Question

A narrow band FM signal is generated using a phase modulator. The maximum deviation at the output of a phase modulator is about________.

• A. \( \pm 250 \text{ Hz} \)
• B. \( \pm 1 \text{ kHz} \)
• C. \( \pm 1 \text{ MHz} \)
• D. \( \pm 25 \text{ Hz} \)

Hint:

Narrowband FM from phase modulation gives small deviation (\( \pm 25 \) Hz typical). Wideband FM gives larger deviation.

Answer 1

The Correct Option is D

In narrowband FM (NBFM), which is typically derived from phase modulation (PM), the modulation index is very small (\( \beta<1 \)). The maximum frequency deviation \( \Delta f \) is small compared to the message bandwidth.
For narrowband FM created using a phase modulator:
- The phase deviation \( \Delta\phi \) is constant, and when modulated by a low frequency message signal (usually in audio range), the frequency deviation becomes very small.
- For standard narrowband FM signals, the deviation typically remains below \( \pm 25 \) Hz.
This is in contrast to wideband FM (like FM radio), where the deviation can be \( \pm 75 \) kHz or more.