Stanford Research Systems SR844 User Manual

User’s Manual

Model SR844
RF Lock-In Amplifier

email: [e-mail] • www.thinkSRS.com

1290-D Reamwood Avenue

Sunnyvale, California 94089

Phone: [phone] • Fax: [phone]

Copyright © 1997, 2007, 2013 by SRS, Inc.

All Rights Reserved.

Revision 2.9 (07/2016)

Certification

Stanford Research Systems certifies that this product met its published specifications at the
time of shipment. Stanford Research Systems further certifies that its calibration
measurements are traceable to the United States National Institute of Standards and
Technology (NIST).

Warranty

This Stanford Research Systems product is warranted against defects in materials and
workmanship for a period of one (1) year from the date of shipment.

Service

For warranty service or repair, this product must be returned to a Stanford Research
Systems authorized service facility. Contact Stanford Research Systems or an authorized
representative before returning this product for repair.

Information in this document is subject to change without notice.

Copyright © Stanford Research Systems, Inc., 1997, 2007, 2013, 2016. All rights reserved.

Stanford Research Systems, Inc.
1290-D Reamwood Avenue
Sunnyvale, California 94089

Printed in U.S.A.

SR844 RF Lock-In Amplifier

i

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 cover is
removed. Do not remove the cover 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 The SR844 operates from a 100V, 120V, 220V, or 240V nominal AC power source
Selection 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
pull 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 the circuit board is pushed firmly into its slot. Push 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 SR844 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.

Fan The fans in the SR844 are required to maintain proper operation. Do not block the vents

in the chassis or the unit may not operate properly.

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 SR844 the stored charge may damage the front-end op
ampls. To avoid this problem, always connect the PMT output to the SR844 input before
turning the PMT on.

SR844 RF Lock-In Amplifier

ii

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)

SR844 RF Lock-In Amplifier

iii

Contents

General Information

Safety and Preparation For Use i
Contents iii
Specifications v

Chapter 1 Getting Started

Quick Start 1-3
The Basic Lock-In 1-5
X, Y, R,
Outputs, Offsets and Expands 1-12
Storing and Recalling Setups 1-18
Aux Outputs and Inputs 1-19

Chapter 2 SR844 Basics

What is a Lock-In Amplifier? 2-3
The Functional SR844 2-8
RF Signal Input Path 2-9
Reference Channel 2-10
I.F. Section 2-11
Inside the DSP 2-12
Analog Outputs and Scaling 2-15
What is Dynamic Reserve ? 2-17
Sources of Error 2-19
Using the SR844 as a Double Lock-In 2-22
Noise Measurements 2-23
Intrinsic (Random) Noise Sources 2-24
External Noise Sources 2-25

θ and dBm 1-9

Chapter 3 Operation

Overview 3-3
Signal Input 3-8
Time Constants 3-11
Sensitivity 3-13
CH1 Display and Output 3-15
CH2 Display and Output 3-20
Reference Section 3-26
Save and Recall 3-30
Interface 3-31
Scan and Rel 3-33
Auto Functions 3-40
Shift Functions 3-41

SR844 RF Lock-In Amplifier

iv Contents

Chapter 4 Programming

Index of Commands 4-2
Introduction 4-7
Command Syntax 4-10
Status Register Definitions 4-31
Example Program 4-34

Chapter 5 Testing

Getting Ready 5-3
Self Test 5-5
Amplitude Response 5-7
Phase Response 5-9
Frequency Accuracy 5-11
Ref Out Amplitude 5-13
DC Outputs and Inputs 5-15
Input Noise 5-17
SR844 Performance Test Record 5-19

Chapter 6 Circuitry

Service 6-3
Circuit Board Locations 6-4
Circuit Descriptions 6-6
Parts Lists 6-25
Schematic Diagrams 6-80

SR844 RF Lock-In Amplifier

v

Specifications

Specifications apply after 30 minutes of warm-up. All specifications are with output filtering enabled (6,
12, 18 or 24 dB/oct) and 2F detection OFF, unless stated otherwise.

Signal Input

Voltage Input single-ended BNC.
Input Impedance 50 Ω or 1 MΩ || 30 pF.
Damage Threshold ±5 V (DC+AC)
Bandwidth 25 kHz to 200 MHz.
Full Scale Sensitivity 100 nV to 1V rms in a 1-3-10 sequence.
Gain Accuracy
< 50 MHz ±0.25 dB
< 200 MHz ±0.50 dB
Gain Stability 0.2%/°C
Coherent Pickup Low Noise Wide Reserve, Sensitivity < 30 mV.
f < 10 MHz < 100 nV (typical)
f < 50 MHz < 2.5 µV (typical)
f < 200 MHz < 25 µV (typical)
Input Noise: 50 Ω Input
100 kHz < f < 100 MHz 2 nV/√Hz (typical), < 4 nV/√Hz (max).
25 kHz < f < 200 MHz < 5 nV/√Hz (typical), < 8 nV/√Hz (max).
Input Noise: 1 MΩ Input
25 kHz < f < 200 MHz 5 nV/√Hz (typical), < 8 nV/√Hz (max).
Dynamic Reserve > 60 dB (expand off)

Reference

External Reference Input 25 kHz to 200 MHz.
Impedance 50 Ω or 10 kΩ || 40 pF.
Level 0.7 Vpp digital or 0 dBm sinusoidal signal.
Pulse Width > 2 ns at any frequency.
Threshold Setting Automatic, set to midpoint of waveform extrema.
Acquisition Time < 10 s (auto-ranging, any frequency).

