SRS Labs SR124 Operation Manual

Operation and Service Manual

Stanford Research Systems

Analog Lock-In Amplifier

SR124

Revision 1.02 • February 3, 2014

Certification

Stanford Research Systems certifies that this product met its published specifications at the time
of shipment.

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.

Copyrightc Stanford Research Systems, Inc., 2012 – 2014. All rights reserved.

Stanford Research Systems, Inc.
1290–D Reamwood Avenue
Sunnyvale, CA 94089 USA
Phone: [phone] • Fax: [phone]

www.thinkSRS.com • e-mail: [e-mail]

Printed in U.S.A. Document number 9-01688-903

SR124 Analog Lock-In Amplifier

Contents

General Information v

Safety and Preparation for Use . . . . . . . . . . . . . . . . v

Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . viii

1 Getting Started 1 – 1

1.1 How to use this manual . . . . . . . . . . . . . . . . . 1 – 2

1.2 Basic instrument check-out . . . . . . . . . . . . . . . 1 – 2

1.2.1 Equipment needed . . . . . . . . . . . . . . . . 1 –2

1.3 Preparations before use . . . . . . . . . . . . . . . . . . 1 – 2

1.4 Signal and input filter . . . . . . . . . . . . . . . . . . . 1– 3

1.5 Phase sensitive detector . . . . . . . . . . . . . . . . . 1 – 4

1.6 Reserve . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 – 5

1.7 Output offset . . . . . . . . . . . . . . . . . . . . . . . . 1– 6

1.8 Reference oscillator external input . . . . . . . . . . . 1– 7

1.9 Reference oscillator output . . . . . . . . . . . . . . . . 1– 8

2 Introduction 2 – 1

2.1 Introduction to the instrument . . . . . . . . . . . . . 2 – 2

2.1.1 Clock stopping architecture . . . . . . . . . . . 2 –2

2.1.2 What does the SR124 measure? . . . . . . . . . 2 – 2

2.2 Instrument overview . . . . . . . . . . . . . . . . . . . 2 – 3

2.2.1 Reference section . . . . . . . . . . . . . . . . . 2 – 3

2.2.2 Signal section . . . . . . . . . . . . . . . . . . . 2 – 6

2.2.3 Input Filter . . . . . . . . . . . . . . . . . . . . 2– 9

2.2.4 Output . . . . . . . . . . . . . . . . . . . . . . . 2 – 10

2.3 Navigating the front panel . . . . . . . . . . . . . . . . 2– 11

2.3.1 Signal input section . . . . . . . . . . . . . . . . 2 –12

2.3.2 Output section . . . . . . . . . . . . . . . . . . 2 – 14

2.3.3 Setup section . . . . . . . . . . . . . . . . . . . 2 – 15

2.3.4 Reference section . . . . . . . . . . . . . . . . . 2 – 17

3 Performance Details 3 – 1

3.1 Sensitivity and gain . . . . . . . . . . . . . . . . . . . . 3– 2

3.1.1 AC Gain . . . . . . . . . . . . . . . . . . . . . . 3 – 2

3.1.2 Scale normalization . . . . . . . . . . . . . . . . 3– 3

i

ii Contents

3.1.3 DC gain . . . . . . . . . . . . . . . . . . . . . . 3– 4

3.2 Dynamic reserve and overloads . . . . . . . . . . . . . 3 – 4

3.3 Input filter details . . . . . . . . . . . . . . . . . . . . . 3– 6

3.4 Attenuators and DC bias constraints . . . . . . . . . . 3 – 9

3.5 Automatic functions . . . . . . . . . . . . . . . . . . . 3 – 9

3.5.1 Auto-phase . . . . . . . . . . . . . . . . . . . . 3– 9

3.5.2 Auto-gain . . . . . . . . . . . . . . . . . . . . . 3 – 10

3.5.3 Auto-offset . . . . . . . . . . . . . . . . . . . . . 3 – 10

3.5.4 Lock assist . . . . . . . . . . . . . . . . . . . . . 3 – 10

4 Remote Operation 4 – 1

4.1 Index of commands . . . . . . . . . . . . . . . . . . . . 4– 2

4.2 Alphabetic list of commands . . . . . . . . . . . . . . . 4 – 4

4.3 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4– 6

4.3.1 Interface configuration . . . . . . . . . . . . . . 4 – 6

4.3.2 Buffers . . . . . . . . . . . . . . . . . . . . . . . 4 – 6

4.3.3 Remote / local . . . . . . . . . . . . . . . . . . . 4 – 6

4.4 Commands . . . . . . . . . . . . . . . . . . . . . . . . . 4 – 7

4.4.1 Command syntax . . . . . . . . . . . . . . . . . 4 – 7

4.4.2 Notation . . . . . . . . . . . . . . . . . . . . . . 4 – 8

4.4.3 Examples . . . . . . . . . . . . . . . . . . . . . 4 – 8

4.4.4 Reference and phase commands . . . . . . . . 4– 9

4.4.5 Input commands . . . . . . . . . . . . . . . . . 4 – 11

