Most of the time you will want as much selectivity as you can get in front of the preamp. The only time that I can think of off hand where you might want a filter behind a preamp is if you are getting a receiver feed from a receiver multicoupler that has a preamp in it, giving a few megs wide signal out of the multicoupler.
Overload of a preamp depends on the total amount of power that gets into it. The wider the window in front of it the more total power that has potential for getting in from many other transmitters. This can cause IM products to be generated in the preamp itself. Once that happens you have opened the barn door and there is not much you can do after the preamp to help the receiver. Sometimes if is good to use a preamp that doesn't necessarily have the best noise figure but maybe has a higher dynamic range (higher intercept point) if your site has lots of nearby transmitters and noise that could overload the preamp. Having a very low noise figure doesn't do you any good if the preamp causes IM to be generated. The second thing is not to run too much gain in the preamp so that the added gain overloads the front end of the receiver. For every Db of gain the preamp provides that reduces the receivers IM rejection ability by the same number of Db. So again if you have lots of strong adjacent signals at your site you don't want lots of preamp gain. Total receiver system noise figure is partially set by the preamp if its noise figure is lower than that of the receiver, which it usually is. Using a preamp with a .5 Db noise figure and a receiver with an 8 Db noise figure won't give you a total noise figure of .5 Db, but somewhere in-between. The more gain the preamp has the lower the overall noise figure will be in this case, unless you have enough gain to cause some of the other low noise figure stages in the receiver to go into compression as Mel eludes to. The stage that goes into compression in the receiver doesn't necessarily have to be the front end of the receiver. The first IF stages in most receivers have a pretty low noise figure and help establish the overall noise figure of the receiver as well as the front end of the receiver. So these stages can be overloaded with too much gain and cause a noise figure reduction. But the biggest problem with too much preamp gain is overloading the mixer in the receiver and causing it to generate IM products. By controlling the gain of the preamp (using attenuators after the preamp) or by other means you can usually find a happy medium of some gain to improve system noise figure (sensitivity) and not too much gain to destroy the IM performance of the receiver. One way to do that is to put in attenuation until the sensitivity just starts to degrade with the preamp in the circuit. That will give you good sensitivity and good IM performance. Any more gain and all you are doing is degrading the receiver IM performance. When shopping for preamps don't only look at gain figures and noise figures, also look at the intercept point to see how much signal it will handle before compression starts. That's where it will start generating IM products. 73 Gary K4FMX > -----Original Message----- > From: Repeater-Builder@yahoogroups.com [mailto:Repeater- > buil...@yahoogroups.com] On Behalf Of Mel Swanberg > Sent: Thursday, November 26, 2009 12:40 PM > To: Repeater-Builder@yahoogroups.com > Subject: Re: [Repeater-Builder] Re: pre-amp placement > > > > > > You might need to add a several DB attenuator > > > > between > > > > the pre amp and the receiver to keep from over > > > > driving > > > > the front end. > > > > Not if you use a good receiver, or not use a preamp with > > too much gain. > > > > Bob NO6B > > What defines "too much gain" can vary wildly. One trick I learned in > building transverters for the microwave bands, and one I now apply to > VHF/UHF preamps is to check the overall noise figure of the system as a > whole. You'd be surprised at what just a few db too much gain can do, and > it doesn't necessarily show up with a quick sensitivity check. > > A preamp can be placed in front of a receiver and, yeah, now the receiver > is more sensitive. But if it's a .5 DB NF preamp, and you're not careful, > your system noise figure can end up going from, say, 6 db for the barefoot > receiver, to 4 db with the preamp - an improvement to be sure, but not > nearly as good as the preamp may be capable of. If that preamp is driving > the receiver front end even just a little bit into compression, you've > lost a lot of potential. Even with a good receiver. Carefully balancing > preamp gain with attenuation on the output can be extremely useful. > > Not everyone has a noise figure meter, though, and measuring NF on an FM > receiver is a pain in the neck. A sinadder can be used to the same effect, > even if the actual noise figure isn't known. It can be interesting to > observe insertion of a few db of pad between the preamp and a receiver, > and watch the sinad sensitivity of a receiver improve by a few tenths of a > microvolt. > > > > > > > > ------------------------------------ > > > > Yahoo! Groups Links > > >
