A phase-locked loop (PLL) is an electronic circuit that synchronizes an output signal's phase with a reference signal. It achieves this by comparing the phase of the two signals and adjusting the output to minimize any phase difference. This technique is widely used in communication systems, such as radio transmitters and receivers, to maintain stable frequencies. PLLs consist of three main components: a phase detector, a low-pass filter, and a voltage-controlled oscillator (VCO). The phase detector identifies the phase difference, the low-pass filter smooths the output, and the VCO generates the adjusted signal. This technology is essential in applications like frequency synthesis and clock recovery.