FM demodulation is commonly performed using which method?

• A. Envelope detector
• B. Discriminator or PLL to recover instantaneous frequency
• C. Mixer
• D. Low-pass filter

Answer: (B)

FM carries information in changes to the carrier’s instantaneous frequency, so you need a circuit that converts those frequency variations into a voltage. A discriminator does this directly: it outputs a voltage proportional to the frequency deviation, using detectors like Foster–Seeley or ratio detectors to recover the audio. A phase-locked loop can also serve as an FM demodulator because the loop’s instantaneous control voltage tracks the input frequency; that error/ control voltage is proportional to the modulation and becomes the baseband signal after proper scaling. An envelope detector responds to amplitude, not frequency, so it’s not suitable for FM. A mixer shifts frequencies and a simple low-pass filter alone doesn’t extract the modulation; they don’t convert instantaneous frequency changes into the baseband signal by themselves. Hence, using a discriminator or a PLL to recover instantaneous frequency is the standard approach for FM demodulation.