*Question E7E01:
Which of the following can be used to generate FM phone signals?*  

*Correct Answer is  B: Reactance modulation of a local oscillator*  

### AI Response: (may contain errors)  

To generate FM (Frequency Modulation) phone signals, the correct method is:

**Correct Answer: Reactance modulation of a local oscillator**

### Explanation:

1. **Why the correct answer is correct:**
   - **Reactance Modulation of a Local Oscillator**: This method involves varying the reactance (inductive or capacitive) in the oscillator circuit, which in turn changes the frequency of the oscillator. This is a direct method of generating FM signals because it directly modulates the frequency of the carrier wave by the audio signal.

2. **Why the other choices are incorrect:**
   - **A: Balanced modulation of the audio amplifier**: Balanced modulation is typically used for generating AM (Amplitude Modulation) signals, not FM. It involves mixing the audio signal with a carrier in such a way that the carrier is suppressed, which is not suitable for FM generation.
   - **C: Reactance modulation of the final amplifier**: Modulating the final amplifier is not practical for FM generation because it would require the entire transmitter to be frequency agile, which is complex and inefficient.
   - **D: Balanced modulation of a local oscillator**: Similar to choice A, balanced modulation is used for AM, not FM. It does not involve frequency modulation of the carrier.

### Useful Concepts:

- **Frequency Modulation (FM)**: In FM, the frequency of the carrier wave is varied in accordance with the amplitude of the input signal.
- **Reactance Modulation**: This technique uses a variable reactance (like a varactor diode) to change the frequency of an oscillator, making it suitable for FM signal generation.

By understanding these concepts, you can see why reactance modulation of a local oscillator is the appropriate method for generating FM signals.