Which statement best describes how FM radio differs from AM radio?

Which statement best describes how FM radio differs from AM radio?

• A. The frequency of the carrier signal is varied.
• B. The amplitude of the carrier signal is varied.
• C. The channel bandwidth is reduced for FM.
• D. The modulation index is fixed for FM.

Answer: (A)

The key idea is that FM encodes information by changing the carrier’s frequency, while keeping its amplitude essentially constant. That makes the statement about varying the frequency of the carrier signal the best description of how FM differs from AM. In contrast, amplitude changes describe AM, not FM, so saying the amplitude of the carrier is varied doesn’t describe FM. As for bandwidth, FM actually tends to require a wider channel bandwidth due to frequency deviations, not a reduced one. And the modulation index in FM isn’t fixed; it depends on how much the frequency deviates for a given modulating signal. So the defining distinction is that FM modulates the carrier frequency rather than its amplitude.

The key idea is that FM encodes information by changing the carrier’s frequency, while keeping its amplitude essentially constant. That makes the statement about varying the frequency of the carrier signal the best description of how FM differs from AM. In contrast, amplitude changes describe AM, not FM, so saying the amplitude of the carrier is varied doesn’t describe FM. As for bandwidth, FM actually tends to require a wider channel bandwidth due to frequency deviations, not a reduced one. And the modulation index in FM isn’t fixed; it depends on how much the frequency deviates for a given modulating signal. So the defining distinction is that FM modulates the carrier frequency rather than its amplitude.