SRS Labs SR124 Operation Manual

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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.

Stanford Research Systems, Inc. 1290–D Reamwood Avenue Sunnyvale, CA 94089 USA Phone: (408) 744-9040 • Fax: (408) 744-9049

www.thinkSRS.com • e-mail: info@thinkSRS.com

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

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.

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

PREAMP

AC GAIN DC GAIN

OUTPUT

REF OUT

SIGNAL

MONITOR

OFFSET

DC BIAS

INTERNAL

TIME

CONSTANT

PSD

FL OAT

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

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

2.2 Instrument overview 2 – 5

In sine mode, this input is AC coupled above 0.016 Hz (10 s time constant), and has an (AC) input impedance of 1 MΩ. A sine wave input greater than 100 mVrms for frequencies above 2 Hz (500mV below 2 Hz) will trigger the input discriminator. Positive zero crossings are detected and considered to be the zero for the reference phase shift. Note that, because the input is AC coupled, the discriminator circuit actually triggers when the input signal crosses the DC average input value in the positive direction.

When a user has a square wave or other TTL-like signals, the Ext. In should be operated in TTL mode. In this conﬁguration, the input is DC coupled, and theinput discriminatortriggers on positive edges as they cross +1 V. In TTL mode, there is no restriction on the reference input duty factor, so long as the input pulses are at least 100 ns wide.

Operating in external mode, the user can select between locking to the fundamental of the input frequency, or either of the ﬁrst two harmonics. Locking to 2f External will cause the reference oscillator to function at twice the external input frequency; locking to 3f External will similarly cause the reference oscillator to operate at three times the external input frequency. Note that, for harmonic operation, the Range setting must correspond to the ﬁnal frequency for the reference oscillator, which might not include the user’s external input frequency.

2.2.1.4 Reference output

The SR124 reference oscillator drives the front-panel Ref. Out BNC signal. This output can be conﬁgured as either sine wave or square wave. The amplitude of the reference output can be set from 10 V to 100nVrms; at several points passive resistive attenuators are switched in to reduce the signal amplitude while keeping a high signal-to-noise ratio on the Ref. Out signal.

The Ref. Out signal can also be DC biased, allowing users to more easily perform experiments such as diﬀerential conductance measurements without additional instrumentation. When enabled, the DC Bias is added to the reference oscillator output; the range of DC Bias is dependent on thereference amplitude, as the Bias and AC both are routed through the same resistive attenuators. See section 3.4 for the detailed interdependence of DC Bias and reference amplitude.

The rear panel Reference Output monitors are not shifted by the DC Bias setting, and they are not attenuated by the amplitude setting; these 4 “quadrant” monitors provide 1 V outputs for auxiliary use.

SR124 Analog Lock-In Ampliﬁer

2 – 6 Introduction

2.2.1.5 Phase shift

A commandable phase shift, between 0◦and 360◦, is applied between the reference oscillator and the phase-sensitive detector. This phase shift determines the phase at which the lock-in will respond. If the Ref. Out signal is directly connected to the Signal Input (A), and the phase is set to 90◦, the output will be near zero; setting the phase to 180◦will result in a negative output signal.

2.2.2 Signal section

The SR124 Signal section provides the ampliﬁcation and signal conditioning of the analog signal that is applied to the phase sensitive detector. Inputs can be either voltage or current, and single-ended or diﬀerential.

2.2.2.1 Voltage preampliﬁer

The SR124 front-end signal input stage consists of a JFET-based lownoise diﬀerential voltage preampliﬁer. Inputs can be conﬁgured as either single-ended (A) or diﬀerential (A–B). To preserve the low noise performance of the input JFET’s, this front-end input stage ampliﬁes the input by a gain of 50× (5×for the largest scale sensitivities).

2.2.2.2 Current preampliﬁer

In single-ended operation, the SR124 internally measures the signal as a voltage on the center pin of the A input, referenced to internal ground in the instrument. This is indicated as A on the front panel. In fully diﬀerential operation, the preampliﬁer measures the voltage diﬀerence between the center pins of the A and B inputs. This conﬁguration is indicated as A–B on the front panel.

When using diﬀerential input mode, it is important that both input cables travel the same path between the experiment and the lock-in. Speciﬁcally, there should not be a large loop area enclosed by the two cables. Such loops are susceptible to magnetic pickup. Ideally, the two coax cables are equal length and fastened to each other along their length.

When used with an SRS external preamp, such as any model from the SR55x series, the SR124 should always be conﬁgured for A–B input, and two equal-length BNC cables should be used to connect the preamp output to the lock-in signal input.

The current input on the SR124 uses the A input BNC. Two internal gain settings are available: 106Volts/Amp and 108Volts/Amp. The current input is always DC coupled and includes an input (burden)

SR124 Analog Lock-In Ampliﬁer

2.2 Instrument overview 2 – 7

resistor of 100 Ω (106range) or 1 kΩ (108range). The maximum fullscale input signal is 500 nA (106range) or 5 nA (108range). Selecting AC input coupling will block the DC output of the current ampliﬁer before it is further ampliﬁed by the voltage preampliﬁer.

Current (transimpedance) ampliﬁers can be susceptible to noise peaking or oscillation when driven with excessive input capacitance. Cable capacitance in particular should be minimized when using the current ampliﬁer by selecting the shortest cables practical. The SR124 current inputs will remain stable for total input capacitances below 12 nF. Note, however, that external input capacitance will increase the voltage noise gain of the current ampliﬁer; input capacitance should always be minimized for best performance.

The overall sensitivity of the SR124 in current mode is dependent on the Sensitivity setting. The current preampliﬁer itself converts the input current signal to a low-level voltage with the speciﬁed transimpedance gain (106or 108); the resulting voltage signal is then ampliﬁed by the entire signal chain including the voltage preampliﬁer. The overall full-scale sensitivity is determined by dividing the Sensitivity setting by the current gain. For example, if the Sensitivity is set to 20 mV and the input is conﬁgured as 108V/A, the full-scale sensitivity will be 200 fA (20 mV / 108V/A).

2.2.2.3 Grounding

In most cases, there is little noise improvement for current inputs by selecting dynamic reserve of Low Noise. The greatest stability is achieved with High Res.

To minimize noise pick-up, it is important to ground the outer shield of the input cable(s). Grounding the input cable at both ends, however, can in some situations introduce unwanted ground loops to the experiment. This has the potential to allow stray magnetic ﬂux to induce ground currents to ﬂow through the shield, creating additional noise and potentially upsetting sensitive measurements.

To help users better manage grounding, the SR124 provides control of the local grounding of the input BNC shields. When set to Ground, the shields on the A and B input connectors are electrically tied to the SR124 ground. When set to Float, however, a 10 kΩ resistor is added in series between the connector shells and instrument ground. This 10 kΩ resistor is large enough to block ﬂux-generated ground currents, while still preventing stray charge from accumulating on the connector shell. If the user’s signal source already provides a good, low-impedance connection between the signal shield and ground, then selecting Float may eliminate potential ground-loop problems.

SR124 Analog Lock-In Ampliﬁer

2 – 8 Introduction

2.2.2.4 AC versus DC coupling

The signal input can be AC or DC coupled. The AC coupling high pass ﬁlter passes signals above 160 mHz and attenuates signals at lower frequencies. Internal gain stages within the signal path are always AC coupled, so any user DC oﬀset will not aﬀect the results at the phase-sensitive detector output (where they would otherwise generate a “1 × f” ripple at the output). However, if the input signal has a signiﬁcant DC oﬀset relative to the AC amplitude, then AC coupling will signiﬁcantly improve the overall dynamic reserve by blocking the DC input before it could saturate the preampliﬁer.

When operating in diﬀerential mode (A–B), AC coupling may reduce the SR124’s eﬀective common-mode rejection and gain accuracy, as the blocking capacitors for the AC coupling are only matched to 5% tolerance. This eﬀect is most pronounced at frequencies below 10 kHz.

2.2.2.5 Dynamic reserve

The total signal gain for the SR124 is distributed between the AC signal path ahead of the PSD, and the DC signal path following the PSD. The product of all gains, AC and DC, combine to provide the selected full-scale Sensitivity. How this allocation is made between AC and DC gain determines the dynamic reserve of the lock-in.

Dynamic reserve is traditionally deﬁned as the ratio of the largest interfering signal that can be tolerated, to the full scale input signal, expressed in dB. For example, if the SR124 is operating at full scale sensitivity of 1 µV, and an interfering signal of up to 1 mV can be rejected before overloading, the dynamic reserve is 60 dB.

Many of the noise advantages of a lock-in ampliﬁer come from the improved noise andstability properties of electronics operating at AC frequencies compared to DC. Overall total gain is determined by the full-scale sensitivity, but the relative allocation of that gain between the AC and DC portions of the instrument rely on a “policy” decision that the user can inﬂuence using the Reserve setting.

For the lowest noise and greatest output stability, set the Reserve mode to Low Noise. This will allocate the maximum gain to the AC portion of the signal path, and apply the minimum DC gain needed for the ﬁnal sensitivity. This conﬁguration minimizes the impact of oﬀsets and drift from the DC circuitry, but also tends to reduce the dynamic reserve since more of the signal gain is applied to the AC path, before the PSD can act to select the signal of interest.

For the greatest dynamic reserve, set the Reserve mode to High Res. This will allocate the maximum gain to the DC portion of the signal

SR124 Analog Lock-In Ampliﬁer

2.2 Instrument overview 2 – 9

path, and apply the minimum AC gain needed for the ﬁnal sensitivity. This conﬁguration will have worse oﬀset and drift behavior compared with Low Noise, since the DC gain stages are providing more of the overall total gain. However, larger interfering signals can be tolerated without overloading the AC circuits, and the PSD will then tend to greatly suppress that interference.

Between these two settings, a compromise Normal mode is also available, which provides more DC gain than the Low Noise setting, but less than the High Reserve setting.

See section 3.2 for more details about the exact gain allocations and overload limits for these three modes.

2.2.3 Input Filter

The phase-sensitive detector is the primary feature for optimizing recovery of small signals in the presence of noise. However, the programmable input ﬁlter can be a helpful supplement in optimizing the SR124’s performance. In applications with signiﬁcant noise or other interference, the magnitude of the interfering signals can limit the total amount of AC gain that can be used before the mixer. The input ﬁlter is available to suppress those interfering signals, allowing greater AC gain to be used for better low-level signal recovery.

