Stanford Research Systems SR810 Data Sheet

MODEL SR810

DSP Lock-In Amplifier

1290-D Reamwood Avenue

Sunnyvale, California 94089

Phone: [phone] • Fax: [phone]

email: [e-mail] • www.thinkSRS.com

Copyright © 1993, 2000 by SRS, Inc.

All Rights Reserved.

Revision 1.8 (01/2005)

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 2-2
X and R 2-5
Outputs, Offsets and Expands 2-7
Storing and Recalling Setups 2-10
Aux Outputs and Inputs 2-11

SR810 BASICS

What is a Lock-in Amplifier? 3-1
What Does a Lock-in Measure? 3-3
The SR810 Functional Diagram 3-4
Reference Channel 3-5
Phase Sensitive Detectors 3-7
Time Constants and DC Gain 3-8
DC Outputs and Scaling 3-10
Dynamic Reserve 3-12
Signal Input Amplifier and Filters 3-14
Input Connections 3-16
Intrinsic (Random) Noise Sources 3-18
External Noise Sources 3-20
Noise Measurements 3-22

OPERATION

Power On/Off and Power On Tests 4-1
Reset 4-1
[Keys] 4-1
Spin Knob 4-1
Local Lockout 4-2
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
Reference 4-15
Auto Functions 4-18
Setup 4-20
Interface 4-21
Warning Messages 4-23

REAR PANEL
Power Entry Module 4-24
IEEE-488 Connector 4-24
RS-232 Connector 4-24

Table of Contents

Aux Inputs (A/D Inputs) 4-24
Aux Outputs (D/A Outputs) 4-24
X and Y Outputs 4-24
Signal Monitor Output 4-25
Trigger Input 4-25
TTL Sync Output 4-25
Preamp Connector 4-25
Using SRS Preamps 4-26

PROGRAMMING

GPIB Communications 5-1
RS-232 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 5-20
Status Reporting 5-21

STATUS BYTE DEFINITIONS
Serial Poll Status Byte 5-23
Service Requests 5-24
Standard Event Status Byte 5-24
LIA Status Byte 5-25
Error Status Byte 5-25

PROGRAM EXAMPLES
Microsoft C, Nationall Instr GPIB 5-27

USING SR510 PROGRAMS 5-31

TESTING

Introduction 6-1
Serial Number 6-1
Firmware Revision 6-1
Preset 6-1
Warm Up 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

1-1

Table of Contents

PERFORMANCE TESTS
Self Tests 6-3
DC Offset 6-4
Common Mode Rejection 6-5
Amplitude Accuracy and Flatness 6-6
Amplitude Linearity 6-8
Frequency Accuracy 6-9
Phase Accuracy 6-10
Sine Output Amplitude 6-11
DC Outputs and Inputs 6-13
Input Noise 6-15
Performance Test Record 6-17

CIRCUITRY

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

PARTS LISTS
CPU and Power Supply Board 7-9
DSP Logic Board 7-13
Analog Board 7-20
Front Panel Display Board 7-27
Miscellaneous and
Chassis Assembly 7-32

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

1-2

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 SR810 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 SR810 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.

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-condensing

WARNING REGARDING USE WITH PHOTO­MULTIPLIERS AND OTHER DETECTORS

The front end amplifier of this instrument is easily
damaged if a photomultiplier is used improperly
with the amplifier. When left completely
unterminated, a cable connected to a PMT can
charge to several hundred volts in a relatively
short time. If this cable is connected to the inputs
of the SR810 the stored charge may damage the
front-end op amps. To avoid this problem, always
discharge the cable and connect the PMT output
to the SR810 input before turning the PMT on.

1-3

Symbols that may be found on SRS products

Symbol Description

Alternating current

Caution - risk of electric shock

Frame or chassis terminal

Caution - refer to accompanying documents

Earth (ground) terminal

Battery

Fuse

On (supply)

Off (supply)

1-4

SR810 DSP Lock In-Amplifier

SPECIFICATIONS

SIGNAL CHANNEL

Voltage Inputs Single-ended (A) or differential (A-B).
Current Input 10
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), ±0.2% Typical
Input Noise 6 nV/√Hz at 1 kHz (typical).
Signal Filters 60 (50) Hz and 120(100) Hz notch filters (Q=4).
CMRR 100 dB at 10 kHz (DC Coupled), decreasing by 6 dB/octave above 10 kHz
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°
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 display has 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 50 Ω 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

6

or 108 Volts/Amp.

external reference.

1-5

SR810 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.
(Y and q are available over the interface only.)
Offset X, Y and R may be offset up to ±105% of full scale. (Y via interface only)
Expand X, Y and R may be expanded by 10 or 100. (Y via interface only)
Reference 4 1/2 digit LED display.
Display and modify reference frequency or phase, sine output amplitude,

harmonic detect, offset percentage (Xor R), or Aux Outputs 1-4.
Data Buffer 8k points from Channel 1 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.
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 RS-232 interfaces standard.
All instrument functions can be controlled through the IEEE-488 and RS-232

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.

1-6

SR810 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
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

RSLP (?) {i} 5-4 Set (Query) the External Reference Slope to Sine(0), TTL Rising

HARM (?) {i} 5-4 Set (Query) the Detection Harmonic to 1 ≤ i ≤ 19999 and i•f ≤ 102

SLVL (?) {x} 5-4 Set (Query) the Sine Output Amplitude to x Vrms. 0.004 ≤ x ≤ 5.000.

INPUT and FILTER page
ISRC (?) {i} 5-5 Set (Query) the Input Configuration to A (0), A-B (1) , I (1 MΩ) (2) or I

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

GAIN and TIME CONSTANT page
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

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

SYNC (?) {i} 5-7 Set (Query) the Synchronous Filter to Off (0) or On below 200 Hz

DISPLAY and OUTPUT page
DDEF (?) { j, k} 5-8 Set (Query) the CH1 display to X, R, Xn, Aux 1or Aux 2 (j=0..4)
and ratio the display to None, Aux1or Aux 2 (k=0,1,2).
FPOP (?) { j} 5-8 Set (Query) the CH1Output Source to X (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 ≤

AOFF i 5-8 Auto Offset X, Y, R (i=1,2,3).

AUX INPUT/OUTPUT page
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

SETUP page
OUTX (?) {i} 5-10 Set (Query) the Output Interface to RS-232 (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).

description

reference mode.

(1), or TTL Falling (2).

kHz.

description

(100 MΩ) (3).

(2), or Both In (3).

description

(1), or Low Noise (2).

(3) dB/oct.

(1).

description

105.00) and Expand to 1, 10 or 100 (j=0,1,2).

description

≤ 10.500.

description

1-7

SR810 DSP Lock In-Amplifier

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
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]

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).

DATA STORAGE page
SRAT (?) {i} 5-13 Set (Query) the DataSample Rate to 62.5 mHz (0) through 512 Hz

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
OUTP? i 5-15 Query the value of X (1), Y (2), R (3) or θ (4). Returns ASCII floating

OUTR? 5-15 Query the value of CH1 Display. 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

SPTS? 5-16 Query the number of points stored in Display i buffer (1,2).
TRCA? j,k 5-16 Read k≥1 points starting at bin j≥0 from CH1 Display buffer in ASCII

TRCB? j,k 5-16 Read k≥1 points starting at bin j≥0 from CH1 Display buffer in IEEE

TRCL? j,k 5-17 Read k≥1 points starting at bin j≥0 from CH1 Display buffer in non-

FAST (?) {i} 5-17 Set (Query) Fast Data Transfer Mode On (1 or 2) or Off (0).On will

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

INTERFACE page
*RST 5-19 Reset the unit to its default configurations.
*IDN? 5-19 Read the SR810 device identification string.
LOCL(?) {i} 5-19 Set (Query) the Local/Remote state to LOCAL (0), REMOTE (1), or

OVRM (?) {i} 5-19 Set (Query) the GPIB Override Remote state to Off (0) or On (1).
TRIG 5-19 Software trigger

STATUS page
*CLS 5-20 Clear all status bytes.
*ESE (?) {i} {,j} 5-20 Set (Query) the Standard Event Status Byte Enable Register to the

*ESR? {i} 5-20 Query the Standard Event Status Byte. If i is included, only bit i is

description

key.

description

(13) or Trigger (14).

description

point value.

value.

floating point.

binary floating point.

normalized binary floating point.

transfer binary X and Y every sample during a scan over the GPIB
interface.

description

LOCAL LOCKOUT (2).

command. Same as trigger input.

description

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.

queried.

1-8

SR810 DSP Lock In-Amplifier

*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.

1-9

SR810 DSP Lock In-Amplifier

STATUS BYTE DEFINITIONS

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)

name usage

bit
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)

name usage

bit
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)

name usage

bit
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

1-10

Getting Started

YOUR FIRST MEASUREMENTS

The sample measurements described in this section are designed to acquaint the first time user with the
SR810 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

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.

3. Press [Phase]

Press the [+90°] key.

Use the knob to adjust the phase shift.

Leave the phase shift at a non-zero value.

Press [Auto Phase]

4. Press [Freq]

Use the knob to adjust the frequency to

10 kHz.

When the power is turned on with the [Setup] key

The lock-in defaults to the internal oscillator reference

Display the reference phase shift in the Reference

Show the internal oscillator frequency in the Reference

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.

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 50 Ω. 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.

display. The phase shift is zero.

This adds 90° to the reference phase shift. The value

of X drops to zero (out of phase).

The knob is used to adjust parameters which are
shown in the Reference display, such as phase,
amplitude and frequency.

Use the Auto Phase function to automatically adjust the
phase to make X a maximum (and Y a minimum). The
phase should be set very close to zero.

display.

The knob now adjusts the frequency. The measured
signal amplitude should stay within 1% of 1 V.

2-2

The Basic Lock-in

Use the knob to adjust the frequency back

to 1 kHz.

5. Press [Ampl]

Use the knob to adjust the amplitude to

0.01 V.

6. Press [Auto Gain] The Auto Gain function will adjust the sensitivity so that

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

Change the sensitivity back to 20 mV.

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

Press [Time Constant Up] to change the

time constant to 3 ms.

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

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.

10. Press [Freq]

Use the knob to adjust the frequency to

55.0 Hz.

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.

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 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 value of X becomes noisy. This is because the 2f

component of the output (at 2 kHz) is no longer
attenuated completely by the low pass filters.

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

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 output is somewhat noisy at this short time
constant and only 1 pole of low pass filtering.

The output is less noisy with 2 poles of filtering.

With 4 poles of low pass filtering, even this short time
constant attenuates the 2f component reasonably 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 110 Hz.

2-3

The Basic Lock-in

11. Press [Sync Filter] This turns on synchronous filtering whenever the
detection frequency is below 200 Hz.

Synchronous filtering effectively removes output
components at multiples of the detection frequency. 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.

2-4

X and R

X, Y, R and q

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 and R.

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.

3. Press [Freq]

Use the knob to change the frequency to

999.8 Hz.

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

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.
The input impedance of the lock-in is 10 MΩ. The

generator may require a terminator. Many generators
have either a 50 Ω or 600 Ω output impedance. Use
the appropriate feed through 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 display 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.

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 output display should now oscillate at
about 0.2 Hz (the accuracy is determined by the
crystals of the generator and 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).

2-5

X and R

Press [Channel 1 Display] to select X again.

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

Press [Source] to turn the INTERNAL led

off.

Press [Trig] to select POS EDGE.

The phase (q) between the reference and the signal
changes by 360° approximately every 5 sec (0.2 Hz
difference frequency). The value of q can read via the
computer interface.

Change the display back to X (slowly oscillating).

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.

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 value of X should be
steady. The actual value 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 display may be stored in the internal data buffer at
a programmable sampling rate. This allows storage of
8k points. See the Programming section for more
details.

2-6

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.

3. Connect the CH1 OUPTUT 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.

5. Press [Channel 1 Auto Offset]

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.

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.

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 (½ of full scale).

X and R may all be offset and expanded separately. (Y

via the interface only). 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.

2-7

Outputs, Offsets and Expands

Press [Channel 1 Offset Modify]
Use the knob to adjust the X offset to 40.0%

Press [Channel 1 Expand] to select x10.

6. Connect the DVM to the X output on the The X and Y outputs on the rear panel always provide

Auto Offset automatically adjusts the X offset (or R)
such that X (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 SR810 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.

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 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.

2-8

Outputs, Offsets and Expands

rear panel. voltages proportional to X and Y (with offset and

expand). The X output voltage should be 10 V, just like
the CH1 output.

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 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 or CH1)
have 100 kHz of bandwidth. The CH1 output, when
configured to be proportional to the displays (even if
the display is X) is updated at 512 Hz and has 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 q.
Thus, Channel 1 (R) should be 0.5V and the CH1
output voltage should be 5V (½ 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.

2-9

Storing and Recalling Setups

STORING and RECALLING SETUPS

The SR810 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.

4. Turn the lock-in off and on while holding
down the [Setup] key. Wait until the power­on tests are complete.

5. Press [Recall]

Use the knob to select setup number 3.
Press [Recall] 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 non-default
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 operation.

Any other key aborts the save.
The current setup is now saved as setup number 3.

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 operation.

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

as those in effect when the setup was saved.

2-10

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.

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.

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.

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.

Only Aux Inputs 1 and 2 can be displayed on the front
panel. The computer interface can read all four inputs.

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

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

The Channel 1 display may be ratio'ed to the Aux Input
1 or 2 voltages. See the Basics section for more about
output scaling.

The display may be stored in the internal data buffers

2-11

Aux Outputs and Inputs

at a programmable sampling rate. This allows storage
of not only the lock-in outputs, X or R, but also the
values of Aux Inputs 1 or 2. See the Programming
section for more details.

2-12

Aux Outputs and Inputs

2-13

SR810 Basics

WHAT IS A LOCK-IN AMPLIFIER?

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 ω

. This might be the

r

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

sin(ωrt + θ

sig

) where V

sig

is the signal amplitude.

sig

The SR810 generates its own sine wave, shown
as the lock-in reference below. The lock-in
reference is V

sin(ωLt + θ

L

).

ref

The SR810 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.

V

= V

psd

sin(ωrt + θ

sigVL

)sin(ωLt + θ

sig

)

ref

= 1/2 V
1/2 V

cos([ωr - ωL]t + θ

sigVL

cos([ωr + ωL]t + θ

sigVL

- θ

) -

sig

ref

+ θ

)

sig

ref

The PSD output is two AC signals, one at the
difference frequency (ω
sum frequency (ω

- ωL) and the other at the

r

+ ωL).

r

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 wr
equals ω

, the difference frequency component will

L

be a DC signal. In this case, the filtered PSD
output will be

= ½ V

V

psd

sigVL

cos(θ

- θ

)

sig

ref

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
lock-in reference frequency. Noise signals at
frequencies far from the reference are attenuated
at the PSD output by the low pass filter (neither

noise-ωref

nor ω

noise+ωref

ω

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

noise

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

are close to DC). Noise at

-ω

| is small). Their attenuation

ref

3-1

SR810 Basics

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. ω

= ωL. Not only do

r

the frequencies have to be the same, the phase
between the signals can not change with time,
otherwise cos(θ

- θ

) will change and V

sig

ref

will not

psd

be a DC signal. In other words, the lock-in
reference needs to be phase-locked to the signal
reference.

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 w

with a fixed phase shift of θ

r

.

ref

Since 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 SR810's internal oscillator may be
used instead. The internal oscillator is just like a
function generator (with variable sine output and a
TTL 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

- θ

sig

). θ is the phase

ref

we can make

ref

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. V

sin(wLt + θ

L

pass filtered output will be

V

= ½ V

psd2

sigVL

sin(θ

- θ

sig

ref

V

~ V

psd2

sinθ

sig

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

X = V

cosθ Y = V

sig

sinθ

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.

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

2

R = (X

+ Y2)½ = Vsig

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 SR810, has two
PSD's, with reference oscillators 90° apart, and
can measure X, Y and R directly. In addition, the
phase q between the signal and lock-in reference,
can be measured according to

θ = tan

-1

(Y/X)

+ 90°), its low

ref

)

cosθ.

sig

sig

3-2

SR810 Basics

WHAT DOES A LOCK-IN MEASURE?

So what exactly does the SR810 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 SR810 measure?

The SR810 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 SR810, 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 SR810, 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

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

0.2546sin(5ωt) + ...

where ω = 2πf. The SR810, 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 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 SR810
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 SR810 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 w (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 w 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.

3-3

SR810 Basics

THE FUNCTIONAL SR810

The functional block diagram of the SR810 DSP
Lock-In Amplifier is shown below. The functions in
the gray area are handled by the digital signal

processor (DSP). We'll discuss the DSP aspects
of the SR810 as they come up in each functional
block description.

Voltage

Current

Reference In
Sine or TTL

A
B

Low Noise

Differential

Amp

I

Discriminator

50/60 Hz

Notch

Filter

PLL

Phase

Locked

Loop

100/120 Hz

Internal

Oscillator

Notch

Filter

Phase

Phase

Shifter

90˚

Shift

Gain

Phase

Sensitive

Detector

Phase

Sensitive

Detector

Low
Pass
Filter

Low
Pass
Filter

DC Gain

Offset

Expand

R and

Θ Calc

DC Gain

Offset

Expand

Y Out

R

Θ

X Out

Sine Out

TTL Out

Discriminator

3-4

SR810 Basics

REFERENCE CHANNEL

A lock-in amplifier requires a reference oscillator
phase-locked to the signal frequency. In general,
this is accomplished by phase-locking an internal
oscillator to an externally provided reference
signal. This reference signal usually comes from
the signal source which is providing the excitation
to the experiment.

Reference Input

The SR810 reference input can trigger on an
analog signal (like a sine wave) or a TTL logic
signal. The first case is called External Sine. The
input is AC coupled (above 1 Hz) and the input
impedance is 1 MΩ. A sine wave input greater
than 200 mV pk will trigger the input discriminator.
Positive zero crossings are detected and
considered to be the zero for the reference phase
shift.

TTL reference signals can be used at all
frequencies up to 102 kHz. For frequencies
below 1 Hz, a TTL reference signal is required.
Many function generators provide a TTL SYNC
output which can be used as the reference. This is
convenient since the generator's sine output might
be smaller than 200 mV or be varied in amplitude.
The SYNC signal will provide a stable reference
regardless of the sine amplitude.

When using a TTL reference, the reference input
trigger can be set to Pos Edge (detect rising
edges) or Neg Edge (detect falling edges). In each
case, the internal oscillator is locked (at zero
phase) to the detected edge.

Internal Oscillator

The internal oscillator in the SR810 is basically a
102 kHz function generator with sine and TTL
sync outputs. The oscillator can be phase-locked
to the external reference.

The oscillator generates a digitally synthesized
sine wave. The digital signal processor, or DSP,
sends computed sine values to a 16 bit digital-to­analog converter every 4 µs (256 kHz). An anti­aliasing filter converts this sampled signal into a
low distortion sine wave. The internal oscillator
sine wave is output at the SINE OUT BNC on the
front panel. The amplitude of this output may be
set from 4 mV to 5 V.

