What is the typical effect of amplitude limiting on demodulation of FM?

• A. It ensures a constant amplitude so that the frequency deviation originates from phase changes, improving demodulation
• B. It increases amplitude variations to enhance AM detection
• C. It reduces receiver sensitivity
• D. It changes the modulation index of FM

Answer: (A)

The key idea is that amplitude limiting makes the FM signal effectively constant in amplitude so the demodulator responds only to frequency (or phase) changes. In FM, the information is carried by how quickly the carrier phase changes over time; the instantaneous frequency is tied to the rate of that phase change. By clipping the signal to a fixed amplitude, the limiter removes amplitude variations that could masquerade as or distort the frequency information. This leaves the demodulator to convert true frequency deviations into the output voltage more cleanly, improving demodulation accuracy.

Increasing amplitude variations would defeat the purpose and harm the detector’s performance, which is why that option isn’t correct. Limiting does not reduce receiver sensitivity in a meaningful way for this context, and it does not alter the FM modulation index, since the index depends on frequency deviation and the modulating signal, not the signal’s amplitude after limiting.