Which circuit is used to detect FM-phone signals?

The frequency discriminator is the correct choice for detecting FM-phone signals due to its fundamental operation in frequency modulation (FM) systems. In FM, the frequency of the carrier wave is varied according to the amplitude of the input signal. A frequency discriminator is specifically designed to convert these frequency variations back into voltage variations that represent the original audio signal, effectively demodulating the FM signal.

This process makes frequency discriminators essential in FM communication systems. They work by taking an incoming FM signal and producing an output that varies in direct proportion to the frequency deviations of the original modulation. As a result, they faithfully recreate the original audio or data information.

While other components like phase-locked loops can also demodulate FM signals, they are typically used for more complex applications like synchronizing signals, rather than straightforward FM-phone signal detection. Signal amplifiers are important for boosting the strength of weak signals but do not specifically demodulate frequency variations. Bandpass filters are used to isolate specific frequency ranges but do not process the modulation itself to recover the original signal.