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.
Essentially the lock-in amplifier measures an AC voltage (or current) and gives an output in the form of a DC voltage proportional to the value of the AC signal being measured. It is called an “amplifier” because the DC level at the output is usually greater than the AC level at the input and is termed “lock-in” because it locks to and measures the particular frequency of interest ignoring all other signals at the input. The heart of the lock-in amplifier is a phase sensitive detector, sometimes known as the demodulator. It is this part of the instrument which demodulates the frequency of interest and it should be noted that its output is also a function of the relative phase angle between the input signal and the associated reference signal. It follows therefore, that the lock-in amplifier can also be used to measure the relative phase relationship of two signals of the same frequency. The signal path of a lock-in amplifier is gain adjusted to achieve a specific input sensitivity when
