Compared to AM, FM encodes information in which way, and how does noise performance compare?

Compared to AM, FM encodes information in which way, and how does noise performance compare?

• A. AM encodes in amplitude; FM encodes in frequency; AM more sensitive to amplitude noise; FM more robust to amplitude noise but requires wider bandwidth
• B. FM encodes in amplitude; AM encodes in frequency; AM more robust to amplitude noise
• C. AM and FM both encode in amplitude; FM more robust to amplitude noise; no bandwidth difference
• D. AM encodes in phase; FM encodes in amplitude; AM more robust to noise

Answer: (A)

The main idea tested is how the modulation method encodes information and how that affects sensitivity to noise. In amplitude modulation, the information is carried by changing the envelope of the carrier—the instantaneous amplitude follows the modulating signal. In frequency modulation, the information is carried by changing the carrier’s instantaneous frequency while keeping the envelope roughly constant. Because AM uses the signal’s amplitude to convey information, any noise that perturbs the envelope directly distorts the received data. That makes AM more susceptible to amplitude noise, which translates into bigger errors in the detected envelope. FM, on the other hand, encodes information in frequency changes; amplitude fluctuations have less impact on the demodulated output, especially after a limiter stage in the receiver that suppresses amplitude variations before the frequency detector. This is why FM is more robust to amplitude noise. The trade-off is bandwidth. To achieve the same modulation in practice, FM typically requires a wider bandwidth than AM (frequency deviation spreads the spectrum more). So the description that AM uses amplitude, FM uses frequency, AM is more sensitive to amplitude noise, and FM is more robust to amplitude noise but needs wider bandwidth aligns with how these modulation schemes operate.

The main idea tested is how the modulation method encodes information and how that affects sensitivity to noise. In amplitude modulation, the information is carried by changing the envelope of the carrier—the instantaneous amplitude follows the modulating signal. In frequency modulation, the information is carried by changing the carrier’s instantaneous frequency while keeping the envelope roughly constant.

Because AM uses the signal’s amplitude to convey information, any noise that perturbs the envelope directly distorts the received data. That makes AM more susceptible to amplitude noise, which translates into bigger errors in the detected envelope. FM, on the other hand, encodes information in frequency changes; amplitude fluctuations have less impact on the demodulated output, especially after a limiter stage in the receiver that suppresses amplitude variations before the frequency detector. This is why FM is more robust to amplitude noise.

The trade-off is bandwidth. To achieve the same modulation in practice, FM typically requires a wider bandwidth than AM (frequency deviation spreads the spectrum more). So the description that AM uses amplitude, FM uses frequency, AM is more sensitive to amplitude noise, and FM is more robust to amplitude noise but needs wider bandwidth aligns with how these modulation schemes operate.