They can be used to demodulate a signal, recover a signal from a noisy communication channel, generate a stable frequency at multiples of an input frequency ( frequency synthesis), or distribute precisely timed clock pulses in digital logic circuits such as microprocessors. Phase-locked loops are widely employed in radio, telecommunications, computers and other electronic applications. These properties are used for computer clock synchronization, demodulation, and frequency synthesis. Consequently, in addition to synchronizing signals, a phase-locked loop can track an input frequency, or it can generate a frequency that is a multiple of the input frequency. Keeping the input and output phase in lockstep also implies keeping the input and output frequencies the same. The oscillator generates a periodic signal of a specific frequency, and the phase detector compares the phase of that signal with the phase of the input periodic signal, to adjust the oscillator to keep the phases matched. The oscillator's frequency and phase are controlled proportionally by an applied voltage, hence the term voltage-controlled oscillator (VCO). There are several different types the simplest is an electronic circuit consisting of a variable frequency oscillator and a phase detector in a feedback loop. A phase-locked loop or phase lock loop ( PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal.
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