phone: (408)744-9040
www.thinkSRS.com
Stanford Research Systems
· 1 mHz to 102.4 kHz frequency range
· >100 dB dynamic reserve
· 5 ppm/°C stability
· 0.01 degree phase resolution
· Time constants from 10 µs to 30 ks
(up to 24 dB/oct rolloff)
· Auto-gain, -phase, -reserve and -offset
· Synthesized reference source
· GPIB and RS-232 interfaces
· SR810 ... $3850
(U.S. list)
· SR830 ... $4750
(U.S. list)
The SR810 and SR830 DSP Lock-In Amplifiers provide high
performance at a reasonable cost. The SR830 simultaneously
displays the magnitude and phase of a signal, while the SR810
displays the magnitude only. Both instruments use digital
signal processing (DSP) to replace the demodulators, output
filters, and amplifiers found in conventional lock-ins. The
SR810 and SR830 provide uncompromised performance with
an operating range of 1 mHz to 102 kHz and 100 dB of drift-
free dynamic reserve.
Input Channel
The SR810 and SR830 have differential inputs with 6 nV/√Hz
input noise. The input impedance is 10 MΩ, and minimum
full-scale input voltage sensitivity is 2 nV. The inputs can
also be configured for current measurements with selectable
current gains of 10
6
and 10
8
V/A. A line filter (50 Hz or
60 Hz) and a 2× line filter (100 Hz or 120 Hz) are provided
to eliminate line related interference. However, unlike
conventional lock-in amplifiers, no tracking band-pass filter
is needed at the input. This filter is used by conventional lock-
ins to increase dynamic reserve. Unfortunately, band pass
filters also introduce noise, amplitude and phase error, and
drift. The DSP design of these lock-ins has such inherently
large dynamic reserve that no band pass filter is needed.
Extended Dynamic Reserve
The dynamic reserve of a lock-in amplifier, at a given full-
scale input voltage, is the ratio (in dB) of the largest interfering
SR810 & SR830 DSP Lock-In Amplifiers
Digital Lock-In Amplifiers
SR810 and SR830 — DSP lock-in amplifiers
SR830 DSP Lock-In Amplifier
WHAT IS A LOCK-IN AMPLIFIER?
Lock-in amplifiers are used to detect and measure
very small AC signals - all the way down to a few
nanovolts! Accurate measurements may be made
even when the small signal is obscured by noise
sources many thousands of times larger.
Lock-in amplifiers use a technique known as
phase-sensitive detection to single out the compo-
nent of the signal at a specific reference frequency
AND phase. Noise signals at frequencies other
than the reference frequency are rejected and do
not affect the measurement.
Let's consider an example. Suppose the signal is a
10 nV sine wave at 10 kHz. Clearly some amplifi-
cation is required. A good low noise amplifier will
have about 5 nV/√Hz of input noise. If the amplifier
bandwidth is 100 kHz and the gain is 1000, then
we can expect our output to be 10 µV of signal
(10 nV x 1000) and 1.6 mV of broadband noise
(5 nV/√Hz x √100 kHz x 1000). We won't have
much luck measuring the output signal unless we
single out the frequency of interest.
If we follow the amplifier with a band pass filter
with a Q=100 (a VERY good filter) centered at
10 kHz, any signal in a 100 Hz bandwidth will be
detected (10 kHz/Q). The noise in the filter pass
band will be 50 µV (5 nV/√Hz x √100 Hz x 1000)
and the signal will still be 10 µV. The output noise
is much greater than the signal and an accurate
measurement can not be made. Further gain will
not help the signal to noise problem.
Now try following the amplifier with a phase-
sensitive detector (PSD). The PSD can detect the
signal at 10 kHz with a bandwidth as narrow as
0.01 Hz! In this case, the noise in the detection
bandwidth will be only 0.5 µV (5 nV/√Hz x √.01 Hz
x 1000) while the signal is still 10 µV. The signal to
noise ratio is now 20 and an accurate measure-
ment of the signal is possible.
What is phase-sensitive detection?
Lock-in measurements require a frequency refer-
ence. Typically an experiment is excited at a fixed
frequency (from an oscillator or function generator)
and the lock-in detects the response from the
SR830 BASICS
experiment at the reference frequency. In the dia-
gram below, the reference signal is a square wave
from a function generator. If the sine output from
the function generator is used to excite the experi-
ment, the response might be the signal waveform
shown below. The signal is V
The SR830 generates its own sine wave, shown
as the lock-in reference below. The lock-in refer-
The SR830 amplifies the signal and then multiplies
it by the lock-in reference using a phase-sensitive
detector or multiplier. The output of the PSD is
simply the product of two sine waves.
The PSD output is two AC signals, one at the dif-
If the PSD output is passed through a low pass
filter, the AC signals are removed. What will be
left? In the general case, nothing. However, if ω
, the difference frequency component
will be a DC signal. In this case, the filtered PSD
. This might be the sync output