4.4.6 Filter commands . . . . . . . . . . . . . . . . . 4 – 12

4.4.7 Gain and time constant commands . . . . . . . 4 – 13

4.4.8 Output commands . . . . . . . . . . . . . . . . 4 – 14

4.4.9 Setup commands . . . . . . . . . . . . . . . . . 4 – 15

4.4.10 Auto commands . . . . . . . . . . . . . . . . . 4 – 16

4.4.11 Data transfer commands . . . . . . . . . . . . . 4– 17

4.4.12 Interface commands . . . . . . . . . . . . . . . 4– 18

4.4.13 Status commands . . . . . . . . . . . . . . . . . 4 – 20

4.5 Status model . . . . . . . . . . . . . . . . . . . . . . . . 4– 23

4.5.1 Status byte (SB) . . . . . . . . . . . . . . . . . . 4– 23

4.5.2 Service request enable (SRE) . . . . . . . . . . 4– 24

4.5.3 Standard event status (ESR) . . . . . . . . . . . 4 – 24

5 Circuits 5 – 1

5.1 Overview of circuits . . . . . . . . . . . . . . . . . . . 5 – 2

5.2 Power Supply . . . . . . . . . . . . . . . . . . . . . . . 5– 2

5.3 CPU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5– 2

5.4 Front Panel . . . . . . . . . . . . . . . . . . . . . . . . . 5 – 2

5.5 Main board . . . . . . . . . . . . . . . . . . . . . . . . . 5 – 2

SR124 Analog Lock-In Amplifier

Contents iii

5.6 Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . 5 –4

5.7 Rear outputs . . . . . . . . . . . . . . . . . . . . . . . . 5 – 5

5.8 Communications . . . . . . . . . . . . . . . . . . . . . 5– 5

5.9 Schematics . . . . . . . . . . . . . . . . . . . . . . . . . 5 – 5

SR124 Analog Lock-In Amplifier

iv Contents

SR124 Analog Lock-In Amplifier

General Information

Safety and Preparation for Use

WARNING

AC line voltage

CAUTION

Line cord

Dangerous voltages, capable of causing injury or death, are
present in this instrument. Do not remove the product covers
or panels. Do not apply power or operate the product without
all covers and panels in place.

The SR124 Analog Lock-In Amplifier operates from a 100 V, 120 V,
220 V, or 240 V nominal AC power source having a line frequency of
50 Hz or 60 Hz. Before connecting the power cord to a power source,
verify that the LINE VOLTAGE SELECTOR, located in the rear panel
power-entry module, is set so that the correct AC line voltage value
is visible.

The SR124 Analog Lock-In Amplifier will be damaged if operated
with the LINE VOLTAGE SELECTOR set for the wrong AC line
voltage, or if the wrong fuses are installed. Verify that the correct
line fuses are installed before connecting the line cord. Fuse size is
5MF “fast blow” (5 × 20 mm). For 100 V/120 V, use 4 A fuses; for
220 V/240 V, use 2 A fuses.

Service

The SR124 Analog Lock-In Amplifier has a detachable, three-wire
power cord for connection to the power source and to a protective
ground. The chassis of the instrument is connected to the outlet
ground to protect against electrical shock. Always use an outlet
which has a properly connected protective ground.

The SR124 Analog Lock-In Amplifier does not have any user service­able parts inside. Refer service to a qualified technician.

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.

v

vi General Information

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)

Symbols you may Find on SRS Products

SR124 Analog Lock-In Amplifier

General Information vii

Notation

The following notation will be used throughout this manual.

WARNING

CAUTION

A warning means that injury or death is possible if the instructions
are not obeyed.

A caution means that damage to the instrument or other equipment
is possible.

Typesetting conventions used in this manual are:

• Front-panel buttons are set as [Button]



• Front-panel knobs are set as

• Front-panel indicators are set as Overload

• Remote command names are set as *IDN?

• Literal text other than command names is set as OFF

Remote command examples will all be set in monospaced font. In
these examples, data sent by the host computer to the SR124 are set
as straight teletype font, while responses received by the host
computer from the SR124 are set as slanted teletype font.