< 1 s (within same octave).
Internal Reference Oscillator 25 kHz to 200 MHz.
Frequency Resolution 3 digits.
Frequency Accuracy ±0.1 in the 3rd digit.
Phase Noise -90 dBc/Hz at f=100 MHz, ∆f=100 Hz.
Reference Outputs Phase locked to either Internal or External reference.
Front Panel Ref Out 25 kHz to 200 MHz square wave, 1.0 Vpp nominal into 50 Ω.
Rear Panel TTL Out
Harmonic Detect
Phase Resolution 0.02°
Absolute Phase Error
< 50 MHz < 2.5°
< 100 MHz < 5.0°
< 200 MHz < 10.0°

25 kHz to 1.5 MHz, 0 to +5 V nominal, ≥ 3 V into 50

Detect at 50 kHz

≤ 2×Reference ≤ 200 MHz.

Ω.

SR844 RF Lock-In Amplifier

vi Specifications

Reference

Relative Phase Error,
Orthogonality
Phase Noise 0.005° rms at 100 MHz, 100 ms time constant.
Phase Drift
< 10 MHz < 0.1°/°C
< 100 MHz < 0.25°/°C
< 200 MHz < 0.5°/°C

< 2.5°

Demodulator

Zero Stability Digital displays have no zero drift.

Analog outputs have < 5ppm/

Filtering
Time Constants 100 µs to 30 ks with 6, 12, 18 or 24 dB/octave roll-off.
None 10 to 20 µs update rate (X and Y outputs), 60 µs (R and θ outputs).
Harmonic Rejection
Odd Harmonics

Other Harmonics and

Sub-harmonics

Spurious Responses

-10 dBc @ 3×Ref, -14 dBc @ 5×Ref, etc.
< -40 dBc

-10 dBc @ Ref ± 2×IF

-23 dBc @ Ref ± 4×IF
< -30 dBc otherwise.

°C drift for all dynamic reserve settings.

Displays

Channel 1 Channel 2 Reference

Type 4½ digit LED 4½ digit LED 4½ digit LED
Displayed X Y Reference Frequency
Quantities R [Volts] θ [degrees] Reference Phase
R [dBm] Y-noise [Volts] Aux Output Voltages
X-noise Y-noise [dBm] Offsets in % of Full Scale
AUX IN 1 AUX IN 2 IF Frequency
Elapsed Settling Time
Ratio The signal may be ratioed with respect to AUX IN 1 or

2. The ratio is applied to both X and Y before
computation of R, R[dBm], X-noise, Y-noise [V, dBm]
and so affects all of these quantities. The ratio input is
normalized to 1 V and has a dynamic range > 100.

Expand The CH1 and CH2 displays and outputs may be

expanded by ×10 or ×100.

SR844 RF Lock-In Amplifier

Specifications vii

CH1 and CH2 Outputs

Connectors

Front Panel BNC.

Voltage Range

10V full scale proportional to X, Y or CH1, CH2 displayed quantity.

±11V full scale for phase

Update Rate

X, Y

48 to 96 kHz

R, θ, Aux Inputs

12 to 24 kHz

X Noise, Y Noise

512 Hz

Aux Inputs and Outputs

Connectors

Rear Panel BNC.

Inputs

2

Type
Range

Differential with 1 MΩ input impedance on both signal and shield.

±10V

Resolution

1/3 mV

Bandwidth

3 kHz

Outputs

2

Range

±10V

Resolution

1 mV

Environmental Conditions

Operating

Temperature: +10°C to +40°C

Relative Humidity: <90% Non-condensing

Non-Operating

Temperature: –25°C to +65°C
Relative Humidity: <95% Non-condensing

General

Furnished Accessories

Power Cord
Operating Manual

Interfaces

IEEE-488.2 and RS232 interfaces standard. All instrument functions
can be controlled and read through either interface.

Power

70 Watts, 100/120/220/240 VAC, 50/60 Hz.

Dimensions

17" W x 5.25" H x 19.5" D

Weight

23 lb.

Warranty

One year parts and labor on materials and workmanship.

±

(Specifications apply over +18°C to +28°C)

SR844 RF Lock-In Amplifier

viii

SR844 RF Lock-In Amplifier

1-1

Chapter 1

Getting Started

The tutorials in this chapter are designed to acquaint the first time user with the SR844 RF Lock-In
Amplifier. The functions and features of the SR844 are grouped together into several short tutorials. You
may choose to do the tutorials selectively depending on your level of experience and your measurement
needs. Do not be concerned that your measurements do not agree exactly with the printed values in the
manual; the focus of these exercises is to learn how to use the instrument.

For all of the tutorials it is expected that you have installed the instrument with the line voltage setting
appropriate to the AC power available. If you have not done so, please see the section Line Voltage
Selection under Safety and Prepar ation for Use (page i) before proceeding further.

The experimental procedures are detailed in two columns. The left column lists the actual steps to be
performed. The right column is an explanation of each step. The front panel Keys, Knob and

READOUTS are denoted in special fonts. Indicators are shown in Bold and connectors in CAPITALS.

