Stanford Research Systems SR530 User Manual

MODEL SR530

LOCK-IN AMPLIFIER

1290-D Reamwood Avenue

Sunnyvale, CA 94089 U.S.A.

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

Email: info@thinkSRS.com • www.thinkSRS.com

Stanford Research Systems, Inc.

Rev. 2.3 (06/2005)

Table of Contents

Condensed Information

SAFETY and Preparation for use 1

Symbols 2 Specifications 3 Front Panel Summary 5 Abridged Command List 7 Status Byte Definition 8 Configuration Switches 8

Guide to Operation

Front Panel 9

Signal Inputs 9 Signal Fi lters 9 Sensitivity 9 Dynamic Reserve 10 Status Indicators 10 Display Select 10 Channel 1 Display 10 R Output 11 Output Channel 1 11 Rel Channel 1 11 Offset Channel 1 11 Expand Channel 1 12 X (RCOSØ) Output 12 Channel 2 Display 12 Ø Output 12 Output Channel 2 13 Rel Channel 2 13 Auto Phase 13 Offset Channel 2 13 Expand Channel 2 14 Y (RSINØ) Output 14 Reference Input 14 Trigger Level 14 Reference Mode 15 Reference Display 15 Phase Controls 15 Time Constants 15 Noise Measurements 15 Power Switch 16 Local/Remote Operation 16 Default Settings 16

Rear Panel 17 AC Power 17 GPIB (IEEE-488) Connector 17 RS232 Connector 17 Signal Monitor Output 17 Pre-Amp Connector 17 A/D Inputs and D/A Outputs 17 Ratio Feature 17 Internal Oscillator 17

Guide to Programming

Communications 19 Command Syntax 19 Status LED's 19 RS232 Echo Feature 20 Try-out with an ASCII Terminal 20

Command List 21 Status Byte 24 Errors 24 Reset Command 25 Trouble-Shooting Interf ac e Probl ems 25 Common Hardware Problems 25 Common Software Problems 25

RS232 Interface

Introduction to the RS232 26 Data Communications Equipment 26 Wait Command 26 Termination Sequence 26

GPIB (IEEE-488) Interface

Introduction to the GPIB 26 GPIB Capabilities 26 Response to Special GPIB comman ds 26 Serial Polls and SRQ's 27 Echo Mode using the RS232 27 Using Both the RS232 & GPIB 27

Lock-in Technique

Introduction to Lock-in Amplifier s 28 Measurement Example 28 Understanding the Specifications 29 Shielding and Ground Loops 29 Dynamic Reserve 30 Current Inputs 30 Bandpass Filter 30 Notch Filters 31 Frequency Range 31 Output Time Constants 31 Noise Measurements 31 Ratio Capability 31 Computer Interfaces 31 Internal Oscillator 31

SR530 Block Diagram

Block Diagram 32 Signal Channel 33 Reference Channel 33 Phase-Sensitive Detector 33 DC Amplifier and System Gain 33 Microprocessor System 33

Circuit Description

Introduction 34 Signal Amplifier 34 Current Amplifier 34 Notch Filters 34 Bandpass Filter 34 Reference Oscillator 35 PSD, LP Filters and DC Amplifier 35 Analog Output 36 A/D's 36 D/A's 36 Expand 36 Front Panel 36 Microprocessor Control 36 RS232 Interface 37 GPIB Interface 37 Power Supplies 37 Internal Oscillator 37

Calibration and Repair

Introduction 38 Multiplier Adjustments 38 Amplifier and Filter Adjustments 38 CMRR Adjustment 38 Line Notch Filter Adjustment 39 2xLine Notch Filter Adjustment 39 Repairing Damaged Front-End 39

IBM PC, Microsoft Basic, via GPIB 51 HP-85, HP Basic, via HPIB 53

Documentation

Parts List, Oscillator Board 55 Parts List, Main Board 56 Parts List, Front Panel Board 70 Parts List, Quad Board 73 Parts List, Miscellaneous 77 Schematic Diagrams 79

Appendix A: Noise Sources and Cures

Johnson Noise 40 '1/f' Noise 40 Noise Spectrum 40 Capacitive Coupling 41 Inductive Coupling 41 Ground Loops 42 Microphonics 42 Thermocouple Effect 42

Appendix B: RS232

Simplest Case Using the RS232 43 Using Control Lines 43 Baud Rates 43 Stop Bits 44 Parity 44 Voltage Levels 44 'Eavesdropping' 44

Appendix C: GPIB

Introduction to the GPIB 45 Bus Description 45

Appendix D: Program Examples

Program Description 46 IBM PC, Microsoft Basic, via RS232 46 IBM PC, Microsoft Fortran, via RS232 47 IBM PC, Microsoft C, via RS232 49

Safety and Preparation for Use

***CAUTION***: This instrument may be damaged if operated with the LINE VOLTAGE SELECTOR set for the wrong applied ac input-source voltage or if the wrong fuse is installed.

LINE VOLTAGE SELECTION

The SR530 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 to position the desired voltage to be 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 and 120V, use a ½ Amp fuse and for 220V and 240V, use a 1/4 Amp fuse.

FURNISHED ACCESSORIES

- Power Cord

- Operating Manual

ENVIRONMENTAL CONDITIONS

OPERATING Temperature: +10° C to +40° C (Specifications apply over +18° C to +28° C) Relative Humidity: < 90% Non-condensing

NON-OPERATING Temperature: -25° C to 65° C Humidity: < 95% Non-condensig

OPERATE WITH COVERS IN PLACE

To avoid personal injury, do not remove the product covers or panels. Do not operate the product without all covers and panels in place.

WARNING REGARDING USE WITH PHOTOMULTIPLIERS

LINE CORD

This instrument has a detachable, three-wire power cord with a three-contact plug for connection to both the power source and protective ground. The protective ground contact connects to the accessible metal parts of the instrument. To prevent electrical shock, always use a power source outlet that has a properly grounded protectiv e-gr ou nd con tac t.

It is relatively easy to damage the signal inputs if a photomultiplier is used improperly with the lock-in amplifier. When left completely unterminated, a PMT will charge a cable to a few hundred volts in a very short time. If this cable is connected to the lockin, the stored charge may damage the front-end transistors. To avoid this problem, provide a leakage path of about 100 KΩ to ground inside the base of the PMT to prevent charge accumulation.

SR530 Specification Summary

General

Power 100, 120, 220, 240 VAC (50/60 Hz); 35 Watts Max Mechanical 17" x 17" x 5.25" (Rack Mount Included) 16 lbs. Warranty Two years parts and labor.