Knob





SR124 Analog Lock-In Amplifier

viii General Information

Specifications

All performance specifications after 1 hour warm-up at 23◦C ±2◦C
ambient.

Signal channel

Parameter Specification
Voltage inputs Single-ended or differential
Sensitivity 100 nV to 500 mV, in 1-2-5 steps
Current input 106V/A or 108V/A
Input impedance

Voltage 10 MΩ + 25 pF, AC or DC coupled
Current 100 Ω (1 kΩ) to virtual ground,

106(108) V/A scale
Gain accurancy ±1% at 1 kHz
Gain stability 100 ppm/◦C

(flat mode, normal reserve)
Input noise, typ. 2.8 nV/√Hz at 1 kHz

0.14 pA/√Hz at 1 kHz (106V/A)

0.014 pA/√Hz at 100 Hz (108V/A)

Input filter (Tunable from 2 Hz to 100 kHz)

Flat Flat within ±1% from 10 Hz to 20 kHz

±5% from 2 Hz to 100 kHz

Band pass Q of 1, 2, 5, 10, 20, 50, and 100
High pass −12 dB/oct rolloff
Low pass −12dB/oct rolloff
Notch Up to 80 dB attenuation

CMRR 90 dB below 10 kHz, DC coupled

decreasing by 6 dB/oct above 10 kHz
Dynamic reserve (without band pass filter)

Low noise 20dB
Normal 40 dB
High reserve 60 dB

SR124 Analog Lock-In Amplifier

General Information ix

Demodulator / Output

Parameter Specification
Output gain stability

Low noise 50 ppm/◦C
Normal 100 ppm/◦C

High reserve 1000 ppm/◦C
Output filter −6 dB/oct or −12 dB/oct
Time constants 1 ms to 300 s in 1–3–10 steps
Output impedance 600 Ω

Reference channel

Parameter Specification
Frequency range 0.2 Hz to 210 kHz
Reference input TTL or sine, 100 mVrms min. f > 2 Hz,

500 mVrms min. f ≤ 2 Hz
locks to positive-going zero crossing (sine),

positive edge (TTL).
Minimum pulse width 100 ns (TTL mode)
Input impedance 1 MΩ

AC coupled, 10 s time constant (sine)

DC coupled (TTL)
Phase resolution 0.01

◦

Phase accuracy ±5◦(2 Hz to 20 kHz)

±10◦(20 kHz to 210 kHz)
Harmonic detection F, 2×F, and 3×F (ext. ref.)

Reference output

SR124 Analog Lock-In Amplifier

Parameter Specification
Range 0.2 Hz to 210 kHz
Waveform sine, square
Frequency accuracy ±0.1% (20 Hz to 21 kHz)
Frequency resolution 3-1/2 digits or 1 mHz
Amplitude range 100 nV to 10 Vrms into high-Z
Amplitude accuracy ±1% at 1 kHz (on 20 Hz to 2.1 kHz range)
Amplitude flatness ±1% for upper decade of each range

±5% for lower decade of each range
Amplitude stability 50 ppm/◦C, typ.
Output impedance 50 Ω
DC bias commandable, to ±10× amplitude,

or ±10 VDC max (amp. dependent;

see section 3.4 for details)

x General Information

Displays and Front Panel

Parameter Specification
Panel meter jeweled bearing, center-zero, mirror-backed
Offset adjustable up to ±1000 % (10×) of full scale
Output 600 Ω output impedance,

Lock-In or AC Volt function

Numeric full static drive (no scanning refresh),

settings or one-time readings

Rear panel inputs and outputs

Parameter Specification
VCO input 10 kΩ input impedance

0 to +10 VDC for f

min

to f

max

of range
VCO output 600 Ω output impedance, 0 to +10 VDC
Quadrant outputs 600 Ω output impedance

Four 0.7 Vrms (2 Vpp) outputs, at 0◦, 90◦, 180◦, 270

Preamp DB-9 connector to power optional

remote preamp

Status TTL outputs, 1 kΩ output, 10 kΩ pullup

−Unlocked low when reference oscillator is unlocked

−Overload low when signal chain is overloaded

Remote Interfaces

RS-232 DB-9, 9600 baud fixed
Optical fiber connection to SX199 Optical Interface

Controller, provides connectivity to
GPIB, RS-232, & Ethernet

◦

General

Parameter Specification
Temperature 0◦C to 40◦C, non-condensing
Power 40 W, 100/120/220/240 VAC, 50/60 Hz
Dimensions 1700W ×500H ×1500D
Weight 23 lbs
Fuse Type 5MF, 5 ×20 mm, “fast blow”

SR124 Analog Lock-In Amplifier

1 Getting Started

In This Chapter

This chapter provides step-by-step instruction to get started quickly
with the SR124 Analog Lock-In Amplifier. Refer to chapter 2 for a
more complete introduction to the SR124.

