Phase-locked loops (PLLs) for analog signals are ubiquitous in today’s physics and engineering applications. This white paper presents their essential functions and operating principles. It also covers several practical use cases that can be conveniently implemented with Zurich Instruments’ lock-in amplifiers.
A PLL is a closed-loop control system with negative feedback that maintains a well-defined phase relation between two periodic signals: its input as a reference and its output as a follower. As a result of phase-locking, the two signals have the same frequency, which enables coherent signal generation and detection across multiple signal sources. In its simplified form, a PLL consists of the following building blocks: