What is a phase-locked loop?

A phase-locked loop (PLL) is an essential control system that generates an output signal whose phase is related to the phase of an input reference signal. It consists of four main building blocks: a stable reference oscillator, a phase detector, a low-pass filter, and a voltage-controlled oscillator (VCO). The phase detector compares the reference signal to the VCO output and generates an error voltage. The low-pass filter smooths this voltage, which is then used to tune the VCO so that its frequency and phase perfectly match the reference. Distractors are incorrect because they either describe completely different circuits, like a monostable multivibrator, or they list the wrong components, such as a ratio detector (used for FM demodulation) instead of a phase detector.