*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 a signal that is phase-locked to a reference signal. The 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.**

### Explanation of the Correct Answer:
- **Phase Detector**: Compares the phase of the input signal (reference) with the output of the voltage-controlled oscillator (VCO) and produces an error signal based on the phase difference.
- **Low-Pass Filter**: Smooths the error signal from the phase detector to remove high-frequency noise, allowing only the desired control signal to pass through.
- **Voltage-Controlled Oscillator (VCO)**: Generates an output frequency that can be adjusted based on the control voltage from the low-pass filter.
- **Stable Reference Oscillator**: Provides a consistent reference frequency for the phase detector to compare against.

This combination allows the PLL to lock onto the frequency and phase of the reference signal, making it useful in various applications like frequency synthesis and demodulation.

### Why the Other Choices are Incorrect:
- **A**: This describes a different type of circuit. A ratio detector is not a standard component of a PLL; instead, a phase detector is used.
- **B**: A monostable multivibrator is a type of pulse generator that produces a single output pulse in response to an input trigger. It does not involve phase locking.
- **D**: A precision push-pull amplifier with a differential phase input does not describe the components or function of a PLL. It lacks the essential elements like the phase detector and VCO.

### Useful Formulas and Rules of Thumb:
- The output frequency of a PLL can be adjusted based on the control voltage, which is derived from the phase difference:
  
$$
f_{out} = f_{ref} + K \cdot V_{control}
$$

where:
- $f_{out}$ = output frequency
- $f_{ref}$ = reference frequency
- $K$ = sensitivity of the VCO (frequency deviation per volt)
- $V_{control}$ = control voltage from the low-pass filter

Understanding these components and their functions will help you grasp the concept of phase-locked loops and their applications in radio communications and electronics.