The input ﬁlter is located in the AC signal path, between the preampliﬁer and the programmable AC gain stage. The input ﬁlter’s function can be selected as Low Pass, High Pass, Band Pass, or Notch ﬁlter. The ﬁlter can also be bypassed by selecting Flat. The ﬁlter is realized as a two-pole state variable circuit, allowing ﬁne control of the ﬁlter tuning parameters. The input ﬁlter is typically used to either selectively pass a frequency range that spans the input signal, or to selectively reject one or more interfering signals at frequencies removed from the signal; these two approaches are sometimes indistinguishable.

The ﬁlter also has a user-conﬁgurable “Q” setting, which controls the relative width of the ﬁlter’s frequency response. In band pass and notch settings, higher Q settings provide a narrower ﬁlter response, allowing more selective frequency selection; lower Q settings wider ﬁlters, with broader frequency selection. Note that for the low pass and high pass settings, the ﬁlter gain is calibrated for unity gain at the peak response—the response across the pass band far from the peak response attenuates the signal by a factor of 1/Q. See section 3.3 for the detailed ﬁlter transfer functions of the SR124 input ﬁlter.

SR124 Analog Lock-In Ampliﬁer

2 – 10 Introduction

2.2.4 Output

The DC portion of the SR124, beginning with the output of the phasesensitive detector, is collectively referred to as the output section.

2.2.4.1 Output ﬁlter

The output of the phase sensitive detector contains many signals. Most of the output signals are at the sum or diﬀerence frequency between an input signal frequency and the reference frequency. Only the component of the input signal whose frequency is exactly equal to the reference frequency will result in a DC output. To remove all the unwanted AC signals, both the “2 × f” (sum of the signal and reference) as well as the noise components, a conﬁgurable low-pass ﬁlter is used.

The output low pass ﬁlter follows the mixer, and comes before the ﬁnal output DC gain is applied. This ﬁlter is key to the performance of the lock-in ampliﬁer, as it selects the DC output of the mixer while rejecting the high frequency ripple artifacts naturally generated by the mixing process. The ﬁlter can be conﬁgured as either 1 pole or 2 poles, with a time constant between 1 ms and 300 s settable in 1–3–10 steps.

The time constant is related to the −3 dB frequency of the ﬁlter by the relation f

= 1/(2πTC), where TC is the time constant in seconds.

-3dB

The low pass ﬁlters are simple 6 dB/octave roll oﬀ, RC-type ﬁlters. A 1 second time constant refers to a ﬁlter whose -3 dB point occurs at

0.16 Hz. In the SR124 the user can select one or two successive stages of output ﬁlter, so that the overall ﬁlter can roll oﬀ is either 6 dB or 12 dB per octave. The time constant refers to the −3 dB point of each ﬁlter stage alone (and not the combined ﬁlter).

The time constant also determines the equivalent noise bandwidth (ENBW) for the measurement. The ENBW is not the ﬁlter −3 dB bandwidth; rather it is the eﬀective bandwidth for Gaussian distributed white noise. When set to 6 dB/octave, theENBW is 1/(4×TC); when set to 12 dB/octave, ENBW = 1/(8 × TC).

It can be useful to consider the frequency domain, in which the output ﬁlter deﬁnes the width of the passband for detection at the reference frequency. By mixing with the lock-in ampliﬁer, this AC band is mixed down to DC for ﬁnal output. The −3 dB bandwidth for signal selection at the input is simply 1/(2π × TC). To eﬀectively reject the mixer AC artifacts, the output ﬁlter should be set so that TC > (2 – 10) × (1/f), where f is the reference oscillator frequency.

SR124 Analog Lock-In Ampliﬁer

2.3 Navigating the front panel 2 – 11

100

200

500

A/I B

Signal

Monitor

Q-factor

Time Constant

µV

AC DC

10 MΩ 10 MΩ 600 Ω 50 Ω

Ground

Flat

Notch

Low Noise

Normal

High Res.

12dB/oct.

6dB/oct.

Low Pass

ne

coarse

High Pass

Band Pass

Float

OVLD

AC Volt

Lock-In

INHIBITED

CPU Activity

TTL

903f0

2.0k 210k

Square Sine

200 21k

20 2.1k

2.0 210

0.2 21

18090

270180

270 360

Unlocked

Remote Activity

OFFSET

OUTPUT

DC Bias

Range (Hz)

Mode

Ref. Out

Ext. In

Quadrant

OVLD

A–B I (10

)

I (10

)

100

200

500

Push to select

Hold to

null

depth

f trim

100

300

All digital clocks and panel controls disabled during lockout

min

2f External f

Internal Rear VCO

kHz

PHASE FREQ AMPL BIAS

OFFSET

kHz V

deg

OVLD

REM

Push to select

Hold to

null

600 Ω

Push for

Ext. In TTL

local

Model SR124 Analog Lock-In Amplier

CONTROL INHIBIT

SETUP

AUTO

Stanford Research Systems

RESERVE

SENSITIVITY

SIGNAL INPUT

INPUT FILTER

OUTPUT

REFERENCE

Input

Ground

Type

Reserve

Slope

On/O

Modify

Mode

Phase

Gain

Oset

Lock Assist

Save Recall

Phase

Freq

Ampl

On/O

Modify

Shape

Couple

BNC

knob

switch

knob

BNC BNC BNC BNC BNC

2.2.4.2 AC Volt mode

When phase-sensitive detection is not possible, the SR124 can be conﬁgured for AC Volt mode. In this setting, the reference oscillator is unused and the phase-sensitive detector (square wave mixer) is controlled directly by the polarity of the ampliﬁed AC input signal. The instrument now acts as an averaging AC voltmeter, where the detection element is an absolute value detector.

For most applications using AC Volt mode, the Reserve setting should be set to Low Noise, since there is no reserve beneﬁt from the mixer in this conﬁguration. If the input signal is not already relatively clean with high signal-to-noise, the input ﬁlter may be used to deﬁne the frequency band for measurement.

2.3 Navigating the front panel

The front panel of the SR124 is organized into distinct functional sections. Knowing this organization will help you to become familiar with its operation. A diagram of the entire front panel is in Figure 2.2, below.

Figure 2.2: The SR124 front panel.

In Figure 2.2, the two large rectangular blocks above “INPUT FILTER” and “REFERENCE” are static numeric displays. The larger white rectangle in the upper center of the drawing is the jewel bearing analog panel meter.

The two large knobs each have a push-button secondary function; in addition to adjusting the parameter currently “in focus” (more

SR124 Analog Lock-In Ampliﬁer

about that later), brieﬂy pressing the knob inward has a secondary function. Also, holding the knob in for several seconds has a tertiary function—clearing or nulling the parameter in focus.

2 – 12 Introduction

100

200

500

A/I B

Signal

Monitor

Q-factor

µV

AC DC

10 MΩ 10 MΩ 600 Ω

Ground

Flat

Notch

Low Noise

Normal

High Res.

Low Pass

High Pass

Band Pass

Float

OVLD

A–B I (10

)

I (10

)

100

200

500

depth

f trim

kHz

Push to select

Hold to

null

Stanford Research Systems

RESERVE

SENSITIVITY

SIGNAL INPUT

INPUT FILTER

knob

BNC BNC BNC

Input

Ground

Type

Reserve

Couple

Near the center of the instrument, in the “CONTROL INHIBIT” block, is a large locking toggle switch. When in the upper position, none of the front-panel (or remote) interface functions are operable. The analog signal processing of the SR124 remains fully functioning while the Inhibit switch is up, but no controls will respond. For normal operation, the Inhibit switch should be left in the lower position.

Where applicable, the corresponding remote command is listed, along with the page where it is deﬁned, in parentheses, like this: (FORM, 4 – 11 ).

2.3.1 Signal input section

The left-hand section of the instrument comprises the “Signal Input”. See Figure 2.3 for detail.

2.3.1.1 Signal input

Figure 2.3: The SR124 front panel signal input section.

User input signals are applied to the SR124 through the “A/I”, or the “A/I” and “B” BNC connectors. The input conﬁguration is controlled

SR124 Analog Lock-In Ampliﬁer

2.3 Navigating the front panel 2 – 13

by the [Input] button. Each successive press of [Input] steps from A, to A–B, to I (106), to I (108), and then back to A. (ISRC, 4 – 11 )

Input coupling can be switched between AC and DC by pressing [Couple]. (ICPL, 4 – 12 )

The outer (shield) terminal of both input BNCs are tied to each other and can be set to Float (through 10 kΩ) or Ground (through 10 Ω) by pressing [Ground]. (IGND, 4 – 12 )

Overloads at the preampliﬁer stage are indicated by the red OVLD indicator in the signal input block.

2.3.1.2 Sensitivity

The overall gain of the SR124 is controlled by setting the sensitivity. Full scale sensitivities from 100 nV to 500 mV, in 1–2–5 steps, can be

selected by turning the



SENSITIVITY knob. (SENS, 4 – 13 )







Overloads in the AC signal path, after the preampliﬁer but before the phase sensitive detector, are indicated by the red OVLD indicator in the sensitivity block.

2.3.1.3 Input ﬁlter

2.3.1.4 Reserve

The input ﬁlter type is selected by pressing [Type]; selections cycle from Band Pass, High Pass, Low Pass, Notch, and Flat. (TYPF, 4 – 12 )

The ﬁlter Q-factor can be adjusted between 1 and 100 by turning the



Q-factor knob. (QFCT, 4 – 12 )



The tuning frequency of the ﬁlter is adjusted by turning the large



INPUT FILTER knob. This knob responds with velocity sensitivity,











so turning the knob more quickly will span larger frequency ranges quickly. The frequency setting is displayed on the numeric display, with either Hz or kHz illuminated. (IFFR, 4 – 12 )

When operating the input ﬁlter as Band Pass or Notch type, and at high Q-factor, it is often necessaryto trim the input ﬁlter performance.

Pressing



INPUT FILTER cycles the knob’s focus between the main







frequency tune, a (dimensionless) trim oﬀset of the frequency(f trim ), and a notch depth adjust (depth ). When adjusting f trim or depth, the units annunciators (Hz and kHz) are both oﬀ. (IFTR, 4 – 12 )

SR124 Analog Lock-In Ampliﬁer

The dynamic reserve setting of the SR124 can be cycled between High Res., Normal, and Low Noise by successive presses of [Reserve].

2 – 14 Introduction

Time Constant

12dB/oct.

6dB/oct.

OVLD

AC Volt

Lock-In

OFFSET

OUTPUT

100

300

300min

OVLD

600 Ω

OUTPUT

knob

BNC

Slope

On/O

Modify

Mode

(RMOD, 4 – 13 )

The output of the entire AC signal chain, just prior to the phase sensitive detector, can be monitored from the “Signal Monitor ” BNC.

2.3.2 Output section

The next section of the instrument is “Output”. See Figure 2.4 for detail.