When an external reference is used, this internal
oscillator sine wave is phase-locked to the
reference. The rising zero crossing is locked to the
detected reference zero crossing or edge. In this
mode, the SINE OUT provides a sine wave phase­locked to the external reference. At low
frequencies (below 10 Hz), the phase locking is
accomplished digitally by the DSP. At higher
frequencies, a discrete phase comparator is used.

The internal oscillator may be used without an
external reference. In the Internal Reference
mode, the SINE OUT provides the excitation for
the experiment. The phase-locked-loop is not
used in this mode since the lock-in reference is
providing the excitation signal.

The TTL OUT on the rear panel provides a TTL
sync output. The internal oscillator's rising zero
crossings are detected and translated to TTL
levels. This output is a square wave.

Reference Oscillators and Phase

The internal oscillator sine wave is not the
reference signal to the phase sensitive detectors.
The DSP computes a second sine wave, phase
shifted by θ

from the internal oscillator (and thus

ref

from an external reference), as the reference input
to the X phase sensitive detector. This waveform
is sin(ω

t + θ

r

). The reference phase shift is

ref

adjustable in .01° increments.

The input to the Y PSD is a third sine wave,
computed by the DSP, shifted by 90° from the

t + θ

second sine wave. This waveform is sin(ω

+

r

ref

90°).

Both reference sine waves are calculated to 20
bits of accuracy and a new point is calculated
every 4 µs (256 kHz). The phase shifts (θ

and

ref

the 90° shift) are also exact numbers and accurate
to better than .001°. Neither waveform is actually
output in analog form since the phase sensitive
detectors are actually multiply instructions inside
the DSP.

Phase Jitter

When an external reference is used, the phase­locked loop adds a little phase jitter. The internal
oscillator is supposed to be locked with zero
phase shift relative the external reference. Phase
jitter means that the average phase shift is zero

3-5

SR810 Basics

but the instantaneous phase shift has a few
millidegrees of noise. This shows up at the output
as noise in phase or quadrature measurements.

Phase noise can also cause noise to appear at the
X and Y outputs. This is because a reference
oscillator with a lot of phase noise is the same as
a reference whose frequency spectrum is spread
out. That is, the reference is not a single
frequency, but a distribution of frequencies about
the true reference frequency. These spurious
frequencies are attenuated quite a bit but still
cause problems. The spurious reference
frequencies result in signals close to the reference
being detected. Noise at nearby frequencies now
appears near DC and affects the lock-in output.

Phase noise in the SR810 is very low and
generally causes no problems. In applications
requiring no phase jitter, the internal reference
mode should be used. Since there is no PLL, the
internal oscillator and the reference sine waves
are directly linked and there is no jitter in the
measured phase. (Actually, the phase jitter is the
phase noise of a crystal oscillator and is very, very
small).

Harmonic Detection

It is possible to compute the two PSD reference
sine waves at a multiple of the internal oscillator
frequency. In this case, the lock-in detects signals
at Nxf
The SINE OUT frequency is not affected. The
SR810 can detect at any harmonic up to N=19999
as long as Nxf

which are synchronous with the reference.

ref

does not exceed 102 kHz.

ref

3-6

SR810 Basics

THE PHASE SENSITIVE DETECTORS (PSD's)

The SR810 multiplies the signal with the reference
sine waves digitally. The amplified signal is
converted to digital form using a 16 bit A/D
converter sampling at 256 kHz. The A/D converter
is preceded by a 102 kHz anti-aliasing filter to
prevent higher frequency inputs from aliasing
below 102 kHz. The signal amplifier and filters will
be discussed later.

This input data stream is multiplied, a point at a
time, with the computed reference sine waves
described previously. Every 4 µs, the input signal
is sampled and the result is multiplied by the two
reference sine waves (90° apart).

Digital PSD vs Analog PSD

The phase sensitive detectors (PSD's) in the
SR810 act as linear multipliers, that is, they
multiply the signal with a reference sine wave.
Analog PSD's (both square wave and linear) have
many problems associated with them. The main
problems are harmonic rejection, output offsets,
limited dynamic reserve and gain error.

The digital PSD multiplies the digitized signal with
a digitally computed reference sine wave.
Because the reference sine waves are computed
to 20 bits of accuracy, they have very low
harmonic content. In fact, the harmonics are at the

-120 dB level! This means that the signal is
multiplied by a single reference sine wave (instead
of a reference and its many harmonics) and only
the signal at this single reference frequency is
detected. The SR810 is completely insensitive to
signals at harmonics of the reference. In contrast,
a square wave multiplying lock-in will detect at all
of the odd harmonics of the reference (a square
wave contains many large odd harmonics).

Output offset is a problem because the signal of
interest is a DC output from the PSD and an
output offset contributes to error and zero drift.
The offset problems of analog PSD's are
eliminated using the digital multiplier. There are no
erroneous DC output offsets from the digital
multiplication of the signal and reference. In fact,
the actual multiplication is totally free from errors.

The dynamic reserve of an analog PSD is limited
to about 60 dB. When there is a large noise signal

present, 1000 times or 60 dB greater than the full
scale signal, the analog PSD measures the signal
with an error. The error is caused by non-linearity
in the multiplication (the error at the output
depends upon the amplitude of the input). This
error can be quite large (10 % of full scale) and
depends upon the noise amplitude, frequency,
and waveform. Since noise generally varies quite
a bit in these parameters, the PSD error causes
quite a bit of output uncertainty.

In the digital lock-in, the dynamic reserve is limited
by the quality of the A/D conversion. Once the
input signal is digitized, no further errors are
introduced. Certainly the accuracy of the
multiplication does not depend on the size of the
numbers. The A/D converter used in the SR810 is
extremely linear, meaning that the presence of
large noise signals does not impair its ability to
correctly digitize a small signal. In fact, the
dynamic reserve of the SR810 can exceed 100 dB
without any problems. We'll talk more about
dynamic reserve a little later.

An analog linear PSD multiplies the signal by an
analog reference sine wave. Any amplitude
variation in the reference amplitude shows up
directly as a variation in the overall gain. Analog
sine wave generators are susceptible to amplitude
drift, especially as a function of temperature. The
digital reference sine wave has a precise
amplitude and never changes. This eliminates a
major source of gain error in a linear analog lock­in.

The overall performance of a lock-in amplifier is
largely determined by the performance of its
phase sensitive detectors. In virtually all respects,
the digital PSD outperforms its analog
counterparts.

We've discussed how the digital signal processor
in the SR810 computes the internal oscillator and
two reference sine waves and handles both phase
sensitive detectors. In the next section, we'll see
the same DSP perform the low pass filtering and
DC amplification required at the output of the
PSD's. Here again, the digital technique eliminates
many of the problems associated with analog lock­in amplifiers.

3-7

SR810 Basics

TIME CONSTANTS and DC GAIN

Remember, the output of the PSD contains many
signals. Most of the output signals have
frequencies which are either the sum or difference
between an input signal frequency and the
reference frequency. Only the component of the
input signal whose frequency is exactly equal to
the reference frequency will result in a DC output.

The low pass filter at the PSD output removes all
of the unwanted AC signals, both the 2F (sum of
the signal and the reference) and the noise
components. This filter is what makes the lock-in
such a narrow band detector.

Time Constants

Lock-in amplifiers have traditionally set the low
pass filter bandwidth by setting the time constant.
The time constant is simply 1/2πf where f is the ­3 dB frequency of the filter. The low pass filters
are simple 6 dB/oct roll off, RC type filters. A 1
second time constant referred to a filter whose ­3 dB point occurred at 0.16 Hz and rolled off at
6 dB/oct beyond 0.16 Hz. Typically, there are two
successive filters so that the overall filter can roll
off at either 6 dB or 12 dB per octave. The time
constant referred to the -3 dB point of each filter
alone (not the combined filter).

The notion of time constant arises from the fact
that the actual output is supposed to be a DC
signal. In fact, when there is noise at the input,
there is noise on the output. By increasing the time
constant, the output becomes more steady and
easier to measure reliably. The trade off comes
when real changes in the input signal take many
time constants to be reflected at the output. This is
because a single RC filter requires about 5 time
constants to settle to its final value. The time
constant reflects how slowly the output responds,
and thus the degree of output smoothing.

The time constant also determines the equivalent
noise bandwidth (ENBW) for noise
measurements. The ENBW is NOT the filter -3 dB
pole, it is the effective bandwidth for Gaussian
noise. More about this later.

Digital Filters vs Analog Filters

The SR810 improves on analog filters in many
ways. First, analog lock-ins provide at most, two
stages of filtering with a maximum roll off of
12 dB/oct. This limitation is usually due to space

and expense. Each filter needs to have many
different time constant settings. The different
settings require different components and
switches to select them, all of which is costly and
space consuming.

The digital signal processor in the SR810 handles
all of the low pass filtering. Each PSD can be
followed by up to four filter stages for up to
24 dB/oct of roll off. Since the filters are digital, the
SR810 is not limited to just two stages of filtering.

Why is the increased roll off desirable? Consider
an example where the reference is at 1 kHz and a
large noise signal is at 1.05 kHz. The PSD noise
outputs are at 50 Hz (difference) and 2.05 kHz
(sum). Clearly the 50 Hz component is the more
difficult to low pass filter. If the noise signal is
80 dB above the full scale signal and we would
like to measure the signal to 1 % (-40 dB), then
the 50 Hz component needs to be reduced by
120 dB. To do this in two stages would require a
time constant of at least 3 seconds. To accomplish
the same attenuation in four stages only requires
100 ms of time constant. In the second case, the
output will respond 30 times faster and the
experiment will take less time.

Synchronous Filters

Another advantage of digital filtering is the ability
to do synchronous filtering. Even if the input signal
has no noise, the PSD output always contains a
component at 2F (sum frequency of signal and
reference) whose amplitude equals or exceeds the
desired DC output depending upon the phase. At
low frequencies, the time constant required to
attenuate the 2F component can be quite long. For
example, at 1 Hz, the 2F output is at 2 Hz and to
attenuate the 2 Hz by 60 dB in two stages requires
a time constant of 3 seconds.

A synchronous filter, on the other hand, operates
totally differently. The PSD output is averaged
over a complete cycle of the reference frequency.
The result is that all components at multiples of
the reference (2F included) are notched out
completely. In the case of a clean signal, almost
no additional filtering would be required. This is
increasingly useful the lower the reference
frequency. Imagine what the time constant would
need to be at 0.001 Hz!

3-8

SR810 Basics

In the SR810, synchronous filters are available at
detection frequencies below 200 Hz. At higher
frequencies, the filters are not required (2F is
easily removed without using long time constants).
Below 200 Hz, the synchronous filter follows either
one or two stages of normal filters. The output of
the synchronous filter is followed by two more
stages of normal filters. This combination of filters
notches all multiples of the reference frequency
and provides overall noise attenuation as well.

Long Time Constants

Time constants above 100 seconds are difficult to
accomplish using analog filters. This is simply
because the capacitor required for the RC filter is
prohibitively large (in value and in size!). Why
would you use such a long time constant?
Sometimes you have no choice. If the reference is
well below 1 Hz and there is a lot of low frequency
noise, then the PSD output contains many very
low frequency components. The synchronous filter
only notches multiples of the reference frequency,
the noise is filtered by the normal filters.

The SR810 can provide time constants as long as
30000 seconds at reference frequencies below
200 Hz. Obviously you don't use long time
constants unless absolutely necessary, but they're
available.

DC Output Gain

How big is the DC output from the PSD? It
depends on the dynamic reserve. With 60 dB of
dynamic reserve, a noise signal can be 1000
times (60 dB) greater than a full scale signal. At
the PSD, the noise can not exceed the PSD's
input range. In an analog lock-in, the PSD input
range might be 5V. With 60 dB of dynamic
reserve, the signal will be only 5 mV at the PSD
input. The PSD typically has no gain so the DC
output from the PSD will only be a few millivolts!
Even if the PSD had no DC output errors,
amplifying this millivolt signal up to 10 V is error
prone. The DC output gain needs to be about the
same as the dynamic reserve (1000 in this case)
to provide a 10 V output for a full scale input
signal. An offset as small as 1 mV will appear as
1 V at the output! In fact, the PSD output offset
plus the input offset of the DC amplifier needs to
be on the order of 10 µV in order to not affect the
measurement. If the dynamic reserve is increased
to 80dB, then this offset needs to be 10 times
smaller still. This is one of the reasons why analog

lock-ins do not perform well at very high dynamic
reserve.

The digital lock-in does not have an analog DC
amplifier. The output gain is yet another function
handled by the digital signal processor. We
already know that the digital PSD has no DC
output offset. Likewise, the digital DC amplifier has
no input offset. Amplification is simply taking input
numbers and multiplying by the gain. This allows
the SR810 to operate with 100 dB of dynamic
reserve without any output offset or zero drift.

What about resolution?

Just like the analog lock-in where the noise can
not exceed the input range of the PSD, in the
digital lock-in, the noise can not exceed the input
range of the A/D converter. With a 16 bit A/D
converter, a dynamic reserve of 60 dB means that
while the noise has a range of the full 16 bits, the
full scale signal only uses 6 bits. With a dynamic
reserve of 80 dB, the full scale signal uses only

2.5 bits. And with 100 dB dynamic reserve, the
signal is below a single bit! Clearly multiplying
these numbers by a large gain is not going to
result in a sensible output. Where does the output
resolution come from?

The answer is filtering. The low pass filters
effectively combine many data samples together.
For example, at a 1 second time constant, the
output is the result of averaging data over the
previous 4 or 5 seconds. At a sample rate of
256 kHz, this means each output point is the
exponential average of over a million data points.
(A new output point is computed every 4 µs and is
a moving exponential average). What happens
when you average a million points? To first order,
the resulting average has more resolution than the
incoming data points by a factor of million . This
represents a gain of 20 bits in resolution over the
raw data. A 1 bit input data stream is converted to
20 bits of output resolution.

The compromise here is that with high dynamic
reserve (large DC gains), some filtering is
required. The shortest time constants are not
available when the dynamic reserve is very high.
This is not really a limitation since presumably
there is noise which is requiring the high dynamic
reserve and thus substantial output filtering will
also be required.

3-9

SR810 Basics

DC OUTPUTS and SCALING

The SR810 has X and Y outputs on the rear panel
and a Channel 1 output on the front panel.

X and Y Rear Panel Outputs

The X and Y rear panel outputs are the outputs
from the two phase sensitive detectors with low
pass filtering, offset and expand. These outputs
are the traditional outputs of an analog lock-in.
The X and Y outputs have an output bandwidth of
100 kHz.

CH1 Front Panel Output

The front panel output can be configured to output
voltages proportional to the CH1 display or X.

If the output is set to X, the output duplicates the
rear panel X output.

If the output is set to Display, the output is updated
at 512 Hz. The CH1 display can be defined as X,
R, X Noise, Aux Input 1 or 2, or any of these
quantities divided by Aux Input 1 or 2. If the
display is defined as simply X, this display, when
output through the CH1 output BNC, will only
update at 512 Hz. It is better in this case to set
output to X directly, rather than the display.

X, Y and R Output scales

The sensitivity of the lock-in is the rms amplitude
of an input sine (at the reference frequency) which
results in a full scale DC output. Traditionally, full
scale means 10 VDC at the X, Y or R BNC output.
The overall gain (input to output) of the amplifier is
then 10 V/sensitivity. This gain is distributed
between AC gain before the PSD and DC gain
following the PSD. Changing the dynamic reserve
at a given sensitivity changes the gain distribution
while keeping the overall gain constant.

The SR810 considers 10 V to be full scale for any
output proportional to simply X, Y or R. This is the
output scale for the X and Y rear panel outputs as
well as the CH1 output when configured to output
X. When the CH1 output is proportional to a
display which is simply defined as X or R, the
output scale is also 10 V full scale.

Lock-in amplifiers are designed to measure the
RMS value of the AC input signal. All sensitivities
and X, Y and R outputs and displays are RMS
values.

Phase is a quantity which ranges from -180° to
+180° regardless of the sensitivity. The measured

phase is only available from the interface.

X, Y and R Output Offset and Expand

The SR810 has the ability to offset the X, Y and R
outputs. This is useful when measuring deviations
in the signal around some nominal value. The
offset can be set so that the output is offset to
zero. Changes in the output can then be read
directly from the display or output voltages. The
offset is specified as a percentage of full scale and
the percentage does not change when the
sensitivity is changed. Offsets up to ±105 % can
be programmed.

The X, Y and R outputs may also be expanded.
This simply takes the output (minus its offset) and
multiplies by an expansion factor. Thus, a signal
which is only 10 % of full scale can be expanded
to provide 10 V of output rather than only 1 V. The
normal use for expand is to expand the
measurement resolution around some value which
is not zero. For example, suppose a signal has a
nominal value of 0.9 mV and we want to measure
small deviations, say 10 µV or so, in the signal.
The sensitivity of the lock-in needs to be 1 mV to
accommodate the nominal signal. If the offset is
set so to 90% of full scale, then the nominal 0.9
mV signal will result in a zero output. The 10 µV
deviations in the signal only provide 100 mV of DC
output. If the output is expanded by 10, these
small deviations are magnified by 10 and provide
outputs of 1 VDC.

The SR810 can expand the output by 10 or 100
provided the expanded output does not exceed full
scale. In the above example, the 10 µV deviations
can be expanded by 100 times before they exceed
full scale (at 1 mV sensitivity).

The analog output with offset and expand is

Output = (signal/sensitivity - offset) x Expand x10V

where offset is a fraction of 1 (50 %=0.5), expand
is 1, 10 or 100, and the output can not exceed 10
V.

In the above example,

Output = (0.91mV/1mV - 0.9) x 10 x 10V = 1V

3-10

SR810 Basics

for a signal which is 10 µV greater than the 0.9 mV
nominal. (Offset = 0.9 and expand =10).

The X and Y offset and expand functions in the
SR810 are output functions, They do NOT
affect the calculation of R or θ. R has its own
output offset and expand.

The X and R offsets and expands may be set from
the front panel. The Y offset and expand may

only be set from the interface.

CH1 Display

The CH1 display can show X, R, X Noise, Aux
Input 1 or 2, or any of these quantities divided by
Aux Input 1 or 2.

Output offsets ARE reflected in the display. For
example, if CH1 is displaying X, it is affected by
the X offset. When the X output is offset to zero,
the displayed value will drop to zero also. Any
display which is showing a quantity which is
affected by a non-zero offset will display a
highlighted Offset indicator below the display.

Output expands do NOT increase the displayed
values of X or R. Expand increases the resolution
of the X or R value used to calculate the displayed
value. For example, CH1 when displaying X does
not increase its displayed value when X is
expanded. This is because the expand function
increases the resolution with which the signal is
measured, not the size of the input signal. The

displayed value will show an increased resolution
but will continue to display the original value
minus the X offset. Any display which is showing a
quantity which is affected by a non-unity expand
will display a highlighted Expand indicator below
the display.

