Just a further note on this: When measuring sensitivity as below don't do it with an isolation T and the antenna connected. The antenna noise will interfere with your measurements. Do it with the antenna disconnected and the signal generator connected where the antenna would normally connect to the duplexer.
Be sure to disable the repeater!! When you are finished making sensitivity measurements THEN place an isolated T in the antenna line and measure the sensitivity with and with out the transmitter on to see if you have any de-sense. Actually you might want to make a de-sense measurement before you put the preamp in to verify things are as they should be so you don't end up chasing two or three problems at the end. Then make a final de-sense measurement when all done. 73 Gary K4FMX > -----Original Message----- > From: Repeater-Builder@yahoogroups.com [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of ldgelectronics > Sent: Thursday, March 22, 2007 11:18 AM > To: Repeater-Builder@yahoogroups.com > Subject: [Repeater-Builder] Re: Preamp and attenuator > > Gary, > > That is very good information. > > We're going back up to the site today and will try that method. > > > > > 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 > > > > > > > > > Yahoo! Groups Links > > >