2.3.2.1 Time constant

Figure 2.4: The SR124 front panel output section.

The panel meter shows the output signal from the SR124. This is the same signal as appears at the output BNC. The meter shows +100% deﬂection when the BNC output is +10 V, and −100% deﬂection when the BNC output is −10 V.

If any part of the signal path is overloaded, the master OVLD annunciator, at the upper right of the panel meter, is lit.

The output ﬁlter time constant is set with the (OFLT, 4 – 14 )



Time Constant knob.







SR124 Analog Lock-In Ampliﬁer

2.3 Navigating the front panel 2 – 15

The choice of one or two poles of output ﬁlter is toggled by pressing [Slope]. (OFSL, 4 – 14 )

If the DC output signal is overloaded, the red OVLD indicator (just above the time constant control) is lit.

2.3.2.2 Offset

The output oﬀset function is enabled and disabled by pressing the [On/Oﬀ] button within the OFFSET block. (OFSE, 4 – 14 )

2.3.2.3 Output

2.3.3 Setup section

2.3.3.1 Automatic functions

Pressing [Modify] within the OFFSET block switches the focus of the REFERENCE block to the oﬀset value; turning the



REFERENCE



knob will then adjust the oﬀset between −1000% and +1000% of full scale. (OFST, 4 – 15 )

The overall functional mode of the SR124 is controlled by the [Mode] button within the OUTPUT block. Pressing [Mode] toggles between Lock-In and AC Volt modes. (OMOD, 4 – 14 )

The output signal from the SR124 is available on the BNC output connector within the OUTPUT block. Full scale at the output is ±10 V.

The next section of the instrument is “Setup”. See Figure 2.5 for detail.

Four built-in automatic functions are available in this block.





SR124 Analog Lock-In Ampliﬁer

Pressing [Phase] within AUTO starts an auto-phase cycle, which adjusts the oscillator phase to maximize the output signal. Autophase is disabled when output oﬀset is enabled. (APHS, 4 – 16 )

Pressing [Gain] starts an auto-gain cycle, which will increase the gain (decrease the sensitivity) step by step to maximize the output without causing a signal overload. (AGAN, 4 – 16 )

Pressing [Oﬀset] starts an auto-oﬀset cycle, which will adjust the OFFSET setting to null the output signal. Note that the auto-oﬀset cycle does not change the enabled/disabled state for OFFSET. Running auto-oﬀset with oﬀset disabled will set and report the oﬀset parameter that would null the output if oﬀset were enabled, but the output is unaﬀected. (AOFF, 4 – 16 )

2 – 16 Introduction

INHIBITED

CPU Activity Remote Activity

All digital clocks and panel controls disabled during lockout

local

CONTROL INHIBIT

SETUP

AUTO

switch

Phase

Gain

Oset

Lock Assist

Save Recall

2.3.3.2 Setup

Figure 2.5: The SR124 front panel setup.

Pressing [Lock Assist] starts a measurement cycle for the Reference oscillator Ext. In, to speed the locking to a user ’s reference when operating in external mode. (ASST, 4 – 17 )

A total of nine (9) separate user conﬁgurations can be saved in the SR124. Pressing [Save] shifts the focus to saving; the



REFERENCE



knob now scrolls between 0-8 to select one of the nine user slots. Pressing [Save] a second time saves the current instrument conﬁguration into that slot; pressing any other key abandons the save request. (SSET, 4 – 15 )

The [Recall] key is used to restore a previously saved conﬁguration. Press [Recall] once to bring up the recall focus, and then turn



REFERENCE to select between user setting 0 through 8. An addi-



tional conﬁguration, the factory defaults, can be restored by turning



REFERENCE to display “dEFLt”. When the desired settings slot is











displayed, press [Recall] again to restore those settings. Pressing any other key will abandon the recall request. (RSET, 4 – 15 )

When the SR124 is in REM mode (remote control), pressing the [Re-





SR124 Analog Lock-In Ampliﬁer

2.3 Navigating the front panel 2 – 17

50 Ω

ne

coarse

TTL

2.0k 210k

Square Sine

200 21k

20 2.1k

2.0 210

0.2 21

18090

270180

270 360

Unlocked

DC Bias

Range (Hz)

Mode

Ref. Out

Ext. In

Quadrant

Push to select

Hold to

null

2f External f

Internal Rear VCO

PHASE FREQ AMPL BIAS

OFFSET

kHz V

deg

REM

Push for

Ext. In TTL

Model SR124 Analog Lock-In Amplier

REFERENCE

knob

BNC BNC

Phase

Freq

Ampl

On/O

Modify

Shape

call] button asserts the “Local” function and returns the instrument to local mode. (LOCL, 4 – 19 )

2.3.3.3 Control inhibit

The SR124 user interface controls can be inhibited by setting the control inhibit switch to the upper position. Note that this is a locking toggle switch, and must be gently pulled outwards while switching upwards or downwards.

When in the upper position, the INHIBITED indicator is illuminated to alert the user that controls and remote commanding are all inhibited.

2.3.4 Reference section

The next section of the instrument is “Reference”. See Figure 2.6 for detail.

SR124 Analog Lock-In Ampliﬁer

Figure 2.6: The SR124 front panel reference.

The numeric display for the Reference section can show the settings for phase, reference oscillator frequency, amplitude, DC bias, output oﬀset, and also the save/recall slot. This selection is generally

2 – 18 Introduction

called the “focus” of the interface, and is indicated by one of the text annunciators at the bottom of the numeric display. For most

of the parameters, turning the large



REFERENCE knob allows that







parameter to be adjusted.

Pressing the



REFERENCE knob inward toggles between coarse and







ﬁne scrolling speeds for adjusting parameters. Holding the knob in for approximately 2 seconds will cause the parameter to be nulled (other than frequency and amplitude, which do not allow zero settings).

Within the REFERENCE section, pressing [Phase] brings the reference phase shift into focus. Note that in AC Volt mode, phase is unused (but may still be adjusted). (PHAS, 4 – 9 )

Pressing [Freq] brings frequency into focus. When operating in In- ternal mode, frequency can be adjusted across the 2 decades range selected by the Range block. (FREQ, 4 – 9 )

Pressing [Freq] when operating either in Rear VCO mode or one of the External modes causes the SR124 to perform a frequency measurement on the actual reference oscillator frequency. That frequency is then displayed on the numeric display.

2.3.4.1 Quadrant

2.3.4.2 Ext. In

If the focus is already on FREQ when the instrument changes mode into either Rear VCO or External, the display will show “- - - - ”; pressing [Freq] again will cause a frequency measurement to be performed and displayed. Each successive press of [Freq] initiates a new measurement.

Pressing [Ampl] brings the reference output amplitude into focus. Turning



REFERENCE adjusts the output amplitude (in Vrms).







(SLVL, 4 – 10 )

Turning the



Quadrant knob adds or subtracts 90◦from the current







value of the reference phase shift. One of the four quadrant indicators is always illuminated to show which quadrant the current value of phase lies in. (QUAD, 4 – 9 )

The Ext. In BNC input is used for locking the SR124 to an external reference; this input is unused when in Internal or Rear VCO mode. The Ext. In circuit can be conﬁgured for either sine inputs or TTL inputs; when in TTL mode, the TTL indicator is illuminated in this section. The user can toggle between sine and TTL mode by pressing



the

Mode knob (see below).







SR124 Analog Lock-In Ampliﬁer

2.3 Navigating the front panel 2 – 19

2.3.4.3 Mode



The

Mode knob selects the reference oscillator mode. Possible selec-







tions are Rear VCO, Internal, f External, 2f External, and 3f External. (FMOD, 4 – 9 )



The

Mode knob also controls the Ext. In conﬁguration between sine



and TTL modes. Pressing







Mode inwards toggles between the two







states, as indicated by the TTL indicator in the Ext. In sub-section. (RSLP, 4 – 10 )

2.3.4.4 Range



The

Range knob selects the reference oscillator frequency range.







Ranges each span 2 decades, and overlap by one decade each. (FRNG, 4 – 10 )

2.3.4.5 DC Bias

Pressing the [On/Oﬀ] button in the DC Bias block toggles the bias on and oﬀ for the reference output signal. (BION, 4 – 11 )

2.3.4.6 Ref. Out

Depending on the current value of the reference amplitude and bias, the bias might not be allowed to turn on. Full details of the interdependence of DC Bias and amplitude can be found in section 3.4.

Pressing the [Modify] button within DC Bias brings the display focus to BIAS. The bias setting can be adjusted by turning



REFERENCE ;



however, the bias will not be added to the reference output unless it is enabled with the [On/Oﬀ] button. (BIAS, 4 – 11 )

The Ref. Out BNC connector provides the refence output signal. The RMS amplitude of the signal is adjusted with the AMPL parameter. Pressing [Shape] toggles between sine output and square output. (FORM, 4 – 11 )





SR124 Analog Lock-In Ampliﬁer

2 – 20 Introduction

SR124 Analog Lock-In Ampliﬁer

3 Performance Details

This chapter provides a detailed discussion of the operating characteristics and architecture of the SR124.

In This Chapter

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

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

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

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

3 – 1

3 – 2 Performance Details

3.1 Sensitivity and gain

While not necessary to operate the SR124, many users will still ﬁnd detailed information about gain allocation helpful for optimizing measurements.

3.1.1 AC Gain

Table 3.1: AC gain allocation

Sensitivity Pre- Post- Mixer Sig. Mon. Total

High Res. Normal Low Noise gain gain gain gain AC gain

100 nV 500 100 10 5 ×10

200 nV 500 50 10 2.5 ×1042.5 ×10 500 nV 500 20 10 1.0 ×1041.0 ×10

100 nV 1 µV 500 100 10 5 ×10

200 nV 2 µV 500 50 10 2.5 ×1042.5 ×10 500 nV 5 µV 500 20 10 1.0 ×1041.0 ×10

100 nV 1µV 10 µV 500 10 10 5 × 10

200 nV 2µV 20 µV 500 5 10 2.5 × 1032.5 ×10 500 nV 5µV 50 µV 500 2 10 1 ×10

1 µV 10 µV 100 µV 500 1 10 5 × 10

2 µV 20 µV 200 µV 500 0.5 10 2.5 × 1022.5 ×10

5 µV 50 µV 500 µV 500 0.2 10 1 ×10 10 µV 100 µV 1 mV 500 1 1 5 ×10 20 µV

200 µV 2 mV 500 0.5 1 2.5 × 1022.5 ×10

50 µV 500 µV 5 mV 500 0.2 1 1 × 10

100 µV 1 mV 10 mV 50 1 1 5 × 10

200 µV 2 mV 20 mV 50 0.5 1 2.5 × 1012.5 ×10 500 µV 5 mV 50 mV 50 0.2 1 1 × 10

1 mV 10 mV 100 mV 5 1 1 5 5 2 mV 20 mV 200 mV 5 0.5 1 2.5 2.5

5 mV 50 mV 500 mV 5 0.2 1 1 1 10 mV 100 mV 5 1 1 5 5 20 mV 200 mV 5 0.5 1 2.5 2.5 50 mV 500 mV 5 0.2 1 1 1

100 mV 5 1 1 5 5 200 mV 5 0.5 1 2.5 2.5 500 mV 5 0.2 1 1 1

5 ×10

1 ×10 5 ×10

1 ×10

Programmable gain stages are located throughout the SR124, in both the AC and DC signal paths. While the actual gain elements can be

SR124 Analog Lock-In Ampliﬁer

3.1 Sensitivity and gain 3 – 3

found in the detailed schematics, it is useful to group the gain stages into several blocks:

Pre-gain : The AC gain preceding the tunable input ﬁlter.

