Hook your signal generator up to your system at the antenna port and measure receiver sensitivity with and without the preamp. Then with the preamp in circuit start adding attenuation between the preamp and the receiver. When you just start to loose sensitivity stop adding attenuation. That should give you near optimum sensitivity without excessive gain. Too much gain in the preamp overloads the receiver mixer and front end amp if it has one.
For every db of gain you add in front of the receiver you reduce the IM performance of the receiver. You only want enough preamp gain to overcome the noise figure of the receiver. Although the noise figure of the receiver and preamp are cumulative the preamp is the biggest contributor in setting system noise figure. In other words putting a hot preamp on a very hot receiver will give you a better overall noise figure than putting that same preamp on a poor receiver but the difference will not be great. You may not be able to realize the full benefit of the preamp if you have excessive IM. You may have to add more attenuation to where it further reduces receiver sensitivity. When you get down to the point that the sensitivity is the same as it was without the preamp, then throw out the preamp. But you may be able to find a happy medium where the preamp does help some without destroying your IM performance. If you still have excess IM problems you can add attenuation ahead of the preamp by raising the insertion loss of the loops on your band pass filter as others have suggested. By raising the insertion loss on the loops it does the same thing as adding an attenuator ahead of the preamp but with the added benefit of steeper skirts on the band pass filter. By the way don't worry about adding adaptors between the preamp and receiver. After all you are looking to add attenuators anyway. But adaptors really make no measurable difference in attenuation at vhf or uhf. They may give a slight impedance mismatch but you probably don't have anything that will measure the small amount of loss from them. 73 Gary K4FMX > -----Original Message----- > From: Repeater-Builder@yahoogroups.com [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of ldgelectronics > Sent: Wednesday, March 21, 2007 9:32 PM > To: Repeater-Builder@yahoogroups.com > Subject: [Repeater-Builder] Re: Preamp and attenuator > > Ken, thanks for the input. > > So if I were to adjust the band pass can to have 3 db loss (it's at > 0.5 db right now) and steeper skirts, it would raise the noise figure > about 2.5 db as well. > > That seemed like a good trick at first, but still raises the noise > figure. > > Obviously lower noise figure is better, but is there some place where > the trade off would be worth it? Maybe I'm not asking the question > properly. > > > > > >The question is how much trouble will be getting into by putting > the > > >6db attenuator on the input side of the preamp? Would it still be > > >better to put it after the preamp even though it would add two > > >adapters? > > > > <---You'd be much better placing the attentuator in the OUTPUT of > the > > preamp (between it and the receiver). Remember that every dB of > loss > > you place ahead of the preamp adds 1 dB of noise figure to the > > receiver. So you'd be adding 6 dB to your overall receiving system > if > > you place the attentuator between the preamp and the duplexer > > > > Ken > > -------------------------------------------------------------------- > ---------- > > President and CTO - Arcom Communications > > Makers of the world famous RC210 Repeater Controller and > accessories. > > http://www.arcomcontrollers.com/ > > Coming soon - the most advanced repeater controller EVER. > > Authorized Dealers for Kenwood and Telewave and > > we offer complete repeater packages! > > AH6LE/R - IRLP Node 3000 > > http://www.irlp.net > > > > > > > > > > Yahoo! Groups Links > > >
