A phase-locked loop (PLL) is an electronic circuit with a voltage or voltage-driven oscillator that constantly adjusts to match the frequency of an input signal. PLLs are used to generate, stabilize, modulate, demodulate, filter or recover a signal from a "noisy" communications channel where data has been interrupted.
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Where phase-locked loops are used
PLLs are used in telecommunications, computers, radio and other electronic applications. They are frequently used in wireless communication, primarily on frequency modulation (FM) or phase modulation (PM) transmissions. Phase-locked loops are more commonly used for digital data transmission than for analog transmission and are more commonly manufactured as integrated circuits, although discrete circuits are used for microwave signal processing.
How phase-locked loops work
PLLs work by constantly adjusting a voltage or current-driven oscillator to match (lock onto) the phase and frequency of an input signal, which typically consists of a voltage-controlled oscillator (VCO) 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, which is set via a crystal-controlled reference oscillator. When the VCO frequency differs from the reference frequency, the phase comparator produces an error voltage. The comparator output is usually run through a low-pass filter (a signal filter that reduces the strength of high-frequency waves) to further reduce noise. The filtered output is fed back to the varactor to continually push the VCO toward the reference frequency.
The filtered output of the comparator also provides the output of the circuit -- the signal found in the transmission (the voice, video or data). Since the signal is encoded by modulating a carrier wave, it can be thought of as the difference between the carrier waveform and the actual transmitted waveform, and can therefore be found in the output of the comparator.
Together, the phase-locked loop, VCO, reference oscillator and phase comparator comprise a frequency synthesizer -- an electronic system that produces a range of frequencies from a single fixed oscillator. Wireless equipment that uses this type of frequency control is said to be frequency-synthesized. Other frequency-synthesized devices include mobile phones, satellite receivers and GPS systems.