Which of the following correctly contrasts AM and FM demodulation methods?

• A. FM demodulation uses envelope detector
• B. AM demodulation uses PLL to recover modulating signal
• C. AM uses envelope detection to recover the modulating signal; FM uses a discriminator or PLL to recover instantaneous frequency
• D. AM and FM both use digital demodulation

Answer: (C)

The main idea is that AM and FM encode information differently, so their demodulation methods reflect what is varying in the signal. In AM, the information is in the amplitude of the carrier, so an envelope detector can extract the modulating signal by following the slowly changing envelope of the RF waveform through rectification and filtering. In FM, the carrier’s amplitude stays nearly constant and the information is in the instantaneous frequency, so a detector that converts frequency deviations into a voltage is used—either a discriminator (a specialized frequency-to-voltage converter) or a phase-locked loop that tracks frequency changes and provides a demodulated output proportional to those changes. That’s why the correct description says AM uses envelope detection and FM uses a discriminator or PLL. The other statements mix up which modulation type uses which method or imply that both are, by default, digitally demodulated, which isn’t the fundamental distinction.