Signal Channel

Inputs Voltage: Single-ended or True Differential

Current: 10

Impedance Voltage: 100 MΩ + 25 pF, ac coupled

Current: 1 kΩ to virtual ground

Full Scale Sensitivity Voltage: 100 nV (10 nV on expand) to 500 mV

Current: 100 fA to 0.5 µA

Maximum Inputs Voltage: 100 VDC, 10 VAC damage threshold

Current: 10 mA damage threshold

Noise Voltage: 7 nV/√Hz at 1 kHz

Current: 0.13 pA/√ Hz at 1 kHz

Common Mode Range: 1 Volt peak; Rejection: 100 dB dc to 1KHz

Gain Accuracy 1% (2 Hz to 100KHz) Gain Stability 200 ppm/°C Signal Filters 60 Hz notch, -50 dB (Q=10, adjustable from 45 to 65 Hz)

120 Hz notch, -50 dB (Q=10, adjustable from 100 to 130 Hz)) Tracking bandpass set to within 1% of ref freq (Q=5)

Dynamic Reserve 20 dB LOW (1 µV to 500 mV sensitivity)

40 dB NORM (100 nV to 50 mV sensitivity) 60 dB HIGH (100 nV to 5 mV sensitivity) Bandpass filter adds 20 dB to dynamic reserve Line Notch filters increase dynamic reserve to 100 dB

Volts/Amp

2 VAC peak-to-peak saturation

1 µA ac peak-to-peak saturation

Above 1KHz the CMRR degrades by 6 dB/Octave

Reference Channel

Frequency 0.5 Hz to 100 kHz Input Impedance 1 MΩ, ac coupled Trigger SINE: 100 mV minimum, 1Vrms nominal

PULSE: ±1 Volt, 1 µsec minimum width

Mode Fundamental (f) or 2nd Harmonic (2f)

Acquisition Time 25 Sec at 1 Hz

6 Sec at 10 Hz

2 Sec at 10 kHz Slew Rate 1 decade per 10 S at 1 kHz Phase Control 90° shifts

Fine shifts in 0.025° steps Phase Noise 0.01° rms at 1 kHz, 100 msec, 12 dB TC Phase Drift 0.1°/°C Phase Error Less than 1° above 10Hz Orthogonality 90° ± 1°

Demodulator

Stability 5 ppm/°C on LOW dy nam ic res erv e

50 ppm/°C on NORM dynamic reserve

500 ppm/°C on HIGH dynamic reserve Time Constants Pre: 1msec to 100 sec (6 dB/Octave)

Post: 1sec, 0.1 sec, none (6 dB/Octave) or none Offset Up to 1X full scale (10X on expand)

Both channels may be offset Harmonic Rej -55 dB (bandpass filter in)

Outputs & Interfaces

Channel 1 Outputs X (RcosØ), X Offset, R (magnitude), R Offset, X Noise, X5 (external D/A) Channel 2 Outputs Y (RsinØ), Y Offset, Ø (phase shift of signal), Y Noise, X6 (external D/A) Output Meters 2% Precision mirrored analo g meter

Output LCD's Four digit auto-ranging LCD display shows same values as the analog meters

Output BNC's ±10 V output corresponds to full scale input, <1Ω output impedance X Output X (RcosØ), ±10 V full scale, < 1Ω output impedance Y Output Y (RsinØ), ±10 V full scale, < 1Ω output imp eda nce Reference LCD Four digit LCD display for reference phase shift or frequency RS232 Interface controls all functions. Baud rates from 300 to 19.2 K GPIB Interface controls all functions. ( IEEE-488 Std ) A/D 4 BNC inputs with 13 bit resolution (±10.24 V) D/A 2 BNC outputs with 13 bit resolution (±10.24 V) Ratio Ratio output equals 10X Channel 1 output divided by the Denominator input. Internal Oscillator Range: 1 Hz to 100 kHz, 1% accuracy

Stability: 150 ppm/°C Distortion: 2% THD Amplitude: 1% accuracy, 500 ppm/°C stability

Front Panel Summary

Signal Inputs Single Ended (A), True Differential (A-B), or Current (I)

Signal Filters Bandpass: Q-of-5 Auto-tracking filter (In or Out)

Line Notch: Q-of-10 Notch Filter at line frequency (In or Out) 2XLine Notch: Q-of-10 Notch Filter at twice line frequency (In or Out)

Sensitivity Full scale sensitivity from 100 nV to 500 mV RMS for voltage inputs

or from 100 fA to 500 nA RMS for current inputs.

Dynamic Reserve Select Dynamic Reserve

LOW 20 dB 5 ppm 1 µV to 500 mV NORM 40 dB 50 ppm 100 nV to 50 mV HIGH 60 dB 500 ppm 100 nV to 5 mV

Status Indicators OVLD Signal Overload

UNLK PLL is not locked to the reference input ERR Illegal or Unrecognized command ACT RS232 or GPIB interface Activity REM Remote mode: front panel has been locked-out

Display Select Channel 1 Channel 2

X (RcosØ) Y (RsinØ) X Offset Y Offset R (Magnitude) Ø (Phase) R Offset Ø (no offset) X Noise Y Noise

X5 (D/A) X6 (D/A) Analog Meters Displays Channel 1 and 2 Outputs as a fraction of full scale Output LCD's Displays the Channel 1 and 2 Outputs in absolute units Output BNC's Channel 1 and 2 Outputs follow Analog Meters, ± 10 V for ± full scale Expand Multiplies the Channel 1 or 2 Analog Meter and Output voltage by a factor X1 or X10.

Stability Sensitivity Ranges

REL Set the Channel 1 or 2 Offset to null the output: subsequent readings are relative

readings. REL with phase display performs auto-phasing. REL with X5, X6 display

zeroes the D/A outputs. Offset Enables or Disables Offset, and allows any offset (up to full scale) to be entered. X, Y,

and R may be offset and X5, X6 may be adjusted. Phase is offset using the reference

phase shift. X BNC X (RcosØ) output, ± 10V full scale Y BNC Y (RsinØ) output, ± 10V full scale Reference Input 1 MΩ Input, 0.5 Hz to 100 KHz, 100 mV minimum Reference Trigger Trigger on rising edge, zero crossing, or falling edge f/2f Mode PLL can lock to either X1 or X2 of the reference input frequency

Phase Controls Adjust phase in smoothly accelerating 0.025° steps, or by

90° steps. Press both 90° buttons to zero the phase. Reference LCD Display reference phase setting or reference frequency Time Constants Pre-filter has time constants from 1 mS to 100 S (6 dB/Octave)

Post-filter has time constants of 0, 0.1 or 1.0 S (6 dB/Octave) ENBW Equivalent Noise Bandwidth. Specifies the bandwidth when making

Noise measurements. (1Hz or 10 Hz ENBW) Power Switch Instrument settings from the last use are recalled on power-up

Abridged Command List

AX Auto offset X AY Auto offset Y AR Auto offset R AP Auto phase

B Return Bandpass Filter Sta tus B0 Take out the Bandpass Filter B1 Put in the Bandpass Filter

C Return the Reference LCD Status C0 Display the Reference Frequency C1 Displ ay the Refere nce Phas e Sh ift

D Return Dynamic Reserve Setting D0 Set DR to LOW range D1 Set DR to NORM range D2 Set DR to HIGH range

En Return Channel n (1 or 2) Expand

Status En,0 Turn Channel n Expand off En,1 Turn Channel n Expand on

F Return the Reference Frequency G Return the Sensitivity Setting

G1 Select 10 nV Full-Scale ... (G1-G3 with SRS preamp only) G24 Select 500 mV Full-Scale

H Return Preamp Status (1=installed) I Return the Remote/Local Status

I0 Select Local: Front panel active I1 Select Remote: Front panel inactive I2 Select Remote with full lock-out

J Set RS232 End-of-Record to <cr> Jn,m,o,p Set End-of-record to n,m,o,p

K1 Simulates Key-press of button #1 ... (see un-abridged command list) K32 Simulates Key-press of button #32

L1 Return Status of Line Notch Filter L1,0 Remove Line Notch Filter L1,1 Insert Line Notch Filter

