Stanford Research Systems SR844 Data Sheet

· 25 kHz to 200 MHz frequency range

· 80 dB dynamic reserve

· Time constants from 100

µµ

s to 30 ks

(6, 12, 18 or 24 dB/oct rolloff)

· "No Time Constant" mode
(10 µµs to 20 µµs update rate)

· Auto gain, phase, reserve and offset

· Two 16-bit DACs and ADCs

· Internal or external reference

· GPIB and RS-232 interfaces

· SR844 ... $7950

(U.S. list)

Stanford Research Systems phone: [phone]

www.thinkSRS.com

The SR844 is the widest bandwidth lock-in amplifier
available. Unlike simple down-converters, no additional
instruments are required. And unlike analog lock-ins, no
manual frequency range switching 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. It includes the many features, ease
of operation and programmability that you've come to expect
from SRS DSP lock-in amplifiers.

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, low zero-drift, accurate RF phase shifts and
orthogonality, and digital output filtering.

Signal Input

The SR844 has both 50 Ω and 1 MΩ inputs. The 1 MΩ input
is used with high source impedances at low frequencies, or
with a standard 10× scope probe. The 50 Ω input provides the
best RF signal matching. Up to 60 dB of RF attenuation or
20 dB of RF gain can be selected in 20 dB increments. Full­scale sensitivities range from 1 Vrms (+13 dBm) to 100 nVrms
(−127 dBm). Gain allocation can be optimized to provide up
to 80 dB of dynamic reserve.

SR844 200 MHz Lock-In Amplifier

High Frequency Lock-In Amplifier

SR844  200 MHz, dual phase RF lock-in amplifier

www.thinkSRS.com

SR844 200 MHz Lock-In Amplifier

Stanford Research Systems phone: [phone]

www.thinkSRS.com

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 auto-threshold
feature which locks to sine, square or pulsed 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 a
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

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 non-volatile 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 auto­offset function and output expand increases the resolution by
up to 100×.

Ratio mode is used to normalize 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 user-defined output for
measuring X, Y, R, R(dBm), θ, and X-noise or Y-noise. Two
user-programmable DACs provide −10.5 V to +10.5 V outputs
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 16-bit 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 (rates to 512 samples/s) the time history of each
displayed measurement. Data may be transferred from the
buffers using either interface. A trigger input is also provided
to synchronize data recording with external events.

Ordering Information

SR844 200 MHz dual phase lock-in $7950

amplifier (w/ rack mount)

SR445A Voltage preamplifier $1100

(350 MHz, 4 channel)

SR844 rear panel

SR844 Specifications

Stanford Research Systems phone: [phone]

www.thinkSRS.com

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
<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 %/ºC
Coherent pickup Low-noise reserve, sens. <30 mV

f <10 MHz <100 nV (typ.)

f <50 MHz <2.5 µV (typ.)

f <200 MHz <25 µV (typ.)
Input noise (50 Ω) 2 nV/√Hz (typ.), <8 nV/√Hz (max.)
Input noise (1 MΩ) 5 nV/√Hz (typ.), <8 nV/√Hz (max.)
Dynamic reserve up to 80 dB

Reference Channel

External reference 25 kHz to 200 MHz

Impedance 50 Ω or 10 kΩ + 40 pF

Level 0.7 Vpp pulse or 0 dBm sine

Pulse width >2 ns at any frequency

Threshold setting Automatic, midpoint of waveform

Acquisition time <10 s (auto-ranging, any frequency)

<1 s (within same octave)

Internal reference 25 kHz to 200 MHz

Freq. resolution 3 digits

Freq. accuracy ±0.1 in the 3

rd

digit
Harmonic detection 2F (50 kHz to 200 MHz)
Reference outputs Phase locked to int./ext. reference

Front panel 25 kHz to 200 MHz square wave

1.0 Vpp nominal into 50 Ω

Rear panel (TTL) 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 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 <5 ppm/°C
drift for all dyn. reserve settings.