In This Chapter

Quick Start 1-3
The Basic Lock-In 1-5
X, Y, R, θθθθ and dBm 1-9
Output s, Off set s and Expands 1-12
Storing and Recalling Setups 1-18
Aux Outputs and Inputs 1-19

SR844 RF Lock-In Amplifier

1-2 G et t ing Start ed

SR844 RF Lock-In Amplifier

Quick S t art 1-3

Quick Start

This section will lead you through the most basic se tup and use of the SR844 RF lock-in amplifier. You
must have selected the line voltage (page i) and connected AC power in order to proceed further.

For this section you will need one BNC cable .

1 D isc onnect all cables from the SR844. Check

that the power cord is connected.

2 Turn the power on while holding down the

Setup key.

3 Wait until the power-on tests a re c ompleted. The instrument first displays SR844 followed by

4 If any of the tests FAIL, try power-on once

more with the Setup button held d own. If the
test FAILs again, note the ROM version and
Serial Number and contact either the factory
or your local representative.

If the Setup key is pressed during power-on, the
instrument perfo rms po wer-on te s t s a nd re tu rns to
its factory preset settings.

the unit’s serial number (CH1 and CH2 displays)
and the firmware revision number (Reference
display).

Several tes ts a re p erformed after this. T he message
DATA TEST PASS follows a read/write test to
the processor RAM. BATT TEST PASS refers to
a test of the battery-backed-up memory. PROG
TEST PASS follows a test of the instrument
program ROM. DSP T EST PASS refers to the
Digital Signal Processor (DSP). RCAL STD SET
is shorthand for Recall Standard Setup (factory
defaults). N ormally, the Setup button is not pressed
during power-up and the last message will instead
be RCAL USER SET , which means that the
previous User’s setup has been recalled.

5 Connect REF OUT on the front panel to the

SIGNAL IN with the BNC cable.

The SR844 defaults to the internal oscillator set at

1.00 MHz. T he reference mode is indicated by the
INTERNAL LED. In this mode the SR844
generates a reference signal at the selected
frequency and de tects input signals in phase and in
quadrature with this reference. A 1. 0 Vpp square
wave reference signal is available at REF OUT.

SR844 RF Lock-In Amplifier

1-4 Quick Start

At 1 MHz, a quarter wavelength is roughly 50

6

Check the readings on the front panel

Press CH1 Display to select R [dBm]. The R[dBm]

meters of BNC cable. Your BNC cable is probably
a lot shorter than this, so the signal input is close to
being in phase with the reference.

displays.

display on CH1 should read +5.6 to +9.6 dBm.

SR844 RF Lock-In Amplifier

Th e Basic Lock-I n 1-5

The Basic Lock-In

This measurement is designed to use the interna l os c illa tor to e xplore some of the basic loc k-in
operations. Yo u s hould alrea dy be familiar with the fund amentals of lock-in detection. See C hapter 2 for a
discuss ion of the bas ics of lock-in me as urements.

Specifically, you will measure the amplitude of the reference oscillator at various frequencies,
sensitivities , time constants and phases. You will need a BNC ca ble for this sec tion.

1 D isc onnect all cables from the SR844.

If the power is off, turn it on. Wait for self­tests to c omplete.

2 Press Shift then Recall (PRESET) to restore

factory presets.

3 Connect REF OUT on the front pa nel to the

SIGNAL IN with the BNC cable.

Turn on the unit.

We will start with the unit in its factory pres e t
configuration.

The factory preset configuration is Internal
Refere nce mode (shown by the INTERNAL LED)
at 1.00 MHz, shown on the Reference display.

The time constant is 100 ms (shown by the time
constant indicators 1, ××××100 and ms above the Time
Constant Up/Do wn keys) and the sensitivity is 1 V
rms (the indicators a re b elo w the Sensitivity
Up/Down keys).

The SR844 reference output (1.0 Vpp nominal
square wave into 50
measurement range (1 Vrms) so we can connect it
directly to the input. The SR844 input impedance is
set to 50 Ω
appropriate for the REF OUT.

(shown by the 50

Ω

) is within the unit’s

ΩΩΩΩ

indicator) which is

The BNC cable has a small phase shift at 1 MHz
(the free-space wavelength is 300 m), so the input
signal should be mostly X (in-phase) with a small Y
(quadrature) component.

The CH1 Display is set to X and should s how
something close to 0.54 V. The CH2 display is set
to Y and should show le s s than

±

0.05 V.

SR844 RF Lock-In Amplifier

1-6 T he Basic Lock-In

Reme mber, the s ignal is a 0.5 Vpk (1.0 Vpp) s qua re
wave. A square wave is composed of signa ls at all
odd harmonics. The SR844 is a square wave
detecting lock-in and detects all of the odd
harmonics of the fundamental. T he amplitude of the
fundame ntal (at 1. 00 MHz) is 4/

π

x Vpk. The

contribution from all odd harmonics is

1 + (1/3)2 + (1/5)2 + (1/7)2 + ... ≈ 1.19

The detected amplitude is 4/

π

x 0.5 Vpk x 1. 19 or

0.759 Vpk. The SR844 reads the s ignal in units of
Vrms (0.707 x Vpk) or 0.537 Vrms .

The CH1 display may not read exa ctly 0. 54 V for a
number of reasons:

• The REF OUT amplitude is only a nominal

specification.

• The basic accuracy of the unit is

±

0.25 dB

(3%).

• Since the phase may not be exactly 0

°

, X=Rcos

is slightly less than R (amplitude ).

4 Press Shift then Phase to perform

AutoPhase.

This adjusts the reference phase inside the
instrument. (The phase at which the signal is
detected changes, but Ref Out remains unchanged.)
This should set the value of Y (on the C H2 display)
to zero.

5 Press Phase. Display the reference phas e. It should be close to

zero.

θ

6

Press the +90° key.

7Use the knob to adjust the reference phase

until Y is zero and X is e qu al to the pos itive
amplitude.

SR844 RF Lock-In Amplifier

This adds 90° to the reference phase. The value of
X should drop to ne ar zero, while Y cha nges to
about –0.54 V (ne gative of the previous X rea ding).

While the reference phase is being displayed, the
knob can be used to change it. The adjustment
described should result in the phase returning to
nearly zero again.

In general, the knob is used to ad jus t the quantity
displayed in the Reference display (if it can be
changed). The keys be low the display are used to
selec t the des ired q ua ntity.

Th e Basic Lock-I n 1-7

8 Press Freq. Now the display shows the reference frequency, still

1.00 MHz.

9 Rotate the knob left to get to 999 kHz and

998 kHz.
Rotate the knob right to get to 1.01 MHz and

1.02 MHz.

The internal frequency may be adjus ted with 3-digit
resolution.

The actual frequ ency is within 1 c ou nt in the 4th
digit of the displayed frequency. For example, when
set to 4. 5 6 MHz, the a ctua l freque ncy is within

0.001 MHz of 4.56 MHz.

10 Use the knob to adju s t the frequency to

The X reading s hould vary less than 10%.

96 kHz.

11 Press Sensitivity Down. The sensitivity changes to 300 mV (indica ted be low

the Sen s itivity Down key). T he OVLD indicators
in the CH1 and CH2 displays indicate that the
readings may be invalid due to an overload
condition. OVLD indicators in the Input, Time
Constant and Sensitivity area s a re us e d to p inpoint
the source of the overload.

12 Press Shift then Se nsitivity Up to perform

AutoSensitivity.

This adjus ts the se nsitivity so that the measured
magnitude, R, is a sizable percentage of full scale.
The instrument should end up on the 1 V s ca le and
the displays showing their previous values.

13 Disconnect the cable at the SIGNAL IN

connector.

Watch the CH1 display bargraph drop down to zero.
The time constant is 100 ms, the bargraph falls
quickly but not instantaneously.

14 Reconnect the cable to SIGNAL IN. Watch the CH1 bargraph come back up.
15 Press Time Constant Down six times until

the time constant is 100

µ

s.

The Time Constant is adjus te d us ing the le ft hand
pair of keys in the T ime Constant area. The

100,

indicated time c onstant should be 1,

x

CH1 and CH2 values remain nea rly unchanged but
may be noisy in the la s t digit.

16 Disconnect the cable at the SIGNAL IN

The bargraph falls and rises nearly instantaneously.

connector, then reconnect it.

17 Press Time Constant Up until the time

const ant i s 3 s.

18 Disconnect the cable at the SIGNAL IN

The indicated time constant should be 3, x1,s. The
CH1 and CH2 displa ys remain nearly unchanged.

The bargra ph falls slowly.
connector. Wait until the C H1 reading drops
to zero.

s . The

µµµµ

SR844 RF Lock-In Amplifier

1-8 T he Basic Lock-In

19 Reconnect the cable to SIGNAL IN. The bargraph rises s lowly. In fact, with a filter slope

of 12 dB/oct, it takes about 5 time constants to get
to within 1% of the final reading. In this case , this
takes more tha n 15 s.

20

Press Slope/Oct DOWN until 24 dB is
selected.

The filter slope is a djusted us ing the right ha nd pair
of keys in the Time Constant area. T he filter rolloff
can be 6, 12, 18 or 24 dB/oct.

With 24 dB/oct rolloff, it take s about 12 time
constants to get within 1% of the final re ad ing.

Remembe r, both the time c onstant and filter slope
affect the output se ttling time.

21

Press Slope/Oct UP until NO FILTER is
selected.

No filtering is als o a vaila ble . In this c ase, the
demodulator outputs are amplified but not filtered.
The high output bandwidth in this ca s e requires that
the outputs be taken from the CH1 or CH2
OUTPUT fro m the front panel a nd not from the
displays.

22

Press Slope/Oct DOWN until 12 dB is

12 dB/oct works well in most s itu ations.

selected.

SR844 RF Lock-In Amplifier

X, Y, R, θθθθ and dBm 1-9

X, Y, R, θθθθ and dBm

This measurement is designed to use the interna l os c illa tor and an external signal source to explore some
of the signal types. You will need a synthe s ized signal generator cable of providing 200 mVrms (0 dBm)
sine waves at 100 kHz into a 50
BNC cables.

Specifically you will display the lock-in outputs when measuring a s ignal tha t has a frequency close to,
but not equal to, the internal reference frequency.

Note: The last few items in this s e ction require that the signal generator have a Sync output; if you are
using a signal generator that has a s ingle output only, you ca n split the output using a BNC T e e (or a
power splitter or a directional coupler).