L2 Return Status of 2XLine Filter L2,0 Remove 2XLine Notch Filter L2,1 Insert 2XLine Notch Filter

M Return the f/2f Status M0 Set reference mode to f M1 Set reference mode to 2f

N Return the ENBW setting N0 Select 1 Hz ENBW N1 Select 10 Hz ENBW

OX Return X Offset Status OX 0 Turn off X Offset OX 1,v Turn on X Offset, v = offset OY Return Y Offset Status OY 0 Turn off Y Offset OY 1,v Turn on Y Offset, v = offset OR Return R Offset Status OR 0 Turn off R Offset OR 1,v Turn on R Offset, v = offset

P Return the Phase Setting Pv Set the Phase to v. Abs(v) <999 deg

Q1 Return the Channel 1 output Q2 Return the Channel 2 output QX Return the X Output QY Return the Y Output

R Return the trigger mode R0 Set the trigger for rising edge R1 Set the trigger for + zero crossing R2 Set the trigger for falling edge

S Return the display status S0 Display X and Y S1 Display X and Y Offsets S2 Display R and Ø S3 Display R Offset and Ø S4 Display X and Y noise S5 Display X5 and X6 (ext D/A)

T1 Return pre-fi lter sett in g T1,1 Set the pre-filter TC to 1 mS ... T1,11 Set the pre-filter TC to 100 S

T2 Return the post-filter setting T2,0 Remove post filter T2,1 Set the post filter TC to 0.1 S T2,2 Set the post filter TC to 1.0 S

V Return the value of the SRQ mask Vn Set the SRQ Mask to the value n

(See the Status Byte definition)

W Return the RS232 wait interval Wn Set RS232 wait interval to nX4mS

Xn Return the voltage at the rear panel

analog port n. (n from 1 to 6)

X5,v Set analog port 5 to voltage v X6,v Set analog port 6 to voltage v

Y Return the Status Byte value Yn Test bit n of the Status Byte

Z Reset to default settings and cancel

all pending commands.

Status Byte Definition

Bit Meaning

0 Magnitude too small to calculate phase 1 Command Parameter is out-of-range 2 No detectable reference input 3 PLL is not locked to the reference 4 Signal Overload 5 Auto-offset failed: signal too large 6 SRQ generated 7 Unrecognized or illegal command

Configuration Switches

There are two banks of 8 switches, SW1 and SW2, located on the rear panel. SW1 sets the GPIB address and SW2 sets the RS232 parameters. The configuration switches are read continuously and any changes will be effective immediately.

SW1:GPIB Mode Switches

Bit Example Function

1 } up GPIB Address Switches 2 } up Address 0 to 30 allowed 3 } up 'up' for bit = 1 4 } down 'down' for bit = 0 5 } up (Most Significant Bit)

6 down 'down' to echo on RS232 (normally 'up')

7 up Not Used 8 up Not Used

If the GPIB mode switches are set as shown in the example column above, then the lockin will be addressed as GPIB device #23, and all GPIB commands and data will be echoed over the RS232 for de-bugging purposes.

SW2:RS232 Mode Switches

Bit 1 Bit 2 Bit 3 Baud Rate

up up up 19200 down up up 9600 up down up 4800 down down up 2400 up up down 1200 down up down 600 up down down 300

Bit Setting Explanation

4 up Odd parity down Even parity

5 up No parity down Parity enabled

6 up No echo (for computer) down Echo mode (for terminal)

7 up Two stop bits down One stop bit

8 unused

Eight data bits are always sent, regardless of the parity setting. The most significant bit is always zero.

Example: Bit 1 'down' and all others 'up' for RS232 communication at 9600 baud, no parity, two stop bits, and no echo or prompts by the SR530.

SR510 Guide to Operation Front Panel

The front panel has been designed to be almost self-explanatory. The effect of each keypress is usually reflected in the change of a nearby LED indicator or by a change in the quantity shown on a digital display. This discussion explains each section of the front panel, proceeding left to right.

Signal Inputs

There are three input connectors located in the SIGNAL INPUT section of the front panel. The rocker switch located above the B input selects the input mode, either single-ended, A, differential, A-B, or current, I.

The A and B inputs are voltage inputs with 100 MΩ, 25 pF input impedance. Their connector shields are isolated from the chassis ground by 10Ω. These inputs are protected to 100V dc but the ac input should never exceed 10V peak. The maximum ac input before overload is 1V peak.

The I input is a current input with an input impedance of 1 KΩ to a virtual ground. The largest allowable dc current before overload is 1 µA. No current larger than 10 mA should ever be

applied to this input. The conversion ratio is 10 V/A, thus, the full scale current sensitivities range from 100 fA to 500 nA with a max ac input before overload of 1 µA peak. You should use short cables when using the current input.

allowable signals at the inputs. The notch frequencies are set at the factory to either 50 Hz or 60 Hz. The user can adjust these frequencies. (See the Maintenance and Repair section for alignment details.) These filters precede the bandpass filter in the signal amplifier.

The bandpass filter has a Q of 5 and a 6 dB roll off in either direction. Thus, the pass band (between 70% pass points) is always equal to 1/5th of the center frequency. The center frequency is continually adjusted to be equal to the internal demodulator frequency. When the reference mode is f, the filter tracks the reference. When the mode is 2f, the filter frequency is twice the reference input frequency. The center frequency tracks as fast as the reference oscillator can slew and may be used during frequency scans. The bandpass filter adds up to 20 dB of dynamic reserve for noise signals outside the pass band, and increases the harmonic rejection by at least 13dB. (2nd harmonic attenuated by 13 dB, higher harmonics attenuated 6dB/octave more.) If not needed to improve the dynamic reserve or the harmonic rejection then the filter should be left

OUT.

Sensitivity

The sensitivity is displayed as a value (1-500) and a scale (nV, µV, mV). When using the current

input, which has a gain of 106 V/A, these scales read fA, pA, and nA. The two keys in the SENSITIVITY section move the sensitivity up and down. If either key is held down, the sensitivity will continue to change in the desired direction four times a second.

Signal Filters

There are three user selectable signal filters available; a line frequency notch, a 2X line frequency notch, and an auto-tracking band pas s . Each of the filters has a pair of indicator LED's and a function key located in the SIGNAL FILTERS section of the front panel. Pressing a key will toggle the status of the appropriate filter. The status of each filter is displayed as IN, filter active, or OUT, filter inactive.

The notch filters have a Q of 10 and a depth of at least 50 dB. Thus, the line frequency notch is 6 Hz wide and the 2X line notch has a width of 12 Hz. Both of these filters can increase the dynamic reserve up to 50 dB at the notch frequencies. The achievable reserve is limited by the maximum

The full scale sensitivity can range from 100 nV to 500 mV. The sensitivity indication is not changed by the EXPAND function. The EXPAND function increases the output sensitivity (Volts out /volts in) as well as the resolution of the digital output display.

Not all dynamic reserves are available at all sensitivities. If the sensitivity is changed to a setting for which the dynamic reserve is not allowed, the dynamic reserve will change to the next setting which is allowed. Sensitivity takes precedence over the dynamic reserve. The sensitivity range of each dynamic reserve is shown below.

Dynamic Reserve Sensitivity Range LOW 1 µV through 500 mV

NORM 100 nV through 50 mV HIGH 100 nV through 5 mV

Dynamic Reserve

The dynamic reserve (DR) is set using the keys in the DYNAMIC RESERVE section. The reserve is displayed by the three indicator LED's, HIGH, NORM, LOW. Only those dynamic reserve settings available for the sensitivity are allowed (see above table). For example, when the sensitivity is 500 mV, the DR will always be LOW.

The dynamic reserve and output stability of each setting are shown below.

the output, i.e. in the ac amplifier or output time constant. In this case, the dynamic reserve, sensitivity, time constant, or ENBW needs to be adjusted.

UNLK indicates that the reference oscillator is not phase locked to the external reference input. This can occur if the reference amplitude is too low, the frequency is out of range, or the trigger mode is incorrect for the reference signal waveform.

ERR flashes when an error occurs on one of the computer interfaces, such as an incorrect command, invalid parameter, etc.

ACT indicates activity on the computer interfaces. This LED blinks every time a character is received or transmitted by the SR530.

Setting Dynamic Reserve (ppm/°C) LOW 20 dB 5 NORM 40 dB 50 HIGH 60 dB 500

Since a higher DR results in degraded output stability, you should use the lowest DR setting for which there is no overload indication. Note that using the Bandpass Filter provides about 20dB of additional DR and so allows you to operate with a lower DR setting.

Output Stability

Status

There are five STATUS LED's. OVLD indicates a signal overload. This condition

can occur when the signal is too large, the sensitivity is too high, the dynamic reserve is too low, the offset is on, the expand is on, the time constant is not large enough, or the ENBW is too large.

The OVLD LED blinks four times a second when an output is overloaded. This occurs if an output exceeds full scale. For example, during a quadrature measurement where X exceeds full scale while Y is near zero, a blinking OVLD indicates that it is safe to take data from the Y output since only the X output is overloaded. The signal path to the Y output is not overloaded. OVLD also blinks if a noise measurement is attempted on an output which exceeds full scale.

If the OVLD LED is on continuously or flashes randomly, then an overload has occurred before

REM indicates that the unit is in the remote state and that the front panel controls are not operat ive. There are two remote states. The Remote-WithLockout will not allow any inputs from the front panel. The Remote-Without-Lockout command allows you to return the front panel to operation by pressing the LOCAL key.

Display Select

The keys in the DISPLAY section sele ct the parameters to be displayed on the OUTPUT METERS and the output of the two OUTPUT BNC connectors. The displayed parameters are indicated by one of the six DISPLAY LED's and can be either the two demodulator outputs (X Y), the demodulator output offsets (X OFST Y OFST), the magnitude and phase (R Ø), the magnitude offset and phase (R OFST Ø), the rms noise on X and Y (X NOISE Y NOISE), or the D/A outputs (X5 D/A X6). When displaying NOISE, the equivalent noise bandwidth is selected in the TIME CONSTANT section. When displaying D/A, the 2 outputs are the X5 and X6 rear panel D/A outputs, allowing the D/A outputs to be set from the front panel. This feature can be used to set the reference frequency when using the internal oscillator.

Channel 1 Display

The channel 1 outputs are summarized below. X is equal to RcosØ where Ø is the phase shift of the signal relative to the reference oscillator of the lock-in.

display CH1 X

output expand? offset? (RCOSØ)

setting

X X+X XOFST X R R+R R OFST R XNOISE X noise yes yes X+X X5 X5 no adjust X+X

The EXPAND and OFFSET conditions for each display are retained when the DISPLAY is changed. Thus, when the DISPLAY is changed from X to R, the EXPAND and OFFSET assume the conditions set the last time the DISPLAY was

R. If the DISPLAY is changed back to X, the EXPAND and OFFSET return to conditions set for X.

yes yes X+X

ofst

yes yes X

ofst

yes yes X+X

ofst

yes yes X+X

ofst

ofst ofst

ofst

(enbw)

R Output

The magnitude, R, is given by the equation:

R = {(X+X

Note that the X and Y offsets affect the value of R while the X and Y expands do not.

The magnitude output has a resolution of 12 bits plus sign and is updated every 3.5 mS. To achieve maximum accuracy, the magnitude should be as large a fraction of full scale as possible.

R is expanded after the calculation. Thus, when R is expanded, the full scale resolution drops by a factor of 10 to about 9 bits.

)2 + (Y+Y

ofst

)2}

1/2

+ R

ofst

The left hand analog meter always displays the CHANNEL 1 OUTPUT voltage. Accuracy is 2% of full scale.

The CHANNEL 1 LCD display provides a read-out of the displayed parameter in real units. The scale of the displayed quantity is indicated by the three scale LED's to the left of the display. This readout auto ranges and will reflect the sensitivity added when the EXPAND function is on. When displaying X5, the scale LED's are off and the units are volts.

Rel Channel 1

Every time the REL key is pressed, the displayed parameter is offset to zero. This is done by loading the displayed parameter's offset with minus one times the present output. If the output is greater than 1.024 times full scale, the REL function will not be able to zero the output. In this case, the OFFSET ON LED will blink and the offset value will be set to its maximum value.

The REL function and the manual OFFSET are both ways to enter the offset value. After using the REL key, the offset may be adjusted using the manual OFFSET.

When the DISPLAY is X, X OFST, or X NOISE, the REL key sets the X OFFSET (which affects the X (RCOSØ) output). If X NOISE is being displayed, the REL function zeroes X and the noise output will require a few seconds to settle again.

When the DISPLAY is R or R OFST, the REL key sets the R OFFSET.

Output Channel 1

The CHANNEL 1 output is available at the left hand OUTPUT BNC connector. The output parameter is selected by the DISPLAY setting and can be X, X OFST, R (magnitude), R OFST, X NOISE, or X5 (external D/A). (Note that X5 is the ratio output at power up. When displaying X5, the ratio output is 10R/X1). All outputs are ±10V full scale when the EXPAND is off. With the EXPAND on, the output is multipled by 10 effectively increasing the full scale sensitivity by 10. (X5 may not be expanded). The output impedance is < 1Ω and the output current is limited to 20 mA.

The REL key zeroes the X5 output when the

DISPLAY is D/A.

Offset Channel 1

The OFFSET buttons control the manual offset. The offset is turned ON and OFF using the upper key in the OFFSET section. When the offset is ON, the lower two keys are used to set the amount of offset. A single key press will advance the offset by 0.025% of full scale. If the key is held down, the offset advances in larger and larger increments, the largest increment being 10% of full scale. When the offset is turned OFF the applied offset returns to zero but the offset value is not lost. The next press of the upper offset key (return

to ON) sets the offset to the previously entered value.

If an attempt is made to advance the offset value beyond full scale, the ON LED will blink. An offset up to 1.024 times the full scale sensitivity may be entered. When the EXPAND is on, this is 10X the full scale output.

Note that the offsets (either manual offset or those generated by the REL function) represent a fraction of the full scale reading, and so their absolute value will change when the sensitivity scale is changed. A signal which has been nulled by an offset will not be nulled when the sensitivity scale is changed. The analog meter and the output BNC indicate the same value given by the equation:

= 10Ae(AvVicosØ+Vos) {if the output is X}

out

in magnitude to the selected sensitivity which is in phase with the reference oscillator will generate a 10V output. The output impedance is <1Ω and the output current is limited to 20 mA.

