In FM demodulation, what are the roles of a slope detector and a PLL?

• A. A slope detector locks to the local oscillator; a PLL uses slope detection.
• B. A slope detector converts frequency deviations into amplitude variations, while a PLL locks to the instantaneous frequency with a VCO and a phase detector.
• C. A slope detector is used for AM; a PLL is used for FM demodulation but not with capture concerns.
• D. A slope detector uses time-domain amplitude; a PLL uses time-domain phase.

Answer: (D)

In FM demodulation you’re trying to recover the instantaneous frequency changes of the carrier. A slope detector does this by exploiting the slope of a tuned circuit’s frequency response: as the carrier’s frequency drifts, the tuned circuit’s output amplitude swings in time, so the instantaneous frequency deviation appears as a time‑varying amplitude signal. In other words, the frequency deviation is converted into an amplitude variation that can be heard as the audio output.

A phase‑locked loop takes a different route. It compares the incoming carrier phase with the phase of a voltage‑controlled oscillator using a phase detector. The loop drives the VCO to follow the input so that the phase error is minimized. The control voltage required to keep the loop in lock is effectively proportional to the instantaneous frequency difference between input and VCO, which mirrors the FM deviation. After suitable filtering, this yields the demodulated audio. So the slope detector encodes Δf as time‑domain amplitude changes, while the PLL uses time‑domain phase information (via phase error and the resulting control signal) to recover Δf.