1.1 How to use this manual . . . . . . . . . . . . . . . . . 1 – 2

1.2 Basic instrument check-out . . . . . . . . . . . . . . . 1 – 2

1.2.1 Equipment needed . . . . . . . . . . . . . . . . 1 –2

1.3 Preparations before use . . . . . . . . . . . . . . . . . 1 – 2

1.4 Signal and input filter . . . . . . . . . . . . . . . . . . 1– 3

1.5 Phase sensitive detector . . . . . . . . . . . . . . . . . 1 – 4

1.6 Reserve . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 – 5

1.7 Output offset . . . . . . . . . . . . . . . . . . . . . . . 1– 6

1.8 Reference oscillator external input . . . . . . . . . . . 1– 7

1.9 Reference oscillator output . . . . . . . . . . . . . . . 1 – 8

1 – 1

1 – 2 Getting Started

1.1 How to use this manual

Two possible starting points are available to new users of the SR124.
Those who want to begin with an overview to the functional layout
of the instrument should turn to Chapter 2.

Users who prefer to jump in and begin using the SR124 first should
continue with this Chapter, where a series of step-by-step procedures
are given to verify the basic performance of the instrument. This will
also provide a quick introduction to the SR124 and how it is operated.

Chapter 3 provides detailed discussions of the subsystems within the
SR124. Technical details, such as the actual gain allocations for each
sensitivity and reserve setting, can be found here.

Chapter 4 discusses remote operation of the SR124, over the optical
fiber or RS-232 interface.

Chapter 5 has a description of the detailed circuit schematics of the
SR124.

1.2 Basic instrument check-out

This chapter provides step-by-step instructions for verifying the basic
operation of the SR124. In addition to confirming proper operation,
it provides a good introduction to operating the lock-in.

1.2.1 Equipment needed

To perform all the steps described in this chapter, you will need:

1. a collection of several BNC cables,

2. a function generator,

3. a general purpose 2-channel oscilloscope.

1.3 Preparations before use

1. Before using the instrument, verify the rear-panel power entry

CAUTION

2. Turn the rear-panel Power switch to off.

3. Plug in the AC line cord to the rear-panel power entry module,

module is properly configured for the power line voltage in
your region. Applying power with improper setting of the
line voltage selector will result in significant damage to the
SR124.

and into a grounded wall outlet.

SR124 Analog Lock-In Amplifier

1.4 Signal and input filter 1 – 3

4. Connect a BNC cable from the front-panel Ref Out BNC (right­hand most connector) to the A/I input of the Signal Input (left­hand most connector).

5. Switch on the AC power. Allow the unit to warm up for 1 hour
for full specified performance.

1.4 Signal and input filter

1. Restore defaults: Press [Recall], and then turn the



REFERENCE knob (large right-hand side knob) clockwise un-



til the display shows ”deFLt”. Press [Recall] a second time to
restore factory defaults.

2. Verify the panel meter shows approximately +20% (positive)
deviation.

3. Turn the
lect 100mV. Verify the meter displays approximately +100%
(positive) deviation.

4. Change the filter to Notch by pressing the [Type] button four
times. The meter should swing to approximately 0% deviation.

5. Turn the
should remain near 0% deviation.







SENSITIVITY knob counterclockwise 2 clicks, to se-





Q-factor knob clockwise until Q=100. The meter











6. Press the [Mode] button in the OUTPUT block to switch to AC
Volt mode. Turn the



SENSITIVITY knob 1 click counterclock-







wise, to select 50 mV. None of the overload indicators should
be illuminated.

7. Slowly (one click at a time) turn the large



INPUT FILTER knob







to minimize the meter display as close to 0 (no deflection) as
possible.

8. Press the



INPUT FILTER knob inward once, to illuminate the







f trim indicator.



9. Adust the frequency fine trim by turning

INPUT FILTER to



mimimize the meter deflection (minimum is not very sensitive
to f trim).

10. Press the



INPUT FILTER inward once more, to illuminate the







depth indicator.

11. Adjust the depth trim by turning



INPUT FILTER to mim-







imize the meter deflection (minimum is somewhat sensitive to
depth).