Ω

load (the DS335 from Stanford Research Systems will s uffice), and

1 D isc onnect all cables from the SR844.

If the power is off, turn it on. Wait for self­tests to c omplete.

2 Press Shift then Recall (PRESET) to restore

factory presets.

3Use the knob to adjust the SR844 reference

frequency to 100 kHz.

4 Press Sensitivity Down.

Press Time Constant Down twice until the
time constant is 10 ms.

5 Tur n on t he ext er nal signal generato r a nd set

the fre quency to 100 kHz exactly, and the
amplitude to 200 mVrms , 0 dBm, or
600 mVpp into 50
real ly matte r. Low-frequency signal generators
may have waveform selection (select sine
wave) a nd DC o ffs et (s e t i t t o zero ). If the
signa l generator offers modulation, make sure
it’s off.

Ω

. The exact value doesn’t

Turn on the unit.

The factory preset configuration is:

1 Vrms s e nsitivity.

100 ms, 12 dB/oct time constant.

Internal Reference at 1.00 MHz.

Signal Input 50

We are using a low reference frequency so that the

intrinsic frequency difference between the SR844

and the signal generator has a smaller abs olute

value.

The SR844 sensitivity should now be 300 mVrms.

We need a shorter time constant to measure the

output signal.

While not phase-locke d, the signal ge nerator and

SR844 should be at very nearly the s ame frequency;

the slight freque ncy difference will be manifested a s

a changing relative phase.

Ω

.

SR844 RF Lock-In Amplifier

1-10 X, Y, R, θθθθ and dBm

6 Connect the signal generator output to the

SR844 SIGNAL IN conne ctor with a BNC
cable.

The CH1 and C H2 readings s hould both vary
between positive and negative values in a correlate d
fashion that reflects the changing relative phase
between the two instrume nts.

7 Adjust the signal gene rator freque ncy if

The extent of adjustment should be les s than 10 Hz.
necessa ry to be tte r match the signal generator
frequency to the SR844.

8 Adjust the signal gene rator freque ncy in steps

of 1 Hz (or less) until the CH1 and CH2

The CH1 and C H2 display bargraphs should now

oscilla te s lo wly.
readings oscillate with a period of a few
seconds.

9 Press CH1 Display once to s ele c t R [V]. R is the signal amplitude and is independent of

2+y2

refere nce p has e ( R=√(x

) ). T he re a d ing of R

does not osc illate .

10 Press C H1 Display to selec t R [dBm]. T he R[dBm] display on CH1 s hould read within a

few dB of 0 dBm (0.224 Vrms) depending upon the

amplitude setting of the s ignal generator.

11 Adjus t the signal gene rator a mplitude to half

The R[dBm] dis play s hould drop by 6 dBm.
the original amplitude (100 mVrms, –6 dBm,
or 300 mVpp).

12 Press C H1 Display se ve ral times until R [V]

is selected once again.

The Display key cycles through the available

choices.

13 Press C H2 Display to se le ct θ. CH2 now shows the signal phase θ. The phase is

changing line arly with a rate equal to the frequency

difference between the signal generator and the

SR844. The readout and bargraph ramp linearly and

smoothly from –180

each period. When displaying

CH2 display is scaled from -180

+180

°

(extreme right).

°

to +180° (or vice-versa) once

θ

, the bargraph on the

°

(extreme left ) to

14 Press the Source key (above REF OUT). Switch the SR844 to External Reference Mode.

Since there is no external reference input connected

yet, the Referenc e D ispla y should read about

19 kHz (the internal oscillator pulls to its lowest

frequency) and the red OUT OF RANGE and

UNLOCK indic a tors s hould be lit.

SR844 RF Lock-In Amplifier

X, Y, R, θθθθ and dBm 1-11

15 Connect the Sync output of the s ignal

generator to the REF IN connector of the
SR844 with a BNC cable.

Note: If you are using a s ignal generator with
a single output, split the output using a BNC
Tee , or a power splitter or 10 to 20 dB
directional coupler. (If you use a directional
coupler the straight-through output should go
to REF IN and the c oupled output should go
to the SIGNAL IN.) You may need to adjust
the signa l generator amplitude to provide the
SR844 with enough signal to lock, and you
may ne ed to a djus t the SR844 sensitivity so
that the signal amplitude, R, is a sizable
fraction of the full s cale range.

16 Change the si gnal gener ato r f req ue ncy t o 1 .00

MHz.

The SR844 locks to the s ignal generator frequency,
and the R and

θ

displays are both stable. Check that

the UNLOCK error indicator (above the knob) is
off.

If the REF IN signal is noisy or too small, the
SR844 may not be able to lock. T he reference
signa l should be greater than 0.6 Vpp. If the signal
generator Sync output cannot drive 50

Ω

to a large

enough amplitude, try changing the Reference Input

ΩΩΩΩ

Impedance to 10 k

by pressing the Ref Z- In key.

The SR844 UNLOCK error indicator comes on
briefly, then goes off to indicate tha t the SR844 has
locked to the ne w frequency. The ne w frequency
should be correctly displayed in the Reference
display.

The displayed value of R should not change
(depending upon the a mplitude flatness of the signal
generator and the accuracy of the SR844). The
value of

θ

may cha nge a few de grees depending
upon the s ignal generator Sync phase and cable
lengths.

