Clark & Roger,
The pre-production CW roofing filter from INRAD (and presumably the standard
one they are offering) has an offset of 700 Hz, not 750 Hz, so the mismatch is
not quite as bad.
Probably the best way to illustrate the problem of mismatch between the 9 MHz
roofing filter and the downstream 9 MHz IF filter is to sketch frequency
response curves of the two filters superimposed. Just a rough sketch,
approximately to scale, showing the upper and lower 6dB points will do.
Doing this for the standard CW roofing filter (BW = 600 Hz and Center Freq. =
700 Hz) and the 221 filter (BW = 250 Hz and CF = 500 Hz) will show that the
lower -6db cutoff frequency of the composite response will fall ABOVE the
lower cutoff of your 221 filter, and that the upper cutoff frequency of the
roofing filter (approx. 1000 Hz) is way above the cutoff frequency of the 221
(approx. 650 Hz). This results in a somewhat asymmetrical overall response
curve. Probably not a huge deal, but not ideal, either.
The way I see it there are several solutions for Omni VI CW ops using the 221
IF filter:
A. Purchase the SSB roofing filter instead of the CW model (it has wider
bandwidth and won't cause the asymmetry, but also isn't as good at the IMD
rejection you are looking for).
B. Use the standard CW roofing filter and listen to CW with a slightly
higher pitch, say 550 Hz. By shifting the center frequency in this way you can
minimize the asymmetry in the overall 9 MHz response.
C. Purchase the CW roofing filter with 500 Hz offset to match the 221.
This is, of course, optimum, and gives you some flexibility for listening to
slightly lower CW pitches, say 400 to 450 Hz.
I've chosen option B for an interim solution until my new roofing filter
arrives.
For CW ops using the standard TT filters with 750 Hz offset, there is no
mismatch problem, or at least it's negligible. For CW ops using the INRAD 753
400 Hz filter with 600 Hz offset, there is a mismatch problem similar to the
221. In this case, the lower 6dB cutoff frequency of the roofing filter is
approximately equal to the cutoff frequency of the 753, which moves the 6dB
cutoff of the composite response upward.
Again, I'd recommend anyone concerned about this to sketch response curves as I
described in order to visualize the composite response of the two filters
working together.
Chuck NI0C
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