*Question E7H06:
Which of these functions can be performed by a phase-locked loop?*  

*Correct Answer is  B: Frequency synthesis and FM demodulation*  

### AI Response: (may contain errors)  

A phase-locked loop (PLL) is a versatile electronic circuit used in various applications, particularly in communication systems. Let's break down the correct answer and the other choices.

### Correct Answer: B: Frequency synthesis and FM demodulation

1. **Frequency Synthesis**: 
   - A PLL can generate a range of frequencies from a single reference frequency. It locks onto the frequency of an input signal and can produce a stable output frequency that is a multiple or fraction of the input frequency.
  
2. **FM Demodulation**: 
   - A PLL can also be used to demodulate frequency-modulated (FM) signals. It does this by tracking the variations in frequency of the incoming FM signal, allowing it to extract the original audio or data signal.

### Why the Other Choices are Incorrect:

- **A: Wide-band AF and RF power amplification**: 
  - While amplifiers can be used in conjunction with PLLs, a PLL itself does not perform power amplification. Its primary functions are frequency control and signal synchronization, not amplification.

- **C: Photovoltaic conversion and optical coupling**: 
  - These processes are related to converting light into electrical energy and transferring light between optical systems, respectively. They do not involve the frequency control or signal processing capabilities of a PLL.

- **D: Comparison of two digital input signals and digital pulse counting**: 
  - While a PLL can be used in digital systems, its main functions do not include direct comparison of digital signals or counting pulses. These tasks are typically handled by other digital circuits, such as comparators or counters.

### Summary

A phase-locked loop is primarily used for:

- **Frequency synthesis**: Generating stable frequencies.
- **FM demodulation**: Extracting information from frequency-modulated signals.

Understanding these functions will help you recognize the applications of PLLs in communication systems and why the other options do not fit.