*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 a device used in radio transmission to produce phase modulation (PM) or frequency modulation (FM) signals. 

### Correct Answer Explanation:
- **Correct Answer: D: Produce PM or FM signals by varying a capacitance**  
  - A reactance modulator works by changing the reactance (which can be capacitive or inductive) in the circuit. When capacitance is varied, it alters the frequency of the oscillator, thus producing FM or PM signals. This is because the frequency of an LC circuit is given by the formula:
  
  $$
  f = \frac{1}{2\pi\sqrt{LC}}
  $$
  
  where:
  - $f$ = frequency
  - $L$ = inductance
  - $C$ = capacitance

### Incorrect Answer Explanations:
- **A: Produce PM or FM signals by varying a resistance**  
  - Varying resistance does not directly affect the frequency of the signal. Instead, it can affect amplitude but not the modulation type (PM or FM).

- **B: Produce AM signals by varying an inductance**  
  - Amplitude modulation (AM) is typically achieved by varying the amplitude of the carrier wave, not by changing inductance. While inductance can affect the circuit's resonant frequency, it is not the primary method for AM.

- **C: Produce AM signals by varying a resistance**  
  - Similar to option B, varying resistance does not create AM signals. AM is produced by varying the amplitude of the carrier wave, not by changing resistance.

### Summary:
- A reactance modulator is specifically designed to create PM or FM signals by varying capacitance, making option D the correct choice. The other options incorrectly associate modulation types with resistance or inductance changes, which do not produce the desired modulation effects.