Phase-locked loop circuits

Phase-locked loop (PLL) circuits are found in a variety of high-frequency applications, from simple clock cleaning circuits to local oscillator (LO) for high-performance radio communication links, and ultra-fast switching frequency synthesizers in vector network analyzers (VNA). A PLL is a feedback system in which a voltage-controlled oscillator and a phase comparator are connected to each other so that the oscillator frequency (phase) can accurately track the applied frequency or the frequency of a phase-modulated signal. PLLS can be used to generate a stable output frequency signal from a fixed low frequency signal. The first PLLS were implemented by the French engineer de Bellescize in the early 1930s. However, it was not until the mid-1960s, when integrated PLL became a relatively low-cost component, that PLLS gained widespread market acceptance.

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