SR124 Analog Lock-In Amplifier

1 – 4 Getting Started



12. Turn the

SENSITIVITY knob counterclockwise 2 clicks, to



10 mV. Verify the meter displays less than ±20% deviation. Re­peat trimming frequency and depth with the







INPUT FILTER



knob to minimize the meter deflection. The meter should be
less than ±10% deviation.

13. Turn



SENSITIVITY 3 clicks clockwise, back to 100 mV. Press







[Mode] to return the output to Lock-In mode.

14. Press the [Type] button twice, to select Band Pass. The meter
should be near −100% deviation.

15. Perform an auto-phase adjustment by pressing the [Phase] but­ton within the AUTO block. After a brief delay, the REFER­ENCE display should show near 180 deg, and the panel meter
should show +100% deviation.

16. Press the [Type] button again to select High Pass filter. The
panel meter should move near 0 deflection.

17. Perform another auto-phase adjustment by pressing [Phase] in
the AUTO block. After the pause, the phase should show a
value near 90 deg, and the meter should return to near +100%
deflection.





18. Press the [Type] button again to select Low Pass filter. The

19. Perform another auto-phase adjustment by pressing [Phase] in

1.5 Phase sensitive detector

This section walks you through a demonstration of how the phase
sensitive detector (square wave mixer) operates. You will need an
oscilloscope to view the output waveforms.

A BNC cable should connect the Reference Output (Ref. Out) to the
A/I signal input. Connect the OUTPUT BNC to the oscilloscope.

1. Restore default settings by pressing [Recall]; turn

2. Change the reference frequency from 1.000 kHz to 100 Hz by

meter should swing to near −100% deflection.

the AUTO block. After the pause, the phase should show a
value near 270 deg, and the meter should return to near +100%
deflection.



REFERENCE if necessary to display ”deFLt”, and then







press [Recall] again.

turning the

the large







Range knob counterclockwise one click. Turn



REFERENCE knob counterclockwise to set the fre-









quency to 47 Hz.

SR124 Analog Lock-In Amplifier

1.6 Reserve 1 – 5

3. Perform an auto-phase adjustment by pressing the [Phase] but­ton within the AUTO block. After a brief delay, the REFER­ENCE display should show near 0 deg.

4. Turn the



Time Constant knob counterclockwise to the min







position. Adjust the scope for 1V per vertical division, and
around 2.5 ms per horizontal division, and adjust the trigger.
You should see a fully-rectified positive sine wave, as shown
in Figure 1.1.

1.6 Reserve

Figure 1.1: Mixer waveform at 0◦(left) and 90◦phase shifts (right).

5. Turn the



Quadrant knob one click clockwise. The signal is







now 90◦out of phase with the reference signal, as shown in
Figure 1.1.

6. Turn the



Quadrant knob one click clockwise. The signal is







now at 180◦relative to the reference signal, as shown in Fig­ure 1.2.

7. Turn the



Quadrant knob one click clockwise. The signal is







now 270◦out of phase with the reference signal, as shown in
Figure 1.2.

1. Restore default settings by pressing [Recall]; turn



REFERENCE if necessary to display ”deFLt”, and then







press [Recall] again.

2. Perform an auto-phase adjustment by pressing [Phase] in the
AUTO block. After the pause, the phase should show a value
near 0 deg.

SR124 Analog Lock-In Amplifier

1 – 6 Getting Started

Figure 1.2: Mixer waveform at 180◦(left) and 270◦phase shifts (right).

3. Turn



SENSITIVITY 2 clicks counterclockwise, to 100 mV. The







meter should read near +100% deflection

4. Turn



Quadrant (in the Reference section) one click clockwise,







to the 90–180 interval. The meter should be near 0 deflection.

5. Turn



SENSITIVITY 3 clicks counterclockwise, to 10 mV. The







meter should still read between −30% and +30% deviation,
with no overload indicators.

6. Turn



SENSITIVITY 1 click counterclockwise, to 5 mV. The sen-







sitivity OVLD indicator should light.

7. Press [Reserve] twice, to select Normal. The OVLD indicator
should turn off. Manually adjust the phase setting (with the



REFERENCE knob) to null the meter. Turn



more clicks counterclockwise, to 1 mV. OVLD should remain
off. Turn







SENSITIVITY once more, to 500 µV. OVLD should









SENSITIVITY 2







light.

8. Press [Reserve] twice again, to select High Res. OVLD should
turn off. Manually adjust the phase setting again to null the

meter. Turn



SENSITIVITY 2 more clicks counterclockwise,







to 100 µV. You may try to further null the signal by manually
adjusting the phase setting again, although the signal may be
unstable with so much gain.

1.7 Output offset

1. Restore default settings by pressing [Recall]; turn



REFERENCE if necessary to display ”deFLt”, and then







press [Recall] again.

SR124 Analog Lock-In Amplifier

1.8 Reference oscillator external input 1 – 7

2. Press [Ampl] in the Reference section, and turn the



REFERENCE knob counterclockwise until the numeric dis-







play shows 95.0 mV.

3. Turn



SENSITIVITY 2 clicks counterclockwise, to 100 mV. The







meter should read near +95% deflection

4. Enable the output offset by pressing [On/Off] in the OFFSET
block. Perform an auto-offset adjustment by pressing [Offset]
in the AUTO block. After the pause, the offset should show a
value near +95% offset. The panel meter should read near zero.

5. Turn



SENSITIVITY 2 more clicks counterclockwise, to 20 mV.







The panel meter should still read near zero, while the REFER­ENCE display shows a value near +475%.

6. Turn



SENSITIVITY 1 click counterclockwise, to 10 mV. The







REFERENCE display should read near +950%, and the panel
meter should still be near null.

7. Turn



SENSITIVITY 1 click further counterclockwise. The dis-







play should show the message “Attn OFFSt”, and the OFFSET
annunciator in the REFERENCE display block should blink.
The sensitivity does not change.

8. Press the [Ampl] button, and reduce the amplitude to 90.0 mV.
The panel meter should show around −50% full scale deflec­tion.

9. Press the [Offset] button in the AUTO block again, to automati­cally re-adjust the offset. The REFERENCE display should now
show near +900%, and the panel meter should be nulled.

1.8 Reference oscillator external input

1. Restore default settings by pressing [Recall]; turn



REFERENCE if necessary to display ”deFLt”, and then



press [Recall] again.

2. Set thefunction generator to produce a 1 Vrms, 2 kHzsine wave.
Connect the function generator output to the SR124 “Ext. In”
BNC, in the REFERENCE section.

3. Turn the
clockwise, to the f External setting. The Unlocked indicator
should illuminate for between 5 and 20 seconds, and then the
SR124 should lock to the external signal.

4. Press [Freq] in the REFERENCE section to perform an oscillator
frequency measurement. After a brief delay, the REFERENCE











Mode knob in the REFERENCE section one click



SR124 Analog Lock-In Amplifier

1 – 8 Getting Started

display should show a value near 2.00 kHz.

5. Turn



Mode back to Internal, and then turn it back to f External.







Immediately after turning the mode to external, press [Lock
Assist] in the AUTO section. The display should show “ASSt
Loc”, and then the SR124 should lock, displaying the external
frequency .

6. Turn



Mode to 2f External, and then press [Lock Assist]. The







display should show “ASSt Loc”, and then “Err outr”, indicat­ing the frequency is out of range for the oscillator.

7. Turn the



Range knob clockwise one click to select 200–21k.







After a few seconds, the oscillator should lock. Press [Freq] to
measure the oscillator; the result should be near 4.000 kHz.

8. Turn



Mode again, to 3f External, and press [Lock Assist]. The







REFERENCE display should show the result near 6.000 kHz.

1.9 Reference oscillator output

1. Restore default settings by pressing [Recall]; turn



REFERENCE if necessary to display ”deFLt”, and then







press [Recall] again.

2. Connect a BNC cable from the rear-panel Reference Output 0
to the Channel 1 of the oscilloscope. Trigger the scope on the
waveform, and verify a 1 kHz, 0.7 Vrms (2 V peak-to-peak) sine
wave.

3. Connect a second BNC cable from the rear-panel Reference
Output 90◦to Channel 2 of the scope. Verify the Channel 2
signal lags Channel 1 by 90◦.

4. Disconnect the cable from the rear-panel 90◦output, and con­nect it to the 180◦output. Verify the Channel 2 signal on the
scope appears inverted relative to Channel 1 (lagging by 180◦).

5. Disconnect the cable from the rear-panel 180◦output, and con­nect it to the 270◦output. Verify the Channel 2 signal on the
scope lags Channel 1 by 270◦(leading by 90◦).

6. Disconnect theChannel 2 BNCfrom the rear-panel of the SR124,
and connect it to the front-panel Ref. Out BNC.

7. Press the [Ampl] button. Press the
once, to select coarse, and then turn



REFERENCE knob in





REFERENCE clockwise











until the display reads 10.00 V. Verify on the oscilloscope that
Channel 2 shows a sine wave of approximately 10 Vrms (28 V
peak to peak). The signal should be in-phase with Channel 1.

