*Question E7H03:
What is a phase-locked loop?*  

*Correct Answer is  C: An electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator*  

### AI Response: (may contain errors)  

A phase-locked loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. It is widely used in radio, telecommunications, and other electronic applications to synchronize signals.

### Correct Answer: C
- **Phase Detector**: Compares the phase of the input signal with the phase of the output signal from the voltage-controlled oscillator (VCO).
- **Low-Pass Filter**: Filters the output of the phase detector to remove high-frequency noise, producing a control voltage.
- **Voltage-Controlled Oscillator (VCO)**: Generates an output signal whose frequency is controlled by the input voltage from the low-pass filter.
- **Stable Reference Oscillator**: Provides a stable frequency reference for the phase detector to compare against.

The PLL works by continuously adjusting the VCO to keep the output signal in phase with the input signal. This is why it is called a "phase-locked" loop.

### Why Other Choices Are Incorrect:
- **Choice A**: Describes components not typically associated with a PLL. A ratio detector and reactance modulator are not part of a PLL.
- **Choice B**: A monostable multivibrator is a different type of circuit used to generate a single output pulse of a specific duration, not related to phase-locking.
- **Choice D**: Describes a circuit that does not match the components or function of a PLL. A precision push-pull amplifier with a differential phase input is not a PLL.

### Key Points:
- A PLL is used to lock the phase of an output signal to an input signal.
- It is essential for applications requiring signal synchronization, such as frequency synthesis and demodulation.
- The main components are the phase detector, low-pass filter, VCO, and reference oscillator.