Generating a reference signal is an important prerequisite for operation of a lock-in amplifier, and a phase-locked loop is usually employed for this purpose. Therefore, we look at this feedback system first and then consider the working principle of the multiplication-and-integration unit in detail.

In the signal path of the lock-in amplifier, the multiplication-and-integration unit is preceded by the front end, which comprises amplifiers and filters. In the reference path, the multiplication-and-integration unit is preceded by the reference channel, which comprises a phase-locked loop and a phase shifter.