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The SR510 and SR530 are analog lock-in amplifiers which can measure
AC signals as small as nanovolts in the presence of much larger
noise levels. Both the single phase SR510 and the dual phase SR530
have low-noise voltage and current inputs, high dynamic reserve,
two stages of time constants, and an internal oscillator. In addition,
both lock-ins come equipped with a variety of features designed
to make them simple to use.
Sine Wave Mixing
The core of the SR510/SR530 is a precision analog sine-wave multiplier.
Lock-ins use a multiplier (demodulator) to translate the signal
input (at reference frequency) down to DC where it can be filtered
and amplified. Many lock-ins use square wave multipliers which introduce
spurious harmonic responses. The SR510/SR530 use clean sine-wave
multipliers which are inherently free of unwanted harmonics.
Signal Input
The SR510/SR530 have differential inputs with 7 nV/√Hz of
input noise and 100 MΩ input impedance. The input can be configured
as a voltage input, or as a current input with 106
V/A gain and an input impedance of 1 kΩ to virtual ground.
Full-scale sensitivities from 500 mV down to 100 nV are available.
Three input prefilters can be selected. The first is a line notch
filter providing 50 dB of rejection at the line frequency. The second
filter similarly provides 50 dB of rejection at the second harmonic
of the line frequency. The third filter is a band pass filter which
automatically tracks the reference frequency. These three filters
can eliminate much of the noise in the signal before it is amplified.
Reference Input
The reference input can be set to lock to sine waves or to either
edge of a pulsed reference. The reference frequency range is 0.5
Hz to 100 kHz, and detection at both the fundamental and second
harmonic of the reference is allowed. A convenient, built-in frequency
meter constantly measures and displays the reference frequency with
4-digit resolution. The reference can be phase shifted with 0.025°
resolution from the front panel, or shifted in 90° increments
for easy measurement of quadrature signals. The SR530 has an auto-phase
feature that lets you quickly determine the phase of the signal
relative to the reference with a single keypress.
Output Time Constants
Two stages of filtering follow the phase sensitive detector. Time
constants can be chosen as long as 100 seconds for maximum noise
reduction, or as short as 1 ms (20 µs with modification) for
use in real-time servo loops. The two filter stages allow a rolloff
of 6 or 12 dB/octave.
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 signal
to the full-scale input voltage. The largest interfering signal
is defined as the amplitude of the largest signal at any frequency
that can be applied to the input before the lock-in cannot measure
a signal with its specified accuracy.
The SR510 and SR530 have a dynamic reserve of between 20 dB and
60 dB, depending on the sensitivity scale. Selecting the band pass
filter adds an additional 20 dB of dynamic reserve, making the maximum
dynamic reserve for these lock-ins 80 dB.
Offset and Expand
The SR510/SR530's offset and expand features make it easy to look
at small changes in a large signal. Output offsets of 0 % to 100
% of full scale can be selected manually, or by using auto-offset,
which automatically selects an offset equal to the signal value.
Once the signal is offset, a 10× expand is available to provide
increased resolution when looking at small changes from a nominal
value.
Analog and Digital Displays
Precision analog meters and 4-digit digital displays are standard
on both lock-ins. On the SR510, you can select displays of the signal
amplitude, the signal offset, or the measured noise. On the SR530,
the first pair of displays show the signal components in rectangular
form (X and Y), polar form (R and Θ), the offset, noise, or
the value of the rear-panel D/A outputs. The other digital display
on both lock-ins can be configured to show either the reference
phase or the reference frequency.
Noise Measurement
The SR510/SR530's noise measurement feature lets you directly measure
the noise in your signal at the reference frequency. Noise is defined
as the rms deviation of the signal from its mean. The SR510/SR530
will report the value of the noise in both a 1 Hz and 10 Hz bandwidth
around the reference frequency.
Internal Oscillator
An internal voltage-controlled oscillator provides both an adjustable-amplitude
sine wave output and a synchronous, fixed-amplitude reference output.
The sine wave amplitude can be set to 0.01, 0.1, or 1 Vrms, and
can drive up to 20 mA. The oscillator frequency is controlled by
a rear-panel voltage input and can be adjusted between 1 Hz and
100 kHz. Typically, the sine wave output is used to excite some
aspect of an experiment, while the reference output provides a frequency
reference to the lock-in.
A/Ds and D/As
There are four A/Ds and two D/As on the rear panel that provide
flexibility in interfacing the SR510/SR530 with external signals.
These input/output ports measure and supply analog voltages with
a range of ±10.24 VDC and a resolution of 2.5 mV. The A/Ds
digitize signals at a rate of 1 kHz. The D/A output is ideal for
controlling the frequency of the SR510/530's internal voltage-controlled
oscillator. A built-in ratio feature allows the SR510/SR530 to calculate
the ratio of its output to a signal at one of the A/D ports. This
feature is important in servo applications to maintain a constant
loop gain, or in experiments that normalize a signal to an intensity
level.
Available Preamplifiers
Although the SR510 and SR530 are completely self contained and
require no preamplification, sometimes an external preamplifier
can be useful. Remote preamplifiers provide gain where it's most
important— right at the detector, before the signal-to-noise
ratio is permanently degraded by cable noise and pickup. The SR550
FET-input preamplifier, the SR552
bipolar-input preamplifier, and the SR554
transformer-input preamplifier are ideally suited for use with the
SR510/SR530 lock-ins. These preamplifiers are especially useful
when measuring extremely low-level signals.
Computer Interfaces
An RS-232 computer interface is standard on both the SR510 and
SR530. An optional GPIB interface is also available. All features
of the instruments can be queried and set via the computer interfaces.
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