*Question E7E02:
What is the function of a reactance modulator?*  

*Correct Answer is  D: Produce PM or FM signals by varying a capacitance*  

### AI Response: (may contain errors)  

A reactance modulator is used to produce phase modulation (PM) or frequency modulation (FM) signals. It does this by varying the reactance in a circuit, which in turn changes the frequency of the carrier signal. Let's break down why the correct answer is correct and why the other choices are incorrect:

### Correct Answer: D
- **Produce PM or FM signals by varying a capacitance**:  
  - A reactance modulator typically uses a variable reactance, such as a capacitance, to change the frequency of an oscillator. By varying the capacitance, the reactance changes, which alters the frequency of the carrier signal, resulting in FM or PM. This is because the frequency of an LC oscillator is given by:
    $$ f = \frac{1}{2\pi\sqrt{LC}} $$
  - By changing the capacitance $C$, the frequency $f$ is modulated, achieving FM or PM.

### Why Other Choices Are Incorrect:
- **Choice A: Produce PM or FM signals by varying a resistance**:  
  - Varying a resistance does not affect the reactance of a circuit. Resistance affects amplitude, not frequency or phase, so it cannot be used to produce FM or PM.

- **Choice B: Produce AM signals by varying an inductance**:  
  - Amplitude Modulation (AM) involves varying the amplitude of the carrier signal, not its frequency or phase. Varying an inductance would affect the frequency, not the amplitude, so it is not used for AM.

- **Choice C: Produce AM signals by varying a resistance**:  
  - While varying a resistance can affect the amplitude of a signal, it is not related to the function of a reactance modulator, which is specifically for FM or PM, not AM.

In summary, a reactance modulator is specifically designed to vary the reactance (capacitance or inductance) to achieve frequency or phase modulation, making option D the correct choice.