Stanford Research Systems SR830 Specifications Sheet

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DSP Lock-In Amplifier

model SR830

1290 D Reamwood Avenue

Sunnyvale, CA94089 USA

Phone: (408) 744-9040 • Fax: (408) 744-9049

www.thinkSRS.com •e-mail: info@thinkSRS.com

Revision 1.5 • 11/99

Stanford Research Systems

TABLE OF CONTENTS

GENERAL INFORMATION

Safety and Preparation for Use 1-3 Specifications 1-5 Abridged Command List 1-7

GETTING STARTED

Your First Measurements 2-1 The Basic Lock-in 2-3 X, Y, R and θ 2-7 Outputs, Offsets and Expands 2-9 Storing and Recalling Setups 2-13 Aux Outputs and Inputs 2-15

SR830 BASICS

What is a Lock-in Amplifier? 3-1 What Does a Lock-in Measure? 3-3 The SR830 Functional Diagram 3-5 Reference Channel 3-7 Phase Sensitive Detectors 3-9 Time Constants and DC Gain 3-11 DC Outputs and Scaling 3-13 Dynamic Reserve 3-15 Signal Input Amplifier and Filters 3-17 Input Connections 3-19 Intrinsic (Random) Noise Sources 3-21 External Noise Sources 3-23 Noise Measurements 3-25

OPERATION

Power On/Off and Power On Tests 4-1 Reset 4-1 [Keys] 4-1 Spin Knob 4-1 Front Panel BNC Connectors 4-2 Key Click On/Off 4-2 Front Panel Display Test 4-2 Display Off Operation 4-2 Keypad Test 4-3 Standard Settings 4-4

FRONT PANEL

Signal Input and Filters 4-5 Sensitivity, Reserve, Time Constants 4-7 CH1 Display and Output 4-12 CH2 Display and Output 4-15 Reference 4-18 Auto Functions 4-21 Setup 4-23 Interface 4-24 Warning Messages 4-26

REAR PANEL

Power Entry Module 4-27 IEEE-488 Connector 4-27 RS232 Connector 4-27 Aux Inputs (A/D Inputs) 4-27 Aux Outputs (D/A Outputs) 4-27 X and Y Outputs 4-27 Signal Monitor Output 4-28 Trigger Input 4-28 TTL Sync Output 4-28 Preamp Connector 4-28

Using SRS Preamps 4-29

PROGRAMMING

GPIB Communications 5-1 RS232 Communications 5-1 Status Indicators and Queues 5-1 Command Syntax 5-1 Interface Ready and Status 5-2 GET (Group Execute Trigger) 5-2

DETAILED COMMAND LIST 5-3 Reference and Phase 5-4 Input and Filter 5-5 Gain and Time Constant 5-6 Display and Output 5-8 Aux Input and Output 5-9 Setup 5-10 Auto Functions 5-11 Data Storage 5-12 Data Transfer 5-15 Interface 5-19 Status Reporting 5-20

STATUS BYTE DEFINITIONS

Serial Poll Status Byte 5-21 Service Requests 5-22 Standard Event Status Byte 5-22 LIA Status Byte 5-23 Error Status Byte 5-23

PROGRAM EXAMPLES

Microsoft C, Nationall Instr GPIB 5-25

USING SR530 PROGRAMS 5-31

Table of Contents

TESTING

Introduction 6-1 Preset 6-1 Serial Number 6-1 Firmware Revision 6-1 Test Record 6-1 If A Test Fails 6-1 Necessary Equipment 6-1 Front Panel Display Test 6-2 Keypad Test 6-2

PERFORMANCE TESTS

Self Tests 6-3 DC Offset 6-5 Common Mode Rejection 6-7 Amplitude Accuracy and Flatness 6-9 Amplitude Linearity 6-11 Frequency Accuracy 6-13 Phase Accuracy 6-15 Sine Output Amplitude 6-17 DC Outputs and Inputs 6-19 Input Noise 6-21

Performance Test Record 6-23

CIRCUITRY

Circuit Boards 7-1 CPU and Power Supply Board 7-3 DSP Logic Board 7-5 Analog Input Board 7-7

PARTS LISTS

DSP Logic Board 7-9 Analog Input Board 7-15 CPU and Power Supply Board 7-21 Front Panel Display Boards 7-24 Miscellaneous 7-30

SCHEMATIC DIAGRAMS

CPU and Power Supply Board Display Board Keypad Board DSP Logic Board Analog Input Board

SAFETY AND PREPARATION FOR USE

WARNING

Dangerous voltages, capable of causing injury or death, are present in this instrument. Use extreme caution whenever the instrument covers

are removed. Do not remove the covers while the unit is plugged into a live outlet.

CAUTION

This instrument may be damaged if operated with the LINE VOLTAGE SELECTOR set for the wrong AC line voltage or if the wrong fuse is installed.

LINE VOLTAGE SELECTION

The SR830 operates from a 100V, 120V, 220V, or 240V nominal AC power source having a line frequency of 50 or 60 Hz. Before connecting the power cord to a power source, verify that the LINE VOLTAGE SELECTOR card, located in the rear panel fuse holder, is set so that the correct AC input voltage value is visible.

Conversion to other AC input voltages requires a change in the fuse holder voltage card position and fuse value. Disconnect the power cord, open the fuse holder cover door and rotate the fuse-pull lever to remove the fuse. Remove the small printed circuit board and select the operating voltage by orienting the printed circuit board so that the desired voltage is visible when pushed firmly into its slot. Rotate the fuse-pull lever back into its normal position and insert the correct fuse into the fuse holder.

LINE FUSE

Verify that the correct line fuse is installed before connecting the line cord. For 100V/120V, use a 1 Amp fuse and for 220V/240V, use a 1/2 Amp fuse.

LINE CORD

The SR830 has a detachable, three-wire power cord for connection to the power source and to a protective ground. The exposed metal parts of the instrument are connected to the outlet ground to protect against electrical shock. Always use an outlet which has a properly connected protective ground.

SERVICE

Do not attempt to service or adjust this instrument unless another person, capable of providing first aid or resuscitation, is present.

Do not install substitute parts or perform any unauthorized modifications to this instrument. Contact the factory for instructions on how to return the instrument for authorized service and adjustment.

