An FM detector's function is to convert what back into voltage variations?

An FM detector's primary function is to demodulate frequency-modulated signals, which means that it translates frequency variations into corresponding voltage changes. In frequency modulation (FM), the information being transmitted is encoded in the frequency of the carrier wave. As the frequency of the wave varies in accordance with the input signal, these frequency changes must be accurately converted back into a utilizable form, typically voltage variations, to retrieve the original information.

When the FM detector performs its operation, it captures the differences in frequency that correspond to the changes in the original signal. This process is critical in ensuring that the information carried by the FM signal is effectively extracted. Therefore, option A is correct, as it directly relates to the core function of an FM detector in converting frequency variations back into voltage variations. Other choices do not pertain to the operation of an FM detector, as they involve different types of modulation or signal processing techniques.