The X (RCOSØ) output is affected by the X offset but may not be expanded. The X (RCOSØ) is not affected by the DISPLAY setting except for two cases. When the DISPLAY is set to X OFST, the

X (RCOSØ) output is the X offset. When the DISPLAY is set to X NOISE, the X (RCOSØ)

output has a bandwidth equal to the ENBW (1 or 10 Hz) instead of the time constant.

Channel 2 Display

The channel 2 outputs are summarized below. Y is equal to RsinØ where Ø is the phase shift of the signal relative to the reference oscillator of the lock-in.

where... Ae= 1 or 10 per the Expand

Av= 1/Sensitivity Vi= magnitude of the signal Ø = phase between signal & reference

Vos= offset (fraction of FS < 1.024)

When the DISPLAY is X, X OFST, or X NOISE, the OFFSET keys adjust the X OFFSET (which affects the X (RCOSØ) output). When the DISPLAY is R or R OFST, the OFFSET keys adjust the R OFFSET. When the DISPLAY is X5, the OFFSET up and down keys set the output voltage of D/A output X5 (also on the rear panel) up to ±10.24 V. Adjusting X5 will cancel the

RATIO output.

Expand Channel 1

The output EXPAND is toggled by pressing the key in the Channel 1 EXPAND section. The expand status is indicated by the X10, expand on, and the X1, expand off, LED's. Only the Channel 1 OUTPUT is affected, the X (RCOSØ) output is not expanded.

display CH2 Y setting output

YY+Y YOFST Y Ø Phase no no Y+Y Ø Phase no no Y+Y YNOISE Y noise yes yes Y+Y X6 X6 no adjust Y+Y

The EXPAND and OFFSET conditions for each display are retained when the DISPLAY is changed. Thus, when the DISPLAY is changed from Y to Ø, the EXPAND and OFFSET turn off. If the DISPLAY is changed back to Y the EXPAND and OFFSET return to conditions set for Y.

expand? offset? (RSINØ)

ofst

yes yes Y+Y yes yes Y

ofst

ofst ofst ofst ofst

(enbw)

Ø Output

The phase, Ø, is given by the equation:

Ø = - tan-1 {(Y+Y

Note that the X and Y offsets affect the value of Ø while the X and Y expands do not.

ofst

)/(X+X

ofst

)}

The X5 D/A output may not be expanded.

X (RCOSØ) Output

The analog output, X+X (RCOSØ) BNC connector. An input signal equal

, is available at the X

ofst

The Phase Output voltage is 50 mV per degree with a resolution of 2.5 mV or 1/20 of a degree. The output range is from -180 to +180 degrees. The phase output is updated every 3.5 mS. To achieve maximum accuracy, the magnitude, R, should be as large a fraction of full scale as

possible. If R is less than 0.5% of full scale, the phase output defaults to zero degrees.

The Phase Output may not be expanded and the OFFSET keys do not offset the Phase Output. However, the Phase Output can be offset using the Reference Phase shift.

The Reference Phase shift, which may be adjusted via the phase controls in the reference section, rotates the lock-in's internal coordinate axes relative to the reference input. The Phase Output is the phase difference between the signal and the lock-in's coordinate system. For example, if a signal exactly in phase with the reference input is being measured and the Reference Phase shift is zero, the Phase Output will be zero also. This is because the lock-i n coordinate system is in phase with the reference input and signal. If the Reference Phase shift is set to +45 degrees, then the lock-in coordinate system rotates to +45 degrees from the reference input. Thus, the reference input is now at -45 degrees from the lock-in coordinate axes. Since the reference and signal are in phase, the signal is now at -45 degrees with respect to the lock-in coordinates and the Phase Output will be -45 degrees.

The sum of the Reference Phase shift and the Phase Output is the absolute phase difference between the signal and the reference input. Therefore, the Phase Output may be offset to zero by adjusting the Reference Phase shift. This is sometimes necessary when the Phase Output is near 180 degrees and varies between +180 and 180 degrees.

Output Channel 2

The CHANNEL 2 output is available at the right hand OUTPUT BNC connector. The output parameter is selected by the DISPLAY setting and can be Y, Y OFST, Ø (phase), Ø (phase), Y NOISE, or X6 (ext D/A). All outputs are ±10V full scale when the EXPAND is off. With the EXPAND on, the output is multipled by 10, effectively increasing the full scale sensitivity by 10. (Ø and X6 may not be expanded). The Ø (phase) output is 50 mV/deg (20 deg per Volt) up to ±9 V (±180 deg). The output impedance is <1Ω and the output current is limited to 20 mA.

The right hand analog meter always displays the CHANNEL 2 OUTPUT voltage. Accuracy is 2% of full scale.

The CHANNEL 2 LCD display provides a read-out of the displayed parameter in real units. The scale of the displayed quantity is indicated by the four scale LED's to the right of the display. This readout auto ranges and will reflect the sensitivity added when the EXPAND function is on. When displaying X6, the scale LED's are off and the units are volts.

Rel Channel 2

The REL function and the manual OFFSET are both ways to enter the offset value. After using the REL key, the offset may be adjusted using the manual OFFSET.

When the DISPLAY is Y, Y OFST, or Y NOISE, the REL key sets the Y OFFSET (which affects the Y (RSINØ) output). If Y NOISE is being displayed, the REL function zeroes Y and the noise output will require a few seconds to settle again.

The REL key zeroes the X6 output when the

DISPLAY is D/A.

Auto Phase

When the DISPLAY is Ø (phase), the REL key sets the Reference Phase Shift to the absolute phase difference between the signal and the reference. This is done by setting the Reference Phase Shift to the sum of the Reference Phase Shift and the present Phase Output. After autophase is performed, the Ø output will be 0 deg, R will be unchanged, X will be maximized, and Y will be minimized.

Offset Channel 2

The OFFSET section controls the manual offset. The offset is turned ON and OFF using the upper key in the OFFSET section. When the offset is ON, the lower two keys are used to set the amount of offset. A single key press will advance the offset by 0.025% of full scale. If the key is held

down, the offset advances in larger and larger increments, the largest increment being 10% of full scale. When the offset is turned OFF the applied offset returns to zero but the offset value is not lost. The next press of the upper offset key (return to ON) sets the offset to the previously entered value.

If an attempt is made to advance the offset value beyond full scale, the ON LED will blink. An offset up to 1.024 times the full-scale sensitivity may be entered. When the EXPAND is on, this is 10X the full scale output.

= 10Ae(AvVisinØ+Vos) {if the output is Y}

out

where... Ae= 1 or 10 per the Expand

Av= 1/Sensitivity Vi= magnitude of the signal Ø = phase between signal & reference Vos= offset (fraction of FS < 1.024)

Y (RSINØ) Output

The analog output, Y+Y (RSINØ) BNC connector. An input signal equal in

magnitude to the selected sensitivity which is 90° out of phase with the reference oscillator will generate a 10V output. The output impedance is <1Ω and the output current is limited to 20 mA.

The Y (RSINØ) output is affected by the Y offset but may not be expanded. The Y (RSINØ) is not affected by the DISPLAY setting except for two cases. When the DISPLAY is set to Y OFST, the

Y (RSINØ) output is the Y offset. When the DISPLAY is set to Y NOISE, the Y (RSINØ) output

has a bandwidth equal to the ENBW (1 or 10 Hz) instead of the time constant.

, is available at the Y

ofst

Reference Input

The REFERENCE INPUT BNC is located in REFERENCE INPUT section. The input is ac

coupled and the impedance is 1 MΩ. The dc voltage at this input should not exceed 100 V and the largest ac signal should be less than 10 V peak.