Ratio displays are displayed as percentages. The
displayed percentage for X/Aux 1 would be

Display % = (signal/sensitivity-offset)xExpandx100
Aux In 1 (in Volts)

where offset is a fraction of 1 (50 %-0.5), expand
is 1, 10 or 100, and the display can not exceed
100 %.

For example, if the sensitivity is 1V and CH1
display is showing X/Aux 1. If X= 500 mV and Aux
1= 2.34 V, then the display value is
(0.5/1.0)x100/2.34 or 21.37 %. This value is
affected by the sensitivity, offset and X expand.

The Ratio indicator below the display is on
whenever a display is showing a ratio quantity.

of X

Display output scaling

What about CH1 outputs proportional to ratio
displays? The output voltage will simply be the
displayed percentage times 10V full scale.

In the above example, the displayed ratio of

21.37 % will output 2.137 V from the CH1 output.

3-11

SR810 Basics

DYNAMIC RESERVE

We've mentioned dynamic reserve quite a bit in
the preceding discussions. It's time to clarify
dynamic reserve a bit.

What is dynamic reserve really?

Suppose the lock-in input consists of a full scale
signal at fref plus noise at some other frequency.
The traditional definition of dynamic reserve is the
ratio of the largest tolerable noise signal to the full
scale signal, expressed in dB. For example, if full
scale is 1 µV, then a dynamic reserve of 60 dB
means noise as large as 1 mV (60 dB greater than
full scale) can be tolerated at the input without
overload.

The problem with this definition is the word
'tolerable'. Clearly the noise at the dynamic
reserve limit should not cause an overload
anywhere in the instrument - not in the input signal
amplifier, PSD, low pass filter or DC amplifier. This
is accomplished by adjusting the distribution of the
gain. To achieve high reserve, the input signal
gain is set very low so the noise is not likely to
overload. This means that the signal at the PSD is
also very small. The low pass filter then removes
the large noise components from the PSD output
which allows the remaining DC component to be
amplified (a lot) to reach 10 V full scale. There is
no problem running the input amplifier at low gain.
However, as we have discussed previously,
analog lock-ins have a problem with high reserve
because of the linearity of the PSD and the DC
offsets of the PSD and DC amplifier. In an analog
lock-in, large noise signals almost always disturb
the measurement in some way.

The most common problem is a DC output error
caused by the noise signal. This can appear as an
offset or as a gain error. Since both effects are
dependent upon the noise amplitude and
frequency, they can not be offset to zero in all
cases and will limit the measurement accuracy.
Because the errors are DC in nature, increasing
the time constant does not help. Most lock-ins
define tolerable noise as noise levels which do not
affect the output more than a few percent of full
scale. This is more severe than simply not
overloading.

Another effect of high dynamic reserve is to
generate noise and drift at the output. This comes
about because the DC output amplifier is running

at very high gain and low frequency noise and
offset drift at the PSD output or the DC amplifier
input will be amplified and appear large at the
output. The noise is more tolerable than the DC
drift errors since increasing the time constant will
attenuate the noise. The DC drift in an analog
lock-in is usually on the order of 1000ppm/°C
when using 60 dB of dynamic reserve. This means
that the zero point moves 1% of full scale over
10°C temperature change. This is generally
considered the limit of tolerable.

Lastly, dynamic reserve depends on the noise
frequency. Clearly noise at the reference
frequency will make its way to the output without
attenuation. So the dynamic reserve at fref is 0dB.
As the noise frequency moves away from the
reference frequency, the dynamic reserve
increases. Why? Because the low pass filter after
the PSD attenuates the noise components.
Remember, the PSD outputs are at a frequency of
|fnoise-fref|. The rate at which the reserve
increases depends upon the low pass filter time
constant and roll off. The reserve increases at the
rate at which the filter rolls off. This is why
24 dB/oct filters are better than 6 or 12 dB/oct
filters. When the noise frequency is far away, the
reserve is limited by the gain distribution and
overload level of each gain element. This reserve
level is the dynamic reserve referred to in the
specifications.

actual reserve

60 dB

40 dB

20 dB

0 dB

The above graph shows the actual reserve vs the
frequency of the noise. In some instruments, the
signal input attenuates frequencies far outside the
lock-in's operating range (f
cases, the reserve can be higher at these

60 dB specified reserve

f

ref

noise

low pass filter
bandwidth

f

noise

>>100 kHz). In these

3-12

SR810 Basics

frequencies than within the operating range. While
this may be a nice specification, removing noise at
frequencies very far from the reference does not
require a lock-in amplifier. Lock-ins are used when
there is noise at frequencies near the signal. Thus,
the dynamic reserve for noise within the operating
range is more important.

Dynamic reserve in the SR810

The SR810, with its digital phase sensitive
detectors, does not suffer from DC output errors
caused by large noise signals. The dynamic
reserve can be increased to above 100 dB without
measurement error. Large noise signals do not
cause output errors from the PSD. The large DC
gain does not result in increased output drift.

In fact, the only drawback to using ultra high
dynamic reserves (>60 dB) is the increased output
noise due to the noise of the A/D converter. This
increase in output noise is only present when the
dynamic reserve is above 60 dB AND set to High
Reserve or Normal. However, the Low Noise
reserve can be very high as we'll see shortly.

To set a scale, the SR810's output noise at 100
dB dynamic reserve is only measurable when the
signal input is grounded. Let's do a simple
experiment. If the lock-in reference is at 1 kHz and
a large signal is applied at 9.5 kHz, what will the
lock-in output be? If the signal is increased to the
dynamic reserve limit (100 dB greater than full
scale), the output will reflect the noise of the signal
at 1 kHz. The spectrum of any pure sine generator
always has a noise floor, i.e. there is some noise
at all frequencies. So even though the applied
signal is at 9.5 kHz, there will be noise at all other
frequencies, including the 1 kHz lock-in reference.
This noise will be detected by the lock-in and
appear as noise at the output. This output noise
will typically be greater than the SR810's own
output noise. In fact, virtually all signal sources will
have a noise floor which will dominate the lock-in
output noise. Of course, noise signals are
generally much noisier than pure sine generators
and will have much higher broadband noise floors.

If the noise does not reach the reserve limit, the
SR810's own output noise may become
detectable at ultra high reserves. In this case,
simply lower the dynamic reserve and the DC gain
will decrease and the output noise will decrease
also. In general, do not run with more reserve than

necessary. Certainly don't use High Reserve when
there is virtually no noise at all.

The frequency dependence of dynamic reserve is
inherent in the lock-in detection technique. The
SR810, by providing more low pass filter stages,
can increase the dynamic reserve close to the
reference frequency. The specified reserve
applies to noise signals within the operating range
of the lock-in, i.e. frequencies below 100 kHz. The
reserve at higher frequencies is actually higher but
is generally not that useful.

Minimum dynamic reserve (Low Noise)

The SR810 always has a minimum amount of
dynamic reserve. This minimum reserve is the
Low Noise reserve setting. The minimum reserve
changes with the sensitivity (gain) of the
instrument. At high gains (full scale sensitivity of
50 µV and below), the minimum dynamic reserve
increases from 37 dB at the same rate as the
sensitivity increases. For example, the minimum
reserve at 5 µV sensitivity is 57 dB. In many
analog lock-ins, the reserve can be lower. Why
can't the SR810 run with lower reserve at this
sensitivity?

The answer to this question is - Why would you
want lower reserve? In an analog lock-in, lower
reserve means less output error and drift. In the
SR810, more reserve does not increase the output
error or drift. More reserve can increase the output
noise though. However, if the analog signal gain
before the A/D converter is high enough, the
5 nV/√Hz noise of the signal input will be amplified
to a level greater than the input noise of the A/D
converter. At this point, the detected noise will
reflect the actual noise at the signal input and not
the A/D converter's noise. Increasing the analog
gain (decreasing the reserve) will not decrease the
output noise. Thus, there is no reason to decrease
the reserve. At a sensitivity of 5 µV, the analog
gain is sufficiently high so that A/D converter noise
is not a problem. Sensitivities below 5 µV do not
require any more gain since the signal to noise
ratio will not be improved (the front end noise
dominates). The SR810 does not increase the
gain below the 5 µV sensitivity, instead, the
minimum reserve increases. Of course, the input
gain can be decreased and the reserve increased,
in which case the A/D converter noise might be
detected in the absence of any signal input.

3-13

SR810 Basics

SIGNAL INPUT AMPLIFIER and FILTERS

A lock-in can measure signals as small as a few
nanovolts. A low noise signal amplifier is required
to boost the signal to a level where the A/D
converter can digitize the signal without degrading
the signal to noise. The analog gain in the SR810
ranges from roughly 7 to 1000. As discussed
previously, higher gains do not improve signal to
noise and are not necessary.

The overall gain (AC plus DC) is determined by
the sensitivity. The distribution of the gain (AC
versus DC) is set by the dynamic reserve.

Input noise

The input noise of the SR810 signal amplifier is
about 5 nVrms/√Hz. What does this noise figure
mean? Let's set up an experiment. If an amplifier
has 5 nVrms/√Hz of input noise and a gain of
1000, then the output will have 5 µVrms/√Hz of
noise. Suppose the amplifier output is low pass
filtered with a single RC filter (6 dB/oct roll off) with
a time constant of 100 ms. What will be the noise
at the filter output?

Amplifier input noise and Johnson noise of
resistors are Gaussian in nature. That is, the
amount of noise is proportional to the square root
of the bandwidth in which the noise is measured.
A single stage RC filter has an equivalent noise
bandwidth (ENBW) of 1/4T where T is the time
constant (RxC). This means that Gaussian noise
at the filter input is filtered with an effective
bandwidth equal to the ENBW. In this example,
the filter sees 5 µVrms/√Hz of noise at its input. It
has an ENBW of 1/(4x100ms) or 2.5 Hz. The
voltage noise at the filter output will be
5 µVrms/√Hz x √2.5Hz or 7.9 µVrms. For
Gaussian noise, the peak to peak noise is about 5
times the rms noise. Thus, the output will have
about 40 µV pk-pk of noise.

Input noise for a lock-in works the same way. For
sensitivities below about 5 µV full scale, the input
noise will determine the output noise (at minimum
reserve). The amount of noise at the output is
determined by the ENBW of the low pass filter.
See the discussion of noise later in this section for
more information on ENBW. The ENBW depends
upon the time constant and filter roll off. For
example, suppose the SR810 is set to 5 µV full
scale with a 100 ms time constant and 6 dB/oct of
filter roll off. The ENBW of a 100 ms, 6 dB/oct filter

is 2.5 Hz. The lock-in will measure the input noise
with an ENBW of 2.5 Hz. This translates to

7.9 nVrms at the input. At the output, this
represents about 0.16 % of full scale (7.9 nV/5
µV). The peak to peak noise will be about 0.8 % of
full scale.

All of this assumes that the signal input is being
driven from a low impedance source. Remember
resistors have Johnson noise equal to

0.13x√R nVrms/√Hz. Even a 50Ω resistor has
almost 1 nVrms/√Hz of noise! A signal source
impedance of 2 kΩ will have a Johnson noise
greater than the SR810's input noise. To
determine the overall noise of multiple noise
sources, take the square root of the sum of the
squares of the individual noise figures. For
example, if a 2 kΩ source impedance is used, the
Johnson noise will be 5.8 nVrms/√Hz. The overall
noise at the SR810 input will be [52 + 5.82]

7.7 nVrms/√Hz.

We'll talk more about noise sources later in this
section.

At lower gains (sensitivities above 50 µV), there is
not enough gain at high reserve to amplify the
input noise to a level greater than the noise of the
A/D converter. In these cases, the output noise is
determined by the A/D noise. Fortunately, at these
sensitivities, the DC gain is low and the noise at
the output is negligible.

Notch filters

The SR810 has two notch filters in the signal
amplifier chain. These are pre-tuned to the line
frequency (50 or 60 Hz) and twice the line
frequency (100 or 120 Hz). In circumstances
where the largest noise signals are at the power
line frequencies, these filters can be engaged to
remove noise signals at these frequencies.
Removing the largest noise signals before the final
gain stage can reduce the amount of dynamic
reserve required to perform a measurement. To
the extent that these filters reduce the required
reserve to either 60 dB or the minimum reserve
(whichever is higher), then some improvement
might be gained. If the required reserve without
these notch filters is below 60 dB or if the
minimum reserve is sufficient, then these filters do
not significantly improve the measurement.

½

or

3-14

SR810 Basics

Using either of these filters precludes making
measurements in the vicinity of the notch
frequencies. These filters have a finite range of
attenuation, generally 10 Hz or so. Thus, if the
lock-in is making measurements at 70 Hz, do not
use the 60 Hz notch filter! The signal will be
attenuated and the measurement will be in error.
When measuring phase shifts, these filters can
affect phase measurements up to an octave away.

Anti-aliasing filter

After all of the signal filtering and amplification,
there is an anti-aliasing filter. This filter is required
by the signal digitization process. According to the
Nyquist criterion, signals must be sampled at a
frequency at least twice the highest signal
frequency. In this case, the highest signal
frequency is 100 kHz and the sampling frequency
is 256 kHz so things are ok. However, no signals
above 128 kHz can be allowed to reach the A/D
converter. These signals would violate the Nyquist
criterion and be undersampled. The result of this
undersampling is to make these higher frequency
signals appear as lower frequencies in the digital
data stream. Thus a signal at 175 kHz would

appear below 100 kHz in the digital data stream
and be detectable by the digital PSD. This would
be a problem.

To avoid this undersampling, the analog signal is
filtered to remove any signals above 154 kHz
(when sampling at 256 kHz, signals above
154 kHz will appear below 102 kHz). This filter has
a flat pass band from DC to 102 kHz so as not to
affect measurements in the operating range of the
lock-in. The filter rolls off from 102 kHz to 154 kHz
and achieves an attenuation above 154 kHz of at
least 100 dB. Amplitude variations and phase
shifts due to this filter are calibrated out at the
factory and do not affect measurements. This filter
is transparent to the user.

Input Impedance

The input impedance of the SR810 is 10 MΩ. If a
higher input impedance is desired, then the
SR550 remote preamplifier must be used. The
SR550 has an input impedance of 100 MΩ and is
AC coupled from 1 Hz to 100 kHz.

3-15

SR810 Basics

INPUT CONNECTIONS

In order to achieve the best accuracy for a given
measurement, care must be taken to minimize the
various noise sources which can be found in the
laboratory. With intrinsic noise (Johnson noise, 1/f
noise or input noise), the experiment or detector
must be designed with these noise sources in
mind. These noise sources are present regardless
of the input connections. The effect of noise
sources in the laboratory (such as motors, signal
generators, etc.) and the problem of differential
grounds between the detector and the lock-in can
be minimized by careful input connections.

There are two basic methods for connecting a
voltage signal to the lock-in - the single-ended
connection is more convenient while the
differential connection eliminates spurious pick-up
more effectively.

Single-Ended Voltage Connection (A)

In the first method, the lock-in uses the A input in a
single-ended mode. The lock-in detects the signal
as the voltage between the center and outer
conductors of the A input only. The lock-in does
not force the shield of the A cable to ground,
rather it is internally connected to the lock-in's
ground via a resistor. The value of this resistor is
selected by the user. Float uses 10 kΩ and
Ground uses 10 Ω. This avoids ground loop
problems between the experiment and the lock-in
due to differing ground potentials. The lock-in lets
the shield 'quasi-float' in order to sense the
experiment ground. However, noise pickup on the
shield will appear as noise to the lock-in. This is
bad since the lock-in cannot reject this noise.
Common mode noise, which appears on both the
center and shield, is rejected by the 100 dB
CMRR of the lock-in input, but noise on only the
shield is not rejected at all.

Experiment

Signal
Source

Grounds may be at different potentials

SR810 Lock-In

A

+

-

R

Differential Voltage Connection (A-B)

The second method of connection is the
differential mode. The lock-in measures the
voltage difference between the center conductors
of the A and B inputs. Both of the signal
connections are shielded from spurious pick-up.
Noise pickup on the shields does not translate into
signal noise since the shields are ignored.

When using two cables, it is important that both
cables travel the same path between the
experiment and the lock-in. Specifically, there
should not be a large loop area enclosed by the
two cables. Large loop areas are susceptible to
magnetic pickup.

Experiment

Signal
Source

Loop

Area

Grounds may be at different potentials

A

B

Common Mode Signals

Common mode signals are those signals which
appear equally on both center and shield (A) or
both A and B (A−B). With either connection
scheme, it is important to minimize both the
common mode noise and the common mode
signal. Notice that the signal source is held near
ground potential in both illustrations above. If the
signal source floats at a nonzero potential, the
signal which appears on both the A and B inputs
will not be perfectly cancelled. The common mode
rejection ratio (CMRR) specifies the degree of
cancellation. For low frequencies, the CMRR of
100 dB indicates that the common mode signal is
canceled to 1 part in 10
100 dB, a 100 mV common mode signal behaves
like a 1 µV differential signal! This is especially
bad if the common mode signal is at the reference
frequency (this happens a lot due to ground
loops). The CMRR decreases by about 6
dB/octave (20 dB/decade) starting at around
1 kHz.

5

. Even with a CMRR of

SR810 Lock-In

+

-

R

3-16

SR810 Basics

Current Input (I)

The current input on the SR810 uses the A input
BNC. The current input has a 1 kΩ input
impedance and a current gain of either 106 or

8

10

Volts/Amp. Currents from 1 µA down to 2 fA

full scale can be measured.

The impedance of the signal source is the most
important factor to consider in deciding between
voltage and current measurements.

For high source impedances, greater than 1 MΩ

6

gain) or 100 MΩ (108 gain), and small

(10
currents, use the current input. Its relatively low
impedance greatly reduces the amplitude and
phase errors caused by the cable capacitance­source impedance time constant. The cable
capacitance should still be kept small to minimize
the high frequency noise gain of the current
preamplifier.

For moderate to low source impedances, or larger
currents, the voltage input is preferred. A small
value resistor may be used to shunt the signal
current and generate a voltage signal. The lock-in
then measures the voltage across the shunt
resistor. Select the resistor value to keep the
shunt voltage small (so it does not affect the
source current) while providing enough signal for
the lock-in to measure.

Which current gain should you use? The current
gain determines the input current noise of the
lock-in as well as its measurement bandwidth.
Signals far above the input bandwidth are
attenuated by 6 dB/oct. The noise and bandwidth
are listed below.

Gain

10
10

Noise Bandwidth

6

130 fA/√Hz 70 kHz

8

13 fA/√Hz 700 Hz

AC vs DC Coupling

The signal input can be either AC or DC coupled.
The AC coupling high pass filter passes signals
above 160 mHz (0.16 Hz) and attenuates signals
at lower frequencies. AC coupling should be used
at frequencies above 160 mHz whenever possible.
At lower frequencies, DC coupling is required.

A DC signal, if not removed by the AC coupling
filter, will multiply with the reference sine wave and
produce an output at the reference frequency.
This signal is not normally present and needs to
be removed by the low pass filter. If the DC
component of the signal is large, then this output
will be large and require a long time constant to
remove. AC coupling removes the DC component
of the signal without any sacrifice in signal as long
as the frequency is above 160 mHz.

