A phase-locked loop (PLL) is an electronic circuit with a voltage- or current-driven oscillator that is constantly adjusted to match in phase (and thus lock on) the frequency of an input signal. In addition to stabilizing a particular communications channel (keeping it set to a particular frequency), a PLL can be used to generate a signal, modulate or demodulate a signal, reconstitute a signal with less noise, or multiply or divide a frequency. PLLs are frequently used in wireless communication, particularly where signals are carried using frequency modulation (FM) or phase modulation (PM). PLLs can also be used in amplitude modulation (AM). PLLs are more commonly used for digital data transmission, but can also be designed for analog information. Phase-locked loop devices are more commonly manufactured as integrated circuits (ICs) although discrete circuits are used for microwave.
A PLL consists of a voltage-controlled oscillator (VCO) that is tuned using a special semiconductor diode called a varactor. The VCO is initially tuned to a frequency close to the desired receiving or transmitting frequency. A circuit called a phase comparator causes the VCO to seek and lock onto the desired frequency, based on the output of a crystal-controlled reference oscillator. This works by means of a feedback scheme. If the VCO frequency departs from the selected crystal reference frequency, the phase comparator produces an error voltage that is applied to the varactor, bringing the VCO back to the reference frequency. The PLL, VCO, reference oscillator, and phase comparator together comprise a frequency synthesizer. Wireless equipment that uses this type of frequency control is said to be frequency-synthesized.
Since a PLL requires a certain amount of time to lock on the frequency of an incoming signal, the intelligence on the signal (voice, video, or data) can be obtained directly from the waveform of the measured error voltage, which will reflect exactly the modulated information on the signal.