RF Lock-In Amplifier

255 Pages·2001·1.24 MB·English

User’s Manual

Model SR844
RF Lock-In Amplifier

1290-D Reamwood Avenue
Sunnyvale, California 94089
Phone: [phone] • Fax: [phone]
email: [email protected] • www.thinkSRS.com

Copyright © 1997 by SRS, Inc.
All Rights Reserved.

Revision 2.6 (10/2003)
Certification
Stanford Research Systems certifies that this product met its published specifications at the
time of shipment. Stanford Research Systems further certifies that its calibration
measurements are traceable to the United States National Institute of Standards and
Technology (NIST).
Warranty
This Stanford Research Systems product is warranted against defects in materials and
workmanship for a period of one (1) year from the date of shipment.
Service
For warranty service or repair, this product must be returned to a Stanford Research
Systems authorized service facility. Contact Stanford Research Systems or an authorized
representative before returning this product for repair.

Information in this document is subject to change without notice.

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

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

Printed in U.S.A.
SR844 RF Lock-In Amplifier
i
Safety and Preparation For Use
WARNING! Dangerous voltages, capable of causing injury or death, are present in this
instrument. Use extreme caution whenever the instrument cover is
removed. Do not remove the cover while the unit is plugged into a live
outlet.

Caution This instrument may be damaged if operated with the LINE VOLTAGE SELECTOR set
for the wrong AC line voltage or if the wrong fuse is installed.

Line Voltage The SR844 operates from a 100V, 120V, 220V, or 240V nominal AC power source
Selection having a line frequency of 50 or 60 Hz. Before connecting the power cord to a power
source, verify that the LINE VOLTAGE SELECTOR card, located in the rear panel fuse
holder, is set so that the correct AC input voltage value is visible.
Conversion to other AC input voltages requires a change in the fuse holder voltage card
position and fuse value. Disconnect the power cord, open the fuse holder cover door and
pull the fuse-pull lever to remove the fuse. Remove the small printed circuit board and
select the operating voltage by orienting the printed circuit board so that the desired
voltage is visible when the circuit board is pushed firmly into its slot. Push the fuse-pull
lever back into its normal position and insert the correct fuse into the fuse holder.
Line Fuse Verify that the correct line fuse is installed before connecting the line cord. For
100V/120V, use a 1 Amp fuse and for 220V/240V, use a 1/2 Amp fuse.

Line Cord The SR844 has a detachable, three-wire power cord for connection to the power source
and to a protective ground. The exposed metal parts of the instrument are connected to
the outlet ground to protect against electrical shock. Always use an outlet which has a
properly connected protective ground.

Service Do not attempt to service or adjust this instrument unless another person, capable of
providing first aid or resuscitation, is present.

Do not install substitute parts or perform any unauthorized modifications to this
instrument. Contact the factory for instructions on how to return the instrument for
authorized service and adjustment.
Fan The fans in the SR844 are required to maintain proper operation. Do not block the vents
in the chassis or the unit may not operate properly.

Warning! Regarding Use With Photomultipliers and Other Detectors
The front end amplifier of this instrument is easily damaged if a photomultiplier is used
improperly with the amplifier. When left completely unterminated, a cable connected to a
PMT can charge to several hundred volts in a relatively short time. If this cable is
connected to the inputs of the SR844 the stored charge may damage the front-end op
ampls. To avoid this problem, always connect the PMT output to the SR844 input before
turning the PMT on.
SR844 RF Lock-In Amplifier
ii
Symbols that may be found on SRS products

Symbol Description
Alternating current
Caution - risk of electric shock
Frame or chassis terminal
Caution - refer to accompanying documents
Earth (ground) terminal
Battery
Fuse
On (supply)
Off (supply)

SR844 RF Lock-In Amplifier
iii
Contents
General Information

Safety and Preparation For Use i
Contents iii
Specifications v
Chapter 1 Getting Started

Quick Start 1-3
The Basic Lock-In 1-5
X, Y, R, θ and dBm 1-9
Outputs, Offsets and Expands 1-12
Storing and Recalling Setups 1-18
Aux Outputs and Inputs 1-19
Chapter 2 SR844 Basics

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

Overview 3-3
Signal Input 3-8
Time Constants 3-11
Sensitivity 3-13
CH1 Display and Output 3-15
CH2 Display and Output 3-20
Reference Section 3-26
Save and Recall 3-30
Interface 3-31
Scan and Rel 3-33
Auto Functions 3-40
Shift Functions 3-41
SR844 RF Lock-In Amplifier
iv Contents
Chapter 4 Programming

Index of Commands 4-2
Introduction 4-7
Command Syntax 4-10
Status Register Definitions 4-31
Example Program 4-34
Chapter 5 Testing

Getting Ready 5-3
Self Test 5-5
Amplitude Response 5-7
Phase Response 5-9
Frequency Accuracy 5-11
Ref Out Amplitude 5-13
DC Outputs and Inputs 5-15
Input Noise 5-17
SR844 Performance Test Record 5-19
Chapter 6 Circuitry

