Which components make up a phase-locked loop (PLL) circuit?

A phase-locked loop (PLL) circuit is a control system that generates a signal with a fixed relationship to the phase of a reference signal. The components that form a standard PLL include a phase detector, a low-pass filter, and a voltage-controlled oscillator.

The phase detector is critical as it compares the phase of the input signal and the output from the voltage-controlled oscillator. It produces an output voltage that is directly proportional to the difference in phase between these two signals.

The low-pass filter then processes this output from the phase detector. Its primary role is to filter out high-frequency noise and to smooth the signal before it reaches the voltage-controlled oscillator. This filtering ensures that the PLL can maintain a stable and accurate output by preventing rapid fluctuations.

Finally, the voltage-controlled oscillator (VCO) generates a frequency that varies based on its input voltage, which comes from the output of the low-pass filter. This allows the PLL to lock onto the frequency and phase of the input signal effectively.

Thus, the combination of these three components works together to enable the PLL to achieve phase locking and frequency synthesis, making option A the correct answer.