The current input current to voltage preamplifier is
always DC coupled. AC coupling can be selected
following the current preamplifier to remove any
DC current signal.

3-17

SR810 Basics

INTRINSIC (RANDOM) NOISE SOURCES

Random noise finds its way into experiments in a
variety of ways. Good experimental design can
reduce these noise sources and improve the
measurement stability and accuracy.

There are a variety of intrinsic noise sources
which are present in all electronic signals. These
sources are physical in origin.

Johnson noise

Every resistor generates a noise voltage across its
terminals due to thermal fluctuations in the
electron density within the resistor itself. These
fluctuations give rise to an open-circuit noise
voltage,

(rms) = (4kTR∆f)½

V

where k=Boltzmann's constant (1.38x10
is the temperature in °Kelvin (typically 300°K), R is
the resistance in Ohms, and ∆f is the bandwidth in
Hz. ∆f is the bandwidth of the measurement.

Since the input signal amplifier in the SR810 has a
bandwidth of approximately 300 kHz, the effective
noise at the amplifier input is Vnoise =
70√R nVrms or 350√R nV pk-pk. This noise is
broadband and if the source impedance of the
signal is large, can determine the amount of
dynamic reserve required.

The amount of noise measured by the lock-in is
determined by the measurement bandwidth.
Remember, the lock-in does not narrow its
detection bandwidth until after the phase sensitive
detectors. In a lock-in, the equivalent noise
bandwidth (ENBW) of the low pass filter (time
constant) sets the detection bandwidth. In this
case, the measured noise of a resistor at the lock­in input, typically the source impedance of the
signal, is simply

The ENBW is determined by the time constant and
slope as shown in the following table. Wait time is
the time required to reach 99 % of its final value.

T= Time Constant

noise

(rms) = 0.13√R√ENBW nV

V

noise

-23

J/°K), T

Slope

ENBW Wait Time
6 dB/oct 1/(4T) 5T
12 dB/oct 1/(8T) 7T
18 dB/oct 3/(32T) 9T
24 dB/oct 5/(64T) 10T

The signal amplifier bandwidth determines the
amount of broadband noise that will be amplified.
This affects the dynamic reserve. The time
constant sets the amount of noise which will be
measured at the reference frequency. See the
SIGNAL INPUT AMPLIFIER discussion for more
information about Johnson noise.

Shot noise

Electric current has noise due to the finite nature
of the charge carriers. There is always some non­uniformity in the electron flow which generates
noise in the current. This noise is called shot
noise. This can appear as voltage noise when
current is passed through a resistor, or as noise in
a current measurement. The shot noise or current
noise is given by

(rms) = (2ql∆f)½

I

noise

where q is the electron charge (1.6x10
Coulomb), I is the RMS AC current or DC current
depending upon the circuit, and ∆f is the
bandwidth.

When the current input of a lock-in is used to
measure an AC signal current, the bandwidth is
typically so small that shot noise is not important.

1/f noise

Every 10 Ω resistor, no matter what it is made of,
has the same Johnson noise. However, there is
excess noise in addition to Johnson noise which
arises from fluctuations in resistance due to the
current flowing through the resistor. For carbon
composition resistors, this is typically 0.1 µV-3 µV
of rms noise per Volt of applied across the
resistor. Metal film and wire-wound resistors have
about 10 times less noise. This noise has a 1/f
spectrum and makes measurements at low
frequencies more difficult.

Other sources of 1/f noise include noise found in
vacuum tubes and semiconductors.

-19

3-18

SR810 Basics

Total noise

All of these noise sources are incoherent. The
total random noise is the square root of the sum of
the squares of all the incoherent noise sources.

3-19

SR810 Basics

EXTERNAL NOISE SOURCES

In addition to the intrinsic noise sources discussed
in the previously, there are a variety of external
noise sources within the laboratory.

Most of these noise sources are asynchronous,
i.e. they are not related to the reference and do
not occur at the reference frequency or its
harmonics. Examples include lighting fixtures,
motors, cooling units, radios, computer screens,
etc. These noise sources affect the measurement
by increasing the required dynamic reserve or
lengthening the time constant.

Some noise sources, however, are related to the
reference and, if picked up in the signal, will add or
subtract from the actual signal and cause errors in
the measurement. Typical sources of synchronous
noise are ground loops between the experiment,
detector and lock-in, and electronic pick up from
the reference oscillator or experimental apparatus.

Many of these noise sources can be minimized
with good laboratory practice and experiment
design. There are several ways in which noise
sources are coupled into the signal path.

Capacitive coupling

An AC voltage from a nearby piece of apparatus
can couple to a detector via a stray capacitance.
Although Cstray may be very small, the coupled
noise may still be larger than a weak experimental
signal. This is especially damaging if the coupled
noise is synchronous (at the reference frequency).

Stray Capacitance

Experiment

Detector

We can estimate the noise current caused by a
stray capacitance by,

i= C

dV/dt = ωC

stray

strayVnoise

where w is 2π times the noise frequency, Vnoise is
the noise amplitude, and Cstray is the stray
capacitance.

Noise

Source

For example, if the noise source is a power circuit,
then f = 60 Hz and V

= 120 V. C

noise

estimated using a parallel plate equivalent
capacitor. If the capacitance is roughly an area of
1 cm2 at a separated by 10 cm, then C

0.009 pF. The resulting noise current will be
400 pA (at 60 Hz). This small noise current can be
thousands of times larger than the signal current.
If the noise source is at a higher frequency, the
coupled noise will be even greater.

If the noise source is at the reference frequency,
then the problem is much worse. The lock-in
rejects noise at other frequencies, but pick-up at
the reference frequency appears as signal!

Cures for capacitive noise coupling include:

1) Removing or turning off the noise source.

2) Keeping the noise source far from the
experiment (reducing C
the signal cables close to the noise
source.

3) Designing the experiment to measure
voltages with low impedance (noise
current generates very little voltage).

4) Installing capacitive shielding by placing
both the experiment and detector in a
metal box.

Inductive coupling

An AC current in a nearby piece of apparatus can
couple to the experiment via a magnetic field. A
changing current in a nearby circuit gives rise to a
changing magnetic field which induces an emf

/dt) in the loop connecting the detector to the

(dØ

B

experiment. This is like a transformer with the
experiment-detector loop as the secondary
winding.

B(t)

Experiment

Detector

can be

stray

). Do not bring

stray

Noise

Source

stray

is

3-20

SR810 Basics

Cures for inductively coupled noise include:

1) Removing or turning off the interfering
noise source.

2) Reduce the area of the pick-up loop by
using twisted pairs or coaxial cables, or
even twisting the 2 coaxial cables used in
differential connections.

3) Using magnetic shielding to prevent the
magnetic field from crossing the area of
the experiment.

4) Measuring currents, not voltages, from
high impedance detectors.

Resistive coupling or ground loops

Currents flowing through the ground connections
can give rise to noise voltages. This is especially a
problem with reference frequency ground currents.

Experiment

Noise Source

In this illustration, the detector is measuring the
signal relative to a ground far from the rest of the
experiment. The experiment senses the detector
signal plus the voltage due to the noise source's
ground return current passing through the finite
resistance of the ground between the experiment
and the detector. The detector and the experiment
are grounded at different places which, in this
case, are at different potentials.

Cures for ground loop problems include:

1) Grounding everything to the same
physical point.

2) Using a heavy ground bus to reduce the
resistance of ground connections.

3) Removing sources of large ground
currents from the ground bus used for
small signals.

Detector

I(t)

Microphonics

Not all sources of noise are electrical in origin.
Mechanical noise can be translated into electrical
noise by microphonic effects. Physical changes in
the experiment or cables (due to vibrations for
example) can result in electrical noise over the
entire frequency range of the lock-in.

For example, consider a coaxial cable connecting
a detector to a lock-in. The capacitance of the
cable is a function of its geometry. Mechanical
vibrations in the cable translate into a capacitance
that varies in time, typically at the vibration
frequency. Since the cable is governed by Q=CV,
taking the derivative, we have

C dV/dt + V dC/dt = dQ/dt = i

Mechanical vibrations in the cable which cause a
dC/dt will give rise to a current in the cable. This
current affects the detector and the measured
signal.

Some ways to minimize microphonic signals are:

1) Eliminate mechanical vibrations near the
experiment.

2) Tie down cables carrying sensitive signals
so they do not move.

3) Use a low noise cable that is designed to
reduce microphonic effects.

Thermocouple effects

The emf created by junctions between dissimilar
metals can give rise to many microvolts of slowly
varying potentials. This source of noise is typically
at very low frequency since the temperature of the
detector and experiment generally changes
slowly. This effect is large on the scale of many
detector outputs and can be a problem for low
frequency measurements, especially in the mHz
range.

Some ways to minimize thermocouple effects are:

1) Hold the temperature of the experiment or
detector constant.

2) Use a compensation junction, i.e. a
second junction in reverse polarity which
generates an emf to cancel the thermal

3-21

SR810 Basics

potential of the first junction. This second
junction should be held at the same

temperature as the first junction.

3-22

SR810 Basics

NOISE MEASUREMENTS

Lock-in amplifiers can be used to measure noise.
Noise measurements are generally used to
characterize components and detectors.

The SR810 measures input signal noise AT the
reference frequency. Many noise sources have a
frequency dependence which the lock-in can
measure.

How does a lock-in measure noise?

Remember that the lock-in detects signals close to
the reference frequency. How close? Input signals
within the detection bandwidth set by the low pass
filter time constant and roll-off appear at the output
at a frequency f=f
appears as noise at the output with a bandwidth of
DC to the detection bandwidth.

For Gaussian noise, the equivalent noise
bandwidth (ENBW) of a low pass filter is the
bandwidth of the perfect rectangular filter which
passes the same amount of noise as the real filter.

The ENBW is determined by the time constant and
slope as shown below. Wait time is the time
required to reach 99 % of its final value.

T= Time Constant

Slope
6 dB/oct 1/(4T) 5T
12 dB/oct 1/(8T) 7T
18 dB/oct 3/(32T) 9T
24 dB/oct 5/(64T) 10T

ENBW Wait Time

. Input noise near fref

sig-fref

Noise estimation

The noise is simply the standard deviation (root of
the mean of the squared deviations)of the
measured X, Y or R .

The above technique, while mathematically sound,
can not provide a real time output or an analog
output proportional to the measured noise. For
these measurements, the SR810 estimates the X
or Y noise directly.

To display the noise of X, for example, simply set
the CH1 display to X noise. The quantity X noise is

computed from the measured values of X using
the following algorithm. The moving average of X
is computed. This is the mean value of X over
some past history. The present mean value of X is
subtracted from the present value of X to find the
deviation of X from the mean. Finally, the moving
average of the absolute value of the deviations is
calculated. This calculation is called the mean
average deviation or MAD. This is not the same as
an RMS calculation. However, if the noise is
Gaussian in nature, then the RMS noise and the
MAD noise are related by a constant factor.

The SR810 uses the MAD method to estimate the
RMS noise of X and Y. The advantage of this
technique is its numerical simplicity and speed.

The noise calculations for X and Y occur at
512 Hz. At each sample, the mean and moving
average of the absolute value of the deviations is
calculated. The averaging time (for the mean and
average deviation) depends upon the time
constant. The averaging time is selected by the
SR810 and ranges from 10 to 80 times the time
constant. Shorter averaging times yield a very
poor estimate of the noise (the mean varies
rapidly and the deviations are not averaged well).
Longer averaging times, while yielding better
results, take a long time to settle to a steady
answer.

To change the settling time, change the time
constant. Remember, shorter settling times use
smaller time constants (higher noise bandwidths)
and yield noisier noise estimates.

X and Y noise are displayed in units of
Volts/√Hz. The ENBW of the time constant is
already factored into the calculation. Thus, the

mean displayed value of the noise should not
depend upon the time constant.

The SR810 performs the noise calculations all of
the time, whether or not X or Y noise are being
displayed. Thus, as soon as X noise is displayed,
the value shown is up to date and no settling time
is required. If the sensitivity is changed, then the
noise estimate will need to settle to the correct
value.

3-23

Front Panel

TIME CONSTANT

6 dB

OVLD

12 dB
18 dB

ks

x100

24 dB

s

3x1x10

ms

SYNC

1

µ

s

< 200 Hz

Slope
/Oct

Sync
Filter

SIGNAL INPUT

OVLD

A
A - B

6

)

FLOAT

I (10

DCACGROUND

I (108)

Input Couple Ground

A/I B

10MΩ/25pF 10MΩ/25pF

Signal Inputs

SENSITIVITY

OVLD

x100

5

x10

2

x1

1

RESERVE

HIGH RESERVE
NORMAL
LOW NOISE

Reserve

FILTERS

LINE
2 x LINE

Notch

CH1 Display

STANFORD RESEARCH SYSTEMS

OVLD

V

µ

A

AUTO

mV

nA

SYNC

µ

V

pA

nV

fA

Offset Ratio

X
R
X noise
AUX IN 1

AUX IN 1

AUX IN 2

AUX IN 2

Display Ratio Expand Output

OFFSET

On/Off Auto Modify

Model SR810 DSP Lock-In Amplifier

%

µ

AV

mVnA

µ

VpA
nVfA
pVaA

Expand

CHANNEL ONE

x10
x100

DISPLAY
X

OUTPUT

<20mA 1M

Analog Output Ref Input Sine Output

Ref Display

TRIG

AxOut1

AxOut2

PHASE

FREQ

Phase Freq Ampl

+90˚ –90˚ Harm #

ZERO

AxOut3 AxOut4

HARM#

AMPL

SINE
POS EDGE
NEG EDGE

Trig

REF IN

REFERENCE

Ω

Offst%

kHz

Hz

DEG

V

AUTO

Phase Gain

Reserve

SETUP

Save Recall

Aux Out

INTERFACE

ERROR

ACTIVE
SRQ
REMOTE

Local Setup

GPIB/RS232
ADDRESS
BAUD
PARITY
QUEUE

UNLOCK

INTERNAL

Source

SINE OUT

50

Ω

Power The power switch is on the rear panel. The SR810 is turned on by

pushing switch up. The serial number (5 digits) and the firmware version
are shown in the displays at power on.

A series of internal tests are performed at this point.

DATA Performs a read/write test to the processor RAM.

BATT The nonvolatile backup memory is tested. Instrument settings are stored

in nonvolatile memory and are retained when the power is turned off.

PROG Checks the processor ROM.

DSP Checks the digital signal processor (DSP).

rCAL If the backup memory check passes, then the instrument returns to the

settings in effect when the power was last turned off (User). If there is a
memory error, then the stored settings are lost and the standard (Std)
settings are used.

Reset To reset the unit, hold down the [Setup] key while the power is turned on.

The unit will use the standard settings. The standard setup is listed on
the next page.

[Keys] The keys are grouped and labeled according to function. This manual

will refer to a key with brackets such as [Key]. A complete description of
the keys follows in this section.

Knob The knob is used to adjust parameters in the Reference display. The

parameters which may be adjusted are internal reference frequency,

4-1

Front Panel

reference phase shift, sine output amplitude, harmonic detect number,
offsets, Aux Output levels, and various Setup parameters.

Local Lockout If the computer interface has placed the unit in the REMOTE state,

indicated by the REMOTE led, then the keys and the knob are disabled.
Attempts to change the settings from the front panel will display the
message 'LOCL LOut' indicating local control is locked out by the
interface.

Reference Input The reference input can be a sine wave (rising zero crossing detected)

or a TTL pulse or square wave (rising or falling edge). The input
impedance is 1 MΩ AC coupled (>1 Hz) for the sine input. For low
frequencies (<1 Hz), it is necessary to use a TTL reference signal. The
TTL input provides the best overall performance and should be used
whenever possible.

Sine Out The internal oscillator output has a 50 Ω output impedance and varies in

amplitude from 4 mVrms to 5 Vrms. The output level is specified into a
high impedance load. If the output is terminated in a low impedance,
such as 50 Ω the amplitude will be less than the programmed amplitude
(half for a 50 Ω load).

This output is active even when an external reference is used. In this

case, the sine wave is phase locked to the reference and its amplitude is
programmable.

A TTL sync output is provided on the rear panel. This output is useful for

triggering scopes and other equipment at the reference frequency. The
TTL sync output is a square wave derived from the zero crossings of the
sine output.

CH1 Output The Channel 1 output can be configured to output a voltage from -10 V

to +10 V proportional to X or the CH1 Display. ±10 V is full scale. The
outputs can source 10 mA maximum.

Signal Inputs The input mode may be single-ended, A, or differential, A-B. The A and

B inputs are voltage inputs with 10 MΩ, 25 pF input impedance. Their
connector shields are isolated from the chassis by 10 Ω (Ground) or
1 kΩ (Float). Do not apply more than 50 V to either input. The shields
should never exceed 1 V. The I (current) input is 1 kΩ to a virtual ground.

Key Click On/Off Press the [Phase] and [Harm#] keys together to toggle the key click on

and off.

Front Panel Display Test To test the front panel displays, press the [Phase] and [Freq] keys

together. All of the LED's will turn on. Press [Phase] to decrease the
number of on LED's to half on, a single LED and no LED's on. Use the
knob to move the turned on LED's across the panel. Press [Freq] to
increase the number of on LED's. Make sure that every LED can be
turned on. Press any other key to exit this test mode.

Display Off Operation To operate with the front panel displays off, press [Phase] and [Freq]

together to enter the front panel test mode. Press [Phase] to decrease

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the number of on LED's until all of the LED's are off. The SR810 is still
operating, the output voltages are updated and the unit responds to
interface commands. To change a setting, press any key other than
[Phase] or [Freq] to return to normal operation, change the desired
parameter, then press [Phase] and [Freq] together to return to the test
mode. Turn the LED's all off with the [Phase] key.

Keypad Test To test the keypad, press the [Phase] and [Ampl] keys together. The

displays will read 'Pad code' and a number of LED indicators will be
turned on. The LED's indicate which keys have not been pressed yet.
Press all of the keys on the front panel, one at a time. As each key is
pressed, the key code is displayed in the Reference display, and nearest
indicator LED turns off. When all of the keys have been pressed, the
display will return to normal. To return to normal operation without
pressing all of the keys, simply turn the knob.

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STANDARD SETTINGS

If the [Setup] key is held down when the power is turned on, the lock-in settings will be set to the defaults
shown below rather than the settings that were in effect when the power was last turned off. The default
settings may also be recalled using the *RST command over the computer interface. In this case, the
communications parameters and status registers are not changed.