Trigger Level

The TRIGGER MODE indicator toggles from POSITIVE to SYMMETRIC to NEGATIVE when

the TRIGGER MODE key is pressed.

When the DISPLAY is Y, Y OFST, or Y NOISE, the OFFSET keys adjust the Y OFFSET (which affects the Y (RSINØ) output). When the DISPLAY is Ø, the OFFSET keys do nothin g. When the DISPLAY is X6, the OFFSET up and down keys set the output voltage of D/A output X6 (also on the rear panel) up to ±10.24V.

Expand Channel 2

The output EXPAND is toggled by pressing the key in the Channel 2 EXPAND section. The expand status is indicated by the X10, expand on, and the X1, expand off, LED's. Only the Channel 2 OUTPUT is affected, the Y (RSINØ) output is not expanded. Ø and X6 may not be expanded.

If the center TRIGGER MODE LED is on, the mode is SYMMETRIC and the reference oscillator will lock to the positive zero crossings of the ac reference input. The ac signal must be symmetric (e.g. sine wave, square wave, etc.) and have a peak to peak amplitude greater than 100 mV. A signal with 1 Vrms amplitude is recommended. The phase accuracy of the reference channel is specified for a 1Vrms sinewave in the symmetric trigger mode.

If the upper TRIGGER MODE LED is on, the mode is POSITIVE. The trigger threshold is +1V and the reference oscillator will lock to the positive going transitions of the reference input. This mode triggers on the rising edges of a TTL type pulse train. The pulse width must be greater than 1 µS.

If the lower TRIGGER MODE LED is on, the mode is NEGATIVE. The trigger threshold is -1V and the reference oscillator will lock to the negative

going transitions of the reference input. This mode triggers on a negative pulse train or on the falling edges of a TTL type pulse train (remembering that the input is ac coupled). The pulse width must be greater than 1 µS.

Reference Mode

The REFERENCE MODE indicator toggles between f and 2f whenever the MODE key is pressed. When the MODE is f, the lock-in will detect signals at the reference input frequency. When the MODE is 2f, the lock-in detects signals at twice the reference input frequency. In either case, the reference oscillator has a maximum frequency of 100 KHz, thus, when in the 2f mode, the reference input frequency may not exceed 50 KHz.

Reference Display

The REFERENCE DIGITAL DISPLAY shows either the reference oscillator frequency or phase shift. The displayed parameter toggles between the two whenever the SELECT key is pressed. The appropriate scale indicator below the display will be on. It is useful to check the frequency display to verify that the lock-in has correctly locked to your reference. The reference frequency is measured to 1 part in 256 resolution at all frequencies. The display reads .000 if there is no reference input and 199.9 kHz if the input frequency exceeds 105 kHz.

Phase Controls

The phase shift between the reference oscillator of the lock-in and the reference input signal is set using the four keys in the PHASE section. The two keys below the FINE label inc r em ent the phase setting in small amounts. A single key press will change the phase by 0.025 degrees in the desired direction. Holding the key down will continue to change the phase with larger and larger steps with the largest step being 10 degrees. The two 90° keys are used to change the phase by 90 degree increments. The upper key will add 90 degrees and the lower key will subtract 90 degrees. Holding both keys down at once sets the phase shift back to zero. The REFERENCE DIGITAL DISPLAY automatically displays the phase whenever any of the PHASE keys are pressed. The phase ranges from -180 degrees to +180 degrees and is the phase delay from the reference input signal.

Time Constant

There are two post demodulator low pass filters, labeled PRE and POST. The PRE filter precedes the POST filter in the output amplifier. Each filter provides 6 dB/oct attenuation.

The PRE filter time constant ranges from 1 mS to

100 S and is selected by the two keys below the PRE filter indicator LED's. Holding down either

key will advance the time constant four times a second in the desired direction.

In many servo applications, no time constant is needed. The SR530 may be modified to reduce the output time constant to about 20 µS. Contact the factory for details.

The POST filter time constant can be set to 1 S or

0.1 S, or can be removed altogether, NONE, using the two keys below the ENBW indicators. When set to NONE, the total attenuation is that of the

PRE filter, or 6 dB/oct. When the POST filter is 1 S or 0.1S, the total attenuation is 12 dB/oct for

frequency components beyond the larger of the POST and PRE filter bandwidths (reciprocal time constant).

Noise Measurements

When the DISPLAY is set to X NOISE Y NOISE, none of the PRE and POST indicator LED's are on. Instead, one of the two ENBW indic ators wil l be on, showing the Equivalent Noise Bandwidth of the rms noise calculation. The ENBW is set using the keys below the ENBW indicator LED's (same keys as used to set the POST filter). The PRE filter keys do nothing in this case. Pressing the upper key when the bandwidth is already 1 Hz will reset the rms noise average (output) to zero, restarting the calculation. Likewise with pressing the lower key when 10 Hz is already selected.

The noise is the rms deviation of the output within a 1 or 10 Hz equivalent noise bandwidth about the reference frequency. A dc output does not contribute to the noise, the noise is determined only by the ac 'wiggles' at the output. By measuring the noise at different frequ enci es , the frequency dependence of the noise density can be found. This usually has the form of v

The noise computation assumes that the noise has a Gaussian distribution (such as Johnson noise). Since the computation takes many time constants (reciprocal ENBW), the noise output

noise ~

1/f.

should be allowed to approach a steady value before a reading is taken. For the 1 Hz ENBW, this time is on the order of 15 to 30 seconds; for the 10 Hz ENBW, the output stabilizes much faster. The noise output will vary slightly since there will always be noise variations that are slow compared to the bandwidth. Any DC component in the output will not contribute to the noise. However, a large DC output will cause the noise computation to initially rise to a large value before approaching the final answer. As a result, the computation will take longer to settle.

If the OVLD indicator is blinking four times a second, then either the X or Y output is overloaded and the corresponding noise calculation should be ignored. If the OVLD LED is on continuously, then the input signal is overloading the ac amplifier or time constant filters. In this case, both noise outputs will be wrong.

To obtain a value for the noise density, the noise reading should be divided by the square root of the ENBW. Thus, when the ENBW is 1 Hz, the noise output is the noise density, and when the ENBW is 10 Hz, the noise density is the noise output divided by √10. For example, if the input noise is measured to be 7 nV with the ENBW set to 1 Hz, the noise density is 7 nV/√Hz. Switching the ENBW to 10 Hz results in a faster measurement and a reading of 22 nV on the output. The noise density is 22 nV/√10 Hz or 7 nV/√Hz. At frequencies » 10 Hz, the noise density should be independent of the ENBW.

Power

This is the instrument's POWER switch. When the power is turned off, the front panel settings are retained so that the instrument will return to the same settings when the power is next turned on. The SR530 always powers up in the LOCAL mode.

The D/A out puts X5 and X6 are not retained during power off. X5 always becomes the RATIO output at power on and X6 is always reset to zero.

the instrument. All displays return to normal after 3 seconds.

Local and Remote

When the instrument is programmed via the computer interface to be in the REMOTE state WITHOUT LOCK-OUT, the LOCAL key will return the instrument to LOCAL front panel control. If the instrument is in the REMOTE WITH LOCK- OUT state, no front panel key will return the status to LOCAL. In this case, a RETURN TO LOCAL command must be sent over the computer interface or the power must be turned off and back on.

