTRUMENTS CORPORATION, INCLUDING, BUT NOT LIMITED TO, SPECIAL
OR CONSEQUENTIAL DAMAGES. NO PERSON, FIRM OR CORPORATION IS AUTHORIZED TO ASSUME FOR EG&G
INSTRUMENTS CORPORATION ANY ADDITIONAL OBLIGATION OR LIABILITY NOT EXPRESSLY PROVIDED FOR HEREIN
EXCEPT IN WRITING DULY EXECUTED BY AN OFFICER OF EG&G INSTRUMENTS CORPORATION.
SHOULD YOUR EQUIPMENT REQUIRE SERVICE
A. Contact your local EG&G Instruments office, agent, representative or distributor to discuss the problem. In many cases it
may be possible to expedite servicing by localizing the problem to a particular plug-in circuit board.
B. We will need the following information, a copy of which should also be attached to any equipment which is returned for
service.
1. Model number and serial number of instrument
2. Your name (instrument user)
3. Your address
6. Symptoms (in detail, including control settings)
7. Your purchase order number for repair charges
(does not apply to repairs in warranty)
8. Shipping instructions (if you wish to authorize
shipment by any method other than normal surface
transportation)
4. Address to which the instrument should be
returned
5. Your telephone number and extension
C. If you experience any difficulties in obtaining service please contact
EG&G Instruments
Signal Recovery
Sorbus House
Phone:
Fax:
+44 (0)118 977 3003
+44 (0)118 977 3493
Mulberry Business Park
WOKINGHAM RG41 2GY
United Kingdom
|