REFERENCE / PHASE

Phase 0.000°
Reference Source Internal
Harmonic # 1
Sine Amplitude 1.000 Vrms
Internal Frequency 1.000 kHz
Ext Reference Trigger Sine

INPUT / FILTERS

Source A
Grounding Float
Coupling AC
Line Notches Out

GAIN / TC

Sensitivity 1 V
Reserve Low Noise
Time Constant 100 ms
Filter dB/oct. 12 dB
Synchronous Off

DISPLAY

CH1 X
Ratio None
Reference Frequency

OUTPUT / OFFSET

CH1 Output X
All Offsets 0.00%
All Expands 1

AUX OUTPUTS

All Output Voltages 0.000 V

SETUP

Output To GPIB
GPIB Address 8
RS-232 Baud Rate 9600
Parity None
Key Click On
Alarms On

DATA STORAGE

Sample Rate 1 Hz
Scan Mode Loop
Trigger Starts No

STATUS ENABLE

REGISTERS Cleared

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Signal Input and Filters

[Input] The [Input] key selects the front end signal input configuration. The input

amplifier can be either a single-ended (A) or differential (A-B) voltage or
a current (I).

The voltage inputs have a 10 MΩ, 25 pF input impedance. Their

connector shields are isolated from the chassis by either 10 Ω (Ground)
or 10 kΩ (Float). Do not apply more than 50 V to either input. The

shields should never exceed 1 V.

The current input uses the A connector. The input is 1 kΩ to a virtual

ground. The largest allowable DC current before overload is 10 µA (1 M
gain) or 100 nA (100 M gain). No current larger than 10 mA should ever
be applied to this input.

The current gain determines the input current noise as well as the input

bandwidth. The 100 MΩ gain has 10 times lower noise but 100 times
lower bandwidth. Make sure that the signal frequency is below the input
bandwidth. The noise and bandwidth are listed below.

Gain
1M 130 fA/√Hz 70 kHz
100M 13 fA/√Hz 700 Hz

The impedance of the current source should be greater than 1 MΩ when

using the 1M gain or 100 MΩ when using the 100M gain.

Changing the current gain does not change the instrument sensitivity.

Sensitivities above 10 nA require a current gain of 1 MΩ. Sensitivities
between 20 nA and 1 µA automatically select the 1 MΩ current gain. At
sensitivities below 20 nA, changing the sensitivity does not change the
current gain.

The message 'IGAn chG' is displayed to indicate that the current gain

has been changed to 1 MΩ as a result of changing the sensitivity.

Noise Bandwidth

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INPUT OVLD The OVLD led in this section indicates an INPUT overload. This occurs

for voltage inputs greater than 1.4Vpk (unless removed by AC coupling)
or current inputs greater than 10 µA DC or 1.4 µA AC (1MΩ gain) or
100 nA DC or 14 nA AC (100MΩ gain). Reduce the input signal level.

[Couple] This key selects the input coupling. The signal input can be either AC or

DC coupled. The current input is coupled after the current to voltage
conversion. The current input itself is always DC coupled (1 kΩ to virtual
ground).

The AC coupling high pass filter passes signals above 160 mHz and

attenuates signals at lower frequencies. AC coupling should be used at
frequencies above 160 mHz whenever possible. At lower frequencies,
DC coupling is required. AC coupling results in gain and phase errors at
low frequencies.

Remember, the Reference Input is AC coupled when a sine

reference is used. This also results in phase errors at low
frequencies.

[Ground] This key chooses the shield grounding configuration. The shields of the

input connectors (A and B) are not connected directly to the lock-in
chassis ground. In Float mode, the shields are connected by 10 kΩ to
the chassis ground. In Ground mode, the shields are connected by
10 Ω to ground. Typically, the shields should be grounded if the signal
source is floating and floating if the signal source is grounded. Do not

exceed 1 V on the shields.

[Notch] This key selects no line notch filters, the line frequency or twice line

frequency notch, or both filters. The line notch filters are pre-tuned to the
line frequency (50 or 60 Hz) and twice the line frequency (100 or 120
Hz).

These filters have an attenuation depth of at least 30 dB. These filters

have a finite range of attenuation, generally 10 Hz or so. If the reference
frequency is 70 Hz, do not use the 60 Hz notch filter! The signal will be
attenuated and the phase shifted. See the SR810 Basics section for a
discussion of when these filters improve a measurement.

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Sensitivity, Reserve and Time Constants

[Sensitivity Up/Dn] The [Sensitivity Up] and [Sensitivity Down] keys select the full scale

sensitivity. The sensitivity is indicated by 1-2-5 times 1, 10 or 100 with
the appropriate units.

The full scale sensitivity can range from 2 nV to 1 V (rms) or 2 fA to 1 µA

(rms). The sensitivity indication is not changed by the X, Y, or R output
expand. The expand functions increase the output scale as well as the
display resolution.

Changing the sensitivity may change the dynamic reserve. Sensitivity

takes precedence over dynamic reserve. See the next page for more
details.

Auto Gain
Pressing the [AUTO GAIN] key will automatically adjust the sensitivity

based upon the detected signal magnitude (R). Auto Gain may take a
long time if the time constant is very long. If the time constant is greater
than 1 second, Auto Gain will abort.

RESERVE OVLD The OVLD led in the Sensitivity section indicates that the signal amplifier

is overloaded. Change the sensitivity or increase the dynamic reserve.

[Reserve] This key selects the reserve mode, either Low Noise, Normal or High

Reserve. The actual reserve (in dB) depends upon the sensitivity. When
the reserve is High, the SR810 automatically selects the maximum
reserve available at the present full scale sensitivity. When the reserve is
Low, the minimum available reserve is selected. Normal is between the
maximum and minimum reserve. Changing the sensitivity may change
the actual reserve, NOT the reserve mode.

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The actual dynamic reserves (in dB) for each sensitivity are listed below.

Sensitivity
1 V 0 0 0
500 mV 6 6 6
200 mV 4 14 14
100 mV 0 10 20
50 mV 6 16 26
20 mV 4 24 34
10 mV 0 20 40
5 mV 6 26 46
2 mV 4 34 54
1 mV 10 40 60
500 µV 16 46 66
200 µV 24 54 74
100 µV 30 60 80
50 µV 36 66 86
20 µV 44 74 94
10 µV 50 80 100
5 µV 56 86 106
2 µV 64 94 114
1 µV 70 100 120
500 nV 76 106 126
200 nV 84 114 134
100 nV 90 120 140
50 nV 96 126 146
20 nV 104 134 154
10 nV 110 140 160
5 nV 116 146 166
2 nV 124 154 174

Do not use ultra high dynamic reserves above 120 dB unless absolutely

necessary. It will be very likely that the noise floor of any interfering
signal will obscure the signal at the reference and make detection
difficult if not impossible. See the SR810 Basics section for more
information.

Auto Reserve
Pressing [AUTO RESERVE] will change the reserve mode to the

minimum reserve required. Auto Reserve will not work if there are low
frequency noise sources which overload infrequently.

[Time Constant Up/Dn] This key selects the time constant. The time constant may be set from

10 µs to 30 s (detection freq.>200 Hz) or 30 ks (detection freq. <200 Hz).
The detection frequency is the reference frequency times the harmonic
detect number. The time constant is indicated by 1 or 3 times 1, 10 or
100 with the appropriate units.

The maximum time constant is 30 s if the detection frequency is above

200 Hz and 30 ks if the detection frequency is below 200 Hz. The actual
range switches at 203.12 Hz when the frequency is increasing and at

199.21 Hz when the frequency is decreasing. The time constant may not
be adjusted beyond the maximum for the present detection frequency. If
the detection frequency is below 200 Hz and 100 s is the time constant

Low Noise Normal High Reserve

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and the frequency increases above 200 Hz, the time constant WILL
change to 30 s. Decreasing the frequency back below 200 Hz will NOT
change the time constant back to 100 s.

The absolute minimum time constant is 10 µs. The actual minimum time

constant depends upon the filter slope and the DC gain in the low pass
filter (dynamic reserve plus expand). The minimum time constant is only
restricted if the dynamic reserve plus expand is high and the filter slope
is low - not a normal operating situation. The tables below list the
minimum time constants for the different filter slopes and gains.

6 dB/oct DC gain (dB)
<45 10 µs
<55 30 µs
<65 100 µs
<75 300 µs
<85 1 ms
<95 3 ms
<105 10 ms
<115 30 ms
<125 100 ms
<135 300 ms
<145 1 s
<155 3 s
<165 10 s
<175 30 s

12 dB/oct DC gain (dB)
<55 10 µs
<75 30 µs
<95 100 µs
<115 300 µs
<135 1 ms
<155 3 ms
<175 10 ms

18 dB/oct DC gain (dB)
<62 10 µs
<92 30 µs
<122 100 µs
<152 300 µs
<182 1 ms

24 dB/oct DC gain (dB
<72 10 µs
<112 30 µs
<152 100 µs
<182 300 µs

To use these tables, choose the correct table for the filter slope in use.

Calculate the DC gain by adding the reserve to the expand (expressed in
dB). Find the smallest DC gain entry which is larger than the gain in use.
Read the minimum time constant for this entry. For example, if the slope
is 12 dB/oct, the reserve is 64 dB, and the X expand is 10 (20 dB), then
the DC gain is 84 dB and the min time constant is 100 µs.

min time constant

min time constant

min time constant

) min time constant

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Time constant is a low priority parameter. If the sensitivity, dynamic

reserve, filter slope, or expand is changed, and the present time constant
is below the new minimum, the time constant WILL change to the new
minimum. Remember, changing the sensitivity may change the reserve
and thus change the time constant.

The message 'tc chnG' will be displayed to indicate that the time

constant has been changed, either by increasing the detection frequency
above 200 Hz, or by changing the sensitivity, dynamic reserve, filter
slope, or expand.

The time constant also determines the equivalent noise bandwidth

(ENBW) of the low pass filter. This is the measurement bandwidth for X
and Y noise and depends upon the time constant and filter slope. (See
the Noise discussion in the SR810 Basics section.)

FILTER OVLD The OVLD led in the Time Constant section indicates that the low pass

filters have overloaded. Increase the time constant or filter roll-off, or
decrease the dynamic reserve.

Analog Outputs with Short Time Constants

When using short time constants below 10 ms, the X and Y analog

outputs from the rear panel or the CH1 output configured to output X
should be used. These outputs have a 100 kHz bandwidth and are
accurate even with short time constants. The CH1 output proportional to
the Display (even if X is displayed) is updated at a 512 Hz rate. This
output does not accurately reflect high frequency outputs.

[Slope /oct] This key selects the low pass filter slope (number of poles). Each pole

contributes 6 dB/oct of roll off. Using a higher slope can decrease the
required time constant and make a measurement faster. The filter slope
affects the minimum time constant (see above). Changing the slope may
change the time constant if the present time constant is shorter than the
minimum time constant at the new filter slope.

[Sync Filter] Pressing this key selects no synchronous filtering or synchronous

filtering on below 200 Hz. In the second case, the synchronous filter is
switched on whenever the detection frequency decreases below 199.21
Hz and switched off when the detection frequency increases above

203.12 Hz. The detection frequency is the reference frequency times the
harmonic detect number. The SYNC indicator in the CH1 display is
turned on whenever synchronous filtering is active.

When the synchronous filter is on, the phase sensitive detectors (PSD's)

are followed by 2 poles of low pass filtering, the synchronous filter, then
2 more poles of low pass filtering. The low pass filters are set by the time
constant and filter slope. If the filter slope requires less then 4 poles
(<24 dB/oct), then the unused poles are set to a minimum time constant.
The poles which are set by the time constant are the ones closest to the
PSD's. For example, if the time constant is 100 ms with 12 dB/oct slope
and synchronous filtering is on, then the PSD's are followed by two poles
of low pass filtering with 100 ms time constant, the synchronous filter,
then two poles of minimum time constant.

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Synchronous filtering removes outputs at harmonics of the reference

frequency, most commonly 2xf. This is very effective at low reference
frequencies since 2xf outputs would require very long time constants to
remove. The synchronous filter does NOT attenuate broadband noise
(except at the harmonic frequencies). The low pass filters remove
outputs due to noise and interfering signals. See the SR810 Basics
section for a discussion of time constants and filtering.

Note:

The synchronous filter averages the outputs over a complete period.

Each period is divided into 128 equal time slots. At each slot, the
average over the previous 128 slots is computed and output. This results
in an output rate of 128xf. This output is then smoothed by the two poles
of filtering which follow the synchronous filter.

The settling time of the synchronous filter is one period of the detection

frequency. If the amplitude, frequency, phase, time constant or slope is
changed, then the outputs will settle for one period. These transients are
because the synchronous filter provides a steady output only if the input
is repetitive from period to period. The transient response also depends
upon the time constants of the regular filters. Very short time constants
(<<period) have little effect on the transient response. Longer time
constants (<period) can magnify the amplitude of a transient. Much
longer time constants (≥period) will increase the settling time far beyond
a period.

Use of the synchronous filter results in a reduction in amplitude

resolution.

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CH1 Display and Output

[Display] This key selects the Channel 1 display quantity. Channel 1 may display

X, R, X Noise, Aux Input 1 or Aux Input 2. The numeric display has the
units of the input signal. The bar graph is ±full scale sensitivity for X, R
and X Noise, and ±10V for the Aux Inputs. Ratio displays are shown in %
and the bar graph is scaled to ±100%. See the SR810 Basics section for
a complete discussion of scaling.

OUTPUT OVLD The OVLD led in the display indicates that the Channel 1 output is

overloaded (greater than 1.09 times full scale). This can occur if the
sensitivity is too low or if the output is expanded such that the output
voltage would exceed 10V.

AUTO This indicator is turned while an auto function is in progress.

SYNC When the synchronous output filter is selected AND the detection

frequency is below 200 Hz, then the SYNC indicator will be on. If the
detection frequency is above 200 Hz, synchronous filtering is not active
and SYNC is off.

[Ratio] This key selects ratio measurements on Channel 1. The Channel 1

display may show X, R, X Noise, Aux Input 1 or Aux Input 2 divided by

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Aux Input 1 or 2. The denominator is indicated by the AUX IN leds above
this key. The Ratio indicator in the display is on to indicate a ratio
measurement. Pressing this key until the AUX IN leds and the Ratio
indicator are off returns the measurement to non-ratio mode.

[Output] This key selects the CH1 OUTPUT source. The Channel 1 Output can

provide an analog output proportional to the Display or X. The output
proportional to X has a bandwidth of 100 kHz (the output is updated at
256 kHz). This output is the traditional X output of a lock-in. Output
proportional to the display (even if the display is simply X) has a
bandwidth of 200 Hz (updated at 512 Hz).

Remember, The X output has 100 kHz of bandwidth. The Display output

should only be used if the time constant is sufficiently long such that
there are no high frequency outputs.

CH1 Offset and Expand The X and R outputs may be offset and expanded separately. Choose

either X or R with the [Display] key to adjust the X or R offset and
expand.

X and R analog outputs are determined by

Output = (signal/sensitivity - offset) x Expand x 10 V

The output is normally 10 V for a full scale signal. The offset subtracts a

percentage of full scale from the output. Expand multiplies the remainder
by a factor from 1, 10 or 100.

Output offsets ARE reflected in displays which depend upon X or R.

X and Y offsets do NOT affect the calculation of R and θ.

Output expands do NOT increase the displayed values of X or R.

Expand increases the display resolution.

If the display is showing a quantity which is affected by an offset or a

non-unity expand, then the Offset and Expand indicators are turned on
below the display.

See the SR810 Basics section for a complete discussion of scaling,

offsets and expands.

[Offset On/Off] Pressing this key turns the X or R offset (as selected by the [Display]

key) on or off. The Offset indicator below the display turns on when the
displayed quantity is offset. This key allows the offset to be turned on
and off without adjusting the actual offset percentage.

[Modify] This key displays the X or R offset percentage (as selected by the

[Display] key) in the Reference Display. Use the knob to adjust the
offset. The Channel 1 display reflects the offset as it is adjusted while the
Reference display shows the actual offset percentage. The offset ranges
from -105.00 % to 105.00 % of full scale. The offset percentage does
not change with sensitivity - it is an output function. To return the

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Reference Display to its original display, press the desired reference
display key ([Phase], [Freq], [Ampl], [Harm #] or [Aux Out]).

[Auto Offset] Pressing this key automatically sets the X or R offset percentage to

offset the selected output quantity to zero.

[Expand] Pressing this key selects the X and R Expand. Use the [Display] key to

select either X or R. The expand can be 1 (no expand), 10 or 100. If the
expand is 10 or 100, the Expand indicator below the display will turn on.
The output can never exceed full scale when expanded. For example, if
an output is 10 % of full scale, the largest expand (with no offset) which
does not overload is 10. An output expanded beyond full scale will be
overloaded.

Short Time Constant Limitations

A short time constant places a limit on the total amount of DC gain

(reserve plus expand) available. If the time constant is short, the filter
slope low and the dynamic reserve high, then increasing the expand may
change the time constant. See the table of time constants and DC gains
in the Gain and Time Constant section.

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Reference

UNLOCK The UNLOCK indicator turns on if the SR810 can not lock to the external

reference.

TRIG The TRIG indicator flashes whenever a trigger is received at the rear

panel trigger input AND internal data storage is triggered.

[Phase] Pressing this key displays the reference phase shift in the Reference

display. The knob may be used to adjust the phase. The phase shift
ranges from -180° to +180° with 0.01° resolution.

When using an external reference, the reference phase shift is the phase

between the external reference and the digital sine wave which is
multiplying the signal in the PSD. This is also the phase between the
sine output and the digital sine wave used by the PSD in either internal
or external reference mode. Changing this phase shift only shifts internal
sine waves. The effect of this phase shift can only be seen at the lock-in
outputs X, Y and θ. R is phase independent.

Auto Phase

Pressing [AUTO PHASE] will adjust the reference phase shift so that the

measured signal phase is 0°. This is done by subtracting the present

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measured value of θ from the reference phase shift. It will take several
time constants for the outputs to reach their new values. Auto Phase
may not result in a zero phase if the measurement is noisy or changing.
If θ is not stable, Auto Phase will abort.

[+90°] and [-90°] The [+90°] and [-90°] keys add or subtract 90.000° from the reference

phase shift. The phase does not need to be displayed to use these keys.

Zero Phase
Pressing the [+90°] and [-90°] keys together will set the reference phase

shift to 0.00°.

[Freq] Pressing this key displays the reference frequency in the Reference

display.

If the reference mode is external, then the measured reference

frequency is displayed. The knob does nothing in this case. If the
harmonic number is greater than 1 and the external reference goes
above 102 kHz/N where N is the harmonic number, then the harmonic
number is reset to 1. The reference will always track the external
reference signal.

If the reference mode is internal, then the internal oscillator frequency is

displayed. The oscillator frequency may adjusted with the knob. The
frequency has 41/2 digits or 0.1 mHz resolution, whichever is larger. The
frequency can range from 0.001 Hz to 102.00 kHz. The upper limit is

decreased if the harmonic number is greater than 1. In this case,
the upper limit is 102 kHz/N where N is the harmonic number.

[Ampl] Pressing this key displays the Sine Output Amplitude in the Reference

display. Use the knob to adjust the amplitude from 4 mVrms to 5 Vrms
with 2 mV resolution. The output impedance of the Sine Out is 50 Ω. If
the signal is terminated in 50 Ω, the amplitude will be half of the
programmed value.