Defaults

If the LOCAL key is held down when the POWER is turned on, the instrument settings will be set to the defaults shown below instead of the settings in effect when the power was turned off.

Parameter Setting BANDPASS OUT

LINE OUT LINE X 2 OUT SENSITIVITY 500 mV DYN RES LOW DISPLAYS X Y EXPANDS OFF OFFSETS OFF (value=0) PRE TIME CONSTANT 100 mS POST TIME CONSTANT 0.1 S ENBW 1 Hz REFERENCE MODE f TRIGGER MODE SYMMETRIC REFERENCE DISPLAY FREQUENCY PHASE SHIFT 0°

Whenever default values are used at power up, the red ERR LED will turn on for about 3 seconds. If the ERR LED is on when the instrument is powered on without the LOCAL key down, then the instrument is ignoring the retained settings. This can be due to a low battery.

When the power is turned on, the CHANNEL 1

OUTPUT DIGITAL DISPLAY will show the SERIAL NUMBER of the instrument and the CHANNEL 2 OUTPUT DIGITAL DISPLAY will show the firmware VERSION. The REFERENCE DIGITIAL DISPLAY shows the model number of

SR530 Guide to Operation

Pin Voltage

Current Available

Rear Panel

AC Power

The ac line voltage selector card, line fuse, and line cord receptacle are located in the fuse holder at the left side of the rear panel. See the section, Preparation for Use at the front of this manual for instructions on setting the ac voltage selector and choosing the correct fuse.

GPIB Connector

The SR530 has an IEEE 488 (GPIB) interface built in. The GPIB address is set using SW1 located to the right of the interface connectors. Refer to page 7 for switch setting details.

RS232 Connector

The SR530 has an RS232 interface. The connector is configured as a DCE. The baud rate, parity, stop bits, and echo mode are selected using SW2 located to the right of the interface connectors. Refer to Page 7 for switch setting details.

Signal Monitor Output

This BNC provides the buffered output of the signal amplifiers and filters. This is the signal just before the demodulator. The output impedance is <1Ω. When a full scale input is applied, the peakto-peak amplitude at this output is 20 mV, 200 mV or 2 V for dynamic reserve settings of high, norm, and low, respectively.

Preamp Connector

This 9 pin "D" connector provides power and control signals to external peripherals such as preamplifiers. The available power is described below.

1 +20 100 mA 2 +5 10 mA 6 -20 100 mA

7 Signal ground 8 Digital ground

General Purpose A/D and D/A

There are four analog input ports, labele d X1 through X4. These inputs may be digitized and read via the computer interfaces. The range is -

10.24 V to +10.24 V and the resolution is 2.5 mV. The input impedance is 1 MΩ. A digitization can be performed in about 3 mS but the result may take longer to transmit over the interface being used.

There are two analog output ports, labele d X5 and X6. The voltages at these ports may be programmed via the computer interfaces. The range is -10.24 V to +10.24 V and the resolution is

2.5 mV. The output impedance is <1Ω and the output current is limited to 20 mA.

Ratio

Output X5 is the ratio output when not programmed by the computer interface or set via the front panel. X5 becomes the ratio output whenever the unit is turned on.

The voltage at X5 is the ratio of the Channel 1 Output to the analog voltage at port X1. An output of 10 V corresponds to a ratio of 1. The ratio is computed by digitizing the Channel 1 Output and the voltage at port X1 and then taking the ratio. The resolution is 2.5 mV. For best accuracy, the sensitivity should be set to provide at least a 50% full scale signal and the analog denominator (X1) should be 5V or greater. The ratio is updated approximately every 3 mS. For the Ratio feature to work, the voltage at the denominator input must exceed 40 mV.

When the DISPLAY is set to D/A, the ratio output is 10 times the magnitude, R, divided by X1.

Internal Oscillator

The INTERNAL OSCILLATOR is a voltage controlled oscillator with a sine wave output . To use the oscillator as the reference source, connect

the REF OUTPUT on the rear panel to the REF INPUT on the front panel. The REF OUTPUT is a

1 Vrms sine wave. The SINE OUTPUT may be used as the stimulus to the experiment. The SINE OUTPUT can be set to three amplitudes, 1 V, 100 mV, and 10 mV (rms) using the amplitude switch. The output impedance is 600Ω. The AMP CAL screw adjusts the amplitude.

The oscillator frequency is controlled by the VCO INPUT voltage. A voltage from 0V to 10V will adjust the frequency according to the VCO RANGE selected. Three ranges are available, 1 Hz/V, 100 Hz/V, and 10 KHz/V. The input impedance is 10 kΩ. The FREQUENCY CAL screw adjusts the frequency.

There are four ways to set the frequency:

1) Connect X5 or X6 (D/A outputs) to the VCO INPUT. The frequency can now be set from the front panel by setting the DISPLAY to D/A and adjusting X5 or X6. The frequency is also controllable via the computer interfaces by programming X5 or X6.

2) If the VCO INPUT is left open, then the oscillator will run at the top of its range (i.e. 10 Hz, 1 KHz, or 100 KHz).

3) A 10 KΩ potentiometer may be connected from the VCO INPUT to ground. This pot will then set the frequency.

4) Connect the VCO INPUT to an external voltage source which can provide 0 to 10V.

In all four cases, if the REF OUTPUT is connected to the REFERENCE INPUT on the front panel, the frequency may be read on the front panel REFERENCE DIGITAL DISPLAY or via the computer interfaces.

SR530 Guide to Programming

An example of a multiple command is: G 5; T 1,4; P 45.10 <cr>

The SR530 Lock-in Amplifier is remotely programmable via both RS232 and GPIB interfaces. It may be used with laboratory computers or simply with a terminal. All front panel features (except signal input selection and power) may be controlled and read via the computer interfaces. The SR530 can also read the analog outputs of other laboratory instruments using its four general purpose analog input ports. There are also two programmable analo g outpu t ports available to provide general purpose control voltages.

Communicating with the SR530

Before using either the RS232 or GPIB interface, the appropriate configuration switches need to be set. There are two banks of 8 switches, SW1 and SW2, located on the rear panel. SW1 sets the GPIB address and SW2 sets the RS232 parameters. The configuration switches are read continuously and any changes will be effective immediately. For details on switch settings, see page 7 at the front of this manual.

Command Syntax

Communications with the SR530 use ASCII characters. Commands to the SR530 may be in either UPPER or lower case.

It is not necessary to wait between commands. The SR530 has a command input buffer of 256 characters and processes the commands in the order received. Likewise, the SR530 has an output buffer (for each interface) of 256 characters.

In general, if a command is sent without parameters, it is interpreted as a request to read the status of the associated function or setting. Values returned by the SR530 are sent as a string of ASCII characters terminated usually by carriage return, line-feed. For example, after the above command is sent, the following read commands would generate the responses shown below.

Command Response from the SR530 G <cr> 5<cr><lf>

T 1 <cr> 4<cr><lf> P <cr> 45.10<cr><lf>

The choice of terminating characters sent by the SR530 is determined by which interface is being used and whether the 'echo' feature is in use. The terminating sequence for the GPIB interface is always <cr><lf> (with EOI). The default sequence for RS232 is <cr> when the echo mode is off, and <cr><lf> when the echo mode is on. The terminating sequence for the RS232 interface may be changed using the J command.

