Q.What is the purpose of a discriminator in an FM broadcast receiver?

Demodulating an FM signal is fundamentally different from demodulating AM: the information is carried in the carrier's frequency, not its amplitude, so the detector must convert changes in frequency into a corresponding change in voltage. Because it responds to — or discriminates between — the frequency variations of the carrier, an FM detector is called a discriminator. Ideally its output voltage is proportional to the instantaneous frequency of the incoming FM signal, so as the carrier swings above and below its centre frequency the output voltage rises and falls in step, reproducing the original message.

Several FM demodulators are in common use:

• Slope detector — the simplest, using the sloping side of a tuned circuit's response to turn frequency changes into amplitude changes, which are then detected.
• Foster-Seeley discriminator — a balanced circuit giving a linear frequency-to-voltage characteristic.
• Ratio detector — similar to the Foster-Seeley but with built-in immunity to amplitude variations, so it needs less limiting.
• Phase-locked-loop (PLL) detector — locks a local oscillator to the incoming frequency; the control voltage that keeps it locked is the recovered message. …