When the reference mode is internal, this is the excitation source

provided by the SR810. When an external reference is used, this sine
output provides a sine wave phase locked to the external reference.

The rear panel TTL Output provides a TTL square wave at the reference

frequency. This square wave is generated by discriminating the zero
crossings of the sine output. This signal can provide a trigger or sync
signal to the experiment when the internal reference source is used. This
signal is also available when the reference is externally provided. In this
case, the TTL Output is phase locked to the external reference.

[Harm #] The SR810 can detect signals at harmonics of the reference frequency.

The SR810 multiplies the input signal with digital sine waves at a multiple
of the reference. Only signals at this harmonic will be detected. Signals
at the original reference frequency are not detected and are attenuated
as if they were noise.

Whenever the harmonic detect number is greater than 1, the

HARM# indicator in the Reference display will flash to remind you

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that the SR810 is detecting signals at a multiple of the reference
frequency.

Always check the harmonic detect number before making any

measurements.

If the harmonic number is set to N, then the internal reference

frequency is limited to 102 kHz/N.

If an external reference is used and the reference frequency

exceeds 102 kHz/N, then N is reset to 1. The SR810 will always track
the external reference.

Pressing this key displays the harmonic number in the Reference

display. The harmonic number may be adjusted using the knob.
Harmonics up to 19999 times the reference can be detected as long as
the harmonic frequency does not exceed 102 kHz. An attempt to
increase the harmonic frequency above 102 kHz will display the
message 'hAr ovEr' indicating harmonic number over range.

[Source] This key selects the reference mode. The normal mode is External

reference (no indicator). The Internal mode is indicated by the
INTERNAL led.

When the reference source is External, the SR810 will phase lock to the

external reference provided at the Reference Input BNC. The SR810 will
lock to frequencies between 0.001 Hz and 102.0 kHz. Use the [Freq]
key to display the external frequency.

When the reference source is Internal, the SR810's synthesized internal

reference is used as the reference. The Reference Input BNC is ignored
in this case. In this mode, the Sine Out or TTL Sync Out provides the
excitation for the measurement. Use the [Freq] key to display and adjust
the frequency.

[Trig] This key selects the external reference input trigger mode.

When either POS EDGE or NEG EDGE is selected, the SR810 locks to

the selected edge of a TTL square wave or pulse train. For reliable
operation, the TTL signal should exceed 3.5 V when high and be less
then 0.5 V when low. The input is directed past the analog discriminator
and is DC coupled into a TTL input gate. This input mode should be
used whenever possible since it is less noise prone than the sine wave
discriminator.

For very low frequencies (<1 Hz), a TTL reference MUST be used.

SINE input mode locks the SR810 to the rising zero crossings of an

analog signal at the Reference Input BNC. This signal should be a clean
sine wave at least 200 mVpk in amplitude. In this input mode, the
Reference Input is AC coupled (above 1 Hz) with an input impedance of
1 MΩ.

Sine reference mode can not be used at frequencies far below 1 Hz.

At very low frequencies, the TTL input modes must be used.

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Auto Functions

Pressing an Auto Function key initiates an auto function which may take

some time. The AUTO led in the CH1 display will be on while the
function is in progress. A multi-tone sound will indicate when the auto
function is complete and the AUTO leds will turn off.

[Auto Reserve] Pressing [AUTO RESERVE] will adjust the dynamic reserve to the

minimum reserve required. To do this, the reserve is decreased until the
analog input amplifier is overloaded. The reserve is then increased to
remove the overload.

Auto Reserve will work only if the overloading noise source has a

frequency greater than a few Hz. Lower frequency noise sources may
overload so infrequently that Auto Reserve can not detect it.

[AUTO RESERVE] does not change the notch prefilter settings.

[Auto Gain] [AUTO GAIN] will adjust the sensitivity so that the detected signal

magnitude is a sizable percentage of full scale. Many time constants are
required to determine whether a particular sensitivity will overload or not.
Auto Gain thus takes a longer time when the time constant is long.

Auto Gain will not run if the time constant is greater than 1 second since

the total time required could be far too long to be useful.

The message 'tc ovEr' will be displayed to indicate that the time constant

is too long for Auto Gain to run.

[Auto Phase] [AUTO PHASE] adjusts the reference phase shift so that the measured

signal phase is 0°. This is done by subtracting the measured value of q
from the programmed reference phase shift. It will take several time
constants for the outputs to reach their new values during which time q
will move towards 0°. Do not press [AUTO PHASE] again until the
outputs have stabilized. When the measurement is noisy or if the outputs
are changing, Auto Phase may not result in a zero phase.

Auto Phase will not run if the value of q is unstable.

The message 'PhAS bAd' will be displayed to indicate that the phase is

unstable and Auto Phase will not run.

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Auto Setup There is no truly reliable way to automatically setup a lock-in amplifier for

all possible input signals. In most cases, the following procedure should
setup the SR810 to measure the input signal.

1. Press [AUTO GAIN] to set the sensitivity.

2. Press [AUTO RESERVE].

3. Adjust the time constant and roll-off until there is no Time Constant
overload.

4. Press [AUTO PHASE] if desired.

5. Repeat if necessary.

At very low frequencies, the auto functions may not function properly.

This is because very low frequency signals overload very infrequently
and the time constants used tend to be very long.

4-19

Front Panel

Setup

[Save] Nine amplifier setups may be stored in non-volatile memory.To save a

setup, press [Save] to display the buffer number (1..9) in the Reference
display. Use the knob to select the desired buffer number. Press [Save]
again to store the setup in the buffer, or any other key to abort the save
process.

The message 'SAvE n donE' is displayed if the setup is successfully

saved. The message 'SAve not donE' is displayed if the save process is
aborted.

[Recall] Nine amplifier setups may be stored in non-volatile memory.To recall a

setup, press [Recall] to display the buffer number (1..9) in the Reference
display. Use the knob to select the desired buffer number. Press [Recall]
again to recall the setup in the buffer, or any other key to abort the
recall process. When a setup is recalled, any data presently in the data
buffer is lost.

The message 'rcal n donE' is displayed if the setup is successfully

recalled. The message 'rcal not donE' is displayed if the recall process is
aborted. The message 'rcal dAtA Err' is displayed if the recalled setup is
not valid. This is usually because a setup has never been saved into the
selected buffer.

[Aux Out] The 4 Aux Outputs may be programmed from the front panel. Press

[Aux Out] until the desired output (1-4) is displayed in the Reference
display. The AxOut indicators below the display indicate which output (1-

4) is displayed. The knob may then be used to adjust the output level
from -10.5 V to +10.5 V. Press [Phase], [Freq], [Ampl] or [Harm#] to
return the display to normal.

4-20

Front Panel

Interface

[Setup] Pressing the [Setup] key cycles through GPIB/RS-232, ADDRESS,

BAUD, PARITY and QUEUE. In each case, the appropriate parameter is
displayed in the Reference display and the knob is used for adjustment.
Press [Phase], [Freq], [Ampl], [Harm#] or [Aux Out] to return the display
to normal and leave Setup.

GPIB/SR-232 The SR810 only outputs data to one interface at a time. Commands may

be received over both interfaces but responses are directed only to the
selected interface. Make sure that the selected interface is set correctly
before attempting to program the SR810 from a computer. The first
command sent by any program should be to set the output to the correct
interface.

Setup GPIB/RS-232 displays the output interface. Use the knob to select

GPIB or RS-232.

ADDRESS Setup ADDRESS displays the GPIB address. Use the knob to select an

address from 0 to 30.

BAUD Setup BAUD displays the RS-232 baud rate. Use the knob to adjust the

baud rate from 300 to 19200 baud.

PARITY Setup PARITY displays the RS-232 parity. Use the knob to select Even,

Odd or None.

QUEUE The last 256 characters received by the SR810 may be displayed to help

find programming errors. Setup QUEUE will display 4 characters (2 per
display) in hexadecimal (see below). Turn the knob left to move farther
back in the buffer, turn the knob right to move towards the most recently
received characters. A '.' is displayed to indicate the ends of the buffer.
All characters are changed to upper case, spaces are removed, and
command delimiters are changed to linefeeds (0A).

To leave this display, press [Setup] to return to GPIB/RS-232 before

pressing [Phase], [Freq], [Ampl], [Harm#] or [Aux Out] to return the
display to normal and leave Setup.

4-21

Front Panel

Hex
2A * 34 4
2B + 35 5
2C , 36 6
2D - 37 7
2E . 38 8
30 0 39 9
31 1 3B ;
32 2 3F ?
33 3

Hex
0A linefeed 50 P
41 A 51 Q
42 B 52 R
43 C 53 S
44 D 54 T
45 E 55 U
46 F 56 V
47 G 57 W
48 H 58 X
49 I 59 Y
4A J 5A Z
4B K
4C L
4D M
4E N
4F O

[Local] When a host computer places the unit in the REMOTE state, no keypad

REMOTE This led is on when the front panel is locked out by a computer interface.

SRQ This indicator is on whenever a GPIB Service Request is generated by

ACTIVE This indicator flashes when there is activity on the computer interface.

ERROR Flashes whenever there is a computer interface error such as an illegal

ASCII Hex ASCII

ASCII Hex ASCII

input or knob adjustment is allowed. The REMOTE indicator is on above
the [Local] key. To return to front panel operation, press the [Local] key.

No front panel adjustments may be made.

the SR810. SRQ stays on until a serial poll is completed.

command or out of range parameter is received.

4-22

Front Panel

WARNING MESSAGES

The SR810 displays various warning messages whenever the operation of the instrument is not obvious. The
two tone warning alarm sounds when these messages are displayed.

Display

LOCL LOut LOCAL LOCKOUT If the computer interface has placed the unit in the REMOTE

IGAn chG IGAIN CHANGE Indicates that the current conversion gain has been changed to

tc chnG TC CHANGE Indicates that the time constant has been changed, either by

hAr ovEr HARMONIC OVER An attempt to increase the harmonic detect frequency above

tc ovEr TC OVER Indicates that the time constant is too long (>1s) for Auto Gain to

PhAS bAd PHASE BAD Indicates that the phase is unstable and Auto Phase will not run.

rcal Err RECALL ERR This message is displayed if the recalled setup is not valid. This

undr UNDR Indicates unit may not be precisely locked at very low frequency.

Warning Message Meaning

state, indicated by the REMOTE led, then the keys and the knob
are disabled. Attempts to change the settings from the front
panel will display this message.

1 MΩ as a result of changing the sensitivity. Sensitivities from
20 nA to 1 µA require 1 MΩ current gain.

increasing the detection frequency from below 200 Hz to above
200 Hz, or by changing the sensitivity, dynamic reserve, filter
slope, or expand.

102 kHz will display this message.

run.

is usually because a setup has never been saved into the
selected buffer.

4-23

Rear Panel

WARNING

!

NO USER SERVICEABLE PARTS INSIDE.
REFER TO USER MANUAL FOR SAFETY NOTICE.
FOR USE BY QUALIFIED PERSONNEL ONLY.

XY

AUX OUT

MONITOR OUT

AUX IN

12

34

PREAMP

FUSE

PULL

1A @100/120V
1/2A @ 220/240V

SRS

MADE IN U.S.A.

TRIG IN TTL OUT

12

34

RS232 (DCE)

IEEE-488 STD PORT

Power Entry Module The power entry module is used to fuse the AC line voltage input, select

the line voltage, and block high frequency noise from entering or exiting
the instrument. Refer to the first page of this manual for instructions on
selecting the correct line voltage and fuse.

IEEE-488 Connector The 24 pin IEEE-488 connector allows a computer to control the SR810

via the IEEE-488 (GPIB) instrument bus. The address of the instrument
is set with the [Setup] key.

RS-232 Connector The RS-232 interface connector is configured as a DCE (transmit on pin

3, receive on pin 2). The baud rate and parity are programmed with the
[Setup] key. To connect the SR810 to a PC serial adapter, which is
usually a DTE, use a straight thru serial cable.

AUX IN 1-4 (A/D Inputs) These are auxiliary analog inputs which can be digitized by the SR810.

The range is -10.5 V to +10.5 V and the resolution is 16 bits (1/3 mV).
The input impedance is 1 MΩ.

The AUX 1 and 2 inputs may be displayed on the CH1 display. These

inputs allow signals other than the lock-in outputs to be acquired (and
stored). Furthermore, ratio quantities such as X/Aux1 may be displayed
(and stored).

AUX OUT 1-4 (D/A Outputs) These are auxiliary analog outputs. The range is -10.5 V to +10.5 V and

the resolution is 1 mV. The output impedance is <1 Ω and the output
current is limited to 10 mA.

These outputs may be programmed from the front panel ([Aux Out])or

via the computer interfaces.

X and Y The X and Y lock-in outputs are always available at these connectors.

The bandwidth of these outputs is 100 kHz. A full scale input signal will
generate ±10V at these outputs. The output impedance is <1 Ω and the
output current is limited to 10 mA.

These outputs are affected by the X and Y offsets and expands. The

actual outputs are

4-24

Rear Panel

X Output = (X/sensitivity - offset)xExpandx10V
Y Output = (Y/sensitivity - offset)xExpandx10V

where the offset is a percentage of full scale and the expand is an

integer from 1, 10 or 100. The X offset and expand are set from the front
panel. The Y offset and expand may only be set from the interface.

Overloads on Y are not reported by the SR810.

MONITOR OUT This BNC provides a buffered output from the signal amplifiers and

prefilters. This is the signal just before the A/D converter and PSD. The
output impedance is <1 Ω and the output current is limited to 10 mA.

The gain from the signal input to the monitor output is the overall gain

minus the dynamic reserve minus 3 dB. The overall gain is 10 V divided
by the sensitivity. The actual dynamic reserve is specified in the
description of the [Reserve] key. For example, if the sensitivity is 10 mV,
the gain is 60 dB. If the dynamic reserve is 20 dB, then the gain from the
input to the monitor output is 60-20-3=37 dB or a gain of 71. A 10 mV
(rms) input will result in a .7 Vrms or1 Vpk output. The gain is only
accurate to about 1.5 dB or 20 %.

This output is useful for determining the cause of input overloads and the

effects of prefiltering. However, because the analog gain never exceeds
2000, very small signals may not be amplified enough to viewed at the
monitor output.

TRIG IN This TTL input may be used to trigger stored data samples and/or to

start data acquisition. If Trigger Start is selected, then a rising edge will
start data storage. If the sample rate is also Trigger, then samples are
recorded at every subsequent trigger. (The first trigger starts the scan
and takes the first data point, subsequent triggers record the rest of the
data points.) When the sample rate is set to Trigger, samples are
recorded whenever there is a rising edge at the Trigger input. The
maximum sample rate is 512 Hz. Data storage is available through the
computer interface only.

TTL OUT This output is the TTL sync output for the internal oscillator. The output is

a square wave whose edges are linked to the sine wave zero crossings.
This is useful when the sine output amplitude is small and a synchronous
trigger is required (to a scope for example). This output is active even
when locked to an external reference.

PREAMP CONNECTOR This 9 pin "D" connector provides power and control signals to external

preamplifiers such as the SR550 and SR552. The power connections
are described below.

Pin
1 +20V
2 +5V
6 -20V
7 Signal Ground
8 Ground

Voltage

4-25

Rear Panel

USING SRS PREAMPS When using either the SR550 or SR552, connect the power cable

(standard 9 pin D connectors) from the preamp to the rear panel preamp
connector on the SR810. Use BNC cables to connect the A output from
the preamp to the A input of the SR810. The B output from the preamp
(preamp ground) may be connected to the B input of the SR810. In this
case, use A-B as the input configuration. Be sure to twist the A and B
cables so that there is no differential noise pickup between the cables.

The SR550 and SR552 are AC coupled from 1 Hz to 100 kHz. Set the

SR810 to AC coupled since the signal must be above 1 Hz. The SR550
has an input impedance of 100 MΩ, the SR552 has 100 kΩ.

The SR810 does NOT compensate for the gain of the preamp. The

SR810 sets both preamps to their maximum gains. Measurements made
by the SR810 with a preamp need to be divided by the gain of the
preamp. The SR550 has a gain of 10 and the SR552 has a gain of 100.

4-26

Remote Programming

INTRODUCTION

The SR810 DSP Lock-in Amplifier may be
remotely programmed via either the RS-232 or
GPIB (IEEE-488) interfaces. Any computer
supporting one of these interfaces may be used to
program the SR810. Both interfaces are receiving
at all times, however, the SR810 will send

responses to only one interface. Specify the
output interface with the [Setup] key or use the
OUTX command at the beginning of every
program to direct the responses to the correct
interface.

COMMUNICATING WITH GPIB

The SR810 supports the IEEE-488.1 (1978)
interface standard. It also supports the required
common commands of the IEEE-488.2 (1987)
standard. Before attempting to communicate with
the SR810 over the GPIB interface, the SR810's
device address must be set. The address is set
with the [Setup] key and may be set between 1
and 30.

COMMUNICATING WITH RS-232

The SR810 is configured as a DCE ( transmit on
pin 3, receive on pin 2) device and supports
CTS/DTR hardware handshaking. The CTS signal
(pin 5) is an output indicating that the SR810 is
ready, while the DTR signal (pin 20) is an input
that is used to control the SR810's data
transmission. If desired, the handshake pins may
be ignored and a simple 3 wire interface (pins 2,3
and 7) may be used. The RS-232 interface baud
rate and parity must be set. These are set with the
[Setup] key. The RS-232 word length is always 8
bits.

STATUS INDICATORS AND QUEUES

To assist in programming, the SR810 has 4
interface status indicators. The ACTIVE indicator
flashes whenever a character is received or
transmitted over either interface. The ERROR
indicator flashes when an error, such as an illegal
command, or parameter out of range, has been
detected. The REMOTE indicator is on whenever
the SR810 is in a remote state (front panel locked
out). The SRQ indicator is on when the SR810
generates a service request. SRQ stays on until a
serial poll is completed.

To help find program errors, the SR810 can
display its receive buffer on the displays. Use the
[Setup] key to access the QUEUE display. The
last 256 characters received by the SR810 may be
displayed in hexadecimal ASCII. See the
OPERATION section for a complete description.

COMMAND SYNTAX

Communications with the SR810 uses ASCII
characters. Commands may be in either UPPER
or lower case and may contain any number of
embedded space characters. A command to the
SR810 consists of a four character command
mnemonic, arguments if necessary, and a
command terminator. The terminator must be a
linefeed <lf> or carriage return <cr> on RS-232, or
a linefeed <lf> or EOI on GPIB. No command
processing occurs until a command terminator is
received. Commands function identically on GPIB
and RS-232 whenever possible. Command
mnemonics beginning with an asterisk "*" are
IEEE-488.2 (1987) defined common commands.
These commands also function identically on RS-

232. Commands may require one or more
parameters. Multiple parameters are separated by
commas (,).

Multiple commands may be sent on one command
line by separating them with semicolons (;). The
difference between sending several commands on
the same line and sending several independent
commands is that when a command line is parsed
and executed, the entire line is executed before
any other device action proceeds.