SR844 RF Lock-In Amplifier

1-12 Output s, Offset s and E xpands

Outputs, Offsets and Expands

This measurement is designed to use the interna l os c illa tor to e xplore some of the basic loc k-in outputs.
You will need BNC cables and a digital voltmeter (DVM).

Specifically, you will me as ure the amplitude of the reference os cilla tor and provide analog outputs
proportional to the measurement. T he effect of offsets and expands on the displayed values and the analog
outputs will be explored.

1 Disconnect all cables from the Lock-In.

If the power is off, turn it on. Wait for self­tests to c omplete.

2 Press Shift then Recall (PRESET) to restore

factory presets.

3 Connect REF OUT on the front panel to the

SIGNAL IN with the BNC cable.

Turn on the unit.

The factory preset configuration is:
1 Vrms s e nsitivity.
100 ms, 12 dB/oct time constant.
Internal Reference at 1.00 MHz.
Signal Input 50

Ω

.

The SR844 reference output (1.0 Vpp nominal
square wave into 50

Ω

) is within the unit’s
measurement range (1 Vrms) so we can connect it
directly to the input. The SR844 input impedance is
set to 50 Ω

(shown by the 50

ΩΩΩΩ

indicator) which is

appropriate for the REF OUT.

The CH1 Display is set to X and should s how
something close to 0.54 V. The CH2 display is set
to Y and should show le s s than

±

0.05 V.

Reme mber, the s ignal is a 0.5 Vpk (1.0 Vpp) s qua re
wave. A square wave is composed of signa ls at all
odd harmonics. The SR844 is a square wave
detecting lock-in and detects all of the odd
harmonics of the fundamental. T he amplitude of the
fundame ntal (at 1. 00 MHz) is 4/

π

x Vpk. The

contribution from all odd harmonics is

SR844 RF Lock-In Amplifier

1 + (1/3)

The detected amplitude is 4/

2

+ (1/5)2 + (1/7)2 + ... ≈ 1.19

π

x 0.5 Vpk x 1. 19 or

0.759 Vpk. The SR844 reads the s ignal in units of
Vrms (0.707 x Vpk) or 0.537 Vrms .

Outp uts, Off set s and E xpands 1-13

The CH1 display may not read exa ctly 0. 54 V for a
number of reasons:

• The Ref Out amplitude is only a nominal

specification.

4 Connect the CH1 OUTPUT to the DVM. Set

the DVM to rea d DC Volts, on the 20 Vdc
scale.

5 Press CH1 Offset Auto (this key is two keys

left of the CH1 OUTPUT connector).

• The basic accuracy of the unit is

±

0.25 dB

(3%).

• Since the phase may not be exactly 0

°

, X=Rcos

is slightly less than R (amplitude ).

The CH1 output is preset to X as indica ted by the X
LED above the CH1 OUTPUT. T he output voltage
is given by the formula:

(X/Sens itiv ity – Xoffset) × Expand × 10V
In this cas e X ≅ 0.54 Vrms, Sensitivity = 1.0 Vrms,

Xoffs e t = 0, Expand = 1 (no output expand), so we
expect the DC output voltage to be about 5.4 V.
The DVM s hould read about this value (depending
upon the exact X reading).

X, Y and R may all be offset and expande d
independently.

Since Channel 1 is dis p lay ing X, the Offset (On/Off,
Auto and Modify) and Expand keys below the
Channel 1 Display set the offset and expand for X.
The dis play s e le ction determines whic h quantity the
Offset and Expand keys operate on.

θ

Offset Auto auto matically a djus ts the offset of the
displaye d qu antity to make the result zero. In this
case, X is o ffset t o ze ro. (Y i s a l s o offset to zero .
See below for an explanation of X and Y offsets.)

The offset affects both the displa yed va lue of X and
the CH1 analog output (X). Thus, after the auto
offset function is performed, both the displayed
value of X and the DVM should s how readings very
close to zero.

The XYOffs indicator in the C hannel 1 Display has
turned on to indica te that the display qua ntity is
affected by XY offsets.

Offsets are useful for making relative measurements
or to cancel the contribution from an unwanted
phase coherent signal. In analog lock-ins, offsets

SR844 RF Lock-In Amplifier

1-14 Output s, Offset s and E xpands

were ge nerally used to remove DC output errors
from the lock-in itself. The SR844 demodulator is
digital and ha s no DC output errors, however, it
does have some coherent pickup at high
frequencies, which can be canceled using offsets.

Important!

Xoffset and Yoffset are applied to the X and Y
demodulator outputs directly. R and

θ

are computed
from the offset values of X and Y. O ffsetting X or
Y changes the measurement of R and

θ

.

In additio n, changing the Reference Pha s e will
modify the values of Xoffset and Yoffset. T hink of
(Xoffset, Yoffset) as a signal vector relative to the
Reference (internal or external) which cancels an
actual s ignal at the input. This cancellation is
preserved even when the detection phase (Reference
Phas e) is changed. This is done by circularly
rotating the values of Xoffset and Yoffset by minus
the Reference Phase. This preserves the phase
rela tio nship b e twee n (Xoffse t, Y o ffs e t ) a nd the
signal input.

6 Press Phase to display the Reference Phase

in the Reference Dis pla y.

Press the +90° key.

Since the vector (Xoffset, Yoffset) is used to cancel
a real signal at the input, Xoffs et and Yoffset are
always turned on and off together. Turning either X
or Y offset on (or off) t urns o n (or off) both offset s .
Auto offsetting either X or Y perfo rms a u to offset
on both X and Y. These statements are true even if
only one of the quantities X or Y is currently being
displayed.

Since auto offset has set (Xoffset, Yoffset) to cancel
the s ignal input, changing the Reference Phase does
not affect the X and Y readings.

X and Y remain zero even as the phase is changed.
This allows phase c oherent signa ls a t the input to be
completely canceled. For example, to cancel
coherent pickup, turn the e xperimental signal off
while leaving all of the signal cabling in place,
perform aut o offset X (or Y ) a nd then turn on the
experimental signal and proceed normally. The
effects of the cohe rent pickup are removed at the
input. The amplitude and phase of the experimental
signal are now measured normally.

Press the Zero key to retu rn the phase to zero.

SR844 RF Lock-In Amplifier

Outp uts, Off set s and E xpands 1-15

7 Press CH1 Offset Modify. The offset of the CH1 display quantity is shown on

the Reference display. The reading is in percent of
full scale. In this case, the Xoffset should be about ­54% (of 1 Vrms).

Note!

The entered offset percentage does not change when
the sensitivity is changed. Howe ve r, it does c hange
if the reference phase is changed (se e above ).

8Use the knob to adju s t the offset until the X

display is 0.1 V.

9 Press CH1 Expand.

The displayed value of X should be close to

0.1 Vrms . T he offset should be about 44% and the
CH1 output voltage (s ee the formula in step 4
above) should be

(0.54 V/1.0 V - 0. 44) × 1 × 10V = 1.0 V

or nearly so.
With an expand of ××××10, the displa y has one more

digit of resolution (100 mV full scale). T he
Expand indicator turns on at the bottom of the
Channel 1 dis pla y to indicate that the displaye d
quantity has been expanded. The output voltage
should now be

(0.54 V/1.0 V - 0. 44) × 10 × 10V = 10 V

or nearly so.
The expand allows the output gain to be increased

by 10 or 100. To use output Expand, it is necessary
to have a displa y rea ding that is les s than 10% or
1% of the full scale se nsitivity. T his ca n be
achieved using offsets if necessary.

The maximum output is limited to 110% of the
display full s c a le (Sensitivity

÷

Expand). Any

greater output will turn o n the OVLD indicator
above the CH1 OUT PUT connector. The OVLD
indicator within the CH1 display will also turn on.

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

Offsets add and subtract from the display. Expand
increases the resolutio n of the display and the gain
of the analog output.

SR844 RF Lock-In Amplifier

1-16 Output s, Offset s and E xpands

10

Press CH1 Output to s e le c t DISPLAY.

11

Press CH1 Display once to select R(V).

This key toggles the CH1 analog output function
between the sele c ted dis pla y qua ntity and X. In
other words, it is poss ible to have a signal
proportional to X on the analog output while the
display s hows R (or some other quantity).

In this cas e , with the d is pla y s e t to X, X remains the
CH1 analog output quantity and the DVM re ad s the
same.

This sele c ts R (Volts) as the CH1 display q uantity.
Since the CH1 Output is set to DISPLAY, R is now
also the CH1 ana log output.

Remember that Xoffset and Yoffset are applied
directly to the demodulator outputs, the value of R
is computed from the offset va lues of X and Y. The
XYOffs indicator in the CH1 display indicates that
the displayed value is affected by XY offsets.

At the present time, Y is offset to zero and X is
offset to 100 mV. T he resultant R is 100 mV and
the CH1 display should read about 0.1 V. Expand is
off since the display quantity R has not been
expanded.

The CH1 ana log output should be

(0.1 V/1.0 V - 0. 0) × 1 × 10V = 1 V

or 10% of full scale.
The Channel 1 Offset and Expand keys now se t the

offse t a nd expa nd for R.

12

Press CH1 Output to s e le c t X. The CH1 ana log output returns to X. The offset and

expand for X are s till in effect, even though R is the
displaye d qu antity. T hus, the DVM reads 10 V.

The CH1 display is unchange d. It still shows R and

0.1 V.

13 Press C H1 Offset On/Off.

This t urns Roffset on. T he ROffs indicator within
the CH1 display turns on to s how that the displayed
quantity is affected by Roffset. (T he XYOffs
indicator within the CH1 display means that XY
offsets are on and also affect the CH1 displayed
quantity.)

14 Press C H1 Offset M o dify. T he offset of the CH1 display q ua ntity, R(V), is

shown on the Reference display. The reading is in
percent of full scale.

SR844 RF Lock-In Amplifier

Outp uts, Off set s and E xpands 1-17

15 Use the knob to adju s t the offset until the

CH1 display is 0.0 V.

The offset should be about -10%. (R was 0. 1 V or
10% of full scale).

Pressing Expand will increase the resolutio n of the
R measurement. T he R offset and expand do not
affect either X or Y. Note that the DVM still reads
10 V for the X output.

16 Press C H1 Offset On/Off again.

This t urns o ff t he R offs e t. T he C H1 ROffs
indicator turns off and the dis playe d R returns to

0.1 V. The XYOffs indicator remains on because
XY offsets are s till on.

17

Press D is pla y four times to return to X
display.

