T4B04

AMATEUR RADIO PRACTICES · Operating controls: frequency tuning, use of filters, squelch function, AGC, memory channels, noise blanker, microphone gain, receiver incremental tuning (RIT), bandwidth selection, scanning function; Digital transceiver configuration; DMR code plugs and talk groups

T4B04T4B →

What does an FM signal sound like when received slightly off frequency?

AThe audio increases in pitch
BThe audio decrease in pitch
CThere is no effect except for reduction in amplitude
DThe audio becomes distorted

Why An FM receiver recovers audio by measuring instantaneous frequency deviation against the center of its discriminator (detector) response curve. That curve is only linear over a limited range centered on the carrier, so if the incoming signal sits off to one side, the deviation swings run into the curved, nonlinear part of the response and the recovered audio comes out distorted, often sounding harsh or clipped on loud peaks.

Watch out Rising or falling pitch is what you hear when you mistune an SSB or CW signal, where the tuning offset shifts the beat frequency directly; FM detection does not work that way, and a simple amplitude drop is what you would expect from a weak signal rather than a mistuned one.

Mistune SSB and the pitch changes; mistune FM and the audio just gets ugly.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T4B04

The rest of T4B