Stanford Research Systems SR844 Specifications Sheet

SR844 RF Lock-In Amplifier
25 kHz - 200 MHz

ier.

Unlike simple down-convert-

ers,

e
required, and unlike analog lock-ins,
no manual frequency range s witch­ing is necessary.

The SR844 provides uncompro­mised performance with a frequency
range of 25 kHz to 200 MHz and up
to 80 dB of drift-free dynamic
reserve. And of course, the SR844
includes the many features, ease of
operation, and programmability that
you've come to expect from SRS
DSP lock-in amplifiers. If your
application involves high frequency
signal detection, there's no better
choice than the SR844.

Digital Technology

The SR844 uses the same advanced
DSP technology found in the SR850,
SR830 and SR810 lock-in
amplifiers. DSP offers many advan­tages over analog instruments - high
dynamic reserve, zero drift, accurate
RF phase shifts and orthogonality,
and digital output filtering.

Most important of all, digital
technology delivers optimum perfor­mance at a price that's thousands
less than other RF lock-in solutions.

SR844 Dual Phase

P

RF Lock-In Amplifier.....$7950

• 25 kHz to 200 MHz frequency range

• DSP technology for high dynamic
reserve, excellent gain stability and
zero drift

• Time constants from 100 µs to 30 ks
(6, 12, 18, 24 dB/oct rolloff)

• No output filter mode (10 to 20 µs
update rate)

• Auto gain, phase, reserve and offset

no additional instruments ar

(U.S. list)

resenting the SR844 Dual
Phase, RF Lock-In Amplif

• Reference output

• Two 16-bit ADCs and DACs

• GPIB and RS-232 interfaces

Signal Input

The SR844 has both 50
inputs. The 1 MΩ input is used with
high source impedances at low
frequencies, or with a standard 10X
scope probe. The 50 Ω input
provides the best RF signal
matching. Full scale sensitivities
range from 1V down to 100 nV.
Three wideband dynamic reserve
selections are available.

Reference

The SR844 offers both external and
internal reference operation. In both
cases, the entire 25 kHz to 200 MHz
frequency range is covered without
any manual range selection. The
external reference input has an auto­threshold feature which locks to
sine, TTL, or NIM signals. The
internal reference is digitally synthe­sized and is adjustable with 3 digit
frequency resolution.

Harmonic detection of the 2F com­ponent is available for both internal
and external reference modes.

A reference output (1.0 Vpp square
wave into 50 Ω) which is phase
synchronous with the lock-in
reference is available in both
external and internal mode.

Output Filters

Time constants from 100 µs to 30 ks
can be selected, with the choice of 6,
12, 18 or 24 dB/oct rolloff. For high
bandwidth, real-time outputs, the
filtering can be by-passed entirely.
In this No Filter mode, the effective
time constant is about 30 µs with the
analog outputs updating every 10 to
20 µs.

Ease of Operation

Unlike other RF lock-in amplifiers,
the SR844 is easy to use. All
instrument functions are set from the

front panel keypad, and the knob is
used to quickly adjust parameters.
Up to nine different instrument
configurations can be stored in non­volatile memory for fast, reliable
instrument setup. Standard RS-232
and GPIB (IEEE-488.2) interfaces
provide connections to your data
acquisition systems.

Useful Features

Auto-functions allow parameters
that are frequently adjusted to be set
automatically. Sensitivity, dynamic
reserve, phase and offset are each
quickly optimized with a simple key
stroke.

The offset and expand features are
useful for evaluating small fluctua­tions in your signal. The input is
nulled with the auto-offset function
and output expand increases the
resolution by up to 100 times.

Ω and 1 MΩ

Ratio mode is used to normalize

ier
from Stanford Research Systems
represents a true breakthrough in
high frequency lock-in amplifier
design. Performance, features, and
value – nothing else compares with
the SR844.

the signal to an externally applied
analog voltage. It is useful to elim­inate the effect of source intensity
fluctuations.

Transfer function measurements
can be easily made from the front
panel by a programmable scan of
up to 11 frequencies. Setups and
offsets are recalled at each
frequency in the scan.

Analog Inputs and
Outputs

The two displays each have a user­defined output for measuring X, Y,
R, R(dBm), θ, and Xnoise or Y
noise. Two user programmable
DACs provide -10.5 V to +10.5 V
voltages with 1 mV resolution.
These outputs may be set from the
front panel or via the computer
interfaces.

In addition, there are two general
purpose analog inputs. These are 16­bit ADCs which can be displayed on
the front panel, read over the
interface, or used to ratio the input
signal.

Internal Memory

The SR844 has two 16,000 point
memory buffers for recording the
time history of each measurement
display at rates up to 512
samples/sec. Data may be trans­ferred from the buffers using either
interface. A trigger input is also
provided to synchronize data

recording with external events.

Outstanding Value

The SR844 RF Lock-In Amplif

Specifications

Signal Channel

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
Sensitivity:

< 1 MHz 100 nVrms to 1 Vrms full scale

(-127 dBm to +13 dBm full scale)

< 50 MHz 1 µVrms to 1 Vrms full scale
≤ 200 MHz 10 µVrms to 1 Vrms full scale

Gain accuracy:

< 50 MHz ± 0.25 dB
≤ 200 MHz ± 0.50 dB

Gain Stability 0.2% /˚C
Coherent Pickup Low Noise Reserve, Sens. < 30 mV

f < 10 MHz < 100 nV (typical)
f < 100 MHz < 1.0 µV (typical)
f < 200 MHz < 2.5 µV (typical)

Input noise: 50 Ω Input

100 kHz < f < 100 MHz 2 nV/√Hz (typ.), 4 nV/√Hz (max.)
25 kHz < f < 200 MHz < 5 nV/√Hz (typ.), < 8 nV/√Hz (max.)

