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 measured.

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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 filter.

• 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 DC offset.
• 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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