Phase-Locked Loop (PLL)

A Phase-locked loop (PLL) is an electronic circuit that controls an oscillator so that the circuit maintains a constant phase angle relative to a reference signal.

The operation of the above circuit is typical of all PLLs. This circuit is a feedback control system that controls the frequency and phase of a voltage-controlled oscillator (VCO). An input signal is applied to a phase detector and the output of the VCO connects to the other phase detector input. As shown in the previous diagram, the frequencies of both signals are the same. The output of the phase detector develops a voltage proportional to the phase difference between the two input signals. The lowpass filter receives this signal from the phase detector and determines the dynamic characteristics of the PLL. This output signal is the filtered signal that controls the VCO.