2 – 2
Introduction
2.1
Introduction to the instrument
Lock-in amplifiers are used to detect and measure very small AC
signals, often in the presence of noise sources that can be orders of
magnitudes larger. Typical lock-in amplifiers today are based on
high-speed digital signal processing (DSP) techniques. This o
ff
ers
outstanding performance and flexibility for many, perhaps most,
applications. For certain demanding situations, however, residual
artifacts from the DSP approach—either due to sampling lag and
real-time response, or RF clock and related noise—can fall short of
users needs.
The SR124 is a modern, all-analog lock-in amplifier that provides out-
standing signal recovery capabilities, without the shortcomings that
can limit the usefulness of more common DSP-based instruments.
The design follows two basic themes. First, the signal path is entirely
built from low-noise analog electronics: the best JFETs, transistors,
op-amps, and discrete components. Second, configuration control
is managed by a microcontroller whose system clock only oscillates
during the brief moments needed to change gains or filter settings.
2.1.1
Clock stopping architecture
This “clock-stopping” architecture, first introduced by SRS in the
SR560 Voltage Preamplifier, eliminates the inconvenience and relia-
bility issues associated with mechanical panel controls, and makes
full remote operation of the SR124 possible. Whenever the microcon-
troller becomes active, the
CPU Activity
indicator illuminates, clearly
showing when the digital clock is running. This occurs in response
to front-panel button presses or remote computer commands.
Sometimes, you need to be certain your experiment will be undis-
turbed: you’ve cooled your sample to a few millikelvin, all your
wiring is still intact, and the best device you’ve built all year is ready
for measurement. A locking toggle switch on the front panel can be
set to “INHIBIT”, forcing the digital clock to remain o
ff
, even if you
press other buttons or knobs. The analog configuration of the SR124
stays steady, letting you run for minutes, hours, days—as long as
you need.
2.1.2
What does the SR124 measure?
In lock-in mode, the SR124 multiplies the input signal by a square
wave at the reference frequency, using a square-wave analog mixer.
The resulting signal is then low-pass filtered to produce an output
proportional to the frequency component of the user’s input signal at
the reference frequency (and also, to a diminishing extent, at it’s odd
SR124
Analog Lock-In Amplifier