◦

SR124 Analog Lock-In Amplifier

1.9 Reference oscillator output 1 – 9

8. Press [Shape] to select Square. Verify the waveform changes to
a square wave with ±10 V (20 V peak to peak).

9. Turn the



REFERENCE knob counterclockwise, to reduce the







amplitude to 1.00 V on the REFERENCE display. Press [Shape]
to return to Sine output.

10. Enable the DC bias by pressing [On/Off] in the DC Bias block.
Press [Modify] in DC Bias, and then adjust the Bias setting

by turning



REFERENCE counterclockwise, to near −2.000 V.







Verify the waveform on the scope is now shifted to an average
of −2 V.

11. Press the DC Bias [On/Off] to turn off the bias, and verify the
waveform returns to zero-centered. Press DC Bias [On/Off]
again to re-enable the −2 V DC Bias.

12. Press the [Ampl] button to select Amplitude again, and then



turn

REFERENCE counterclockwise until you reach the limit.







Turn the knob slow counterclockwise to reach 10.0 mV. Notice
that the SR124 will not allow the Amplitude to decrease below

10.0 mV with the DC Bias enabled and set to −2 V.

13. Press DC Bias [On/Off] to disable the Bias. With the REFER­ENCE focus still on Amplitude, turn



REFERENCE counter-







clockwise to 9.99 mV. Notice the sound of the relay click.

14. Now press DC Bias [On/Off] to attempt to re-enable the Bias.
The SR124 should beep, and the AMPL annunciator will flash,
indicating the present value of Amplitude is incompatable with
(too small for) the requested (−2 V) DC Bias.

15. Press DC Bias [Modify]. The old value (−2 V) should be dis­played, but the SR124 beeps in warning that this value is

presently unaccessable. Turn the



REFERENCE knob in ei-







ther direction; the displayed BIAS value will jump to −100 mV.
This is the limiting value for DC Bias when the amplitude is be­tween 0.1 mV and 9.99 mV. Full details of the interdependence
of DC bias and Amplitude can be found in section 3.4.

SR124 Analog Lock-In Amplifier

1 – 10 Getting Started

SR124 Analog Lock-In Amplifier

2 Introduction

In This Chapter

This chapter provides a basic overview of the SR124 Analog Lock-In
Amplifier.

2.1 Introduction to the instrument . . . . . . . . . . . . . 2 – 2

2.1.1 Clock stopping architecture . . . . . . . . . . . 2 –2

2.1.2 What does the SR124 measure? . . . . . . . . . 2 – 2

2.2 Instrument overview . . . . . . . . . . . . . . . . . . . 2– 3

2.2.1 Reference section . . . . . . . . . . . . . . . . . 2 – 3

2.2.2 Signal section . . . . . . . . . . . . . . . . . . . 2 – 6

2.2.3 Input Filter . . . . . . . . . . . . . . . . . . . . 2– 9

2.2.4 Output . . . . . . . . . . . . . . . . . . . . . . . 2 – 10

2.3 Navigating the front panel . . . . . . . . . . . . . . . 2 – 11

2.3.1 Signal input section . . . . . . . . . . . . . . . . 2 –12

2.3.2 Output section . . . . . . . . . . . . . . . . . . 2 – 14

2.3.3 Setup section . . . . . . . . . . . . . . . . . . . 2 – 15

2.3.4 Reference section . . . . . . . . . . . . . . . . . 2 – 17

2 – 1

2 – 2 Introduction

2.1 Introduction to the instrument

Lock-in amplifiers are used to detect and measure very small AC
signals, often in the presence of noise sources that can be orders of
magnitudes larger. Typical lock-in amplifiers today are based on
high-speed digital signal processing (DSP) techniques. This offers
outstanding performance and flexibility for many, perhaps most,
applications. For certain demanding situations, however, residual
artifacts from the DSP approach—either due to sampling lag and
real-time response, or RF clock and related noise—can fall short of
users needs.

The SR124 is a modern, all-analog lock-in amplifier that provides out­standing signal recovery capabilities, without the shortcomings that
can limit the usefulness of more common DSP-based instruments.
The design follows two basic themes. First, the signal path is entirely
built from low-noise analog electronics: the best JFETs, transistors,
op-amps, and discrete components. Second, configuration control
is managed by a microcontroller whose system clock only oscillates
during the brief moments needed to change gains or filter settings.

2.1.1 Clock stopping architecture

This “clock-stopping” architecture, first introduced by SRS in the
SR560 Voltage Preamplifier, eliminates the inconvenience and relia­bility issues associated with mechanical panel controls, and makes
full remote operation of the SR124 possible. Whenever the microcon­troller becomes active, the CPU Activity indicator illuminates, clearly
showing when the digital clock is running. This occurs in response
to front-panel button presses or remote computer commands.