There is no need to wait between commands. The
SR810 has a 256 character input buffer and
processes commands in the order received. If the
buffer fills up, the SR810 will hold off handshaking
on the GPIB and attempt to hold off handshaking
on RS-232. Similarly, the SR810 has a 256
character output buffer to store outputs until the
host computer is ready to receive. If either buffer
overflows, both buffers are cleared and an error
reported.

The present value of a particular parameter may
be determined by querying the SR810 for its
value. A query is formed by appending a question
mark "?" to the command mnemonic and omitting
the desired parameter(s) from the command.

5-1

Remote Programming

Values returned by the SR810 are sent as a string
of ASCII characters terminated by a carriage
return <cr> on RS-232 and by a line-feed <lf> on
GPIB. If multiple queries are sent on one
command line (separated by semicolons, of
course) the answers will be returned individually,
each with a terminator.

Examples of Command Formats

FMOD 1 <lf> Set reference source to

internal

FREQ 10E3 <lf> Set the internal reference

frequency to 10000 Hz
(10 kHz)

*IDN? <lf> Queries the device

identification
STRT <lf> Starts data acquisition
OUTP? 1 <lf> Queries the value of X

INTERFACE READY AND STATUS

The Interface Ready bit (bit 1) in the Serial Poll
Status Byte signals that the SR810 is ready to
receive and execute a command. When a
command is received, this bit is cleared indicating
that an operation is in progress. While the
operation is in progress, no other commands will
be processed. Commands received during this
time are stored in the buffer to be processed later.
Only GPIB serial polling will generate a response
while a command is in progress. When the
command execution terminates, the Interface

Ready bit is set again and new commands will be
processed. Since most commands execute very
quickly, the host computer does not need to
continually check the Interface Ready bit.
Commands may be sent one after another and
they will be processed immediately.

When using the GPIB interface, serial polling may
be used to check the Interface Ready bit in the
Serial Poll Byte while an operation is in progress.
After the Interface Ready bit becomes set,
signalling the completion of the command, then
the ERR or ESB bit may be checked to verify
successful completion of the command.

If the RS-232 interface is used, or serial polling is
not available, then the *STB?, *ESR?, ERRS?,
and LIAS? status query commands may be used
to query the Status Bytes. Since the SR810
processes one command at a time, the status
query will not be processed until the previous
operation is finished. Thus a response to the
status query in itself signals that the previous
command is finished. The query response may
then be checked for various errors.

GET (GROUP EXECUTE TRIGGER)

The GPIB interface command GET is the same as
the TRIG command. GET is the same as a trigger
input. GET only has an effect if the sampling rate
is triggered or if triggers start a scan.

5-2

Remote Programming

DETAILED COMMAND LIST

The four letter mnemonic in each command sequence specifies the command. The rest of the sequence
consists of parameters. Multiple parameters are separated by commas. Parameters shown in { } are optional
or may be queried while those not in { } are required. Commands that may be queried have a question mark
in parentheses (?) after the mnemonic. Commands that may ONLY be queried have a ? after the mnemonic.
Commands that MAY NOT be queried have no ?. Do not send ( ) or { } as part of the command.

The variables are defined as follows.
i, j, k, l, m integers
x, y, z real numbers
f frequency
s string

All numeric variables may be expressed in integer, floating point or exponential formats ( i.e., the number five
can be either 5, 5.0, or .5E1). Strings are sent as a sequence of ASCII characters.

Remember!
All responses are directed only to the selected output interface!

Use the OUTX command to select the correct interface at the beginning of every program.

5-3

Remote Programming

REFERENCE and PHASE COMMANDS

PHAS (?) {x} The PHAS command sets or queries the reference phase shift. The

parameter x is the phase (real number of degrees). The PHAS x
command will set the phase shift to x. The value of x will be rounded to

0.01°. The phase may be programmed from -360.00 ≤ x ≤ 729.99 and
will be wrapped around at ±180°. For example, the PHAS 541.0
command will set the phase to -179.00° (541-360=181=-179). The
PHAS? queries the phase shift.

FMOD (?) {i} The FMOD command sets or queries the reference source. The

parameter i selects internal (i=1) or external (i=0).

FREQ (?) {f} The FREQ command sets or queries the reference frequency. The

FREQ? query command will return the reference frequency (in internal or
external mode).

The FREQ f command sets the frequency of the internal oscillator. This

command is allowed only if the reference source is internal. The
parameter f is a frequency (real number of Hz). The value of f will be
rounded to 5 digits or 0.0001 Hz, whichever is greater. The value of f is
limited to 0.001 ≤ f ≤ 102000. If the harmonic number is greater than 1,
then the frequency is limited to nxf ≤ 102 kHz where n is the harmonic
number.

RSLP (?) {i} The RSLP command sets or queries the reference trigger when using

the external reference mode. The parameter i selects sine zero crossing
(i=0), TTL rising edge (i=1), or TTL falling edge (i=2). At frequencies
below 1 Hz, the TTL reference must be used.

HARM (?) {i} The HARM command sets or queries the detection harmonic. This

parameter is an integer from 1 to 19999. The HARM i command will set
the lock-in to detect at the ith harmonic of the reference frequency. The
value of i is limited by ixf ≤ 102 kHz. If the value of i requires a detection
frequency greater than 102 kHz, then the harmonic number will be set to
the largest value of i such that ixf ≤ 102 kHz.

SLVL (?) {x} The SLVL command sets or queries the amplitude of the sine output.

The parameter x is a voltage (real number of Volts). The value of x will
be rounded to 0.002V. The value of x is limited to 0.004 ≤ x ≤ 5.000.

5-4

Remote Programming

INPUT and FILTER COMMANDS

ISRC (?) {i} The ISRC command sets or queries the input configuration. The

parameter i selects A (i=0), A-B (i=1), I (1 MΩ) (i=2) or I (100 MΩ) (i=3).

Changing the current gain does not change the instrument sensitivity.

Sensitivities above 10 nA require a current gain of 1 MΩ. Sensitivities
between 20 nA and 1 µA automatically select the 1 MΩ current gain. At
sensitivities below 20 nA, changing the sensitivity does not change the
current gain.

IGND (?) {i} The IGND command sets or queries the input shield grounding. The

parameter i selects Float (i=0) or Ground (i=1).

ICPL (?) {i} The ICPL command sets or queries the input coupling. The parameter i

selects AC (i=0) or DC (i=1).

ILIN (?) {i} The ILIN command sets or queries the input line notch filter status. The

parameter i selects Out or no filters (i=0), Line notch in (i=1), 2xLine
notch in (i=2) or Both notch filters in (i=3).

5-5

Remote Programming

GAIN and TIME CONSTANT COMMANDS

SENS (?) {i} The SENS command sets or queries the sensitivity. The parameter i

selects a sensitivity below.

i
0 2 nV/fA 13 50 µV/pA
1 5 nV/fA 14 100 µV/pA
2 10 nV/fA 15 200 µV/pA
3 20 nV/fA 16 500 µV/pA
4 50 nV/fA 17 1 mV/nA
5 100 nV/fA 18 2 mV/nA
6 200 nV/fA 19 5 mV/nA
7 500 nV/fA 20 10 mV/nA
8 1 µV/pA 21 20 mV/nA
9 2 µV/pA 22 50 mV/nA
10 5 µV/pA 23 100 mV/nA
11 10 µV/pA 24 200 mV/nA
12 20 µV/pA 25 500 mV/nA
26 1 V/µA

RMOD (?) {i} The RMOD command sets or queries the reserve mode. The parameter i

OFLT (?) {i} The OFLT command sets or queries the time constant. The parameter i

i
0 10 µs 10 1 s
1 30 µs 11 3 s
2 100 µs 12 10 s
3 300 µs 13 30 s
4 1 ms 14 100 s
5 3 ms 15 300 s
6 10 ms 16 1 ks
7 30 ms 17 3 ks
8 100 ms 18 10 ks
9 300 ms 19 30 ks

Time constants greater than 30s may NOT be set if the

OFSL (?) {i} The OFSL command sets or queries the low pass filter slope. The

sensitivity i sensitivity

selects High Reserve (i=0), Normal (i=1) or Low Noise (minimum) (i=2).
See the description of the [Reserve] key for the actual reserves for each
sensitivity.

selects a time constant below.

time constant i time constant

harmonic x ref. frequency (detection frequency) exceeds 200 Hz. Time
constants shorter than the minimum time constant (based upon the filter
slope and dynamic reserve) will set the time constant to the minimum
allowed time constant. See the Gain and TIme Constant operation
section.

parameter i selects 6 dB/oct (i=0), 12 dB/oct (i=1), 18 dB/oct (i=2) or
24 dB/oct (i=3).

5-6

Remote Programming

SYNC (?) {i} The SYNC command sets or queries the synchronous filter status. The

parameter i selects Off (i=0) or synchronous filtering below 200 Hz (i=1).
Synchronous filtering is turned on only if the detection frequency
(reference x harmonic number) is less than 200 Hz.

5-7

Remote Programming

DISPLAY and OUTPUT COMMANDS

DDEF (?) { j, k} The DDEF command selects the CH1 display. The DDEF j, k command

sets the CH1 display to parameter j with ratio k as listed below.

CH1 (i=1)
j
0 X
1 R
2 X Noise
3 Aux In 1
4 Aux In 2

k
0 none
1 Aux In 1
2 Aux In 2

The DDEF? command queries the display and ratio of the display. The

display

ratio

returned string contains both j and k separated by a comma. For
example, if the DDEF? command returns "1,0" then the CH1 display is R
with no ratio.

FPOP (?) {j} The FPOP command sets or queries the CH1 front panel output source.

The FPOP j command sets the output to quantity j where j is listed
below.

CH1 (i=1)
j
0 CH 1 Display
1 X

output quantity

OEXP (?) i {, x, j} The OEXP command sets or queries the output offsets and expands.

The parameter i selects X (i=1), Y (i=2) or R (i=3) and is required. The
parameter x is the offset in percent (-105.00 ≤ x ≤ 105.00). The
parameter j selects no expand (j=0), expand by 10 (j=1) or 100 (j=2). The
OEXP i, x, j command will set the offset and expand for quantity i. This
command requires BOTH x and j. The OEXP? i command queries the
offset and expand of quantity i. The returned string contains both the
offset and expand separated by a comma. For example, if the OEXP? 2
command returns "50.00,1" then the Y offset is 50.00 % and the Y
expand is 10.

Setting an offset to zero turns the offset off. Querying an offset which is

off will return 0% for the offset value.

The Y offset and expand may only be set using the OEXP command.

The Yoffset and expand only affect the rear panel Y output. Overloads
on Y are not reported by the SR810.

AOFF i The AOFF i command automatically offsets X (i=1), Y (i=2) or R (i=3) to

zero. The parameter i is required. This command is equivalent to
pressing the [Auto Offset] keys.

5-8

Remote Programming

AUX INPUT and OUTPUT COMMANDS

OAUX? i The OAUX? command queries the Aux Input values. The parameter i

selects an Aux Input (1, 2, 3 or 4) and is required. The Aux Input
voltages are returned as ASCII strings with units of Volts. The resolution
is 1/3 mV. This command is a query only command.

AUXV (?) i {, x} The AUXV command sets or queries the Aux Output voltage when the

output. The parameter i selects an Aux Output (1, 2, 3 or 4) and is
required. The parameter x is the output voltage (real number of Volts)
and is limited to -10.500 ≤ x ≤ 10.500. The output voltage will be set to
the nearest mV.

5-9

Remote Programming

SETUP COMMANDS

OUTX (?) {i} The OUTX command sets the output interface to RS-232 (i=0) or GPIB

(i=1). The OUTX i command should be sent before any query
commands to direct the responses to the interface in use.

OVRM i In general, every GPIB interface command will put the SR810 into the

REMOTE state with the front panel deactivated. To defeat this feature,
use the OVRM 1 command to override the GPIB remote. In this mode,
the front panel is not locked out when the unit is in the REMOTE state.
The OVRM 0 command returns the unit to normal remote operation.

KCLK (?) {i} The KCLK command sets or queries the key click On (i=1) or Off (i=0)

state.

ALRM (?) {i} The ALRM command sets or queries the alarm On (i=1) or Off (i=0)

state.

SSET i The SSET i command saves the lock-in setup in setting buffer i (1≤i≤9).

The setting buffers are retained when the power is turned off.

RSET i The RSET i command recalls the lock-in setup from setting buffer i

(1≤i≤9). Interface parameters are not changed when a setting buffer is
recalled with the RSET command. If setting i has not been saved prior to
the RSET i command, then an error will result.

5-10

Remote Programming

AUTO FUNCTIONS

AGAN The AGAN command performs the Auto Gain function. This command is

the same as pressing the [Auto Gain] key. Auto Gain may take some
time if the time constant is long. AGAN does nothing if the time constant
is greater than 1 second. Check the command execution in progress bit
in the Serial Poll Status Byte (bit 1) to determine when the function is
finished.

ARSV The ARSV command performs the Auto Reserve function. This

command is the same as pressing the [Auto Reserve] key. Auto Reserve
may take some time. Check the command execution in progress bit in
the Serial Poll Status Byte (bit 1) to determine when the function is
finished.

APHS The APHS command performs the Auto Phase function. This command

is the same as pressing the [Auto Phase] key. The outputs will take
many time constants to reach their new values. Do not send the APHS
command again without waiting the appropriate amount of time. If the
phase is unstable, then APHS will do nothing. Query the new value of
the phase shift to see if APHS changed the phase shift.

AOFF i The AOFF i command automatically offsets X (i=1), Y (i=2) or R (i=3) to

zero. The parameter i is required. This command is equivalent to
pressing the [Auto Offset] keys.

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Remote Programming

DATA STORAGE COMMANDS

Data Storage

The SR810 can store up to 8191 points from the Channel 1 display in an internal data buffer. The data buffer
is NOT retained when the power is turned off. The data buffer is accessible only via the computer interface.

Configure the display to show the desired quantity (with appropriate ratio, offset and expand). The data buffer
stores the quantity which is displayed. Only the quantity which is displayed on the CH1 display can be stored.
Frequency, for example, can not be stored.

Data Points and Bins

Data points stored in the buffer are sometimes referred to by their bin position within the buffer. The oldest
data point is bin0, the next point is bin1, etc. A buffer with N points numbers them from 0 to N-1.

Sample Rate

The Sample Rate can be varied from 512 Hz down to 62.5 mHz (1 point every 16 sec). The sample rate sets
how often points are added to the storage buffer. Both displays are sampled at the same rate (and at the
same times).

In addition to the internal sample rates, samples can be triggered by an external TTL trigger. In this mode, a
sample is recorded within 2 ms of a rising edge trigger on the rear panel Trigger input. Triggers which occur
faster than 512 Hz are ignored.

Storage Time

The buffer holds 8191 samples taken at the sample rate. The entire storage time is 8191 divided by the
sample rate.

End of Scan

When the buffer becomes full, data storage can stop or continue.

The first case is called 1 Shot (data points are stored for a single buffer length). At the end of the buffer, data
storage stops and an audio alarm sounds.

The second case is called Loop. In this case, data storage continues at the end of the buffer. The data buffer
will store 8191 points and start storing at the beginning again. The most recent 8191 points will be contained
in the buffer. Once the buffer has looped around, the oldest point (at any time) is at bin#0 and the most recent
point is at bin#8190.

The default mode is Loop.

Starting and Stopping a Scan

The STRT, PAUS and REST commands are used to control data storage. Basically, the STRT command
starts data storage after a reset or pause. The PAUS command pauses data storage but does not reset the
buffer. The REST stops data storage and resets the buffer data.

In addition, the rear panel Trigger input can be used to start data storage. To select this mode, use the TSTR
command. In this mode, a rising TTL trigger will act the same as the STRT command. The sample rate can
be either internal or Triggered. In the first case, the trigger starts the storage and data is sampled at the
programmed sample rate (up to 512 Hz). In the latter case, the first trigger will start the storage and data will
be sampled at every subsequent trigger.

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Remote Programming

Aliasing Effects

In any sampled data stream, it is possible to sample a high frequency signal such that it will appear to be a
much lower frequency. This is called aliasing.

For example, suppose the lock-in is detecting a signal near 1 Hz with a relatively short time constant. The X
output will have a DC component and a 2 Hz component (2xf). If the sample rate is 2 Hz, then the samples
may be taken as illustrated below.

The samples represent a sine wave much slower than 2 Hz that isn't actually present in the output! In this
case, a much higher sampling rate will solve the problem.

Aliasing occurs whenever the output signal being sampled contains signals at frequencies greater
than ½ the sample rate. The effect is most noticeable when trying to sample an output frequency at an

integer multiple of the sample rate (as above). The above aliasing problem will be the same for a 1 kHz
output (500 times the sample rate) as for the 2 Hz output.

Generally, the highest possible sample rate should be used given the desired storage time. The lock-in time
constant and filter slope should be chosen to attenuate signals at frequencies higher than ½ the sample rate
as much as possible.

SRAT (?) {i} The SRAT command sets or queries the data sample rate. The

parameter i selects the sample rate listed below.

i
0 62.5 mHz 7 8 Hz
1 125 mHz 8 16 Hz
2 250 mHz 9 32 Hz
3 500 mHz 10 64 Hz
4 1 Hz 11 128 Hz
5 2 Hz 12 256 Hz
6 4 Hz 13 512 Hz
14 Trigger

quantity i quantity

SEND (?) {i} The SEND command sets or queries the end of buffer mode. The

parameter i selects 1 Shot (i=0) or Loop (i=1). If Loop mode is used,
make sure to pause data storage before reading the data to avoid
confusion about which point is the most recent.

TRIG The TRIG command is the software trigger command. This command

has the same effect as a trigger at the rear panel trigger input.

TSTR (?) {i} The TSTR command sets or queries the trigger start mode. The

parameter i=1 selects trigger starts the scan and i=0 turns the trigger
start feature off.

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Remote Programming

STRT The STRT command starts or resumes data storage. STRT is ignored if

storage is already in progress.

PAUS The PAUS command pauses data storage. If storage is already paused

or reset then this command is ignored.

REST The REST command resets the data buffers. The REST command can

be sent at any time - any storage in progress, paused or not, will be
reset. This command will erase the data buffer.

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Remote Programming

DATA TRANSFER COMMANDS

OUTP ? i The OUTP? i command reads the value of X, Y, R or θ. The parameter

i selects X (i=1), Y (i=2), R (i=3) or q (i=4). Values are returned as ASCII
floating point numbers with units of Volts or degrees. For example, the
response might be "-1.01026". This command is a query only command.

OUTR ? The OUTR? command reads the value of the CH1 display. Values are

returned as ASCII floating point numbers with units of the display. For
example, the response might be "-1.01026". This command is a query
only command.

SNAP ? i,j {,k,l,m,n} The SNAP? command records the values of either 2, 3, 4, 5 or 6

parameters at a single instant. For example, SNAP? is a way to query
values of X and Y (or R and θ) which are taken at the same time. This is
important when the time constant is very short. Using the OUTP? or
OUTR? commands will result in time delays, which may be greater than
the time constant, between reading X and Y (or R and θ).

