A simple method of testing for ISP/Spread Spectrum Interference on your 900 repeater.
Because ISP and Spread Spectrum equipment is so difficult to see on an analyzer, I came up with a simple way of testing your repeater's input channel for the interference. 
First, I would like to say that many people dismiss ISP interference as "mobile flutter" or just bad propogation on 900.  THIS IS NOT THE CASE!....usually.  Click on THIS to hear what ISP sounds like on a repeater's input.  See!  Sounds just like mobile flutter!  This was taken from a repeater's input on 902.250 from the N6ICW repeater on Mt. Vaca CA.  At the same location, other 900 repeaters below 902.100 are completely quiet!  The signal under all that noise would be nearly a full quieting signal.  This is an extreme case, but it comes in all shapes and sizes!
On a spectrum analyzer, it simply looks like a bunch of spikes bouncing around the band...if you can see them at all.  The newer digital analyzers are not usually fast enough to actually catch the bursts, but my old analog Avcom sees them just fine! 
The best way I have found is to get a radio capable of receiving at 902 MHz like a converted Maxtrac and program in a few freq's between 902.0 and 902.9875.  I have mine programmed with 902.0125, .0500, .100, .150, .200 etc.  Then connect it as shown below with a directional coupler, antenna and signal generator.  The value of the DC is not important.  Anything between 10-30db will work fine.
Now, set your signal generator to the same frequency as your receiver and generate a slightly noisy signal into the receiver.  Then connect and disconnect the antenna and compare the difference!  I also use this method with a yagi to find the direction of the interference.