1-3

1-4

SR830 DSP LOCK-IN AMPLIFIER

SPECIFICATIONS

SIGNAL CHANNEL

Voltage Inputs Single-ended (A) or differential (A-B). Current Input 106 or 108 Volts/Amp. Full Scale Sensitivity 2 nV to 1 V in a 1-2-5-10 sequence (expand off). Input Impedance Voltage: 10 MΩ+25 pF, AC or DC coupled.

Current: 1 kΩ to virtual ground. Gain Accuracy ±1% from 20°C to 30°C (notch filters off). Input Noise 6 nV/√Hz at 1 kHz (typical). Signal Filters 60 (50) Hz and 120(100) Hz notch filters (Q=4). CMRR 90 dB at 100 Hz (DC Coupled). Dynamic Reserve Greater than 100 dB (with no signal filters). Harmonic Distortion -80 dB.

REFERENCE CHANNEL

Frequency Range 1 mHz to 102 kHz Reference Input TTL (rising or falling edge) or Sine.

Sine input is1 MΩ, AC coupled (>1 Hz). 400 mV pk-pk minimum signal. Phase Resolution 0.01° Absolute Phase Error <1° Relative Phase Error <0.01° Orthogonality 90° ± 0.001° Phase Noise External synthesized reference: 0.005° rms at 1 kHz, 100 ms, 12 dB/oct.

Internal reference: crystal synthesized, <0.0001° rms at 1 kHz. Phase Drift <0.01°/°C below 10 kHz

<0.1°/°C to 100 kHz Harmonic Detect Detect at Nxf where N<19999 and Nxf<102 kHz. Acquisition Time (2 cycles + 5 ms) or 40 ms, whichever is greater.

DEMODULATOR

Zero Stability Digital displays have no zero drift on all dynamic reserves.

Analog outputs: <5 ppm/°C for all dynamic reserves. Time Constants 10 µs to 30 s (reference > 200 Hz). 6, 12, 18, 24 dB/oct rolloff.

up to 30000 s (reference < 200 Hz). 6, 12, 18, 24 dB/oct rolloff.

Synchronous filtering available below 200 Hz. Harmonic Rejection -80 dB

INTERNAL OSCILLATOR

Frequency 1 mHz to 102 kHz. Frequency Accuracy 25 ppm + 30 µHz Frequency Resolution 4 1/2 digits or 0.1 mHz, whichever is greater. Distortion f<10 kHz, below -80 dBc. f>10 kHz, below -70 dBc.1 Vrms amplitude. Output Impedance 50 Ω Amplitude 4 mVrms to 5 Vrms (into a high impedance load) with 2 mV resolution.

(2 mVrms to 2.5 Vrms into 50Ω load). Amplitude Accuracy 1% Amplitude Stability 50 ppm/°C Outputs Sine output on front panel. TTL sync output on rear panel.

When using an external reference, both outputs are phase locked to the

external reference.

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SR830 DSP Lock-In Amplifier

DISPLAYS

Channel 1 4 1/2 digit LED display with 40 segment LED bar graph.

X, R, X Noise, Aux Input 1 or 2. The display can also be any of these quantities divided by Aux Input 1 or 2.

Channel 2 4 1/2 digit LED display with 40 segment LED bar graph.

Y, θ, Y Noise, Aux Input 3 or 4. The display can also be any of these

quantities divided by Aux Input 3 or 4. Offset X, Y and R may be offset up to ±105% of full scale. Expand X, Y and R may be expanded by 10 or 100. Reference 4 1/2 digit LED display.

Display and modify reference frequency or phase, sine output amplitude,

harmonic detect, offset percentage (X, Y or R), or Aux Outputs 1-4. Data Buffer 16k points from both Channel 1 and Channel 2 display may be stored

internally. The internal data sample rate ranges from 512 Hz down to 1

point every 16 seconds. Samples can also be externally triggered. The data

buffer is accessible only over the computer interface.

INPUTS AND OUTPUTS

Channel 1 Output Output proportional to Channel 1 display, or X.

Output Voltage: ±10 V full scale. 10 mA max output current. Channel 2 Output Output proportional to Channel 2 display, or Y.

Output Voltage: ±10 V full scale. 10 mA max output current. X and Y Outputs Rear panel outputs of cosine (X) and sine (Y) components.

Output Voltage: ±10 V full scale. 10 mA max output current. Aux. Outputs 4 BNC Digital to Analog outputs.

±10.5 V full scale, 1 mV resolution. 10 mA max output current. Aux. Inputs 4 BNC Analog to Digital inputs.

Differential inputs with1 MΩ input impedance on both shield and center

conductor. ±10.5 V full scale, 1 mV resolution. Trigger Input TTL trigger input triggers stored data samples. Monitor Output Analog output of signal amplifiers (before the demodulator).

GENERAL

Interfaces IEEE-488 and RS232 interfaces standard.

All instrument functions can be controlled through the IEEE-488 and RS232

interfaces. Preamp Power Power connector for SR550 and SR552 preamplifiers. Power 40 Watts, 100/120/220/240 VAC, 50/60 Hz. Dimensions 17"W x 5.25"H x 19.5"D Weight 30 lbs. Warranty One year parts and labor on materials and workmanship.

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SR830 DSP Lock-In Amplifier

COMMAND LIST

VARIABLES i,j,k,l,m Integers

f Frequency (real) x,y,z Real Numbers s String

REFERENCE and PHASE page description PHAS (?) {x} 5-4 Set (Query) the Phase Shift to x degrees. FMOD (?) {i} 5-4 Set (Query) the Reference Source to External (0) or Internal (1). FREQ (?) {f} 5-4 Set (Query) the Reference Frequency to f Hz.Set only in Internal reference mode. RSLP (?) {i} 5-4 Set (Query) the External Reference Slope to Sine(0), TTL Rising (1), or TTL Falling (2). HARM (?) {i} 5-4 Set (Query) the Detection Harmonic to 1 ≤ i ≤ 19999 and i•f ≤ 102 kHz. SLVL (?) {x} 5-4 Set (Query) the Sine Output Amplitude to x Vrms. 0.004 ≤ x ≤5.000.

