Srs RS810 User Manual

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

Storing and Recalling Setups

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

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