A Phase-Locked Loop (PLL) is a feedback control system that generates an output signal whose phase is related to the phase of an input signal. In frequency synthesis, a PLL is used to generate an output signal that has a frequency that is a multiple or fraction of the input frequency. The PLL compares the phase of the input signal with the phase of a voltage-controlled oscillator (VCO) output, and adjusts the VCO frequency to match the input signal frequency. This allows the PLL to synthesize an output frequency that is a multiple or fraction of the input frequency, making it useful in applications such as clock generation, frequency modulation, and frequency shifting.