INPUT and FILTER page description ISRC (?) {i} 5-5 Set (Query) the Input Configuration to A (0), A-B (1) , I (1 MΩ) (2) or I (100 MΩ) (3). IGND (?) {i} 5-5 Set (Query) the Input Shield Grounding to Float (0) or Ground (1). ICPL (?) {i} 5-5 Set (Query) the Input Coupling to AC (0) or DC (1). ILIN (?) {i} 5-5 Set (Query) the Line Notch Filters to Out (0), Line In (1) , 2xLine In (2), or Both In (3).

GAIN and TIME CONSTANT page description SENS (?) {i} 5-6 Set (Query) the Sensitivity to 2 nV (0) through 1 V (26) rms full scale. RMOD (?) {i} 5-6 Set (Query) the Dynamic Reserve Mode to HighReserve (0), Normal (1), or Low Noise (2). OFLT (?) {i} 5-6 Set (Query) the Time Constant to 10 µs (0) through 30 ks (19). OFSL (?) {i} 5-6 Set (Query) the Low Pass Filter Slope to 6 (0), 12 (1), 18 (2) or 24 (3) dB/oct. SYNC (?) {i} 5-7 Set (Query) the Synchronous Filter to Off (0) or On below 200 Hz (1).

DISPLAY and OUTPUT page description DDEF (?) i {, j, k} 5-8 Set (Query) the CH1 or CH2 (i=1,2) display to XY, Rθ, XnYn, Aux 1,3 or Aux 2,4 (j=0..4)

and ratio the display to None, Aux1,3 or Aux 2,4 (k=0,1,2). FPOP (?) i {, j} 5-8 Set (Query) the CH1 (i=1) or CH2 (i=2) Output Source to X or Y (j=1) or Display (j=0). OEXP (?) i {, x, j} 5-8 Set (Query) the X, Y, R (i=1,2,3) Offset to x percent ( -105.00 ≤ x ≤ 105.00)

and Expand to 1, 10 or 100 (j=0,1,2). AOFF i 5-8 Auto Offset X, Y, R (i=1,2,3).

AUX INPUT/OUTPUT page description OAUX ? i 5-9 Query the value of Aux Input i (1,2,3,4). AUXV (?) i {, x} 5-9 Set (Query) voltage of Aux Output i (1,2,3,4) to x Volts. -10.500 ≤ x ≤ 10.500.

SETUP page description OUTX (?) {i} 5-10 Set (Query) the Output Interface to RS232 (0) or GPIB (1). OVRM (?) {i} 5-10 Set (Query) the GPIB Overide Remote state to Off (0) or On (1). KCLK (?) {i} 5-10 Set (Query) the Key Click to Off (0) or On (1). ALRM (?) {i} 5-10 Set (Query) the Alarms to Off (0) or On (1). SSET i 5-10 Save current setup to setting buffer i (1≤i≤9). RSET i 5-10 Recall current setup from setting buffer i (1≤i≤9).

AUTO FUNCTIONS page description AGAN 5-11 Auto Gain function. Same as pressing the [AUTO GAIN] key. ARSV 5-11 Auto Reserve function. Same as pressing the [AUTO RESERVE] key. APHS 5-11 Auto Phase function. Same as pressing the [AUTO PHASE] key. AOFF i 5-11 Auto Offset X,Y or R (i=1,2,3).

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SR830 DSP Lock-In Amplifier

DATA STORAGE page description SRAT (?) {i} 5-13 Set (Query) the DataSample Rate to 62.5 mHz (0) through 512 Hz (13) or Trigger (14). SEND (?) {i} 5-13 Set (Query) the Data Scan Mode to 1 Shot (0) or Loop (1). TRIG 5-13 Software trigger command. Same as trigger input. TSTR (?) {i} 5-13 Set (Query) the Trigger Starts Scan modeto No (0) or Yes (1). STRT 5-13 Start or continue a scan. PAUS 5-13 Pause a scan. Does not reset a paused or done scan. REST 5-14 Reset the scan. All stored data is lost.

DATA TRANSFER page description OUTP? i 5-15 Query the value of X (1), Y (2), R (3) or θ (4). Returns ASCII floating point value. OUTR? i 5-15 Query the value of Display i (1,2). Returns ASCII floating point value. SNAP?i,j{,k,l,m,n} 5-15 Query the value of 2 thru 6 paramters at once. OAUX? i 5-16 Query the value of Aux Input i (1,2,3,4). Returns ASCII floating point value. SPTS? 5-16 Query the number of points stored in Display buffer. TRCA? i,j,k 5-16 Read k≥1 points starting at bin j≥0 from Display i (1,2) buffer in ASCII floating point. TRCB? i,j,k 5-16 Read k≥1 points starting at bin j≥0 from Display i (1,2) buffer in IEEE binary floating point. TRCL? i,j,k 5-17 Read k≥1 points starting at bin j≥0 from Display i (1,2) buffer in non-normalized binary floating

point.

FAST (?) {i} 5-17 Set (Query) Fast Data Transfer Mode On (1) or Off (0).On will transfer binary X and Y every

sample during a scan over the GPIB interface.

STRD 5-18 Start a scan after 0.5sec delay. Use with Fast Data Transfer Mode. INTERFACE page description

❋RST 5-19 Reset the unit to its default configurations. ❋IDN? 5-19 Read the SR830 device identification string.

LOCL(?) {i} 5-19 Set (Query) the Local/Remote state to LOCAL (0), REMOTE (1), or LOCAL LOCKOUT (2). OVRM (?) {i} 5-19 Set (Query) the GPIB Overide Remote state to Off (0) or On (1). TRIG 5-19 Software trigger command. Same as trigger input.

STATUS page description

❋CLS 5-20 Clear all status bytes. ❋ESE (?) {i} {,j} 5-20 Set (Query) the Standard Event Status Byte Enable Register to the decimal value i (0-255).

❋ESE i,j sets bit i (0-7) to j (0 or 1). ❋ESE? queries the byte. ❋ESE?i queries only bit i. ❋ESR? {i} 5-20 Query the Standard Event Status Byte. If i is included, only bit i is queried. ❋SRE (?) {i} {,j} 5-20 Set (Query) the Serial Poll Enable Register to the decimal value i (0-255). ❋SRE i,j sets bit i (0-

7) to j (0 or 1). ❋SRE? queries the byte, ❋SRE?i queries only bit i. ❋STB? {i} 5-20 Query the Serial Poll Status Byte. If i is included, only bit i is queried. ❋PSC (?) {i} 5-20 Set (Query) the Power On Status Clear bit to Set (1) or Clear (0).

ERRE (?) {i} {,j} 5-20 Set (Query) the Error Status Enable Register to the decimal value i (0-255). ERRE i,j sets bit i

(0-7) to j (0 or 1). ERRE? queries the byte, ERRE?i queries only bit i. ERRS? {i} 5-20 Query the Error Status Byte. If i is included, only bit i is queried. LIAE (?) {i} {,j} 5-20 Set (Query) the LIA Status Enable Register to the decimal value i (0-255). LIAE i,j sets

bit i (0-7) to j (0 or 1). LIAE? queries the byte, LIAE?i queries only bit i. LIAS? {i} 5-20 Query the LIA Status Byte. If i is included, only bit i is queried.

1-8

STATUS BYTE DEFINITIONS

SR830 DSP Lock-In Amplifier

SERIAL POLL STATUS BYTE (5-21)

bit name usage 0 SCN No data is being acquired 1 IFC No command execution in progress 2 ERR Unmasked bit in error status byte set 3 LIA Unmasked bit in LIA status byte set 4 MAV The interface output buffer is non-empty 5 ESB Unmasked bit in standard status byte set 6 SRQ SRQ (service request) has occurred 7 Unused

STANDARD EVENT STATUS BYTE (5-22)

bit name usage 0 INP Set on input queue overflow 1 Unused 2 QRY Set on output queue overflow 3 Unused 4 EXE Set when command execution error occurs 5 CMD Set when an illegal command is received 6 URQ Set by any key press or knob rotation 7 PON Set by power-on

LIA STATUS BYTE (5-23)

bit name usage 0 RSRV/INPT Set when on RESERVE or INPUT overload 1 FILTR Set when on FILTR overload 2 OUTPT Set when on OUTPT overload 3 UNLK Set when on reference unlock 4 RANGE Set when detection freq crosses 200 Hz 5 TC Set when time constant is changed 6 TRIG Set when unit is triggered 7 Unused

ERROR STATUS BYTE (5-23)

bit name usage 0 Unused 1 Backup Error Set when battery backup fails 2 RAM Error Set when RAM Memory test finds an error 3 Unused 4 ROM Error Set when ROM Memory test finds an error 5 GPIB Error Set when GPIB binary data transfer aborts 6 DSP Error Set when DSP test finds an error 7 Math Error Set when an internal math error occurs

1-9

SR830 DSP Lock-In Amplifier

1-10

GETTING STARTED

YOUR FIRST MEASUREMENTS

The sample measurements described in this section are designed to acquaint the first time user with the SR830 DSP Lock-In Amplifier. Do not be concerned that your measurements do not exactly agree with these exercises. The focus of these measurement exercises is to learn how to use the instrument.

It is highly recommended that the first time user step through some or all of these exercises before attempting to perform an actual experiment.

The experimental procedures are detailed in two columns. The left column lists the actual steps in the experiment. The right column is an explanation of each step.

[Keys] Front panel keys are referred to in brackets such as [Display] where

'Display' is the key label.

Knob The knob is used to adjust parameters which are displayed in the

Reference display.

2-1

Getting Started

2-2

The Basic Lock-in

THE BASIC LOCK-IN

This measurement is designed to use the internal oscillator to explore some of the basic lock-in functions. You will need BNC cables.

Specifically, you will measure the amplitude of the Sine Out at various frequencies, sensitivities, time constants and phase shifts.

1. Disconnect all cables from the lock-in. Turn the power on while holding down the [Setup] key. Wait until the power-on tests are completed.

2. Connect the Sine Out on the front panel to the A input using a BNC cable.

When the power is turned on with the [Setup] key pressed, the lock-in returns to its standard default settings. See the Standard Settings list in the Operation section for a complete listing of the settings.

The Channel 1 display shows X and Channel 2 shows Y.

The lock-in defaults to the internal oscillator reference set at 1.000 kHz. The reference mode is indicated by the INTERNAL led. In this mode, the lock-in generates a synchronous sine output at the internal reference frequency.

The input impedance of the lock-in is 10 MΩ. The Sine Out has an output impedance of 50Ω. Since the Sine Output amplitude is specified into a high impedance load, the output impedance does not affect the amplitude.

The sine amplitude is 1.000 Vrms and the sensitivity is 1 V(rms). Since the phase shift of the sine output is very close to zero, Channel 1 (X) should read close to 1.000 V and Channel 2 (Y) should read close to 0.000 V.

3. Press [Auto Phase]

4. Press [Phase]

5. Press the [+90°] key.

Automatically adjust the reference phase shift to eliminate any residual phase error. This should set the value of Y to zero.

Display the reference phase shift in the Reference display. The phase shift should be close to zero.

This adds 90° to the reference phase shift. The value of X drops to zero and Y becomes minus the magnitude (-1.000 V).

2-3

The Basic Lock-in

Use the knob to adjust the phase shift until Y is zero and X is equal to the positive amplitude.

Press [Auto Phase]

6. Press [Freq]

Use the knob to adjust the frequency to 10 kHz.

Use the knob to adjust the frequency back to 1 kHz.

The knob is used to adjust parameters which are shown in the Reference display, such as phase, amplitude and frequency. The final phase value should be close to zero again.

Use the Auto Phase function to return Y to zero and X to the amplitude.

Show the internal oscillator frequency in the Reference display.

The knob now adjusts the frequency. The measured signal amplitude should stay within 1% of 1 V and the phase shift should stay close to zero (the value of Y should stay close to zero).

The internal oscillator is crystal synthesized with 25 ppm of frequency error. The frequency can be set with 4 1/2 digit or 0.1 mHz resolution, whichever is greater.

7. Press [Ampl]

Use the knob to adjust the amplitude to

0.01 V.

8. Press [Auto Gain]

9. Press [Sensitivity Up] to select 50 mV full scale.

Change the sensitivity back to 20 mV.

10. Press [Time Constant Down] to change the time constant to 300 µs.

Show the sine output amplitude in the Reference display.

As the amplitude is changed, the measured value of X should equal the sine output amplitude. The sine amplitude can be set from 4 mV to 5 V rms into high impedance (half the amplitude into a 50 Ω load).