Post-gain : The AC gain following the tunable input ﬁlter.

Mixer gain : The AC gain applied at the input to the phase sensitive detector.

DC gain : The DC gain applied following the mixer, and following the

output low-pass ﬁlter.

The front panel “Signal Monitor” BNC (located in the Reserve block) provides a buﬀered copy of the signal after it is ampliﬁed by the Pre-gain and Post-gain. The total AC gain, for all sensitivities and reserve settings, is given in Table 3.1.

The nominal overall gain, AC and DC combined, can be calculate from the equation

Nominal Gain =

10 V

(3.1)

where VFSis the full-scale sensitivity, in volts. For example, if the sensitivity is set to 200 µV, the nominal overall gain is (10 V)/(200 µV)=50,000.

3.1.2 Scale normalization

The SR124 is calibrated for RMS units, but the square wave demodulator actually measures an absolute value average. As a result, there can be confusion about the precise values of gain used in the instrument.

On a properly calibrated unit, the SR124 will output exactly 10.00 V when the input is VFS, a full-scale (RMS) sine wave, properly phased with the oscillator.

The input function V(t) for a full-scale sine wave is

V(t) =√2V

sin(2π f t + φ)

When properly phased for maximum signal, the PSD multiplies by the square-wave function

(

PSD(t) =

+1 : 0 ≤ (2π ft + φ) mod 2π < π

−1 : π ≤ (2π ft + φ) mod 2π < 2π

The time-averaged value of the product V(t)×PSD(t) can be evaluated by integrating the ﬁrst half-cycle of the input sine wave

√

2VFSsin(θ)dθ =

2√2

SR124 Analog Lock-In Ampliﬁer

3 – 4 Performance Details

The factor, 2√2/π = 0.9003, would cause the SR124 to read about 10% too low if the circuitry were tuned to provide a total gain as given by Equation (3.1). Instead, the actual total gain is set about 11% greater, to correct for this factor.

The correct formula for the actual overall gain is

3.1.3 DC gain

Actual Gain =

10 V

2√2

(3.2)

Continuing the example from above, for a sensitivity of 200 µV, the actual overall gain, given by Equation (3.2), is 55,536.

A natural consequence of this calibration is that, when the input waveform is a square wave, the output reads approximately 11% greater than the actual RMS value of the (square wave) input.

The DC gain is programmed by the SR124 to make up the diﬀerence between the total AC gain and the required Overall Gain from Equation (3.2). Continuing the example above, with full-scale sensitivity of 200 µV, we can see from Table 3.1 three possible AC gain conﬁgurations, based on the dynamic reserve setting. At Low Noise, the total AC gain is 2.5 × 103; for Normal reserve, the total AC gain is 250; while at High Res., we see the total AC gain is 25. Since in each case, the overall Actual Gain must be 55,536, we ﬁnd the DC gain is:

Reserve DC Gain

Low Noise 22.21

Normal 222.1

High Res. 2221

Low Noise : DC Gain is 222.1 for sensitivitys 100 nV, 200 nV, and 500 nV.

Normal : DC gain is 22.21 for sensitivities 100 mV, 200 mV, and 500 mV.

High Res. : DC gain is 222.1 for sensitivities 10 mV, 20 mV, 50 mV; and DC

3.2 Dynamic reserve and overloads

These three DC Gains are used for most of the sensitivity settings of the SR124. The exceptions are at the extreme values of sensitivity, and are as follows:

gain is 22.21 for sensitivities 100 mV, 200 mV, and 500 mV.

The SR124 provides signiﬁcant ﬂexibilty to recover small signals in the presence of noise and other interference: besides overall sensitivity control, the input ﬁlter and dynamic reserve settings can both

SR124 Analog Lock-In Ampliﬁer

3.2 Dynamic reserve and overloads 3 – 5

Table 3.2: Maximum input signals before Input Filter overload

Sensitivity Maximum

High Res. Normal Low Noise input (RMS)

100 nV 14.5 mV 200 nV 14.5 mV

500 nV 14.5 mV 100 nV 1 µV 14.5mV 200 nV 2 µV 14.5mV 500 nV 5 µV 14.5mV

100 nV 1µV 10 µV 14.5 mV 200 nV 2µV 20 µV 14.5 mV 500 nV 5µV 50 µV 14.5 mV

1 µV 10 µV 100 µV 14.5 mV 2 µV 20 µV 200 µV 14.5 mV

5 µV 50 µV 500 µV 14.5 mV 10 µV 100 µV 1 mV 14.5mV 20 µV 200 µV 2 mV 14.5mV 50 µV 500 µV 5 mV 14.5mV

100 µV 1 mV 10 mV 145 mV 200 µV 2 mV 20 mV 145 mV 500 µV 5 mV 50 mV 145 mV

1 mV 10 mV 100 mV 1.28 V 2 mV 20 mV 200 mV 1.28 V

5 mV 50 mV 500 mV 1.28 V 10 mV 100 mV 1.28 V 20 mV 200 mV 1.28 V 50 mV 500 mV 1.28 V

100 mV 1.28 V 200 mV 1.28 V 500 mV 1.28 V

SR124 Analog Lock-In Ampliﬁer

be used to optimize out of band rejection. However, to recover the signal of interest, no stage in the signal path can be permitted to overload.

Table 3.2 gives the maximum sinewave input (in Vrms) that can be applied to the signal input without overloading the pre-gain signal chain, which preceeds the input ﬁlter. For signals passed by the input ﬁlter (all signals, when in Flat ﬁlter type), Table 3.3 gives the maximum sinewave inputs that can be applied to the signal input without overloading any part of the AC signal path including the PSD (mixer).

3 – 6 Performance Details

Table 3.3: Maximum input signals before PSD overload

Sensitivity Maximum

High Res. Normal Low Noise input (RMS)

100 nV 7 µV 200 nV 24 µV

500 nV 63 µV 100 nV 1 µV 7 µV 200 nV 2 µV 24 µV 500 nV 5 µV 63 µV

100 nV 1µV 10 µV 130 µV 200 nV 2µV 20 µV 250 µV 500 nV 5µV 50 µV 650 µV

1 µV 10 µV 100 µV 1.3 mV 2 µV 20 µV 200 µV 2.5 mV

5 µV 50 µV 500 µV 6.5 mV 10 µV 100 µV 1 mV 12.5mV 20 µV 200 µV 2 mV 14 mV 50 µV 500 µV 5 mV 14 mV

100 µV 1 mV 10 mV 129 mV 200 µV 2 mV 20 mV 160 mV 500 µV 5 mV 50 mV 160 mV

1 mV 10 mV 100 mV 1.25 V 2 mV 20 mV 200 mV 1.25 V

5 mV 50 mV 500 mV 1.25 V 10 mV 100 mV 1.25 V 20 mV 200 mV 1.25 V 50 mV 500 mV 1.25 V

100 mV 1.25 V 200 mV 1.25 V 500 mV 1.25 V

3.3 Input ﬁlter details

The input ﬁlter is constructed as a state-variable ﬁlter with usersettable cutoﬀ frequency f0and Q-factor. In the SR124, the peak gain of the input ﬁlter is calibrated to be unity. The nominal transfer functions are shown in the following ﬁgures.

Of particular note is that the pass-band portion of the low pass and high pass ﬁlters have a gain of 1/Q far from the resonance. For most applications requiring low pass or high pass input ﬁltering, it is eﬀective to leave Q=1.

SR124 Analog Lock-In Ampliﬁer

3.3 Input ﬁlter details 3 – 7

-2

-1

-60

-50

-40

-30

-20

-10

Band Pass

Normalized frequency (f/f

)

Filter response (dB)

Q = 1

100

-2

-1

-60

-50

-40

-30

-20

-10

High Pass

Normalized frequency (f/f

)

Filter response (dB)

Q = 1

100

SR124 Analog Lock-In Ampliﬁer

Figure 3.1: The SR124 band pass input ﬁlter gain

Figure 3.2: The SR124 high pass input ﬁlter gain

3 – 8 Performance Details

-2

-1

-60

-50

-40

-30

-20

-10

Normalized frequency (f / f0)

Filter response (dB)

Low Pass

Q = 1

100

-0.3

-0.2

-0.1

0.1

0.2

0.3

-60

-50

-40

-30

-20

-10

Notch Filter

Normalized frequency (f/f

)

Filter response (dB)

Q = 1

100

Figure 3.3: The SR124 low pass input ﬁlter gain

Figure 3.4: The SR124 notch input ﬁlter gain

SR124 Analog Lock-In Ampliﬁer

3.4 Attenuators and DC bias constraints 3 – 9

3.4 Attenuators and DC bias constraints

The SR124 Reference Output includes several resistive output attenuators to provide low excitations for sensitive experiments. The architecture imposes constraints on the magnitude of DC bias available, depending on the current reference amplitude setting. This constraint is because the DC bias signal is passed through the same resistive attenuator network as the AC reference signal.

Table 3.4 shows the four attenuator ranges, set by the reference output amplitude. Note that any time one of the attenuators is switched in or out (when the amplitude is commanded across one of the bounderies listed in the table), the actual output waveform is zeroed for approximately 20 ms. Also note that, for large values of DC bias and amplitude, the actual output voltage will saturate outside of the range ±14.5 V DC.

Table 3.4: Available DC Bias values by Amplitude setting

AC Amplitude DC Bias DC Bias

(RMS) Range Resolution

10 mV – 10 V ±10 V 1 mV

100 µV – 9.99 mV ±100 mV 10 µV

1 µV – 99.9 µV ±1 mV 100nV

100 nV – 990 nV ±100 µV 10 nV

3.5 Automatic functions

The various automatic functions of the SR124 all involve the instrument performing some internal measurement and then adjusting a parameter as a result. This section discusses the constraints and required time for the functions.

3.5.1 Auto-phase

The automatic phase cycle will adjust the phase setting of the SR124 to maximize the detected signal in Lock-in mode. Three steps occur in an auto-phase cycle:

1. The phase shift is internally set to 0◦, and the instrument then

2. The phse shift is then internally set to 90◦, and the instrument

SR124 Analog Lock-In Ampliﬁer

pauses for 8 output Time Constant periods (for settling). At the end of the settling time, the voltage at the output is internally measured.

again waits 8 Time Constants for settling. At the end of this settling time, the output voltage is again measured.

3 – 10 Performance Details

3. From the four-quadrant arctangent of the two preceding measurements, the SR124 determines the nominal phase shift required for the user input signal, and programs the PHASsetting accordingly.0

The entire auto-phase cycle requires 16× the output Time Constant setting to execute. For example, if the Time Constant is set to 3 s, the auto-phase will require 48 seconds.

3.5.2 Auto-gain

The automatic gain cycle adjusts the sensitivity setting of the SR124 to maximize the output signal without causing an overload.

If the instrument is not in an overload state when auto-gain is started, then the unit will begin decreasing the sensitivity (increasing the gain), one step at a time, until an overload is detected. At each setting, the SR124 pauses for 5 output Time Constants, or 500 ms (whichever is longer), for settling.

If no overload is ever detected, the instrument stops at 100 nV sensitivity. If an overload is detected, the sensitivity is increased one setting back to the point overload was not ﬁring.

3.5.3 Auto-offset

3.5.4 Lock assist

If the SR124 was already in Overload when auto-gain is started, then the sensitivity is increased (gain decreased), one step at a time, until the overload is cleared. The same delay of the longer of 5 time constants or 500 ms is used at each setting.

The automatic oﬀset cycle measures the current output voltage, and adjusts the output oﬀset setting to attempt to null the current output. Since the measurement and oﬀset driver both occur after the output ﬁlter time constant is applied, this function always executes promptly. Note that for proper results, the SR124 output should be stable before starting an auto-oﬀset cycle.

The lock assist function is only used in external mode and can speed the PLL acquisition of a user’s reference signal.

The lock assist ﬁrst begins a frequency measurement of the Ext. In signal to determine the user input frequency. This step requres the greater of 2 s or 2 periods of the input signal.

The SR124 then determines if the frequency is in-range for the current setting of the reference Range (accounting for 2 × f or 3× f operation,

SR124 Analog Lock-In Ampliﬁer

3.5 Automatic functions 3 – 11

if necessary).

If the frequency is valid for the current range, then the reference oscillator is temporarily halted, and the PLL tuning ﬁlter is “precharged” to the correct voltage for the desired frequency.

The ﬁnal step in the process is to arm a comparator that waits for the next positive zero-crossing of the external frequency reference. When that comparator ﬁres, the VCO oscillator is un-haulted, and the oscillation begins (approximately) in-phase with the external reference.

SR124 Analog Lock-In Ampliﬁer

3 – 12 Performance Details

SR124 Analog Lock-In Ampliﬁer

4 Remote Operation

In This Chapter

This chapter describes operating the SR124 over the remote interfaces.

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

4 – 1

4 – 2 Remote Operation

4.1 Index of commands

Symbol Deﬁnition

f, g Floating-point value i, j Unsigned integer z Literal token

(?) Required for queries; illegal for set commands

var Parameter always required

{var} Required parameter for set commands; illegal for queries [var] Optional parameter for both set and query forms

Reference and phase

PHAS(?) {f } 4 – 9 Reference phase QUAD(?) {z} 4 – 9 Phase quadrant FMOD(?) {z} 4 – 9 Reference mode FREQ(?) {f } 4 – 9 Reference oscillator frequency SLVL(?) {g} 4 – 10 Reference output amplitude RSLP(?) {z} 4 – 10 Reference slope shape FRNG(?) {z} 4 – 10 Reference oscillator range BION(?) {z} 4 – 11 Reference DC bias enable BIAS(?) {g} 4 – 11 Reference DC bias magnitude FORM(?) {z} 4 – 11 Reference output waveform

Input

ISRC(?) {z} 4 – 11 Input source IGND(?) {z} 4 – 12 Input shield grounding ICPL(?) {z} 4 – 12 Input coupling

Filter

TYPF(?) {z} 4 – 12 Input ﬁlter type QFCT(?) {z} 4 – 12 Input ﬁlter Q-factor IFFR(?) {f } 4 – 12 Input ﬁlter frequency IFTR(?) {g} 4 – 12 Input ﬁlter frequency trim NCHD(?) {g} 4 – 13 Input ﬁlter notch depth trim

Gain and time constant

SENS(?) {z} 4 – 13 Input sensitivity RMOD(?) {z} 4 – 13 Reserve mode OFLT(?) {z} 4 – 14 Output ﬁlter time constant OFSL(?) {z} 4 – 14 Output ﬁlter slope

Output

OMOD(?) {z} 4 – 14 Output mode

SR124 Analog Lock-In Ampliﬁer

4.1 Index of commands 4 – 3

OFSE(?) {z} 4 – 14 Output oﬀset enable OFST(?) {g} 4 – 15 Output oﬀset magnitude

Setup

KCLK(?) {z} 4 – 15 Key clicks ALRM(?) {z} 4 – 15 Audible alarms SSET(?) {z} 4 – 15 Save user settings RSET(?) {z} 4 – 15 Recall user settings

Auto

AGAN(?) [z] 4 – 16 Auto gain APHS(?) {z} 4 – 16 Auto phase AOFF(?) {z} 4 – 16 Auto oﬀset AREF(?) {z} 4 – 17 Measure reference frequency ASST(?) {z} 4 – 17 External lock-assist

Data transfer

OUTR? 4 – 17 Output ORTI? 4 – 17 Output RTI

Interface

*IDN? 4 – 18 Identify TOKN(?) {z} 4 – 18 Token Mode *OPC(?) 4 – 18 Operation complete LOCL(?) {z} 4 – 19 Local lockout *RST 4 – 19 Reset

Status

LOCK? 4 – 20 Lock status OVLD? 4 – 20 Overload *STB? [i] 4 – 21 Status byte *SRE(?) [i,] {j} 4 – 21 Service request enable *ESR? [i] 4 – 21 Standard event status *ESE(?) [i,] {j} 4 – 21 Standard event status enable *CLS 4 – 21 Clear status LEXE? 4 – 21 Last execution error LCME? 4 – 22 Last command error

SR124 Analog Lock-In Ampliﬁer

4 – 4 Remote Operation

4.2 Alphabetic list of commands

*CLS 4 – 21 Clear status *ESE(?) [i,] {j} 4 – 21 Standard event status enable *ESR? [i] 4 – 21 Standard event status *IDN? 4 – 18 Identify *OPC(?) 4 – 18 Operation complete *RST 4 – 19 Reset *SRE(?) [i,] {j} 4 – 21 Service request enable *STB? [i] 4 – 21 Status byte

AGAN(?) [z] 4 – 16 Auto gain ALRM(?) {z} 4 – 15 Audible alarms AOFF(?) {z} 4 – 16 Auto oﬀset APHS(?) {z} 4 – 16 Auto phase AREF(?) {z} 4 – 17 Measure reference frequency ASST(?) {z} 4 – 17 External lock-assist

BIAS(?) {g} 4 – 11 Reference DC bias magnitude BION(?) {z} 4 – 11 Reference DC bias enable

FMOD(?) {z} 4 – 9 Reference mode FORM(?) {z} 4 – 11 Reference output waveform FREQ(?) {f } 4 – 9 Reference oscillator frequency FRNG(?) {z} 4 – 10 Reference oscillator range

ICPL(?) {z} 4 – 12 Input coupling IFFR(?) {f } 4 – 12 Input ﬁlter frequency IFTR(?) {g} 4 – 12 Input ﬁlter frequency trim IGND(?) {z} 4 – 12 Input shield grounding ISRC(?) {z} 4 – 11 Input source

KCLK(?) {z} 4 – 15 Key clicks

LCME? 4 – 22 Last command error LEXE? 4 – 21 Last execution error

SR124 Analog Lock-In Ampliﬁer

4.2 Alphabetic list of commands 4 – 5

LOCK? 4 – 20 Lock status LOCL(?) {z} 4 – 19 Local lockout

NCHD(?) {g} 4 – 13 Input ﬁlter notch depth trim

OFLT(?) {z} 4 – 14 Output ﬁlter time constant OFSE(?) {z} 4 – 14 Output oﬀset enable OFSL(?) {z} 4 – 14 Output ﬁlter slope OFST(?) {g} 4 – 15 Output oﬀset magnitude OMOD(?) {z} 4 – 14 Output mode ORTI? 4 – 17 Output RTI OUTR? 4 – 17 Output OVLD? 4 – 20 Overload

PHAS(?) {f } 4 – 9 Reference phase

QFCT(?) {z} 4 – 12 Input ﬁlter Q-factor QUAD(?) {z} 4 – 9 Phase quadrant

RMOD(?) {z} 4 – 13 Reserve mode RSET(?) {z} 4 – 15 Recall user settings RSLP(?) {z} 4 – 10 Reference slope shape

SENS(?) {z} 4 – 13 Input sensitivity SLVL(?) {g} 4 – 10 Reference output amplitude SSET(?) {z} 4 – 15 Save user settings

TOKN(?) {z} 4 – 18 Token Mode TYPF(?) {z} 4 – 12 Input ﬁlter type

SR124 Analog Lock-In Ampliﬁer

4 – 6 Remote Operation

4.3 Introduction

Remote operation of the SR124 is through a simple command language documented in this chapter. Both set and query forms of most commands are supported, allowing the user complete control of the lock-in from a remote computer through RS-232, or through the optical ﬁber and the SX199 interface to GPIB, RS-232, or ethernet interfaces.

Where applicable, the corresponding front-panel interface to each command is also indicated. Most instrument settings are retained in non-volatile memory. Upon power-on, these settings are restored to their values before the power was turned oﬀ. Where appropriate, the default value for parameters is listed in boldface in the command descriptions.

Note that remote commanding does not function when the “Control Inhibit” toggle switch is in the upper position.

4.3.1 Interface conﬁguration

4.3.2 Buffers

4.3.3 Remote / local

Both RS-232 and optical ﬁber interfaces are ﬁxed conﬁguration, 9600 baud, 8-bit, with no parity or ﬂow control.

