A phase-locked loop or phase lock loop (PLL) is a control system that generates an outputsignal whose phase is related to the phase of an input \"reference\" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector. This circuit compares the phase of the input signal with the phase of the signal derived from its output oscillator and adjusts the frequency of its oscillator to keep the phases matched. The signal from the phase detector is used to control the oscillator in a feedback loop.