The Auto Gain function will adjust the sensitivity so that the measured magnitude (R) is a sizable percentage of full scale. Watch the sensitivity indicators change.

Parameters which have many options, such as sensitivity and time constant, are changed with up and down keys. The sensitivity and time constant are indicated by leds.

The values of X and Y become noisy. This is because the 2f component of the output (at 2 kHz) is no longer attenuated completely by the low pass filters.

Press [Time Constant Up] to change the time constant to 3 ms.

Let's leave the time constant short and change the filter slope.

2-4

11. Press the [Slope/Oct] key until 6 dB/oct is

selected.

The Basic Lock-in

Parameters which have only a few values, such as filter slope, have only a single key which cycles through all available options. Press the corresponding key until the desired option is indicated by an led.

The X and Y outputs are somewhat noisy at this short time constant and only 1 pole of low pass filtering.

The outputs are less noisy with 2 poles of filtering. Press [Slope/Oct] again to select 12 dB/oct. Press [Slope/Oct] twice to select 24 db/oct.

Press [Slope/Oct] again to select 6 db/oct.

12. Press [Freq]

Use the knob to adjust the frequency to

55.0 Hz.

13. Press [Sync Filter]

With 4 poles of low pass filtering, even this short

time constant attenuates the 2f component rea-

sonably well and provides steady readings.

Let's leave the filtering short and the outputs noisy

for now.

Show the internal reference frequency on the

Reference display.

At a reference frequency of 55 Hz and a 6 db/oct,

3 ms time constant, the output is totally dominated

by the 2f component at 100 Hz.

This turns on synchronous filtering whenever the

detection frequency is below 200 Hz.

Synchronous filtering effectively removes output

components at multiples of the detection frequen-

cy. At low frequencies, this filter is a very effective

way to remove 2f without using extremely long

time constants.

The outputs are now very quiet and steady, even

though the time constant is very short. The

response time of the synchronous filter is equal to

the period of the detection frequency (18 ms in this

case).

This concludes this measurement example. You

should have a feeling for the basic operation of the

front panel. Basic lock-in parameters have been

introduced and you should be able to perform

simple measurements.

2-5

The Basic Lock-in

2-6

X, Y, R and θ

This measurement is designed to use the internal oscillator and an external signal source to explore some of the display types. You will need a synthesized function generator capable of providing a 100 mVrms sine wave at 1.000 kHz (the DS335 from SRS will suffice), BNC cables and a terminator appropriate for the generator function output.

Specifically, you will display the lock-in outputs when measuring a signal close to, but not equal to, the internal reference frequency. This setup ensures changing outputs which are more illustrative than steady outputs.

The displays will be configured to show X, Y, R and θ.

1. Disconnect all cables from the lock-in. Turn the power on while holding down the [Setup] key. Wait until the power-on tests are completed.

2. Turn on the function generator, set the frequency to 1.0000 kHz (exactly) and the amplitude to 500 mVrms.

Connect the function output (sine wave) from the synthesized function generator to the A input using a BNC cable and appropriate terminator.

When the power is turned on with the [Setup] key pressed, the lock-in returns to its standard settings. See the Standard Settings list in the Operation section for a complete listing of the settings.

The Channel 1 display shows X and Channel 2 shows Y.

The input impedance of the lock-in is 10 MΩ. The generator may require a terminator. Many generators have either a 50Ω or 600Ω output impedance. Use the appropriate feedthrough or T termination if necessary. In general, not using a terminator means that the function output amplitude will not agree with the generator setting.

The lock-in defaults to the internal oscillator reference set at 1.000 kHz. The reference mode is indicated by the INTERNAL led. In this mode, the internal oscillator sets the detection frequency.

The internal oscillator is crystal synthesized so that the actual reference frequency should be very close to the actual generator frequency. The X and Y displays should read values which change very slowly. The lock-in and the generator are not phase locked but they are at the same frequency with some slowly changing phase.

3. Press [Freq]

Use the knob to change the frequency to

999.8 Hz.

Show the internal oscillator frequency on the Reference display.

By setting the lock-in reference 0.2 Hz away from the signal frequency, the X and Y outputs are

0.2 Hz sine waves (frequency difference between reference and signal). The X and Y output displays

2-7

X, Y, R and θ

should now oscillate at about 0.2 Hz (the accuracy is determined by the crystals of the generator and the lock-in).

4. Press [Channel 1 Display] to select R.

5. Press [Channel 2 Display] to select θ.

6. Press [Freq] Use the knob to adjust the frequency slowly to

try to stop the rotation of the phase.

7. Use a BNC cable to connect the TTL SYNC output from the generator to the Reference Input of the lock-in.

The default Channel 1 display is X. Change the display to show R. R is phase independent so it shows a steady value (close to 0.500 V).

The default Channel 2 display is Y. Change the display to show θ. The phase between the reference and the signal changes by 360° approximately every 5 sec (0.2 Hz difference frequency).

The bar graph in this case is scaled to ±180°. The bar graph should be a linear phase ramp at

0.2 Hz.

Show the internal oscillator frequency. As the internal reference frequency gets closer to

the signal frequency, the phase rotation gets slower and slower. If the frequencies are EXACTLY equal, then the phase is constant.

By using the signal generator as the external reference, the lock-in will phase lock its internal oscillator to the signal frequency and the phase will be a constant.

Press [Source] to turn the INTERNAL led off.

Press [Trig] to select POS EDGE.

Select external reference mode. The lock-in will phase lock to the signal at the Reference Input.

With a TTL reference signal, the slope needs to be set to either rising or falling edge.

The phase is now constant. The actual phase depends upon the phase difference between the function output and the sync output from the generator.

The external reference frequency (as measured by the lock-in) is displayed on the Reference display. The UNLOCK indicator should be OFF (successfully locked to the external reference).

The displays may be stored in the internal data buffers at a programmable sampling rate. This allows storage of 16000 points of both displays.

2-8

Outputs, Offsets and Expands

OUTPUTS, OFFSETS and EXPANDS

This measurement is designed to use the internal oscillator to explore some of the basic lock-in outputs. You will need BNC cables and a digital voltmeter (DVM).

Specifically, you will measure the amplitude of the Sine Out and provide analog outputs proportional to the measurement. The effect of offsets and expands on the displayed values and the analog outputs will be explored.

1. Disconnect all cables from the lock-in. Turn

the power on while holding down the [Setup] key. Wait until the power-on tests are completed.

2. Connect the Sine Out on the front panel to the

A input using a BNC cable.

When the power is turned on with the [Setup] key pressed, the lock-in returns to its standard settings. See the Standard Settings list in the Operation section for a complete listing of the settings.

The Channel 1 display shows X and Channel 2 shows Y.

3. Connect the CH1 OUTPUT on the front panel

to the DVM. Set the DVM to read DC Volts.

4. Press [Ampl]

Use the knob to adjust the sine amplitude to

0.5 V.

The CH1 output defaults to X. The output voltage is simply (X/Sensitivity - Offset)xExpandx10V. In this case, X = 1.000 V, the sensitivity = 1 V, the offset is zero percent and the expand is 1. The output should thus be 10 V or 100% of full scale.

Display the sine output amplitude. Set the amplitude to 0.5 V. The Channel 1 display

should show X=0.5 V and the CH1 output voltage should be 5 V on the DVM (1/2 of full scale).

2-9

Outputs, Offsets and Expands

5. Press [Channel 1 Auto Offset]

X, Y and R may all be offset and expanded separately. Since Channel 1 is displaying X, the OFFSET and [Expand] keys below the Channel 1 display set the X offset and expand. The display determines which quantity (X or R) is offset and expanded.

Auto Offset automatically adjusts the X offset (or Y or R) such that X (or Y or R) becomes zero. In this case, X is offset to zero. The offset should be about 50%. Offsets are useful for making relative measurements. In analog lock-ins, offsets were generally used to remove DC output errors from the lock-in itself. The SR830 has no DC output errors and the offset is not required for most measurements.

The offset affects both the displayed value of X and any analog output proportional to X. The CH1 output voltage should be zero in this case.

The Offset indicator turns on at the bottom of the Channel 1 display to indicate that the displayed quantity is affected by an offset.

Press [Channel 1 Offset Modify]

Use the knob to adjust the X offset to 40.0%

Press [Channel 1 Expand] to select x10.

Show the Channel 1 (X) offset in the Reference display.

Change the offset to 40% of full scale. The output offsets are a percentage of full scale. The percentage does not change with the sensitivity. The displayed value of X should be 0.100 V (0.5 V - 40% of full scale). The CH1 output voltage is (X/Sensitivity - Offset)xExpandx10V.

CH1 Out = (0.5/1.0 - 0.4)x1x10V = 1 V With an expand of 10, the display has one more

digit of resolution (100.00 mV full scale). The Expand indicator turns on at the bottom of the

Channel 1 display to indicate that the displayed quantity is affected by a non-unity expand.

The CH1 output is (X/Sensitivity - Offset)xExpandx10V. In this case, the output voltage is

CH1 Out = (0.5/1.0 - 0.4)x10x10V = 10V The expand allows the output gain to be increased

by up to 100. The output voltage is limited to

10.9 V and any output which tries to be greater will

2-10

Outputs, Offsets and Expands

turn on the OVLD indicator in the Channel 1 display.

With offset and expand, the output voltage gain and offset can be programmed to provide control of feedback signals with the proper bias and gain for a variety of situations.

Offsets add and subtract from the displayed values while expand increases the resolution of the display.

6. Connect the DVM to the X output on the rear

panel.

7. Connect the DVM to the CH1 OUTPUT on the

front panel again.

Press [Channel 1 Output] to select Display.

Press [Channel 1 Display] to select R.

The X and Y outputs on the rear panel always provide voltages proportional to X and Y (with offset and expand). The X output voltage should be 10 V, just like the CH1 output.

The front panel outputs can be configured to output different quantities while the rear panel outputs always output X and Y.

NOTE: Outputs proportional to X and Y (rear panel, CH1 or CH2) have 100 kHz of bandwidth. The CH1 and CH2 outputs, when configured to be proportional to the displays (even if the display is X or Y) are updated at 512 Hz and have a 200 Hz bandwidth. It is important to keep this in mind if you use very short time constants.

CH1 OUTPUT can be proportional to X or the display. Choose Display. The display is X so the CH1 output should remain 10.0 V (but its bandwidth is only 200 Hz instead of 100 kHz).

Let's change CH1 to output R. The X and Y offset and expand functions are

output functions, they do NOT affect the calculation of R or θ. Thus, Channel 1 (R) should be 0.5V and the CH1 output voltage should be 5V (1/2 of full scale).

The Channel 1 offset and expand keys now set the R offset and expand. The X offset and expand are still set at 40% and x10 as reflected at the rear panel X output.

See the DC Outputs and Scaling discussion in the Lock-In Basics section for more detailed information on output scaling.

2-11

Outputs, Offsets and Expands

2-12

Storing and Recalling Setups

STORING and RECALLING SETUPS

The SR830 can store 9 complete instrument setups in non-volatile memory.

1. Turn the lock-in on while holding down the

[Setup] key. Wait until the power-on tests are completed. Disconnect any cables from the lock-in.

2. Press [Sensitivity Down] to select 100 mV.

Press [Time Constant Up] to select 1 S.

3. Press [Save]

Use the knob to select setup number 3. Press [Save] again.

When the power is turned on with the [Setup] key pressed, the lock-in returns to its standard settings. See the Standard Settings list in the Operation section for a complete listing of the settings.

Change the lock-in setup so that we have a nondefault setup to save.

Change the sensitivity to 100 mV. Change the time constant to 1 second.

The Reference display shows the setup number (1-9).

The knob selects the setup number. Press [Save] again to complete the save opera-

tion. Any other key aborts the save. The current setup is now saved as setup number

4. Turn the lock-in off and on while holding down

the [Setup] key. Wait until the power-on tests are complete.

5. Press [Recall]

Use the knob to select setup number 3. Press [Recall] again.

Change the lock-in setup back to the default setup. Now let's recall the lock-in setup that we just saved.

Check that the sensitivity and time constant are 1V and 100 ms (default values).

The Reference display shows the setup number. The knob selects the setup number. Press [Recall] again to complete the recall opera-

tion. Any other key aborts the recall. The sensitivity and time constant should be the

same as those in effect when the setup was saved.

