Which statement is true about FM in terms of spectrum usage?

Which statement is true about FM in terms of spectrum usage?

• A. FM uses more RF Spectrum due to wider bandwidth
• B. FM uses less RF Spectrum due to narrower bandwidth
• C. FM uses the same RF Spectrum as AM
• D. FM does not use RF Spectrum

Answer: (A)

FM broadens the signal’s bandwidth because the carrier frequency actually varies with the modulating signal. The required bandwidth isn’t just the two sidebands around a fixed carrier; it grows with how far the carrier can deviate (the peak frequency deviation, Δf) and with the highest modulating frequency (f_m). Carson’s rule captures this: B ≈ 2(Δf + f_m). Since Δf is typically large for audio signals, the resulting FM bandwidth is much wider than AM’s, which is about 2f_m for standard double-sideband AM with a carrier. So FM uses more RF spectrum due to its wider bandwidth. The other statements—that FM uses less spectrum, uses the same spectrum as AM, or doesn’t use RF spectrum—don’t align with how FM actually occupies a broader band.

FM broadens the signal’s bandwidth because the carrier frequency actually varies with the modulating signal. The required bandwidth isn’t just the two sidebands around a fixed carrier; it grows with how far the carrier can deviate (the peak frequency deviation, Δf) and with the highest modulating frequency (f_m). Carson’s rule captures this: B ≈ 2(Δf + f_m). Since Δf is typically large for audio signals, the resulting FM bandwidth is much wider than AM’s, which is about 2f_m for standard double-sideband AM with a carrier. So FM uses more RF spectrum due to its wider bandwidth. The other statements—that FM uses less spectrum, uses the same spectrum as AM, or doesn’t use RF spectrum—don’t align with how FM actually occupies a broader band.