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The SR810 Lock-In Amplifier and SR830 Lock-In Amplifier provide
high performance at a reasonable cost. The SR830 simultaneously
displays the magnitude and phase of a signal, while the SR810 displays
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 Lock-In Amplifiers 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 input
can also be configured for current measurements with selectable
current gains of 106 and 108
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 based design of these lock-ins has such inherently large dynamic
reserve that no tracking 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 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.
Conventional lock-in amplifiers use an analog demodulator to mix
an input signal with a reference signal. Dynamic reserve is limited
to about 60 dB, and these instruments suffer from poor stability,
output drift, and excessive gain and phase error. Demodulation in
the SR810 Lock-In Amplifier and SR830 Lock-In Amplifier is accomplished
by sampling the input signal with a high-precision A/D converter,
and multiplying the digitized input by a synthesized reference signal.
This digital demodulation technique results in more than 100 dB
of true dynamic reserve (no prefiltering) and is free of the errors
associated with analog instruments.
Digital Filtering
The digital signal processor also handles the task of output filtering,
allowing time constants from 10 µsec to 30,000 s, with a choice
of 6, 12, 18 and 24 dB/oct rolloff. For low frequency measurements
(below 200 Hz), synchronous filters can be engaged to notch out
multiples of the reference frequency. Since the harmonics of the
reference have been eliminated (notably 2F), effective output filtering
can be achieved with much shorter time constants.
Digital Phase Shifting
Analog phase shifting circuits have also been replaced with a DSP
calculation. Phase is measured with 0.01° resolution, and the
X and Y outputs are orthogonal to 0.001°.
Frequency Synthesizer
The built-in direct digital synthesis (DDS) source generates a very
low distortion (-80 dBc) reference signal. Single frequency
sine waves can be generated from 1 mHz to 102 kHz with 4½
digits of resolution. Both frequency and amplitude can be set from
the front panel or from a computer. When using an external reference,
the synthesized source is phase locked to the reference signal.
Auto Functions
Auto-functions allow parameters that are frequently adjusted to
automatically be set by the instrument. Gain, phase, offset and
dynamic reserve are each quickly optimized with a single key press.
The offset and expand features are useful when examining small fluctuations
in a measurement. The input signal is quickly nulled with the auto-offset
function, and resolution is increased by expanding around the relative
value by up to 100×. Harmonic detection is no longer limited
to only the 2F component. Any harmonic (2F, 3F, ... nF) up to 102
kHz can now be measured without changing the reference frequency.
Analog Inputs and Outputs
Both instruments have a user-defined output for measuring X, R,
X-noise, Aux1, Aux 2, or the ratio of the input signal to an external
voltage. The SR830 has a second, user-defined output that measures
Y, Θ, Y-noise, Aux 3, Aux 4 or ratio. The SR810 and SR830
both have X and Y analog outputs (rear panel) that are updated at
256 kHz. Four auxiliary inputs (16-bit ADCs) are provided for
general purpose use—like normalizing the input to source intensity
fluctuations. Four programmable outputs (16-bit DACs) provide voltages
from -10.5 V to +10.5 V and are settable via the front panel or
computer interfaces.
Internal Memory
The SR810 Lock-In has an 8,000 point memory buffer for recording
the time history of a measurement at rates up to 512 samples/s.
The SR830 has two 16,000 point buffers to simultaneously record
two measurements. Data is transferred from the buffers using the
computer interfaces. A trigger input is also provided to externally
synchronize data recording.
Easy Operation
The SR810 Lock-In Amplifier and SR830 Lock-In Amplifier are simple
to use. All instrument functions are set from the front-panel keypad,
and a spin knob is provided to quickly adjust parameters. Up to
nine different instrument configurations can be stored in non-volatile
RAM for fast and easy instrument setup. Standard RS-232 and GPIB
(IEEE-488.2) interfaces allow communication with computers. All
functions can be controlled and read through the interfaces.
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