Input noise: 1 MΩ Input

25 kHz < f < 200 MHz 5 nV/√Hz (typ.), < 8 nV/√Hz (max.)

Dynamic reserve up to 80 dB

Reference

External Reference Input 25 kHz to 200 MHz

Impedance 50 Ω or 10 kΩ + 40 pF
Level 0.7 Vpp pulse or 0 dBm sine
Pulse Width > 2ns at any frequency
Threshold Setting Automatic, midpoint of waveform
Acquisition Time < 10s (auto-ranging, any frequency)

< 1s (within same octave)

Internal Reference 25 kHz to 200 MHz

Frequency Resolution 3 digits

Frequency Accuracy ± 0.1 in the 3rd digit
Harmonic Detection 2F (signal from 50 kHz to 200 MHz)
Reference Outputs Phase locked to Int. or Ext. 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

Phase Resolution 0.02°
Absolute Phase Error:

< 50 MHz < 2.5°

< 100 MHz < 5.0°

< 200 MHz < 10.0°
Rel. Phase Error, Orthog. < 2.5°
Phase Noise (external) 0.005° rms at 100 MHz, 100 ms TC
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 < 5 ppm/°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

No Filter 10-20 µs update rate (X and Y outputs)

Harmonic Rejection

Odd Harmonics -9.5 dBc @3xRef, -14 dBc @ 5xRef,

etc. (20log 1/n where n = 3, 5, 7, 9...)

Other Harmonics < -40 dBc

and sub-harmonics

Spurious Responses -10 dBc @Ref ±2 x IF

-23 dBc @Ref ±4 x IF
< -30 dBc otherwise.
(2 kHz < IF < 12 kHz)

Displays

Channel 1:

Type 4.5 digit LED with 40 seg. bar graph
Quantities X,R[V], R[dBm], Xnoise, AuxIn 1

Channel 2:

Type 4.5 digit LED with 40 seg. bar graph
Quantities Y, θ[deg], Ynoise[V], Ynoise[dBm],

AuxIn 2

Expand x10 or x100 for Ch1, Ch2 displays and

outputs

Ratio X and Y ratioed with respect to

AuxIn 1 or AuxIn 2 before filtering
and computation of R. The ratio input
is normalized to 1 V and has a
dynamic range greater than 100.

Reference:

Type 4.5 digit LED
Quantities Ref Freq, Phase, Offsets, AuxOut, IF

Freq, Elapsed Time

Channel 1 and Channel 2 Outputs

Voltage Range ±10 V full scale proportional to X, Y

or CH1, CH2 displayed quantity

Update Rate:

X,Y ≥ 48 kHz
R, θ, Aux Inputs ≥ 10 kHz
X noise, Ynoise 512 Hz

Auxiliary Inputs and Outputs

Inputs 2

Type Differential, 1 MΩ
Range ±10 V
Resolution 1/3 mV
Bandwidth 3 kHz

Outputs 2

Range ±10 V
Resolution 1 mV

Data Buffers Two 16,000 point buffers. Data is

recorded at rates up to 512 Hz and is
read using the computer interfaces.

General

Interfaces IEEE-488 and RS-232 interfaces are

standard. All instrument functions can
be controlled and read through the
interfaces.

Power 70 Watts, 100/120/220/240 VAC,

50/60 Hz.
Dimensions 17” W x 5.25” H x 19.5” D
Weight 23 lbs.
Warranty One year parts and labor on materials

and workmanship

Inside the SR844

D to A Converter

Front Panel

Outputs

A to D ConverterVariable IF Gain180 kHz LPF

Mixer

DSP

CH2

CH1

Configurable Attenuation

and Gain

XTAL Reference Synthesizer

Loop Filter

VCO

Divider Chain

X Ref

Y Ref

Ref

IF Chop

Phase

Comparator

RF Signal Path

Reference Channel

Signal Input

External

Reference Input

IF Section

Int Ref

Ext Ref

X-IF

Y-IF

Ordering Information

SR844

RF Lock-In Amplifier $7950 (U.S. list price)
(includes rackmount hardware)

At the RF operating frequencies of the SR844,
analog mixers must be used to mix the reference
with the signal. However, these analog mixers, while
providing excellent RF performance, suffer from
serious DC output errors. In a lock-in amplifier, the
desired output from the mixer is often much smaller
than these DC errors. In an analog lock-in, large RF
gains are necessary to overcome this problem, at the
expense of accuracy and dynamic reserve. The
SR844 solves this problem by 'chopping', or mixing,
the reference signal with a lower intermediate
frequency (IF). When this chopped reference is
mixed with the signal, the desired mixer output is at

the IF frequency, not at DC. This entirely avoids the
DC problem associated with the mixers. The final
conversion from the IF is performed digitally using
SRS's DSP lock-in technology. The most important
benefits of this architecture are lower RF gain
resulting in higher accuracy and dynamic reserve,
and precise phase shifting. In addition, DSP
processing provides digital output filtering, fast
computation of magnitude and phase, signal ratio
measurement, and many other features. The SR844
combines the best analog and digital techniques,
resulting in state-of-the-art performance at an afford­able price.

1290-D Reamwood Avenue • Sunnyvale, CA 94089 email: [e-mail]
Telephone: [phone] • FAX: [phone] WWW: www.srsys.com

Printed in USA ©1997 Stanford Research Systems, Inc. All specifications and prices subject to change (4/97)