After more experimenting and measuring, I think the RF is done enough. I
found that the main 20 MHz and the 300 MHz LPFs were letting a lot of HF
above 1200 MHz straight through. I believe the choke I used in the 300
MHz LPF has a self-resonance in that range - it's a wire coil molded in
plastic type, which may have a lot of inter-turn capacitance, spoiling
the HF. I had used the same kind in the diplexer, and also as a helper
choke at the 20 MHz LPF input.
It looked pretty bad above 1200 MHz, but some quick experiments with the
HP8568B and its TG, let me figure it out up to 1800 MHz and correct it
without having to change a bunch of parts, and possible complications.
The diplexer is now a different HPF that looks like a short (instead of
a termination), reflecting most back to the mixer, and an added 475 R
damping resistor across the helper choke took out the overall filter
resonance above 1200 MHz - all the way down and flat as the MF portion
of the stop-band, up to 1800 MHz. It turned out that using the HPF as a
short is beneficial - the minimum signal levels appear at the LPF, and
it cut the DC offset nearly in half..The possible cable resonances do
not seem to be a factor, and it looks like most of the >1200 MHz
spurious content was going straight through the filter system, and not
around it.
All of the LO harmonic spurs are now -100 dBm or less, basically in the
noise floor, and only discernible by zooming in around the spot
frequencies. I finally settled on 20 nF for the CM capacitance, so
ground loop CMR should be very good. Now that the RF is "done," I can
wrap up the DC offset control - unless I discover some new tangent to go
off on.
Ed
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