The SR124 stores incoming bytes from the remote interfaces in separate 128-byte input buﬀers. Characters accumulate in the input buﬀer until a command terminator (hCRior hLFi) is received, at which point the message is parsed and enqueued for execution. Query responses from the SR124 are buﬀered in interface-speciﬁc 256-byte output queues. Queries are returned to the interface from which they were received (RS-232 or optical).

If an input buﬀer overﬂows, then all data in the input buﬀer are discarded, and an error is recorded in the ESR status register.

Any time the SR124 receives a remote command terminator (hCRi or hLFi), the instrument transitions into the “Remote” state. When in Remote (indicated by the REM annunciator in the Reference display block), no keypad input or knob adjustment is allowed. To return to front panel operation, press the [Recall] button (which also functions as the “local” key).

Alternatively, the SR124 can be returned to Local mode by sending the LOCL LOCAL command.

SR124 Analog Lock-In Ampliﬁer

4.4 Commands 4 – 7

4.4 Commands

This section provides syntax and operational descriptions for remote commands.

4.4.1 Command syntax

The four letter mnemonic (shown in CAPS) in each command sequence speciﬁes the command. The rest of the sequence consists of parameters.

Commands may take either set or query form, depending on whether the “?” character follows the mnemonic. Set only commands are listed without the “?”, query only commands show the “?” after the mnemonic, and optionally query commands are marked with a “(?)”.

Parameters shown in { } and [ ] are not always required. Parameters in { } are required to set a value, and should be omitted for queries. Parameters in [ ] are optional in both set and query commands. Parameters listed without surrounding characters are always required.

Do not send ( ) or { } or [ ] as part of the command.

Multiple parameters are separated by commas. Multiple commands may be sent on one command line by separating them with semicolons (;) so long as the input buﬀer does not overﬂow. Commands are terminated by either hCRi or hLFi characters. Null commands and whitespaces are ignored. Execution of the command does not begin until the command terminator is received.

Token parameters (generically shown as z in the command descrip-tokens tions) can be speciﬁed either as a keyword or as an integer value. Command descriptions list the valid keyword options, with each keyword followed by its corresponding integer value. For example, to set the reference mode to internal, the following two commands are equivalent:

FMOD INTERNAL —or— FMOD 1

For queries that return token values, the return format (keyword or integer) is speciﬁed with the TOKN command.

SR124 Analog Lock-In Ampliﬁer

4 – 8 Remote Operation

4.4.2 Notation

The following table summarizes the notation used in the command descriptions:

Symbol Deﬁnition

f, g Floating-point value i, j Unsigned integer z Literal token

(?) Required for queries; illegal for set commands

var Parameter always required

{var} Required parameter for set commands; illegal for queries [var] Optional parameter for both set and query forms

4.4.3 Examples

Each command is provided with a simple example illustrating its usage. In these examples, all 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.

The usage examples vary with respect to set/query, optional parameters, and token formats. These examples are not exhaustive, and are intended to provide a convenient starting point for user programming.

SR124 Analog Lock-In Ampliﬁer

4.4 Commands 4 – 9

4.4.4 Reference and phase commands

Reference phasePHAS(?) {f }

Set (query) the reference phase shift {to f }, in degrees. Phase must be in the range 0 ≤f < 360. The default value is PHAS 0.00.

PHAS 30.25Example:

Phase quadrantQUAD(?) {z}

Set (query) the phase quadrant {to quadrant z=(I 1, II 2, III 3, IV

4)}. Note that quadrants tokens are Roman numerals.

The quadrants are deﬁned so that QUAD I corresponding to 0◦≤ φ < 90◦, QUAD II to 90◦≤ φ < 180◦, and so on.

Commanding a new value of QUAD adds orsubtracts multiples of 90 to the current PHAS setting to bring the phase into the commanded quadrant.

PHAS 105.25Example: QUAD?

◦

Reference modeFMOD(?) {z}

Set (query) the reference mode {to z=(EXT1F 0, INTERNAL 1, EXT2F

2, EXT3F 3, RVCO 4)}.

FMOD INTERNALExample:

Reference oscillator frequencyFREQ(?) {f }

Set (query) the reference oscillator frequency {to f}, in hertz. The default value is FREQ 1000.00.

The set form of the command is only allowed when the oscillator mode is FMOD INTERNAL. The parameter f must be in the range deﬁned by by the FRNG command. Queries while in FMOD INTERNAL mode return the commanded oscillator frequency.

Queries in either external or rear-VCO modes return the most recently measured value of the oscillator frequency. Note that the FREQ? query itself does not initiate a new measurement of the oscillator frequency. That action is performed with the AREF command.

FREQ?Example:

137.036000000

SR124 Analog Lock-In Ampliﬁer

4 – 10 Remote Operation

Reference output amplitudeSLVL(?) {g}

Set (query) the reference oscillator output amplitude {to g}, in volts RMS. The default value is SLVL 0.100.

Allowed values for SLVL may be restricted if the DC Bias is enabled. See section 3.4 for details.

SLVL 1.5E-6Example:

Reference slope shapeRSLP(?) {z}

Set (query) the external reference input slope mode {to z=(SINE 0, TTL 1)}.

RSLP is used to conﬁgure the external reference input trigger circuit. When set to RSLP SINE, the Ext. In BNC input is AC-coupled to a discriminator circuit armed to trigger on positive-going zero crossings. When set to RSLP TTL, the Ext. In connection is DC-coupled, and the discriminator looks for positive-going transitions crossing a +1 V threshold, compatible with TTL and CMOS logic.

RSLP SINEExample:

Reference oscillator rangeFRNG(?) {z}

Set (query) the reference oscillator range {to z=(FRNG P2 0, FRNG 2 1, FRNG 20 2, FRNG 200 3, FRNG 2K 4)}.

In all modes, the oscillator range FRNG must ﬁrst be set to the correct span for the desired oscillator frequency. If operating in one of the harmonic modes (FMOD EXT2F or EXT3F), the FRNG setting must span the harmonic frequency, which may not necessarily include the (lower) external input frequency. For example, if operating in second harmonic mode (FMOD EXT3F) and the external input is

1.5 kHz, the oscillator will lock to 4.5 kHz and the range must be set to FRNG FRNG

200 or FRNG FRNG 2K; it will not lock if set to

FRNG FRNG 20.

Each range spans 2 decades, and overlaps its adjacent ranges by one decade each:

z Value Frequency Range (Hz)

FRNG P2 0 0.2 – 21

FRNG 2 1 2 – 210

FRNG 20 2 20 – 2.1 k

FRNG 200 3 200 – 21 k

FRNG 2K 4 2 k – 210 k

FRNG 3.Example:

SR124 Analog Lock-In Ampliﬁer

4.4 Commands 4 – 11

Reference DC bias enableBION(?) {z}

Set (query) the reference output DC bias {to z=(OFF 0, ON 1)}.

Turning the bias on enables DC biasing of the referenceoutput signal.

BION 1Example:

Reference DC bias magnitudeBIAS(?) {g}

Set (query) the bias magnitude {to g}, in volts. The default value is BIAS 0.00.

Allowed values for BIAS may be restricted based on the value of SLVL. See section 3.4 for details.

BIAS?Example:

0.001250000

Reference output waveformFORM(?) {z}

Set (query) the reference output waveform {to z=(SQUARE 0, SINE

1)}.

4.4.5 Input commands

FORM SINEExample:

Input sourceISRC(?) {z}

Set (query) the input source conﬁguration {to z=(A 0, AMINUSB 1, CUR1E6 2, CUR1E8 3)}.

The ﬁrst two settings (ISRC A and ISRC AMINUSB) select the voltage input preamp, conﬁgured for single-ended A input, or diﬀerential AMINUSB input.

The last two settings (ISRC CUR1E6 and ISRC CUR1E8) select the current (transimpedance) ampliﬁer. The current ampliﬁer has a selectable gain of either 1 MΩ (106V/A) or 100 MΩ (108V/A). In either current conﬁguration, the transimpedance gain is in addition to the overall voltage gain set by the SENS command.

For example, if ISRC CUR1E8 and SENS S50MV, then the overall sensitivity corresponding to a full-scale (10 V) output is

50 mV ×

108V/A

= 500 pA.

SR124 Analog Lock-In Ampliﬁer

ISRC?Example:

4 – 12 Remote Operation

Input shield groundingIGND(?) {z}

Set (query) the input shield grounding {to z=(FLOAT 0, GROUND 1)}.

IGND 1Example:

Input couplingICPL(?) {z}

Set (query) the input source coupling {to z=(AC 0, DC 1)}.

ICPL 1Example:

4.4.6 Filter commands

Input ﬁlter typeTYPF(?) {z}

Set (query) the input ﬁlter conﬁguration {to z=(BANDPASS 0,

HIGHPASS 1, LOWPASS 2, NOTCH 3, FLAT 4)}.

TYPF FLATExample:

Input ﬁlter Q-factorQFCT(?) {z}

Set (query) the input ﬁlter Q-factor {to z=(Q1 0, Q2 1, Q5 2, Q10 3,

Q20 4, Q50 5, Q100 6 )}.

QFCT Q5Example: QFCT?

Input ﬁlter frequencyIFFR(?) {f }

Set (query) the input ﬁlter frequency {to f }, in Hz. The default value is IFFR 1000.00.

The allowed range is 2.0 to 110000.0 (2 Hz ≤ f ≤110 kHz).

IFFR 1000Example:

Input ﬁlter frequency trimIFTR(?) {g}

Set (query) the input ﬁlter frequency trim value {to g}. The default value is IFTR 0.

The parameter g can range from −999 ≤ g ≤ +999, in dimensionless units.

IFTR -128Example:

SR124 Analog Lock-In Ampliﬁer

4.4 Commands 4 – 13

Input ﬁlter notch depth trimNCHD(?) {g}

Set (query) the input ﬁlter notch depth trim value {to g}. The default value is NCHD 0.

The parameter g can range from −999 ≤ g ≤ +999, in dimensionless units.

NCHD?Example:

125.000000000

4.4.7 Gain and time constant commands

Input sensitivitySENS(?) {z}

Set (query) the full-scale input sensitivity setting {to z}. Allowable values are:

z Value Full-scale z Value Full-scale

sensitivity sensitivity

S100NV 0 100 nV S500UV 11 500 µV S200NV 1 200 nV S1MV 12 1 mV S500NV 2 500 nV S2MV 13 2 mV

S1UV 3 1 µV S5MV 14 5 mV S2UV 4 2 µV S10MV 15 10 mV

S5UV 5 5 µV S20MV 16 20 mV S10UV 6 10 µV S50MV 17 50 mV S20UV 7 20 µV S100MV 18 100 mV S50UV 8 50 µV S200MV 19 200 mV

S100UV 9 100 µV S500MV 20 500 mV

S200UV 10 200 µV

SR124 Analog Lock-In Ampliﬁer

See ISRC command discription for a discussion of sensitivity when conﬁgured for current inputs.

