At 3/25/2009 19:33, you wrote: >About the only device that has a sharp enough response is a front-end >crystal filter. While very effective, they are lossy and cost between $350 >and $500 each. Here is more info: > ><www.mtronpti.com/pdf/21334133Filter_datasheet81007.pdf> > >73, Eric Lemmon WB6FLY
IMO front-end crystals filters do little to eliminate adjacent-channel interference when the interfering signal center is only 15 kHz away. The specs for the narrowest filter above are only 20 dB down @ 30 kHz, so 15 kHz away the rejection will be rather low, maybe only a few dB higher than the insertion loss. The key to reducing adjacent channel interference is to apply the correct amount & bandwidth of IF filtering, and to use a receiver of good IF design so that distortion does not occur in the IF amplifier stages. I've had good success with G.E. MVP & Mastr II receivers. They have 10 poles of 11.2 MHz crystal IF filtering (the G.E. needs to have a missing IF filter retrofitted, otherwise it's only 8 poles) which often gets the job done, but for tougher adjacent channel problems I go with the ComSpec filter retrofit kit (<http://www.com-spec.com/narrow.htm>). I typically don't use the whole kit, but only install the filters needed to get the receiver down to -50 dB @ +/- 10 kHz, which is TASMA's specification for receiver IF response. BTW, you can try a simple test to see if your receiver meets the above spec: open squelch & inject an on-channel unmodulated signal that results in 20 dB quieting, then move your sig. gen. + or - 10 kHz off frequency & see how much more signal you have to put into the receiver to obtain the same amount of quieting. You may be unpleasantly surprised. If it doesn't meet spec, you need to consult with the manufacturer to see if they offer any narrower IF filters for it, or check the above link to see if any of those filters can be swapped for your wider IF filter. I once retrofitted a 15 pole 455 kHz IF filter in a Yaesu FT-227R (the old "Memorizer"). It's -70 dB point was +/- 8 kHz. I could park that RX 10 kHz away from most repeater outputs & hear very little; absolutely nothing bothered it 15 kHz away. Bob NO6B
