Thanks, Chuck. I knew someone would have a better analogy of explaining this
situation than I did at 4:30AM after being up nearly 22 hours and spending
the past 10 hours working the 160 Meter CW contest. I thought I still "had
it together" but I shouldn't have been trying to answer this message when I
was so over tired.
You did a much better job with far less words and more accuracy. Thanks for
your input, Chuck.
vy 73, -=Rog-K9RB=- (personally, FWIW, I prefer about 400 hz.pitch,
especially for very weak CW signals)
----- Original Message -----
From: "Chuck Guenther" <ni0c@earthlink.net>
To: <tentec@contesting.com>
Sent: Sunday, January 30, 2005 8:47 AM
Subject: Re: [TenTec] Inrad taking orders for Omni VI roofing filter***500hz
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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