In FM modulation, what does Δf represent?

• A. Average carrier frequency.
• B. Maximum carrier frequency deviation.
• C. Modulating signal amplitude.
• D. Signal bandwidth.

Answer: (B)

In FM, Δf is the maximum amount the carrier frequency can swing away from its unmodulated value f_c. It represents the peak frequency deviation caused by the modulating signal, determined by the modulation sensitivity and the peak amplitude of the modulation. For a single-tone modulator, Δf equals the modulation sensitivity times the peak modulating voltage, illustrating how larger input swings push the carrier further from f_c. It’s not the average carrier frequency, which stays centered at f_c, and it’s not the modulating signal amplitude itself (though the amplitude controls how large the deviation is). The bandwidth, described by rules like Carson’s (approximately 2[Δf + f_m]), depends on Δf but is a separate concept from the deviation itself.