What is the function of a phase-locked loop (PLL) circuit?

A phase-locked loop (PLL) circuit plays a critical role in synchronizing the frequency of an output signal to that of a reference frequency. Specifically, it compares the output of a voltage-controlled oscillator (VCO) to a stable frequency reference. The PLL adjusts the frequency of the VCO to ensure that its output matches the frequency of the reference signal. This synchronization is essential in various applications, such as frequency synthesis, demodulation, and clock recovery in communication systems.

The operation of a PLL involves feedback control; when there is a phase difference between the VCO output and the reference frequency, the PLL generates a control voltage that adjusts the VCO to minimize that phase difference, effectively locking the two frequencies together. This ability to maintain a consistent phase relationship makes PLLs invaluable for maintaining stable frequency outputs in electronic devices.

The other options do not describe the primary function of a phase-locked loop effectively. For instance, amplifying signals is a function that typically falls under the purview of amplifiers rather than PLLs, and converting analog to digital signals is the role of analog-to-digital converters (ADCs). Filtering unwanted frequencies is a characteristic of filters but is not the main role of a PLL. Thus, the identification of the