The CH1 display retu rns to showing X with Xoffset
and Expand on.

18 Press Expand twice to turn Expand off. Turn off X expand. T he Expand key cyc les through

none, x10 and x100.

19 Press C H1 Offset On/Off once. Turn off X offset. This a lso tu rns o ff Y o ffset. T he

XYOffs indicators turn off and the displays s how
the original meas urement of the REF OUT signal.

This completes this exercise . For more informatio n
see Chapter 3, CH1 D isplay and O utput.

SR844 RF Lock-In Amplifier

1-18 Storing and Recalling Setups

Storing and Recalling Setu ps

The SR844 can store 9 complete instrument s etups in non-volatile memory.
1 Press Shift then Recall (PRESET) to restore

factory presets.

2 Press Sensitivity Down twice.

Press Time Constant Up twice.

3 Press Save. The CH1 and C H2 displays s how SAVE n where n

4Use the knob to select setup number 3. The knob selects the setup number (shown in the

5 Press Save again. The second time Save is press e d c ompletes the

This restores the SR844 to its factory presets.
The factory preset configuration is:

1 Vrms s e nsitivity.
100 ms, 12 dB/oct time constant.
Internal Reference at 1.00 MHz.
Signal Input 50

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

These keypres se s s ele c t 100 mV sensitivity and 1 s
time constant.

is a number from 1 to 9.

CH2 display).

operati on. The me ssage DONE is appe ars briefly in
the Re ference display. A ny other key pressed a t this
time aborts the Save.

Ω

.

The current setup is now saved as setup number 3.

6 Press Shift then Recall to restore factory

presets.

7 Press Recall. The CH1 and C H2 displays s how RCAL n where n

8Use the knob to select setup number 3. The knob selects the setup number.
9 Press Recall again. The second time Recall is p res s e d c ompletes the

The sensitivity and time constant revert to 1 V and
100 ms respectively. Now let’s recall the setup that
we just saved.

is a number from 1 to 9.

operati on. The me ssage DONE is a ppea rs briefly in
the Re ference display. A ny other key pressed a t this
time aborts the Recall.

The time constant and sensitivity should have
reverted back to their saved values of 100 mV a nd
1 s respectively.

SR844 RF Lock-In Amplifier

Aux Outputs and Inputs 1-19

Aux Outputs and Inputs

This measurement is designed to illustrate the use o f the Aux Outputs and Inputs o n the rear panel. You
will ne e d BNC c ab les a nd 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 Press Shift then Recall to restore factory

presets.

2 Connect AUX OUT 1 on the rear panel to the

DVM. Set the DVM to re ad DC Volts, either
auto-ranging or on the 20 Vdc scale.

3 Press AuxOut once or until the Reference

Display shows the leve l of AUX OUT 1, as
shown by the AxOut1 indicator beneath the
display.

4Use the knob to adjust the level to 10.000 V. Change the output to 10 V. The DVM s hould read

5Use the knob to adjust the level to –5.000 V. Change the output to –5 V. The DVM s hould read

This restores the SR844 to its factory presets.

The Aux Outputs can provide programmable DC
voltages between –10. 5 a nd 10.5 Volts. The outputs
may be set from the front pa nel, or via the c omputer
interface.

Show the level of AUX OUT 1 on the Reference
display. T he default value is 0. 000 V.

very close to 10.000 V.

very close to –5.000 V.

The auxiliary outputs are us e ful for controlling other
parameters in an experiment, such as pressure,
temperature, wavelength, etc. The AuxOut voltages
may be set remotely over the GPIB or RS-232
interface.

6 Press CH1 Display four times or until

AUX IN 1 is selected.

7 Disconnect AUX OUT 1 from the DVM. We will use AUX OUT 1 to provide an analog

8 C onnect AUX OUT 1 to AUX IN 1 on the

rear panel.

Pressing Display cycles the CH1 Display through
the five available q ua ntities. AUX IN 1 s hows the
voltage at AUX IN 1. The two Aux inputs can each
read an analog voltage in the
inputs may be us ed for monitoring and meas uring
other parameters i n an experiment, su c h as p re s sure ,
temperature, position, etc. T he Aux In voltages may
be read remotely over the GPIB or RS-232
interface.

voltage to measure.

The CH1 display shows the voltage at AUX IN 1
(close to -5.000 V).

±

10.5 V range. These

SR844 RF Lock-In Amplifier

1-20 Aux Outputs and Inputs

9 Use the knob to adjust AUX OUT 1 to

-6.500 V.

The CH1 display should now read close to

-6.500 V.

Besides reading basic DC voltages, the Aux In voltages may be used to normalize the signal. In Ratio
mode, the X and Y signals are multiplied by (1.000V/AUX IN 1) or (1.000V/AUX IN 2) prior to time
constant filtering. Ratio mode is fully e xplained in Chapter 3, CH 1 Display and Output.

Another application of the AUX IN voltages is to provide a se cond de modulation, sometimes known a s
the Double Lock-In Technique. This is de s c ribed in Chapter 2.

The displays may be stored in the internal data buffers at a programmable sampling rate. This allows
storage of not only the lock-in outputs (X, Y, R or

θ

) but also the values of the AUX IN voltages. See

Chapter 4, Data Storage, for more information.

SR844 RF Lock-In Amplifier