I managed to build a filter, using the values for a 9th order
Butterworth, 50 ohms, 25 MHz fc. The caps were fairly straightforward to
get nearly right on in values, with one or two (paralleled) selected
micas for each spot. The chokes were tricky. I decided to use IF-can
style adjustable ones, since I managed to scrounge up a few that were
close enough. The whole thing is built on special double ground plane
board (with 0.15" via grid shorting the sides together) stock, which
took a lot of hand crafting to mount the cans and lay everything out right.
I checked it with the TG, and it looks like a filter, kind of as
expected. After much tweaking of the chokes, I got it to look fairly
good, but it's all open-loop, part-wise - the chokes are set for
appearance of the response, not necessarily right values. So, it's some
kind of LPF, but that's about all I can say. The chokes are the weak
link, since they're hard to measure accurately.
I put the filter into the noise project, and the result looks pretty
good. Measuring the actual noise output on the SA, and zooming in, I
found it was flat to less than half a dB p-p, looking at 1 dB/div. Not
bad considering my eyeball-controlled adjustment using 10 dB/div and the
TG beforehand. This flatness is the net effect of the noise output
itself, and the filter, and a little bit the SA, so pretty decent. The
fc is around 22 MHz at the "best appearance" setting, and the Z-match
seems OK. There's no pad at the filter input, and about 3 dB at the
output, then that same 20 feet of cable to the SA. The high frequency
rejection looks pretty good too, with the 70 MHz and 140 MHz (the worst
offenders) below -85 dBm. This can be improved with more grounding
enhancement, and possibly adding shielding - it's kind of open
construction now, just on the board. The chokes are fairly well
contained and shielded in the cans, but the caps are exposed.
Anyway, for this purpose, it's way better than the original filter,
which can now be returned to its other project. I'm fairly happy with it
so far, but expect it to be one of those never ending projects - always
room for improvement.
Ed
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