Service 6-3
Circuit Board Locations 6-4
Circuit Descriptions 6-6
Parts Lists 6-25
Schematic Diagrams 6-73
SR844 RF Lock-In Amplifier
v
Specifications
Specifications apply after 30 minutes of warm-up. All specifications are with output filtering enabled (6,
12, 18 or 24 dB/oct) and 2F detection OFF, unless stated otherwise.
Signal Input
Voltage Input single-ended BNC.
Input Impedance 50 Ω or 1 MΩ || 30 pF.
Damage Threshold ±5 V (DC+AC)
Bandwidth 25 kHz to 200 MHz.
Full Scale Sensitivity 100 nV to 1V rms in a 1-3-10 sequence.
Gain Accuracy
< 50 MHz ±0.25 dB
< 200 MHz ±0.50 dB
Gain Stability 0.2%/°C
Coherent Pickup Low Noise Wide Reserve, Sensitivity < 30 mV.
f < 10 MHz < 100 nV (typical)
f < 50 MHz < 2.5 µV (typical)
f < 200 MHz < 25 µV (typical)
Input Noise: 50 Ω Input
100 kHz < f < 100 MHz 2 nV/√Hz (typical), < 4 nV/√Hz (max).
25 kHz < f < 200 MHz < 5 nV/√Hz (typical), < 8 nV/√Hz (max).
Input Noise: 1 MΩ Input
25 kHz < f < 200 MHz 5 nV/√Hz (typical), < 8 nV/√Hz (max).
Dynamic Reserve > 60 dB (expand off)

Reference
External Reference Input 25 kHz to 200 MHz.
Impedance 50 Ω or 10 kΩ || 40 pF.
Level 0.7 Vpp digital or 0 dBm sinusoidal signal.
Pulse Width > 2 ns at any frequency.
Threshold Setting Automatic, set to midpoint of waveform extrema.
Acquisition Time < 10 s (auto-ranging, any frequency).
< 1 s (within same octave).
Internal Reference Oscillator 25 kHz to 200 MHz.
Frequency Resolution 3 digits.
Frequency Accuracy ±0.1 in the 3rd digit.
Phase Noise -90 dBc/Hz at f=100 MHz, ∆f=100 Hz.
Reference Outputs Phase locked to either Internal or External reference.
Front Panel Ref Out 25 kHz to 200 MHz square wave, 1.0 Vpp nominal into 50 Ω.
Rear Panel TTL Out 25 kHz to 1.5 MHz, 0 to +5 V nominal, ≥ 3 V into 50 Ω.
Harmonic Detect Detect at 50 kHz ≤ 2×Reference ≤ 200 MHz.
Phase Resolution 0.02°
Absolute Phase Error
< 50 MHz < 2.5°
< 100 MHz < 5.0°
< 200 MHz < 10.0°
SR844 RF Lock-In Amplifier
vi Specifications
Reference
Relative Phase Error, < 2.5°
Orthogonality
Phase Noise 0.005° rms at 100 MHz, 100 ms time constant.
Phase Drift
< 10 MHz < 0.1°/°C
< 100 MHz < 0.25°/°C
< 200 MHz < 0.5°/°C

Demodulator
Zero Stability Digital displays have no zero drift.
Analog outputs have < 5ppm/°C drift for all dynamic reserve settings.
Filtering
Time Constants 100 µs to 30 ks with 6, 12, 18 or 24 dB/octave roll-off.
None 10 to 20 µs update rate (X and Y outputs), 60 µs (R and θ outputs).
Harmonic Rejection
Odd Harmonics -10 dBc @ 3×Ref, -14 dBc @ 5×Ref, etc.
Other Harmonics and < -40 dBc
Sub-harmonics
Spurious Responses -10 dBc @ Ref ± 2×IF
-23 dBc @ Ref ± 4×IF
< -30 dBc otherwise.

Displays
Channel 1 Channel 2 Reference
Type 4½ digit LED 4½ digit LED 4½ digit LED
Displayed X Y Reference Frequency
Quantities R [Volts] θ [degrees] Reference Phase
R [dBm] Y-noise [Volts] Aux Output Voltages
X-noise Y-noise [dBm] Offsets in % of Full Scale
AUX IN 1 AUX IN 2 IF Frequency
Elapsed Settling Time
Ratio The signal may be ratioed with respect to AUX IN 1 or
2. The ratio is applied to both X and Y before
computation of R, R[dBm], X-noise, Y-noise [V, dBm]
and so affects all of these quantities. The ratio input is
normalized to 1 V and has a dynamic range > 100.
Expand The CH1 and CH2 displays and outputs may be
expanded by ×10 or ×100.

SR844 RF Lock-In Amplifier
Specifications vii
CH1 and CH2 Outputs
Connectors Front Panel BNC.
Voltage Range ±10V full scale proportional to X, Y or CH1, CH2 displayed quantity.
±11V full scale for phase
Update Rate
X, Y 48 to 96 kHz
R, θ, Aux Inputs 12 to 24 kHz
X Noise, Y Noise 512 Hz

Aux Inputs and Outputs
Connectors Rear Panel BNC.
Inputs 2
Type Differential with 1 MΩ input impedance on both signal and shield.
Range ±10V
Resolution 1/3 mV
Bandwidth 3 kHz
Outputs 2
Range ±10V
Resolution 1 mV

Environmental Conditions
Operating Temperature: +10°C to +40°C
(Specifications apply over +18°C to +28°C)
Relative Humidity: <90% Non-condensing
Non-Operating Temperature: –25°C to +65°C
Relative Humidity: <95% Non-condensing

General
Furnished Accessories Power Cord
Operating Manual
Interfaces IEEE-488.2 and RS232 interfaces standard. All instrument functions
can be controlled and read through either interface.
Power 70 Watts, 100/120/220/240 VAC, 50/60 Hz.
Dimensions 17" W x 5.25" H x 19.5" D
Weight 23 lb.
Warranty One year parts and labor on materials and workmanship.
SR844 RF Lock-In Amplifier
viii

SR844 RF Lock-In Amplifier