SENS?Example:

S100UV

Reserve modeRMOD(?) {z}

Set (query) the input sensitivity setting {to z=(HIGH 0, NORMAL 1, LOWNOISE 2)}.

RMOD 1Example:

4 – 14 Remote Operation

Output ﬁlter time constantOFLT(?) {z}

Set (query) the output ﬁlter time constant setting {to z}. Allowable values are:

z Value Full-scale sensitivity

TCMIN 0 < 500 µs TC1MS 1 1 ms TC3MS 2

TC10MS 3 10 ms TC30MS 4 30 ms

TC100MS 5 100 ms

TC300MS 6 300 ms

TC1S 7 1 s

TC3S 8 3 s TC10S 9 10 s

TC30S 10 30 s TC100S 11 100 s TC300S 12 300 s

3 ms

OFLT 7Example:

4.4.8 Output commands

Output ﬁlter slopeOFSL(?) {z}

Set (query) the output ﬁlter slope (rolloﬀ) {to z=(SLOPE6DB 0,

SLOPE12DB 1)}.

OFSL SLOPE12DBExample:

Output modeOMOD(?) {z}

Set (query) the output mode {to z=(LOCKIN 0, ACVOLT 1)}.

OMOD?Example:

LOCKIN

Output oﬀset enableOFSE(?) {z}

Set (query) the output oﬀset mode {to z=(OFF 0, ON 1)}.

OFSE 1Example:

SR124 Analog Lock-In Ampliﬁer

4.4 Commands 4 – 15

Output oﬀset magnitudeOFST(?) {g}

Set (query) the output oﬀset magnitude {to g}, in percent full scale. The default value is OFST 0.00.

The allowed range for g is −1000≤g≤ +1000 (±10× full-scale).

OFST 300.253Example:

4.4.9 Setup commands

Key clicksKCLK(?) {z}

Set (query) audible key clicks {to z=(OFF 0, ON 1)}.

Note there is no corresponding front-panel method to access this command; it is exclusive to the remote interface.

Audible alarmsALRM(?) {z}

Set (query) audible alarms {to z=(OFF 0, ON 1)}.

Note that all sounds that are not “key clicks” are considered “alarms” for the purpose of the ALRM command. There is no corresponding front-panel methdo to access this command; it is exclusive to the remote interface.

Save user settingsSSET(?) {z}

Save (query) the user settings {to non-volatile block z=(USER0 0, USER1 1, . . . , USER8 8)}.

The set version of SSET saves user settings to parameter block z. The query version SSET? returns the most block name or number z most recently saved into.

Recall user settingsRSET(?) {z}

Retrieve (query) the user settings {to non-volatile block z=(USER0 0, USER1 1, . . . , USER8 8, DEFAULT 9)}.

The set version of RSET retreives user settings from non-volatile block z and reconﬁgures the SR124 to those settings. The query form simply returns the name/number of the most-recently retreived memory block.

SR124 Analog Lock-In Ampliﬁer

Note that RSET DEFAULT is equivalent to *RST.

4 – 16 Remote Operation

4.4.10 Auto commands

The following commands all cause the SR124 to perform a series of intermal measurements and adjustments. See section 3.5 for a discussion of how much time these functions may require.

Auto gainAGAN(?) [z]

Set (query) the auto gain function {to z=(OFF 0, ON 1)}. Setting AGAN with no parameter will initiate an auto gain cycle, and is equivalent to AGAN 1.

The set version of AGAN can initiate an auto gain cycle by commanding AGAN to ON (1). If a currently-executing auto gain must be cancelled in-progress, send the command AGAN OFF.

Quering AGAN will respond with one of the following 5 token values:

z value Deﬁnition

OFF 0 Auto gain not running

ON 1 Auto gain adjust in progress

NOTREADY 2 Not currently possible to start

SUCCESS 3 Auto gain cycle concluded successfully

FAILED 4 Auto gain cycle failed

Auto phaseAPHS(?) {z}

Set (query) the auto phase function {to z=(OFF 0, ON 1)}. Setting APHS with no parameter will initiate an auto phase cycle, and is equivalent to APHS 1.

The set version of APHS can initiate an auto phase cycle by commanding APHS to ON (1). If a currently-executing auto phase must be cancelled in-progress, send the command APHS OFF.

Quering APHS will respond with one of the same 5 tokens as described above, for AGAN.

Auto oﬀsetAOFF(?) {z}

Set (query) the auto oﬀset function {to z=(OFF 0, ON 1)}. Setting AOFF with no parameter will initiate an auto oﬀset cycle, and is equivalent to AOFF 1.

The set version of AOFF can initiate an auto oﬀset cycle by commanding AOFF to ON (1). If a currently-executing auto oﬀset must be cancelled in-progress, send the command AOFF OFF.

The AOFF command can be executed independent of the OFFO state (whether the oﬀset is enabled or not). The command AOFF 1 measures the current value of the output, and adjusts the OFST parameter to best null the output voltage.

SR124 Analog Lock-In Ampliﬁer

4.4 Commands 4 – 17

Quering AOFF will respond with one of the same 5 tokens as described above, for AGAN.

AOFF 1Example:

Measure reference frequencyAREF(?) {z}

Set (query) the “measure the reference oscillator frequency” function {to z=(OFF 0, ON 1)}. Setting AREF with no parameter will initiate a reference frequency measurement cycle, and is equivalent to AREF 1.

The set version of AREF can initiate an auto measurement cycle by commanding AREF to ON (1). If a currently-executing auto oﬀset must be cancelled in-progress, send the command AREF OFF.

Quering AREF will respond with one of the same 5 tokens as described above, for AGAN.

External lock-assistASST(?) {z}

Set (query) the reference oscillator lock-assist function {to z=(OFF 0, ON 1)}. Setting ASST with no parameter will initiate an auto lock- assist cycle, and is equivalent to ASST 1.

The set version of ASST can initiate a lock-assist cycle by commanding ASST to ON (1). If a currently-executing auto lock-assist must be cancelled in-progress, send the command ASST OFF.

Quering ASST will respond with one of the same 5 tokens as described above, for AGAN.

4.4.11 Data transfer commands

OutputOUTR?

Query the output value, in volts. The range of OUTR? responses is always in the range −10 ≤ OUTR? ≤ +10.

Output RTIORTI?

Query the output value, in volts, referenced to input.

The relationship betwen OUTP? and ORTI? is

where VFSis the full-scale voltage sensitivity, in volts. For example, if SENS=8 (S50UV), VFS= 50 × 10−6V. Then if OUTP?=+3.14, then

ORTI? returns +0.000015700 (15.7 µV input).

ORTI? = (OUTP?/10 V) × V

SR124 Analog Lock-In Ampliﬁer

4 – 18 Remote Operation

4.4.12 Interface commands

Identify*IDN?

Query the SR124 identiﬁcation string.

The response is formatted as:

Stanford Research Systems,SR124,s/n******,ver#.##

where ****** is the 6-digit serial number, and #.## is the ﬁrmware revision level.

*IDN?Example:

Stanford Research Systems,SR124,s/n098023,ver1.00

Token ModeTOKN(?) {z}

Set (query) the token response mode {to z=(OFF 0, ON 1)}.

Token response mode controls the formatting of response messages generated by the SR124 to remote queries oftoken-type values. When TOKN OFF, the SR124 responds with the numeric version of the token quantity. When TOKN ON, the text version is returned.

TOKN?Example:

Operation complete*OPC(?)

The set form, *OPC, will set the OPC bit in the Standard Event Status register; the query form, *OPC?, will return the value 1.

*OPC is useful for pacing streams of remote commands; the *OPC command will not be processed by the command execution of the SR124 until all preceding commands have been executed. This includes “slow” commands such as auto gain (AGAN).

Note, however, that commands are considered completed once all hardware settings they require are made; analog settling times are not part of the normal “execution” process. As a result, *OPC should not be used to indicate that new instrument settings have settled; rather, the usefulness of *OPC is in assuring that the remote interface does not overﬂow or lose synchronization with a user’s application program.

*OPC?Example:

SR124 Analog Lock-In Ampliﬁer

4.4 Commands 4 – 19

Local lockoutLOCL(?) {z}

Set (query) the local lockout {to z=(LOCAL 0, REMOTE 1, LOCKOUT 2)}.

The LOCL command provides control over user access to front-panel control of the SR124. When LOCL REMOTE or LOCK LOCKOUT, the REM indicator will be lit; in this state, the user cannot control any instrument functions from the front panel.

When in LOCL REMOTE, the [Recall/Local] button acts as the “local” function, bringing the SR124 back to the LOCL LOCAL state with full front-panel control. Note, however, that in LOCL LOCKOUT, even the [Recall/Local] button is disabled: only a remote command returning the instrument to LOCL LOCAL, or a power cycle, will restore frontpanel control.

Note that receipt of aa remote command terminator character places the instrument into the LOCL REMOTE state.

LOCL LOCKOUTExample:

Reset*RST

Reset the SR124 to its default conﬁguration.

The following commands are internally excecuted upon receipt of the *RST command:

• PHAS 0.0

• FMOD

• FREQ 1000.0

• SLVL 0.100

• FRNG FRNG 20

• BION OFF

• BIAS 0.0

• FORM SINE

• ISRC A

• IGND GROUND

• ICPL DC

• TYPF FLAT

• QFCT Q1

• IFFR 1000.0

• IFTR 0

• NCHD 0

• SENS S500MV

• RMOD LOWNOISE

• OFLT TC100MS

SR124 Analog Lock-In Ampliﬁer

4 – 20 Remote Operation

• OFSL SLOPE6DB

• OMOD LOCKIN

• OFSE OFF

• OFST 0.0

• KCLK ON

• ALRM ON

4.4.13 Status commands

Lock statusLOCK?

Reads the current value of the reference oscillator lock status. Returns a token, with values z=(UNLOCKED 0, LOCKED 1, or NOTPLL 2).

If the SR124 reference oscillator is operating in one of the external reference modes (FMOD EXT1F, FMOD EXT2F, or FMOD EXT3F), then the query LOCK? will return either UNLOCKED 0 or LOCKED 1 based on the current status.

If the SR124 reference oscillator is conﬁgured for either

FMOD INTERNAL or FMOD RVCO, then LOCK? responds with NOTPLL

LOCK?Example:

LOCKED

OverloadOVLD?

Reads the current value of the signal overload status. Returns an integer between 1 and 15 if an overload is detected, or 0 if there is no overload.

