What part of the AFC system is responsible for sampling the modulator output?

The correct answer is the phase-locked loop. In an Automatic Frequency Control (AFC) system, the phase-locked loop (PLL) plays a crucial role in ensuring that the output frequency remains stable and locked to the desired signal. It continuously monitors the modulator output by comparing its phase with that of a reference signal. If there is any phase difference between these signals, the PLL adjusts the frequency to reduce the difference, effectively maintaining synchronization.

The other components mentioned, such as the voltage-controlled oscillator and the crystal-controlled reference oscillator, contribute to different functions within the AFC system. The voltage-controlled oscillator generates frequencies based on voltage input and can be adjusted, while the crystal-controlled reference oscillator provides a stable reference frequency. However, it is the phase-locked loop that directly samples the modulator output and makes real-time adjustments to maintain frequency stability. This aspect is essential for the functioning of the AFC system. Therefore, the phase-locked loop is integral to sampling the modulator output effectively.