Unless your test banch is inside a Fariday Cage you can'y rule out extraneous signals. If there is a strong transmitter near by it's still questionable. But you're right about off frequency or off "channel" signals causing quieting in the receiver. This is another reason for pre filtering the input to a preamp. preamps are generally broad banded devices. It's better to control what you are amplifying by 17db (read, just what's on your input frequency) than to amplify everything in the band by 17db. This sort of thing can amplify a strong co-channel signal to the point of desensing or overloading the receiver (hint, on 2 meters (VHF)your transmitter is 600Kc away) even in a "quiet" area. 73 MERRY CHRISTMAS AC0Y
--- In [email protected], Kevin Custer <[EMAIL PROTECTED]> wrote: > > > >>Think about this for a minute, if the noise level increased, the squelch > >>circuit would have seen MORE noise and rammed the squelch closed. > >>What actually happened is, adding a better device in front of the > >>receiver lowered the receivers total noise figure, decreasing the noise > >>in the squelch circuit, requiring the squelch pot to be set tighter. > >>Very common effect. > >> > >> > > > >Common? Must be another explanation, as you're talking apples & oranges > >here w.r.t. noise. Remember this is FM, so more noise power at the front > >end doesn't mean more noise at the discriminator unless the limiter isn't > >being driven into limiting, which is probably what's happening. But adding > >a preamp can only add total noise power, never subtract. > > > >Bob NO6B > > > > If what you are saying is correct, adding the preamplifier should have > placed a receiver lacking in overall gain into full (or at least more) > limiting. If so, this would have raised the noise level to the > discriminator, thus tightening the squelch. He commented that he needed > to set the squelch pot tighter, which means there was less noise present > after the installation of the preamp. Maybe I didn't explain it well, > but I have seen this effect before, even on the bench where extraneous > signals quieting the receiver can be ruled out. > > Kevin > Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