The response integer is binary-weighted based on the four (4) separate signal stages that can generate an overload detect:

• 1 Preamp overload

• 2 Current amp overload

• 4 Intermediate (AC) amp overload

• 8 Output (DC) amp overload

For example, if the preamp and the intermediate amp are both overloading, OVLD? will respond with 5 (= 1 + 4).

OVLD?Example:

SR124 Analog Lock-In Ampliﬁer

4.4 Commands 4 – 21

Status byte*STB? [i]

Reads the Status Byte register [bit i].

*STB?Example:

Service request enable*SRE(?) [i,] {j}

Set (query) the Service Request Enable register [bit i] {to j}.

*SRE 0,1Example:

Standard event status*ESR? [i]

Reads the Standard Event Status Register [bit i].

Upon executing *ESR?, the returned bit(s) of the ESR register are cleared.

*ESR?Example:

Standard event status enable*ESE(?) [i,] {j}

Set (query) the Standard Event Status Enable Register [bit i] {to j}.

*ESE 6,1Example: ESE?

Clear status*CLS

*CLS immediately clears the ESR register, and the UNLOCK and

OVERLOAD bits in the Status Byte.

*CLSExample:

Last execution errorLEXE?

Query the last execution error code. A query of LEXE? always clears the error code, so a subsequent LEXE? will return 0. Valid codes are:

Value Deﬁnition

0 No execution error since last LEXE? 1 Illegal value 2 Wrong token 3 Invalid bit 4 Queue full 5 Not compatible

SR124 Analog Lock-In Ampliﬁer

4 – 22 Remote Operation

IFFR 1234567; LEXE?; LEXE?Example:

1;0

The error (1, “Illegal value,”) is because the parameter value (1234567) is too large for IFFR. The second read of LEXE? returns 0.

Last command errorLCME?

Query the last command error code. A query of LCME? always clears the error code, so a subsequent LCME? will return 0. Valid codes are:

Value Deﬁnition

0 No execution error since last LCME? 1 Illegal command 2 Undeﬁned command

Illegal query

3 4 Illegal set 5 Missing parameter(s) 6 Extra parameter(s) 7 Null parameter(s) 8 Parameter buﬀer overﬂow

9 Bad ﬂoating-point 10 Bad integer 11 Bad integer token 12 Bad token value 13 Bad hex block 14 Unknown token

*IDNExample: LCME?

The error (4, “Illegal set”) is due to the missing “?”.

SR124 Analog Lock-In Ampliﬁer

4.5 Status model 4 – 23

undefined

MSS: Master Summary Status

ESB: Event Status Bit

undefined

Status Byte

SB SRE

OPC: Operation c omplete

undef

DDE: Device error

EXE: Execution error

CME: Command error

undef

QYE: Query error

ESR ESE

Standard Event Status

4.5 Status model

The SR124 status registers follow the hierarchical IEEE–488.2 format.status registers A block diagram of the status register array is given in Figure 4.1.

Figure 4.1: Status Model for the SR124 Analog Lock-In Ampliﬁer

Event Registers : These read-only registers record the occurrence of deﬁned

Enable Registers : These read/write registers deﬁne a bitwise mask for their cor-

Status Byte : This read-only register represents the top of the status model,

4.5.1 Status byte (SB)

SR124 Analog Lock-In Ampliﬁer

There are three categories of registers in the status model of the lockin:

events within the lock-in. If the event occurs, the corresponding bit is set to 1. Upon querying an event register, any set bits within it are cleared. These are sometimes known as “sticky bits,” since once set, a bit can only be cleared by reading its value. Event register names end with SR or EV.

responding event register. If any bit position is set in an event register while the same bit position is also set in the enable register, then the corresponding summary bit message is set in the Status Byte. Enable register names end with SE or EN.

and is populated with summary bit messages and interface condition bits. Enabled bits within the Status Byte generate the remote Request Service event.

At power-on, all status registers are cleared.

The Status Byte is the top-level summary of the SR124 status model. When enabled by the Service Request Enable register, a bit set in the Status Byte causes the MSS (Master Summary Status) bit to be set.

4 – 24 Remote Operation

Weight Bit Flag

1 0 undef (0) 2 1 undef (0) 4 2 undef (0)

8 3 undef (0) 16 4 undef (0) 32 5 ESB 64 6 MSS

128 7 undef (0)

ESB : Event Status Bit. Indicates whether one or more of the enabled

events in the Standard Event Status Register is true.

MSS : Master Summary Status. Indicates whether one or more of the

enabled status messages in the Status Byte register is true.

This register is read with the *STB? query.

4.5.2 Service request enable (SRE)

Each bit in the SRE corresponds one-to-one with a bit in the SB register, and acts as a bitwise AND of the SB ﬂags to generate MSS. Bit 6 of the SRE is undeﬁned—setting it has no eﬀect, and reading it always returns 0. This register is set and queried withthe *SRE(?) command.

At power-on, this register is cleared.

4.5.3 Standard event status (ESR)

The Standard Event Status Register consists of 8 event ﬂags. These event ﬂags are all “sticky bits” that are set by the corresponding events, and cleared only by reading or with the *CLS command. Reading a single bit (with the *ESR? i query) clears only Bit i.

Weight Bit Flag

1 0 OPC

2 1 undef (0)

4 2 QYE

8 3 DDE 16 4 EXE 32 5 CME 64 6 undef (0)

128 7 undef (0)

OPC : Operation Complete. Set by the *OPC command.

QYE : Query Error. Indicates data in the output queue has been lost.

DDE : Device-Dependent Error. Indicates an internal command

queue overﬂow.

SR124 Analog Lock-In Ampliﬁer

4.5 Status model 4 – 25

EXE : Execution Error. Indicates the error in a command that was

successfully parsed. Out-of-range parameters are an example.

CME : Command Error. Indicates a command parser-detected error.

4.5.3.1 Standard event status enable (ESE)

The ESE acts as a bitwise AND with the ESR register to produce the single-bit ESB message in the Status Byte Register (SB). The register can be set and queried with the *ESE(?) command.

At power-on, this register is cleared.

SR124 Analog Lock-In Ampliﬁer

4 – 26 Remote Operation

SR124 Analog Lock-In Ampliﬁer

5 Circuits

In This Chapter

This chapter presents a brief description of the SR124 circuit design. A complete parts list and circuit schematics are included.

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

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

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

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

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

5 – 1

5 – 2 Circuits

5.1 Overview of circuits

The following sections correspond to schematic pages at the end of the manual.

5.2 Power Supply

The SR124 power supply board uses low-voltage AC power from the shielded transformer to power the instrument. Separate power nets are created for the oscillator, the main signal board, and the CPU / Front-panel boards. A single “star ground” is established on the power supply board, where it is connected to the instrument chassis.

5.3 CPU

The CPU board contains the microcontroller that conﬁgures all instrument hardware of the SR124. Non-volatile memory on the CPU board stores user settings. The fully stopable CPU oscillator is also on this board.

5.4 Front Panel

5.5 Main board

The front panel board contains the LED indicators and button controls. All numeric displays are driven from static shift registers, whose values are only updated in response to a user command.

The Main boardcontains the front-end voltage and current preampliﬁers, programmable ﬁlters, the PSD (square-wave mixer), the output ﬁlters, and the reference output attenuator.

The input JFET, Q1101, is quite sensitive and may be damaged by overvoltage, including overvoltage from a static discharge (ESD). Symptoms of a damaged JFET can be excess noise or signal loss.

Instructions for replacing the input JFET are as follows:

1. Equipment needed:

• Replacement JFET (obtained from SRS)

• BNC grounding cap or 50 Ω terminator

• Voltmeter capable of showing millivolts

• Test leads with small clips, to attach to test point loops

• Small adjusting screwdriver

SR124 Analog Lock-In Ampliﬁer

5.5 Main board 5 – 3

2. Work should be performed at an ESD-controlled workstation. Be sure the technician is grounded before opening the instrument.

3. Turn oﬀ power to the unit, and remove the AC power cord. Disconnect all other cables from the SR124. Remove the top lid of the instrument by removing the 2 large black screws from either side of the lid, and the 6 small black screws from the top of the lid. Slide the lid slightly backwards, and then lift away from the instrument.

4. Locate the input JFET, marked as Q1101 on the main circuit board. You can ﬁnd Q1101 directly behind the A/I and B inputs, about 5 cm back from the edge of the board. Q1101 is socketed, as shown in Figure 5.1. Using your thumb and foreﬁnger, carefully remove the JFET.

SR124 Analog Lock-In Ampliﬁer

Figure 5.1: The front-end JFET (Q1101), installed.

5. The replacement JFET will be installed in the same socket. Please note that two of the socket positions, aligned with the front and back of the instrument, are left unpopulated. It is important that the pins of the new JFET all be seated correctly in the remaining positions. Refer to Figure 5.2 for the unpopulated positions. When fully seated, the JFET should look like Figure 5.1.

6. After replacing the JFET, the oﬀset voltage must be trimmed to near zero. Power must be applied to the SR124 with the top

5 – 4 Circuits

Figure 5.2: The front-end JFET (Q1101) socket, un-installed.

5.6 Oscillator

cover removed. Be sure to have a second person nearby for safety, and do not proceed if the metal shroud covering the AC power entry module has been removed.

7. Place a shorting cap or 50 Ω terminator onto the A/I input, and turn the instrument on. Press [Recall], and turn the



REFERENCE knob to show “dEFLt”. Press [Recall] a sec-



ond time to restore defaults. Turn the







SENSITIVITY knob to







5 mV.

8. Attach the voltmeter test leads to Test Points TP1104 and TP1105 (see Figure 5.3). Slowly adjust trimmer R1102 to null the voltage between TP1104 and TP1105. You should be able to reduce the voltage to < 5 mV.

9. Return the top cover and reinstall all the screws.

The Oscillator board contains the quadrature sinewave voltagecontrolled oscillator. This board is mounted vertically, at the righthand side of the SR124 chassis.

SR124 Analog Lock-In Ampliﬁer

5.7 Rear outputs 5 – 5

TRIMMER

Test Points

Figure 5.3: Trimming the replacement JFET

5.7 Rear outputs

5.8 Communications

5.9 Schematics

The Rear outputs board contains the fully-diﬀerential buﬀer ampliﬁers to receive the oscillator quadrant signals and drive them relative to chassis ground.

The small Communications board, located near the power-entry module, contains the ﬁber optic transmit and receive components. A simple RS-232 driver circuit is also on this board.

Circuit schematic diagrams follow this page.

SR124 Analog Lock-In Ampliﬁer

SRS Labs SR124 Operation Manual

Specifications and Main Features

Frequently Asked Questions

User Manual