Phase-locked loop

A feedback system that adjusts an oscillator to maintain a chosen phase or frequency relationship with a reference signal. It is used for synchronization, frequency synthesis, and signal recovery.

Phase detector: A circuit or algorithm that produces an error signal related to the phase difference between two signals. It measures the reference-to-oscillator phase error that drives correction.
Frequency synthesizer: An electronic system that generates selectable frequencies from a stable reference. A phase-locked loop can multiply or divide a reference to produce precise output frequencies.
Phase (waves): The position of a point within a repeating cycle, commonly expressed as an angle or fraction of a period. The loop’s error is defined by comparing the phases of its reference and oscillator signals.
Injection locking: Synchronization of an oscillator by applying an external periodic signal directly to it. It can synchronize oscillators without the explicit phase detector and feedback filter of a conventional loop.
Loop filter: A filter that shapes a feedback loop’s response, including its stability, bandwidth, and transient behavior. It smooths the phase-error signal and sets how quickly the loop responds.
Clock recovery: The extraction of timing information from a data signal to recreate a synchronized clock. A loop can follow transitions in received data and reconstruct its sampling timing.
Frequency: The number of cycles of a repeating event completed per unit time. A persistent phase difference changes when the two signals have different frequencies.
Frequency-locked loop: A feedback system that adjusts an oscillator using frequency error rather than phase error. It can aid frequency acquisition, while phase locking provides precise phase synchronization.
Voltage-controlled oscillator: An oscillator whose output frequency varies with an applied control voltage. It changes frequency in response to the filtered error signal.
Carrier recovery: The estimation and reconstruction of a carrier’s frequency and phase from a modulated signal. A phase-locked loop can synchronize a receiver’s local oscillator to the transmitted carrier.

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