brian@sdcsvax.UUCP (Brian Kantor) (07/18/85)
In article <204@redwood.UUCP> rpw3@redwood.UUCP (Rob Warnock) writes: >| > (Note to ham radio newcomers: the ``squelch tail'' is the burst of white > >Actually, with some fancy coupling between the squelch and AGC circuits, >it should be possible to eliminate most or all of the "squelch tail". >The squelch tail occurs because the AGC gain opens up faster than the >squelch closes. By using a longer time-constant ... It doesn't quite work that way.... In most FM receivers, there is little or no AGC (if any, its just used to prevent overloading of early stages). Instead, the signal is amplified to levels that clip out all amplitude variations in the signal. Since the modulation on the signal being received is in the form of frequency deviations, the amplitude is irrelevent. It is precisely because most natural noise the intrudes upon a transmitted signal is amplitude in nature that this clipping process (called 'limiting') eliminates the noise background on FM signals. Since the modulation can still be recovered even after the limiting has disposed of most of the amplitude variations, you have a very quiet communications channel with little natural interference. It even gets rid of a lot of electron noise generated in the receiver circuits. The receiver hears a lot of this noise from random electrons when there is no signal being received. The squelch circuit in most receivers works by measuring the amount of high-frequency audio content in the received signal (typically, for a 300-3000 Hz communications channel, the squelch will sample at above 16 KHz). When this noise energy drops (when the receiver is quieted by a received signal), the squelch opens. Some more sophisticated receivers and in the received signal strength to prevent the squelch from blowing if the receive loses gain for some reason (lower supply voltage, for example, will reduce the gain and thus the noise, but won't simulate a received signal). All such noise squelches have a slight delay before opening to eliminate popping cause by noise impulses that might momentarily quiet the receiver. Some delay in closing the squelch is a good idea, as when a signal fades quickly due to reinforcement and cancellation of multiple reflections from the moving mobile transmitter or receiver (called ``chopping'' or ``popcorn'' depending upon its severity), the squelch should not close as it cannot open quickly enough to avoid cutting off small parts of the received audio. But on a strong signal that is not choppy, there is no need for a closing delay. One of the first radios on the market with dual-rate noise squelch was the Motorola MICOR, which had a moderate delay when the signal's average quieting level was below some specific threshold, and was much faster when the signal was strong. The effect was that weak signals had a squelch tail but did not chop off voices, and strong ones went away without a noise burst. Hope this clears up some of the confusion. Brian Kantor WB6CYT UC San Diego decvax\ brian@ucsd.arpa akgua >--- sdcsvax --- brian ucbvax/ Kantor@Nosc