A command to the SR530 consists of one or two command letters, arguments or parameters if necessary, and an ASCII carriage return (<cr>) or line-feed (<lf>) or both. The different parts of the command do not need to be separated by spaces. If spaces are included, they will be ignored. If more than one parameter is required by a command, the parameters must be separated by a comma. Examples of commands are:

G 5 <cr> set the sensitivity to 200 nV T 1,4 <cr> set the pre filter to 30 mS F <cr> read the reference frequency P 45.10 <cr> set phase shift to 45.10¡ X 5,-1.23E-1 <cr> set port X5 to -0.123 V

Multiple commands may be sent on a single line. The commands must be separated by a semicolon (;) character. The commands will not be executed until the terminating carriage return is sent.

Note that the terminating characters are sent with each value returned by the SR530. Thus, the response to the command string G;T1;P<cr> while using the RS232 non-echo mode would be 5<cr>4<cr>45.10<cr>.

Front Panel Status LED's

The ACT LED flashes whenever the SR530 is sending or receiving characters over the computer interfaces.

The ERR LED flashes whenever an error has occurred, such as, an illegal command has been received, a parameter is out of range, or a communication buffer has exceeded 240 characters. This LED flashes for about three seconds on power-up if the battery voltage is insufficient to retain previous instrument settings.

The REM LED is on whenever the SR530 is programmed to be in the remote state.

RS232 Echo and No Echo Operation

In order to allow the SR530 to be operated from a terminal, an echo feature has been included which causes the unit to echo back commands received over the RS232 port. This feature is enabled by setting switch 6 on SW2 to the DOWN position. In this mode, the SR530 will send line-feeds in addition to carriage returns with each value returned and will also send the prompts 'OK>' and '?>' to indicate that the previous command line was either processed or contained an error. Operating the SR530 from a terminal is an ideal way to learn the commands and responses before attempting to program a computer to control the SR530. When the unit is controlled by a computer, the echo feature should be turned off to prevent the sending of spurious characters which the computer is not expecting.

Try-Out with an ASCII Terminal

Before attempting any detailed programming with the SR530, it is best to try out the commands using a terminal. Connect a terminal with an RS232 port to the RS232 connector on the rear panel of the SR530. A 'straight' RS232 cable is required since the SR530 is a DCE and the terminal is a DTE. Set the baud rate, parity, and stop bits to match the terminal by setting SW2 per the switch setting table given on page 7. The echo mode should be enabled (switch 6 DOWN). After setting SW2 and connecting the terminal, hold down the LOCAL key while turning the unit on. This causes the SR530 to assume its default settings so that the following discussion will agree with the actual responses of the SR530. The ACT and ERR LED's on the front panel will flash for a second and the sign-on message will appear on the terminal. Following the message, the prompt 'OK>' will be displayed. This indicates that the SR530 is ready to accept commands.

DIGITAL DISPLAY. Typing the phase read command, P<cr>, will now return the string 45.00 to the terminal.

Now read the gain using the sensitivity read command, G<cr>. The response should be 24 meaning that the sensitivity is at the 24th setting or 500 mV. Change the sensitivity by typing G19<cr>. The sensitivity should now be 10 mV. Check the front panel to make sure this is so.

The Channel 1 Output of the lock-in is read by typing the command, Q1<cr>. The response is a signed floating point number with up to 5 significant digits plus a signed exponent. Change the gain to 10 uV using the G10 command. The response to the Q1 command will now be similar to the previous one except that the exponent is different.

Attach a DC voltmeter to the X6 output on the rear panel. The range should allow for 10V readings. The voltage at the X6 output can be set using the X6 command. Type X6,5.0<cr> and the X6 output will change to 5.0V. To read this back to the terminal, just type X6<cr>. When setting the X6 voltage, the voltage may be sent as an integer (5), real (5.000), or floating point (0.500E1) number. Now connect the X6 output to the X1 input (a ls o on the rear panel). X1 through X4 are analog input ports. To read the voltage on X1, simply type X1<cr>. The response 5.000 should appear on the terminal. The analog ports X1 through X6 can be used by your computer to read outputs of other instruments as well as to control other laboratory parameters.

At this point, the user should experiment with a few of the commands. A detailed command list follows.

Type the letter 'P' followed by a carriage return (P<cr>). The SR530 responds by sending to the terminal the characters 0.00 indicating that the phase is set to 0 degrees. In general, a command with no arguments or parameters reads a setting of the unit. To set the phase to 45 degrees, type the command, P45<cr>. To see that the phase did change, use the SELECT key on the front panel to display the phase on the REFERENCE

SR530 Command List

The leading letters in each command sequence specify the command. The rest of the sequence consists of parameters. Multiple parameters are separated by a comma. Those parameters shown in {} are optional while those without {} are required. The variables m and n represent integers while v represents a real number. Parameters m and n must be expressed in integer format while v may be in integer, real, or floating point format.

AX AY AR AP

The A command causes the auto offset (rel) function to execute. Auto offset is performed by reading the output and using that value as the appropriate offset. Every time an "AX" command is received, the auto offset function is executed on the X output. The "AY" command auto offsets the Y output. The "AR" command auto offsets the R output. Note that "AX" and "AY" will affect the R output but "AR" will not affect X and Y. The "AP" command will execute the auto-phase routine. This is done by setting the reference phase shift with the present phase difference between the signal and the reference input. The ¯ output then reads zero and the reference display reads the signal phase shift. "AP" maximizes X and minimizes Y but R is unaffected. The A commands may be issued at any time, regardless of the DISPLAY setting.

B {n}

If n is "1", the B command sets the bandpass filter in. If n is "0", the bandpass filter is taken out. If n is absent, then the bandpass filter status is returned.

C {n} If n is "1", the C command sets the reference LCD display to show the phase setting. If n is "0", the

LCD will display the reference frequency. If n is absent, the parameter being displayed (frequency or phase) is returned. Note that the P and F commands are used to read the actual values of the phase and frequency.

D {n} If n is included, the D command sets the dynamic reserve. If n is absent, the dynamic reserve

setting is returned.

nDyn 0LOW 1 NORM 2 HIGH

Note that not all dynamic reserve settings are allowed at every sensitivity.

E m {,n}

The E command sets and reads the status of the output expands. If m is "1", then Channel 1 is selected, if m is "2", Channel 2 is selected. The parameter m is required. If n is "1", the E command expands the selected output channel. If n is "0", the expand is turned off for the selected channel. If n is absent, the expand status of the selected channel is returned. Note that the expands do not affect the X and Y BNC outputs, only the Channel 1 and 2 outputs.

The F command reads the reference frequency. For example, if the reference frequency is 100 Hz, the F command returns the string "100.0". If the reference frequency is 100.0 kHz, the string "100.0E+3" is returned. The F command is a read only command.

G {n}

If n is included, the G command sets the gain (sensitivity). If n is absent, the gain setting is returned.

n Sensitivity 1 10 nV 2 20 nV 3 50 nV 4 100nV 5 200nV 6 500nV 71 µV 82 µV 95 µV 10 10 µV 11 20 µV 12 50 µV 13 100µV 14 200µV 15 500µV 16 1 mV 17 2 mV 18 5 mV 19 10 mV 20 20 mV 21 50 mV 22 100mV

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Stanford Research Systems SR530 User Manual

Specifications and Main Features

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User Manual