Time constants 100 µs to 30 ks with 6, 12, 18 or

24 dB/octave rolloff

”No Filter” mode 10 to 20 µs update rate (X and Y)

Harmonic rejection

Odd harmonics −9.5 dBc @ 3 × ref, −14 dBc @ 5 × ref,

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

Even harmonics < −40 dBc
Sub-harmonics < −40 dBc
Spurious responses −10 dBc @ref ±2 × IF

−23 dBc @ref ±4 × IF
< −30 dBc otherwise

Displays

Channel 1

Type 4½-digit LED and 40-seg. bar graph

Quantities X, R (V or dBm), X-noise, Aux In 1
Channel 2

Type 4½-digit LED and 40-seg. bar graph

Quantities Y, θ, Y-noise (V or dBm), Aux In 2
Expand ×10 or ×100 for Ch1 and Ch2
Ratio X and Y ratioed with respect to

Aux In 1 or Aux In 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 4½-digit LED

Quantities Ref Freq, Phase, Offsets, Aux Out,

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 to 96 kHz

R, θ, Aux inputs 12 to 24 kHz

X-noise, Y-noise 512 Hz

Auxiliary Inputs and Outputs

Inputs 2

Type Differential, 1 MΩ

Range ±10 V

Resolution 0.33 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 computer interfaces.

General

Interfaces IEEE-488.2 and RS-232 interfaces

are standard.

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

50/60 Hz

Dimensions 17" × 5.25" × 19.5" (WHD)
Weight 23 lbs.
Warranty One year parts and labor on defects

in materials and workmanship

The SR844 RF lock-in amplifier utilizes a combination of
analog and digital techniques to obtain maximum
performance over a wide frequency range. Since it is not
feasible to use pure digital techniques at the SR844's
maximum 200 MHz operating frequency, analog down­conversion is used to transform the signal frequency to a range
suitable for DSP processing.

A block diagram of the SR844 lock-in amplifier is shown
below. The RF input signal passes through adjustable RF
attenuators and gain stages depending on the selected input
sensitivity. The signal is then mixed with two reference
signals which differ in phase by 90°.

The reference signals are generated from either the external
reference input or the built-in frequency synthesizer, using a
phase-lock loop (PLL) circuit. If the reference frequency
were exactly at the signal frequency, the output of the mixers
would be at DC. Since it is difficult to build drift and offset
free amplifier chains, the SR844 chops the reference signals at
a chopping frequency (f

c

) which is chosen to be fast relative
to the fastest time constants, yet slow relative to the input
signal frequency. The IF amplifier and filter chain can now be
AC coupled, eliminating DC offset and drift problems.

Once the in-phase and quadrature IF signals have been
amplified and low-pass filtered, the signals are digitized by
two precision 16-bit analog-to-digital converters. The digital
IF signals are ratioed (if ratio mode is selected) and digitally

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low-pass filtered (with 6, 12, 18 or 24 dB/oct filter slopes)
allowing the original signal amplitude and phase to be
recovered.

Note that the SR844 uses a square wave mixer, not a sine
wave mixer like other SRS lock-in amplifiers. This is because
precision sine generation is impractical with current
technology over the SR844's operating range. The effect of
using a square wave mixer is that the lock-in will respond not
only at the reference frequency, but at all the Fourier
components of the square wave reference. Since a square
wave consists of odd harmonics with amplitudes 1/3, 1/5, 1/7,
etc., the SR844 will respond at odd multiples of the reference
frequency as well as at the reference frequency as itself. This
usually does not present a problem as long as it is understood.

SR844 200 MHz Lock-In Amplifier

About RF Lock-In Amplifiers

SR844 block diagram

Signal Input

Reference Input

RF Section IF Section

50Ω

RF

Preamp

1MΩ

Auto Threshold

Comparator

Attenuator

Internal Synthesizer

Reference Section

In Phase Ref.

Quadrature Ref.

ChopperPLL

RF
Gain

Chopped In Phase

Ref. Signal

Quadrature Ref. Signal

Mixer

Gain

Gain

DSP

Processor

LPF

LPF

16 bit ADC

16 bit ADC

16 bit DAC

16 bit DAC

Ch1/X

Ch2/Y