Which statement correctly describes the bandwidth difference between AM and FM transmissions?

Which statement correctly describes the bandwidth difference between AM and FM transmissions?

• A. AM has a wider bandwidth than FM
• B. FM has a wider bandwidth than AM
• C. They have the same bandwidth
• D. AM and FM bandwidth depends on the signal power

Answer: (B)

Bandwidth depends on how the information is encoded in the carrier. In amplitude modulation, the information is carried by changes in amplitude while the carrier stays fixed in frequency. The spectrum then consists of the carrier plus two sidebands, each mirroring the modulating signal’s spectrum. The total bandwidth is simply twice the highest modulating frequency, B_AM = 2 f_m. In frequency modulation, the information is carried by changes in the carrier’s frequency, not its amplitude. Those frequency variations push energy into many surrounding frequencies, creating a broad set of sidebands. A practical way to estimate FM bandwidth is Carson’s rule: B_FM ≈ 2(Δf + f_m), where Δf is the maximum deviation of the carrier frequency. Since Δf is typically large for audio signals, and you add the modulating bandwidth f_m, FM ends up spreading its energy over a much wider slice of spectrum than AM. Putting it together with a simple example helps: if the modulating signal has 5 kHz bandwidth, AM needs about 10 kHz. If FM has a peak deviation of, say, 75 kHz, Carson’s rule gives about 160 kHz of bandwidth. That illustrates why FM is normally much wider than AM. So, the statement that FM has a wider bandwidth than AM is the best description.

Bandwidth depends on how the information is encoded in the carrier. In amplitude modulation, the information is carried by changes in amplitude while the carrier stays fixed in frequency. The spectrum then consists of the carrier plus two sidebands, each mirroring the modulating signal’s spectrum. The total bandwidth is simply twice the highest modulating frequency, B_AM = 2 f_m.

In frequency modulation, the information is carried by changes in the carrier’s frequency, not its amplitude. Those frequency variations push energy into many surrounding frequencies, creating a broad set of sidebands. A practical way to estimate FM bandwidth is Carson’s rule: B_FM ≈ 2(Δf + f_m), where Δf is the maximum deviation of the carrier frequency. Since Δf is typically large for audio signals, and you add the modulating bandwidth f_m, FM ends up spreading its energy over a much wider slice of spectrum than AM.

Putting it together with a simple example helps: if the modulating signal has 5 kHz bandwidth, AM needs about 10 kHz. If FM has a peak deviation of, say, 75 kHz, Carson’s rule gives about 160 kHz of bandwidth. That illustrates why FM is normally much wider than AM.

So, the statement that FM has a wider bandwidth than AM is the best description.