On 25 June 2012 23:44, Sean Breheny wrote: > Hi Dave, > > If you take a look at the selts.org link you posted, it mentions the > method involving AGC as being used sometimes for FM receivers. > However, I did not realize that the noise squelch method seems to be > more common. I've been an amateur radio operator for 21 years and > didn't know that! Thanks for learnin' me something! > > Sean > > > On Mon, Jun 25, 2012 at 12:50 AM, David VanHorn wro= te: >> You don't have AGC in an FM radio, in fact you have the opposite, a >> limiter that removes as much of the AM component as possible. >> >> Here's a link that explains squelch well: >> http://en.wikipedia.org/wiki/Squelch >> >> Two approaches: >> http://www.taha.islam-piers.org/circuits/2M-Circuits/squelch-descrip.htm >> http://www.seits.org/repeater/bilevel.htm >> >> FWIW the AC audio output of an FM receiver is at maximum when there is >> no signal. >> >> I have had occasion to demonstrate that typical FM signals carry a >> fair AM component a time or two. >> >> There was one time when a transmitter PLL became unlocked, and the >> transmitter was emitting nicely in the 2M FM band AND on the local ATC >> tower frequency. =A0It was clear audio into their AM recievers. =A0 The >> thing is that the stages past the modulator, including the antenna >> system, usually add some AM component to the FM transmission since >> they have a finite bandwidth. >> >> A suitably narrow tuned circuit can be the foundation of a crystal >> radio that will reproduce FM signals. >> -- At one stage quite a few years ago we were looking at putting an FM derived noise squelch circuit into an AM VHF radio because the noise squelch system worked more reliably than the AGC based one. I think the tap-off point was ahead of the AGC controlled IF amplifier stage so it was only subject to AGC on the RF side. RP --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .