What Is a Lock-In Amplifier?
Lock-in amplifiers are used to measure the amplitude and phase
of signals buried in noise. They achieve this by acting as a
narrow bandpass filter which removes much of the unwanted noise
while allowing through the signal which is to be measured.
The frequency of the signal to be measured and hence the
passband region of the filter is set by a reference signal, which
has to be supplied to the lock-in amplifier along with the
unknown signal. The reference signal must be at
the same frequency as the modulation of the signal to be
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Lock-In Amplifiers Internal Workings
A basic lock-in amplifier can be split into 4 stages: an input
gain stage, the reference circuit, a demodulator and a low pass
- Input Gain Stage: The variable gain
input stage pre-processes the signal by amplifying it to
a level suitable for the demodulator. Nothing complicated
here, but high performance amplifiers are required.
- Reference Circuit: The reference circuit
allows the reference signal to be phase shifted.
- Demodulator: The demodulator is a
multiplier. It takes the input signal and the reference
and multiplies them together. When you multiply two
waveforms together you get the sum and difference
frequencies as the result. As the input signal to be
measured and the reference signal are of the same
frequency, the difference frequency is zero and you get a
DC output which is proportional to the amplitude of the
input signal and the cosine of the phase difference
between the signals. By adjusting the phase of the
reference signal using the reference circuit, the phase
difference between the input signal and the reference can
be brought to zero and hence the DC output level from the
multiplier is proportional to the input signal. The noise
signals will still be present at the output of the
demodulator and may have amplitudes 1000x larger than the
- Low Pass Filter: As the various noise
components on the input signal are at different
frequencies to the reference signal, the sum and
difference frequencies will be non zero and will not
contribute to the DC level of the output signal. This DC
level (which is proportional to the input signal) can now
be recovered by passing the output from the demodulator
through a low pass filter.
The above gives an idea of how a basic lock-in amplifier
works. Actual lock-in amplifiers are more complicated, as there
are instrument offsets that need to be removed, but the basic
principle of operation is the same.
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