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
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