phaselocking

Phaselocking is the synchronization of the phase of an oscillator to the phase of an external reference signal or to another oscillator. In a locked state, the phase difference between the driven signal and the reference becomes constant (or constrained within a small bound), and the oscillator’s frequency adjusts to match the reference or a harmonic of it. Phase locking can arise from mutual coupling in a network of oscillators or from feedback in a control system.

In electronics, the canonical implementation is a phase-locked loop (PLL), which comprises a phase detector, a loop filter, and a controllable oscillator. The phase detector compares the reference with the oscillator output to generate an error signal; the loop filter shapes the response, and the oscillator is adjusted to minimize phase error. A PLL can capture and lock to a reference frequency, track frequency changes, and reject phase noise to improve timing accuracy. Lock ranges describe conditions under which the loop can achieve and maintain lock, including the capture range and hold-in range.

Outside electronics, phaselocking can describe injection locking, where a slave oscillator is forced by a strong external signal, or optical phase locking in lasers where a master laser stabilizes the phase of a slave laser. In physics, phaselocking and entrainment refer to synchronization phenomena in coupled oscillators, described by models such as the Kuramoto model, and realized in mechanical, electrical, and biological systems.

Applications include communications, frequency synthesis, clock recovery in digital systems, and precision metrology. Noise, delay, and nonlinear dynamics can cause intermittent slips of the phase or loss of lock.