2-13

Storing and Recalling Setups

2-14

Aux Outputs and Inputs

AUX OUTPUTS and INPUTS

This measurement is designed to illustrate the use of the Aux Outputs and Inputs on the rear panel. You will need BNC cables and a digital voltmeter (DVM).

Specifically, you will set the Aux Output voltages and measure them with the DVM. These outputs will then be connected to the Aux Inputs to simulate external DC voltages which the lock-in can measure.

1. Disconnect all cables from the lock-in. Turn

the power on while holding down the [Setup] key. Wait until the power-on tests are completed.

2. Connect Aux Out 1 on the rear panel to the

DVM. Set the DVM to read DC volts.

3. Press [Aux Out] until the Reference display

shows the level of Aux Out 1( as indicated by the AxOut1 led below the display).

Use the knob to adjust the level to 10.00 V.

Use the knob to adjust the level to -5.00 V.

When the power is turned on with the [Setup] key pressed, the lock-in returns to its standard settings. See the Standard Settings list in the Operation section for a complete listing of the settings.

The 4 Aux Outputs can provide programmable voltages between -10.5 and +10.5 volts. The outputs can be set from the front panel or via the computer interface.

Show the level of Aux Out 1 on the Reference display.

Change the output to 10V. The DVM should display 10.0 V.

Change the output to -5V. The DVM should display -5.0 V.

The 4 outputs are useful for controlling other parameters in an experiment, such as pressure, temperature, wavelength, etc.

4. Press [Channel 1 Display] to select AUX IN 1.

5. Disconnect the DVM from Aux Out 1. Connect

AuxOut 1 to Aux In 1 on the rear panel.

Change the Channel 1 display to measure Aux Input 1.

The Aux Inputs can read 4 analog voltages. These inputs are useful for monitoring and measuring other parameters in an experiment, such as pressure, temperature, position, etc.

We'll use Aux Out 1 to provide an analog voltage to measure.

Channel 1 should now display -5 V (Aux In 1).

2-15

Aux Outputs and Inputs

6. Press [Channel 2 Display] to select AUX IN 3.

7. Connect Aux Out 1 to Aux In 3 on the rear panel.

Change the Channel 2 display to measure Aux Input 3.

Channel 2 should now display -5 V (Aux In 3). The Channel 1 and 2 displays may be ratio'ed to

the Aux Input voltages. See the Basics section for more about output scaling.

The displays may be stored in the internal data buffers at a programmable sampling rate. This allows storage of not only the lock-in outputs, X,Y, R or θ, but also the values of the Aux Inputs. See the Programming section for more details.

2-16

WHAT IS A LOCK-IN AMPLIFIER?

SR830 BASICS

Lock-in amplifiers are used to detect and measure very small AC signals - all the way down to a few nanovolts! Accurate measurements may be made even when the small signal is obscured by noise sources many thousands of times larger.

Lock-in amplifiers use a technique known as phase-sensitive detection to single out the component of the signal at a specific reference frequency AND phase. Noise signals at frequencies other than the reference frequency are rejected and do not affect the measurement.

Why use a lock-in?

Let's consider an example. Suppose the signal is a 10 nV sine wave at 10 kHz. Clearly some amplification is required. A good low noise amplifier will have about 5 nV/√Hz of input noise. If the amplifier bandwidth is 100 kHz and the gain is 1000, then we can expect our output to be 10 µV of signal (10 nV x 1000) and 1.6 mV of broadband noise (5 nV/√Hz x √100 kHz x 1000). We won't have much luck measuring the output signal unless we single out the frequency of interest.

If we follow the amplifier with a band pass filter with a Q=100 (a VERY good filter) centered at 10 kHz, any signal in a 100 Hz bandwidth will be detected (10 kHz/Q). The noise in the filter pass band will be 50 µV (5 nV/√Hz x √100 Hz x 1000) and the signal will still be 10 µV. The output noise is much greater than the signal and an accurate measurement can not be made. Further gain will not help the signal to noise problem.

Now try following the amplifier with a phasesensitive detector (PSD). The PSD can detect the signal at 10 kHz with a bandwidth as narrow as

0.01 Hz! In this case, the noise in the detection bandwidth will be only 0.5 µV (5 nV/√Hz x √.01 Hz x 1000) while the signal is still 10 µV. The signal to noise ratio is now 20 and an accurate measurement of the signal is possible.

What is phase-sensitive detection?

Lock-in measurements require a frequency reference. Typically an experiment is excited at a fixed frequency (from an oscillator or function generator) and the lock-in detects the response from the

experiment at the reference frequency. In the diagram below, the reference signal is a square wave at frequency ωr. This might be the sync output from a function generator. If the sine output from the function generator is used to excite the experiment, the response might be the signal waveform shown below. The signal is V where V

The SR830 generates its own sine wave, shown as the lock-in reference below. The lock-in reference is VLsin(ωLt + θ

The SR830 amplifies the signal and then multiplies it by the lock-in reference using a phase-sensitive detector or multiplier. The output of the PSD is simply the product of two sine waves.

psd

The PSD output is two AC signals, one at the difference frequency (ωr - ωL) and the other at the sum frequency (ωr + ωL).

If the PSD output is passed through a low pass filter, the AC signals are removed. What will be left? In the general case, nothing. However, if ω equals ωL, the difference frequency component will be a DC signal. In this case, the filtered PSD output will be

psd

is the signal amplitude.

sig

Reference

Signal

Lock-in Reference

= V = 1/2 V

= 1/2 V

sigVL

1/2 V

sin(ωrt + θ

cos([ωr - ωL]t + θ

sigVL

cos([ωr + ωL]t + θ

sigVL

cos(θ

sigVL

ref

sig

ref

sig

)sin(ωLt + θ

- θ

ref

sig

)

sin(ωrt + θ

)

ref

- θ + θ

ref

) -

ref

sig

)

3-1

SR830 Basics

This is a very nice signal - it is a DC signal proportional to the signal amplitude.

