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Definition: PLL

(Phase-Locked Loop) An electronic circuit that compares an input frequency and phase to a reference signal. It then generates a voltage proportional to the difference between the input and the reference. PLLs are used in myriad digital and mixed mode (analog and digital) applications as regulators, demodulators, synchronizers and frequency multipliers and dividers. For example, in a superheterodyne FM radio, a PLL is used to lock the local oscillator to an accurate frequency reference such as a crystal or ceramic resonator. It can also be used to demodulate the FM audio and stereo subcarrier signals from the intermediate frequency (IF) stage. See heterodyning.

PLLs in an FM Radio
This example shows how PLLs are used for regulation and demodulation in an FM radio tuned to 101.5 MHz. The FM frequency range is 88-108 MHz, and this example uses a 10.7 MHz intermediate frequency (IF) and a 1 MHz reference frequency.