Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site petrus.UUCP Path: utzoo!watmath!clyde!burl!ulysses!gamma!epsilon!zeta!sabre!bellcore!petrus!karn From: karn@petrus.UUCP (Phil R. Karn) Newsgroups: net.ham-radio Subject: Re: "PL" and tweedles Message-ID: <401@petrus.UUCP> Date: Mon, 15-Jul-85 14:16:14 EDT Article-I.D.: petrus.401 Posted: Mon Jul 15 14:16:14 1985 Date-Received: Thu, 18-Jul-85 03:33:30 EDT References: <86@biomed.UUCP> <957@sdcsvax.UUCP> <11397@brl-tgr.ARPA> <204@redwood.UUCP> Organization: Bell Communications Research, Inc Lines: 21 > 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. You're forgetting that FM is a nonlinear modulation method. An ideal discriminator reads signal PHASE and is completely insensitive to RF level. Moreover, there is limiting ahead of the discriminator, so the "gain" in an FM receiver up to this point depends instantaneously on the signal level. When the input signal to an "ideal" FM receiver goes away, the pre-discriminator gain instantaneously goes to infinity. (In a real receiver, it goes just high enough for front-end thermal noise to saturate the limiters). Thus, you have squelch noise that starts immediately upon loss of signal. The AGC is there just as an engineering convenience to keep the front-end intermod products down when receiving a very strong signal. It plays no part in keeping the audio output level constant, as it does in linear modulation reception such as SSB. Phil