Narrow band detection

Now suppose the input is made up of signal plus noise. The PSD and low pass filter only detect signals whose frequencies are very close to the lockin reference frequency. Noise signals at frequencies far from the reference are attenuated at the PSD output by the low pass filter (neither ω

ref

nor ω

noise+ωref

are close to DC). Noise at fre-

noise

quencies very close to the reference frequency will result in very low frequency AC outputs from the PSD (|ω

noise-ωref

| is small). Their attenuation depends upon the low pass filter bandwidth and roll-off. A narrower bandwidth will remove noise sources very close to the reference frequency, a wider bandwidth allows these signals to pass. The low pass filter bandwidth determines the bandwidth of detection. Only the signal at the reference frequency will result in a true DC output and be unaffected by the low pass filter. This is the signal we want to measure.

Where does the lock-in reference come from?

We need to make the lock-in reference the same as the signal frequency, i.e. ωr = ωL. Not only do the frequencies have to be the same, the phase between the signals can not change with time, otherwise cos(θ be a DC signal. In other words, the lock-in reference needs to be phase-locked to the signal reference.

sig

- θ

) will change and V

ref

psd

will not

sync) which is always phase-locked to the reference oscillator.

Magnitude and phase

Remember that the PSD output is proportional to V

cosθ where θ = (θ

sig

difference between the signal and the lock-in reference oscillator. By adjusting θ equal to zero, in which case we can measure V

(cosθ=1). Conversely, if θ is 90°, there will be no output at all. A lock-in with a single PSD is called a single-phase lock-in and its output is V

This phase dependency can be eliminated by adding a second PSD. If the second PSD multiplies the signal with the reference oscillator shifted by 90°, i.e. VLsin(ωLt + θ tered output will be

V V

psd2

= 1/2 V ~ V

sig

sigVL

sinθ

Now we have two outputs, one proportional to cosθ and the other proportional to sinθ. If we call the first output X and the second Y,

X = V

cosθ Y = V

sig

these two quantities represent the signal as a vector relative to the lock-in reference oscillator. X is called the 'in-phase' component and Y the 'quadrature' component. This is because when θ=0, X measures the signal while Y is zero.

- θ

sig

ref

sin(θ

sig

). θ is the phase

ref

we can make θ

ref

+ 90°), its low pass fil-

- θ

ref

)

sig

sinθ

sig

cosθ.

Lock-in amplifiers use a phase-locked-loop (PLL) to generate the reference signal. An external reference signal (in this case, the reference square wave) is provided to the lock-in. The PLL in the lock-in locks the internal reference oscillator to this external reference, resulting in a reference sine wave at ωr with a fixed phase shift of θ

. Since

ref

the PLL actively tracks the external reference, changes in the external reference frequency do not affect the measurement.

All lock-in measurements require a reference signal.

In this case, the reference is provided by the excitation source (the function generator). This is called an external reference source. In many situations, the SR830's internal oscillator may be used instead. The internal oscillator is just like a function generator (with variable sine output and a TTL

By computing the magnitude (R) of the signal vector, the phase dependency is removed.

R = (X2 + Y2)

1/2

= V

sig

R measures the signal amplitude and does not depend upon the phase between the signal and lock-in reference.

A dual-phase lock-in, such as the SR830, has two PSD's, with reference oscillators 90° apart, and can measure X, Y and R directly. In addition, the phase θ between the signal and lock-in reference, can be measured according to

θ = tan-1 (Y/X)

3-2

WHAT DOES A LOCK-IN MEASURE?

SR830 Basics

So what exactly does the SR830 measure? Fourier's theorem basically states that any input signal can be represented as the sum of many, many sine waves of differing amplitudes, frequencies and phases. This is generally considered as representing the signal in the "frequency domain". Normal oscilloscopes display the signal in the "time domain". Except in the case of clean sine waves, the time domain representation does not convey very much information about the various frequencies which make up the signal.

What does the SR830 measure?

The SR830 multiplies the signal by a pure sine wave at the reference frequency. All components of the input signal are multiplied by the reference simultaneously. Mathematically speaking, sine waves of differing frequencies are orthogonal, i.e. the average of the product of two sine waves is zero unless the frequencies are EXACTLY the same. In the SR830, the product of this multiplication yields a DC output signal proportional to the component of the signal whose frequency is exactly locked to the reference frequency. The low pass filter which follows the multiplier provides the averaging which removes the products of the reference with components at all other frequencies.

The SR830, because it multiplies the signal with a pure sine wave, measures the single Fourier (sine) component of the signal at the reference frequency. Let's take a look at an example. Suppose the input signal is a simple square wave at frequency f. The square wave is actually composed of many sine waves at multiples of f with carefully related amplitudes and phases. A 2V pk-pk square wave can be expressed as

frequencies is removed by the low pass filter following the multiplier. This "bandwidth narrowing" is the primary advantage that a lock-in amplifier provides. Only inputs at frequencies at the reference frequency result in an output.

RMS or Peak?

Lock-in amplifiers as a general rule display the input signal in Volts RMS. When the SR830 displays a magnitude of 1V (rms), the component of the input signal at the reference frequency is a sine wave with an amplitude of 1 Vrms or

2.8 V pk-pk. Thus, in the previous example with a 2 V pk-pk

square wave input, the SR830 would detect the first sine component, 1.273sin(ωt). The measured and displayed magnitude would be 0.90 V (rms) (1/√2 x 1.273).

Degrees or Radians?

In this discussion, frequencies have been referred to as f (Hz) and ω (2πf radians/sec). This is because people measure frequencies in cycles per second and math works best in radians. For purposes of measurement, frequencies as measured in a lock-in amplifier are in Hz. The equations used to explain the actual calculations are sometimes written using ω to simplify the expressions.

Phase is always reported in degrees. Once again, this is more by custom than by choice. Equations written as sin(ωt + θ) are written as if θ is in radians mostly for simplicity. Lock-in amplifiers always manipulate and measure phase in degrees.

S(t) = 1.273sin(ωt) + 0.4244sin(3ωt) +

0.2546sin(5ωt) + ...

where ω = 2πf. The SR830, locked to f will single out the first component. The measured signal will be 1.273sin(ωt), not the 2V pk-pk that you'd measure on a scope.

In the general case, the input consists of signal plus noise. Noise is represented as varying signals at all frequencies. The ideal lock-in only responds to noise at the reference frequency. Noise at other

3-3

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Stanford Research Systems SR830 Specifications Sheet

Specifications and Main Features

Frequently Asked Questions

User Manual