Sometimes, you need to be certain your experiment will be undis­turbed: you’ve cooled your sample to a few millikelvin, all your
wiring is still intact, and the best device you’ve built all year is ready
for measurement. A locking toggle switch on the front panel can be
set to “INHIBIT”, forcing the digital clock to remain off, even if you
press other buttons or knobs. The analog configuration of the SR124
stays steady, letting you run for minutes, hours, days—as long as
you need.

2.1.2 What does the SR124 measure?

In lock-in mode, the SR124 multiplies the input signal by a square
wave at the reference frequency, using a square-wave analog mixer.
The resulting signal is then low-pass filtered to produce an output
proportional to the frequency component of the user’s input signal at
the reference frequency (and also, to a diminishing extent, at it’s odd

SR124 Analog Lock-In Amplifier

2.2 Instrument overview 2 – 3

harmonics). This process is also known as phase-sensitive detection,
and the square-wave mixer is also referred to as the phase-sensitive
detector (PSD) or the synchronous detector.

A separate AC Volt mode allows the square-wave mixer to be con-
trolled by the signal polarity of the input signal itself, essentially con­verting the SR124 into an absolute-value averaging measurement.
When a phase reference signal is not available, the AC Volt mode
(together with the input filter) can be used to recover specific AC sig­nals as well, although the lock-in mode will almost always produce
superior results when a reference is available.

The final measured value is displayed on the jeweled panel meter,
and output as a proportional DC voltage. Full-scale sensitivity is
set from the front panel in 1 – 2 – 5 steps from 100 nV to 500 mV. A
full scale input in-phase with the reference will generate +10 V at the
output BNC, while a full scale signal 180◦out of phase will generate

−10 V. Lock-in amplifiers as a general rule display the input signal
in volts RMS, and this is the basis for the SR124 calibration as well.
For example, if the SR124 is configured for 2 mV sensitivity and a
1 mVrms sine wave is input, at the reference frequency, then the BNC
output will read +5 V and the panel meter will show 50% positive
deflection.

2.2 Instrument overview

2.2.1 Reference section

An overview of the SR124 with its main sections is given below.
Further details of each block are in chapter 3. A block diagram of
the SR124 is given in Figure 2.1.

Operating in Lock-In mode, the SR124 requires the reference oscil­lator to control the frequency and phase of the square-wave mixer.
The analog reference oscillator of the SR124 generates a stable sine
wave with outputs at 0◦, 90◦, 180◦, and 270◦(all four outputs can
be monitored from rear panel BNC connectors). The oscillator is
based on an analog voltage-controlled oscillator (VCO) that can tune
across a factor of 100 in frequency; 5 overlapping frequency ranges
are available for operation from0.2 Hz – 21 Hz, up to 2 kHz – 210 kHz.
The oscillator is controlled in one of three ways: internal, rear-panel
VCO, and external reference. When the SR124 operates in AC Volt
mode, the reference oscillator is available for excitation outputs, but
is not routed to the PSD.

SR124 Analog Lock-In Amplifier

2 – 4 Introduction

A / I

B

PREAMP

AC GAIN DC GAIN

OUTPUT

REF OUT

SIGNAL

MONITOR

OFFSET

DC
BIAS

INTERNAL

TIME

CONSTANT

PSD

FL OAT

A

A–B

LOCK-IN

AC VOLT

INPUT
FILTER

PHASE

SHIFT

REF

ATTEN

+

–

+

–

×10

+

–

×1

REAR VCO

EXT. IN

PLL

VCO

90°

0°

SQUARE

600 Ω

600 Ω

50 Ω

2.2.1.1 Internal mode

2.2.1.2 Rear-panel VCO mode

2.2.1.3 External mode

Figure 2.1: The SR124 block diagram.

Operating in Internal mode, the SR124 oscillator is programmed by
an internally-generated DC voltage source. Within the oscillator

frequency range (set by the



Range knob), the oscillator frequency is



controlled with the front-panel control knob or remote command.

When configured for Rear VCO operation, the SR124 oscillator pro­gramming voltage is directly controlled by the user through a rear­panel BNC input. Input voltage from 0 to +10 V will set the oscillator
frequency between the lower and upper limits set by the range, with
an approximately linear transfer function.

The SR124 oscillator can lock to an external reference signal applied
to the Ext. In BNC connector. This input operates in two distinct
modes, sine input and TTL input (indicated by the TTL indicator
just above the connector).





SR124 Analog Lock-In Amplifier