Which of the following is NOT a method for generating FM?

Question

Which of the following is NOT a method for generating FM?

1. A. By reactance modulator
2. B. By varactor diode
3. C. By switching modulator (Correct answer)
4. D. By Armstrong modulator

Correct Answer

Option C — By switching modulator

Detailed Solution & Explanation

The correct answer is By switching modulator.

Key Points

• A switching modulator is a method of generating amplitude modulation, not frequency modulation, so it is the odd one out.
• The genuine FM generation methods among the options are:
  • Reactance modulator — a direct FM method that varies the oscillator's effective reactance
  • Varactor diode — a direct method using a voltage-dependent capacitance to pull the oscillator frequency
  • Armstrong modulator — the classic indirect FM method, generating narrowband FM via phase modulation and then multiplying it up

Additional Information

• Direct FM varies the oscillator frequency itself, giving large deviation but poorer frequency stability.
• Indirect (Armstrong) FM starts from a highly stable crystal oscillator, so its stability is excellent, at the cost of needing frequency multipliers to reach the required deviation.