Prompted by Roger, N1RJ, I decided to do some digging around. Aided by isolating specific stages of my receive signal path as much as possible, and using precision 3 and 6 dB pads at various points to see where they dropped the IMD more than the attenuation of the pad(s), I have come up with a list of suspects.
1) The preamp. This was oversight on my part. A review of the original construction article clearly states a filter will likely be required if using a Beverage. Evidence: I simplified the signal path to Beverage>Preamp>Receiver. Placing a 6 dB pad before the preamp dropped the IMD level more than 12 dB (may be closer to 18, I did not attempt to verify). Placing the 6 dB pad after the preamp dropped the IMD products 6 dB. 2) The ICE 196 receiver protector. Evidence: 6 dB pad before the 196 reduced IMD more than 12 dB. Placing the pad after the 196 reduced IMD 6 dB. Bypassing the 196 reduced IMD more than 12 dB. 3) A relay used to switch the preamp in and out. This relay had apparently "failed", since replacing it with another of the same type made a big difference. This likely explains why my IMD problem when using the preamp went from annoying to severe a couple years ago. (I am aware relays in a receive signal path can be problematic due to insufficient current to clean the contacts). Based on results of this investigation my proposed plan (once I figure out how to fund it) is: 1) Replace the preamp. I'm not in the mood to build another one from scratch so I'm leaning heavily toward the Clifton Labs Z10040B. 2) Use a HPF in front of the preamp. Whether actually necessary or not it seems like good practice. I am leaning toward the Clifton Labs Z10022A, although I am torn between that and the Par Electronics BCST-HPF. The latter has more attenuation at the high end of the BC band, but not as much at the middle and low end. 3) Get rid of the ICE 196. I've had it with these things. This is not the first problem I have had with these devices generating crud. I have not as yet come up with a plan I am entirely comfortable with for receiver protection in its absence but I am leaning toward using a relay to short the receive signal path to ground when transmitting. Rather than put an additional relay contact in series with the signal path, I am thinking about having a normally open contact connected to the signal path, taking it to ground when transmitting. I welcome any comments. 73, Paul N1BUG _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
