What degree of selectivity is ideal in the IF circuitry of a wideband FM phone receiver?

In the context of wideband FM (Frequency Modulation) phone receivers, the ideal degree of selectivity in the intermediate frequency (IF) circuitry is determined by balancing the need to discriminate between closely spaced signals and the requirement to maintain fidelity of the received audio.

Wideband FM typically has a deviation of 5 kHz to 15 kHz for the audio frequency range, which corresponds to the bandwidth of the signal being processed. For effective reception, the selectivity of the receiver needs to be sufficiently narrow to isolate the desired signal from adjacent channels without introducing significant distortion or loss of quality.

A selectivity of 15 kHz strikes a balance here. It is adequate to reject potential interference from nearby channels while still allowing the fidelity of the signal to be maintained, as it aligns more closely with the modulation characteristics of the wideband FM signal. This ensures that the received audio is clear and free from excessive noise or distortion that could result from either too broad or too narrow a selectivity.

In contrast, selectivity settings that are too wide (such as options suggesting 30 kHz or 20 kHz) may allow interference from adjacent channels, while a setting that is too narrow (like 5 kHz) may compromise the reception quality of