The SNAP? command requires at least two parameters and at most six

parameters. The parameters i, j, k, l, m, n select the parameters below.

i,j,k,l,m,n
1 X
2 Y
3 R
4 θ
5 Aux In 1
6 Aux In 2
7 Aux In 3
8 Aux In 4
9 Reference Frequency
10 CH1 display

The requested values are returned in a single string with the values

separated by commas and in the order in which they were requested.
For example, the SNAP?1,2,9,5 will return the values of X, Y, Freq and

Aux In 1. These values will be returned in a single string such as
"0.951359,0.0253297,1000.00,1.234".
The first value is X, the second is Y, the third is f, and the fourth is

Aux In 1.

The values of X and Y are recorded at a single instant. The values of R

and θ are also recorded at a single instant. Thus reading X,Y OR R, θ

yields a coherent snapshot of the output signal. If X,Y,R and θ are all

read, then the values of X,Y are recorded approximately 10µs apart from

R,q. Thus, the values of X and Y may not yield the exact values of R and

q from a single SNAP? query.

The values of the Aux Inputs may have an uncertainty of up to 32µs. The

frequency is computed only every other period or 40 ms, whichever is

longer.

parameter

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Remote Programming

The SNAP? command is a query only command. The SNAP? command

is used to record various parameters simultaneously, not to transfer data
quickly.

OAUX? i The OAUX? command reads the Aux Input values. The parameter i

selects an Aux Input (1, 2, 3 or 4) and is required. The Aux Input
voltages are returned as ASCII strings with units of Volts. The resolution
is 1/3 mV. This command is a query only command.

SPTS ? The SPTS? command queries the number of points stored in the

Channel 1 buffer. If the buffer is reset, then 0 is returned. Remember,
SPTS? returns N where N is the number of points - the points are
numbered from 0 (oldest) to N-1 (most recent). The SPTS?command
can be sent at any time, even while storage is in progress. This
command is a query only command.

TRCA ? j, k The TRCA? command queries the points stored in the Channel 1 buffer.

The values are returned as ASCII floating point numbers with the units of
the trace. Multiple points are separated by commas and the final point is
followed by a terminator. For example, the response with two points
might be "-1.234567e-009,+7.654321e-009,".

Points are read from the buffer starting at bin j (j≥0). A total of k bins are

read (k≥1). To read a single point, set k=1. Both j and k are required. If
j+k exceeds the number of stored points (as returned by the SPTS?
query), then an error occurs. Remember, SPTS? returns N where N is
the total number of bins - the TRCA? command numbers the bins from 0
(oldest) to N-1 (most recent). If data storage is set to Loop mode, make
sure that storage is paused before reading any data. This is because the
points are indexed relative to the most recent point which is continually
changing.

TRCB ? j, k The TRCB? command queries the points stored in the Channel 1 buffer.

The values are returned as IEEE format binary floating point numbers
(with the units of the trace). There are 4 bytes per point. Multiple points
are not separated by any delimiter. The bytes can be read directly into a
floating point array (in most languages).

Do not query the IFC (no command in progress) status bit after sending

the TRCB command. This bit will not be set until the transfer is complete.

When using the GPIB interface, EOI is sent with the final byte. The

points must be read using a binary transfer (see your GPIB interface
card software manual). Make sure that the software is configured to NOT
terminate reading upon receipt of a CR or LF.

When using the RS-232 interface, the word length must be 8 bits. The

points must be read as binary bytes (no checking for linefeeds, carriage
returns or other control characters). Most serial interface drivers are
designed for ASCII text only and will not work here. In addition, the data
transfer does not pause between bytes. The receiving interface must
always be ready to receive the next byte. In general, using binary
transfers on the RS-232 interface is not recommended.

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Remote Programming

Points are read from the buffer starting at bin j (j≥0). A total of k bins are

read (k≥1) for a total transfer of 4k bytes. To read a single point, set k=1.

Both j and k are required. If j+k exceeds the number of stored points (as

returned by the SPTS? query), then an error occurs. Remember, SPTS?

returns N where N is the total number of bins - the TRCB? command

numbers the bins from 0 (oldest) to N-1 (most recent). If data storage is

set to Loop mode, make sure that storage is paused before reading any

data. This is because the points are indexed relative to the most recent

point which is continually changing.

TRCL ? j, k The TRCL? command queries the points stored in the Channel 1 buffer.

The values are returned in a non-normalized floating point format (with

the units of the trace). There are 4 bytes per point. Multiple points are not

separated by any delimiter. The bytes CANNOT be read directly into a

floating point array.

Each point consists of four bytes. Byte 0 is the LSB and Byte 3 is the

MSB. The format is illustrated below.

The mantissa is a signed 16 bit integer (-32768 to 32767). The exponent

is a signed integer whose value ranges from 0 to 248 (thus byte 3 is

always zero). The value of a data point is simply,

value = m x 2

where m is the mantissa and exp is the exponent.

The data within the SR810 is stored in this format. Data transfers using

this format are faster than IEEE floating point format. If data transfer

speed is important, the TRCL? command should be used.

Do not query the IFC (no command in progress) status bit after sending

the TRCL command. This bit will not be set until the transfer is complete.

When using the GPIB interface, EOI is sent with the final byte. The

points must be read using a binary transfer (see your GPIB interface

card software manual). Make sure that the software is configured to NOT

terminate reading upon receipt of a CR or LF.

When using the RS-232 interface, the word length must be 8 bits. The

points must be read as binary bytes (no checking for linefeeds, carriage

returns or other control characters). Most serial interface drivers are

designed for ASCII text only and will not work here. In addition, the data

transfer does not pause between bytes. The receiving interface must

always be ready to receive the next byte. In general, using binary

transfers on the RS-232 interface is not recommended.

Points are read from the buffer starting at bin j (j≥0). A total of k bins are

read (k≥1) for a total transfer of 4k bytes. To read a single point, set k=1.

Both j and k are required. If j+k exceeds the number of stored points (as

16 bits 16 bits

0

exp mantissa

byte3 byte2 byte1 byte0

(exp-124)

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Remote Programming

returned by the SPTS? query), then an error occurs. Remember, SPTS?
returns N where N is the total number of bins - the TRCB? command
numbers the bins from 0 (oldest) to N-1 (most recent). If data storage is
set to Loop mode, make sure that storage is paused before reading any
data. This is because the points are indexed relative to the most recent
point which is continually changing.

FAST (?) {i} The FAST command sets the fast data transfer mode on and off. The

parameter i selects:

i=0: Off
i=1: On (DOS programs or other dedicated data collection computers)
i=2: On (Windows Operating System Programs)

When the fast transfer mode is on, whenever data is sampled (during a

scan), the values of X and Y are automatically transmitted over the GPIB
interface (this mode is not available over RS-232). The sample rate sets
the frequency of the data transfers. It is important that the receiving
interface be able to keep up with the transfers.

To use the FAST2 mode, a ROM version of 1.06 or higher is required in

the SR810. If you need a ROM upgrade, please contact Stanford
Research Systems. The FAST2 version uses the lock-in transmit queue
to buffer the GPIB data being sent to the host. Since the transmit queue
can buffer a maximum of 63 X and Y data pairs, the host can only be
diverted for short periods of time (e.g. 120mS at 512Hz sample rate)
without causing the lock-in to "time out" and abort the FAST mode data
transfer.

The values of X and Y are transferred as signed integers, 2 bytes long

(16 bits). X is sent first followed by Y for a total of 4 bytes per sample.
The values range from -32768 to 32767. The value ±30000 represents
±full scale (i.e. the sensitivity).

Offsets and expands are included in the values of X and Y. The

transferred values are (raw data - offset) x expand. The resulting value
must still be a 16 bit integer. The value ±30000 now represents ±full
scale divided by the expand factor.

At fast sample rates, it is important that the receiving interface be able to

keep up. If the SR810 finds that the interface is not ready to receive a
point, then the fast transfer mode is turned off.

The fast transfer mode may be turned off with the FAST0 command.

The transfer mode should be turned on (using FAST1 or FAST 2) before

a scan is started. Then use the STRD command (see below) to start a
scan. After sending the STRD command, immediately make the SR810
a talker and the controlling interface a listener. Remember, the first
transfer will occur with the very first point in the scan. If the scan is
started from the front panel or from the trigger input, then make sure that
the SR810 is a talker and the controlling interface a listener BEFORE the
scan actually starts.

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Remote Programming

STRD After using FAST1 or FAST 2 to turn on fast data transfer, use the STRD

command to start the data storage. STRD starts data storage after a

delay of 0.5 sec. This delay allows the controlling interface to place itself

in the read mode before the first data points are transmitted. Do not use

the STRT command to start the scan. See the programming examples

at the end of this section.

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Remote Programming

INTERFACE COMMANDS

*RST The *RST command resets the SR810 to its default configurations. The

communications setup is not changed. All other modes and settings are
set to their default conditions and values. This command takes some
time to complete. This command resets any data scan in progress. Data
stored in the buffers will be lost.

*IDN? The *IDN? query returns the SR810's device identification string. This

string is in the format

"Stanford_Research_Systems,SR810,s/n00111,ver1.000".

In this example, the serial number is 00111 and the firmware version is

1.000.

LOCL (?) {i} The LOCL command sets the local/remote function. If i=0 the SR810 is

LOCAL, if i=1 the SR810 will go REMOTE, and if i=2 the SR810 will go
into LOCAL LOCKOUT state. The states duplicate the GPIB local/remote
states. In the LOCAL state both command execution and keyboard input
are allowed. In the REMOTE state command execution is allowed but
the keyboard and knob are locked out except for the [LOCAL] key which
returns the SR810 to the LOCAL state. In the LOCAL LOCKOUT state all
front panel operation is locked out, including the [LOCAL] key.

The REMOTE indicator is directly above the [LOCAL] key.

The Overide Remote mode must be set to No in order for the front panel

to be locked out. If Overide Remote is Yes, then the front panel is active
even in the REMOTE state.

OVRM (?) {i} The OVRM command sets or queries the GPIB Overide Remote Yes/No

condition. The parameter i selects No (i=0) or Yes (i=1). When Overide
Remote is set to Yes, then the front panel is not locked out when the unit
is in the REMOTE state. The REMOTE indicator will still be on and the
[LOCAL] key will still return the unit to the Local state.

The default mode is Overide Remote Yes. To lock-out the front panel,

use the OVRM0 command before local lock-out.

TRIG The TRIG command is the software trigger command. This command

has the same effect as a trigger at the rear panel trigger input.

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Remote Programming

STATUS REPORTING COMMANDS

The Status Byte definitions follow this section.

*CLS The *CLS command clears all status registers. The status enable

registers are NOT cleared.

*ESE (?) {i} {,j} The *ESE i command sets the standard event enable register to the

decimal value i (0-255). The *ESE i,j command sets bit i (0-7) to j (0 or

1). The *ESE? command queries the value (0-255) of the status byte

enable register. The *ESE? i command queries the value (0 or 1) of bit i.

*ESR? {i} The *ESR? command queries the value of the standard event status

byte. The value is returned as a decimal number from 0 to 255. The

*ESR? i command queries the value (0 or 1) of bit i (0-7). Reading the

entire byte will clear it while reading bit i will clear just bit i.

*SRE (?) {i} {,j} The *SRE i command sets the serial poll enable register to the

decimal value i (0-255). The *SRE i,j command sets bit i (0-7) to j (0 or

1).The *SRE? command queries the value (0-255) of the serial poll

enable register. The *SRE? i command queries the value (0 or 1) of bit i.

*STB? {i} The *STB? command queries the value of the serial poll status byte.

The value is returned as a decimal number from 0 to 255. The *STB? i

command queries the value (0 or 1) of bit i (0-7). Reading this byte has

no effect on its value.

*PSC (?) {i} The *PSC command sets the value of the power-on status clear bit. If

i=1 the power-on status clear bit is set and all status registers and enable

registers are cleared on power up. If i=0 the bit is cleared and the status

enable registers maintain their values at power down. This allows a

service request to be generated at power up.

ERRE (?) {i} {,j} The ERRE i command sets the error status enable register to the

decimal value i (0-255). The ERRE i,j command sets bit i (0-7) to j (0 or

1). The ERRE? command queries the value (0-255) of the error status

enable register. The ERRE? i command queries the value (0 or 1) of bit i.

ERRS? {i} The ERRS? command queries the value of the error status byte. The

value is returned as a decimal number from 0 to 255. The ERRS? i

command queries the value (0 or 1) of bit i (0-7). Reading the entire byte

will clear it while reading bit i will clear just bit i.

LIAE (?) {i} {,j} The LIAE command sets the lock-in (LIA) status enable register to the

decimal value i (0-255). The LIAE i,j command sets bit i (0-7) to j (0 or 1).

The LIAE? command queries the value of the LIA status enable register.

The LIAE? i command queries the value (0 or 1) of bit i.

LIAS? {i} The LIAS? command queries the value of the lock-in (LIA) status

byte. The value is returned as a decimal number from 0 to 255. The

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Remote Programming

LIAS? i command queries the value (0 or 1) of bit i (0-7). Reading the
entire byte will clear it while reading bit i will clear just bit i.

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Remote Programming

STATUS BYTE DEFINITIONS

The SR810 reports on its status by means of four status bytes: the Serial Poll Status byte, the Standard Event
Status byte, the LIA Status byte, and the Error Status byte.

The status bits are set to 1 when the event or state described in the tables below has occurred or is present.

SERIAL POLL bit
STATUS BYTE

0 SCN No scan in progress (Stop or Done). A Paused

1 IFC No command execution in progress.

2 ERR An enabled bit in the error status byte has been

3 LIA An enabled bit in the LIA status byte has been

4 MAV The interface output buffer is non-empty.

5 ESB An enabled bit in the standard status byte has

6 SRQ SRQ (service request) has occurred.

7 Unused

The ERR, LIA, and ESB bits are set whenever any bit in both their respective status bytes AND enable
registers is set. Use the SSRE, SESE, ERRE and LIAE commands to set enable register bits. The ERR, LIA
and ESB bits are not cleared until ALL enabled status bits in the Error, LIA and Standard Event status bytes
are cleared (by reading the status bytes or using SCLS).

Using SSTB? to read the Serial Poll Status Byte

A bit in the Serial Poll status byte is NOT cleared by reading the status byte using SSTB?. The bit stays set
as long as the status condition exists. This is true even for SRQ. SRQ will be set whenever the same bit in
the serial poll status byte AND enable register is set. This is independent of whether a serial poll has occurred
to clear the service request.

Using SERIAL POLL

Except for SRQ, a bit in the Serial Poll status byte is NOT cleared by serial polling the status byte. When
reading the status byte using a serial poll, the SRQ bit signals that the SR810 is requesting service. The SRQ
bit will be set (1) the first time the SR810 is polled following a service request. The serial poll automatically
clears the service request. Subsequent serial polls will return SRQ cleared (0) until another service request
occurs. Polling the status byte and reading it with SSTB? can return different values for SRQ. When polled,
SRQ indicates a service request has occurred. When read, SRQ indicates that an enabled status bit is set.

name usage

scan is considered to be in progress.

set.

set.

been set.

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Remote Programming

SERVICE REQUESTS (SRQ)

A GPIB service request (SRQ) will be generated whenever a bit in both the Serial Poll Status byte AND Serial
Poll Enable register is set. Use SSRE to set bits in the Serial Poll Enable register. A service request is only
generated when an enabled Serial Poll Status bit becomes set (changes from 0 to 1). An enabled status bit
which becomes set and remains set will generate a single SRQ. If another service request from the same
status bit is desired, the requesting status bit must first be cleared. In the case of the ERR, LIA and ESB bits,
this means clearing the enabled bits in the ERR, LIA and ESB status bytes (by reading them). Multiple
enabled bits in these status bytes will generate a single SRQ. Another SRQ (from ERR, LIA or ESB) can only
be generated after clearing the ERR, LIA or ESB bits in the Serial Poll status byte. To clear these bits, ALL
enabled bits in the ERR, LIA or ESB status bytes must be cleared.

The controller should respond to the SRQ by performing a serial poll to read the Serial Poll status byte to
determine the requesting status bit. Bit 6 (SRQ) will be reset by the serial poll.

For example, to generate a service request when a RESRV overload occurs, bit 0 in the LIA Status Enable
register needs to be set (LIAE 0,1 command) and bit 3 in the Serial Poll Enable register must be set
(*SRE 3,1 command). When a reserve overload occurs, bit 0 in the LIA Status byte is set. Since bit 0 in the
LIA Status byte AND Enable register is set, this ALSO sets bit 3 (LIA) in the Serial Poll Status byte. SInce bit
3 in the Serial Poll Status byte AND Enable register is set, an SRQ is generated. Bit 6 (SRQ) in the Serial Poll
Status byte is set. Further RESRV overloads will not generate another SRQ until the RESRV overload status
bit is cleared. The RESRV status bit is cleared by reading the LIA Status byte (with LIAS?). Presumably, the
controller is alerted to the overload via the SRQ, performs a serial poll to clear the SRQ, does something to
try to remedy the situation (change gain, experimental parameters, etc.) and then clears the RESRV status bit
by reading the LIA status register. A subsequent RESRV overload will then generate another SRQ.

STANDARD EVENT bit
STATUS BYTE

0 INP Set on input queue overflow (too many

1 Unused

2 QRY Set on output queue overflow (too many

3 Unused

4 EXE Set when a command can not execute correctly

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.

The bits in this register remain set until cleared by reading them or by the jCLS command.

name usage

commands received at once, queues cleared).

responses waiting to be transmitted, queues
cleared).

or a parameter is out of range.

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Remote Programming

LIA STATUS BYTE bit name usage

0 INPUT/RESRV Set when an Input or Amplifier overload is

detected.

1 FILTR Set when a Time Constant filter overload is

detected.

2 OUTPT Set when an Output overload is detected.

3 UNLK Set when a reference unlock is detected.

4 RANGE Set when the detection frequency switches

ranges (harmonic x ref. frequency decreases
below 199.21 Hz or increases above

203.12 Hz). Time constants above 30 s and
Synchronous filtering are turned off in the upper
frequency range.

5 TC Set when the time constant is changed

indirectly, either by changing frequency range,
dynamic reserve, filter slope or expand.

6 TRIG Set when data storage is triggered. Only if

samples or scans are in externally triggered
mode.

7 unused

The LIA Status bits stay set until cleared by reading or by the *CLS command.

ERROR STATUS BYTE bit

0 Unused

1 Backup Error Set at power up when the battery backup has

2 RAM Error Set when the RAM Memory test finds an error.

3 Unused

4 ROM Error Set when the ROM Memory test finds an error.

5 GPIB Error Set when GPIB fast data transfer mode aborted.

6 DSP Error Set when the DSP test finds an error.

7 Math Error Set when an internal math error occurs.

The Error Status bits stay set until cleared by reading or by the *CLS command.

name usage

failed.

5-25

Remote Programming

5-26