Which statement about FM bandwidth relative to AM is true?

Which statement about FM bandwidth relative to AM is true?

• A. It uses the same bandwidth as AM
• B. It requires a wider bandwidth than AM
• C. It is more sensitive to amplitude noise than AM
• D. It has poorer frequency stability than AM

Answer: (B)

FM transmits information by varying the carrier frequency, not its amplitude. This causes the spectrum to spread because the instantaneous frequency shifts create many frequency components around the carrier. The amount of spread is set by how far the frequency can swing (the deviation) and how fast the modulating signal changes (its highest frequency). A practical rule of thumb, Carson’s rule, says the FM bandwidth is about 2 times the sum of the maximum deviation and the highest modulating frequency. In AM, the information is carried in amplitude and the bandwidth is basically twice the highest modulating frequency, with energy concentrated in the carrier and two sidebands. Since FM’s bandwidth grows with deviation and modulation content, it is typically wider than AM. That’s why the statement about FM needing a wider bandwidth is true. The other ideas don’t fit the usual behavior: FM isn’t more sensitive to amplitude noise and, with proper design, can offer solid frequency stability, while AM’s bandwidth remains tied to the modulating content.

FM transmits information by varying the carrier frequency, not its amplitude. This causes the spectrum to spread because the instantaneous frequency shifts create many frequency components around the carrier. The amount of spread is set by how far the frequency can swing (the deviation) and how fast the modulating signal changes (its highest frequency). A practical rule of thumb, Carson’s rule, says the FM bandwidth is about 2 times the sum of the maximum deviation and the highest modulating frequency. In AM, the information is carried in amplitude and the bandwidth is basically twice the highest modulating frequency, with energy concentrated in the carrier and two sidebands. Since FM’s bandwidth grows with deviation and modulation content, it is typically wider than AM. That’s why the statement about FM needing a wider bandwidth is true. The other ideas don’t fit the usual behavior: FM isn’t more sensitive to amplitude noise and, with proper design, can offer solid frequency stability, while AM’s bandwidth remains tied to the modulating content.