Roger and Chuck -
Thanks for the explanations, it is good to see the basics of the
argument for a different center frequency. I believe I was the first
guy to have the 221 (after bugging Ten Tec along with a few other
diehards :-) and it is my favorite filter for CW when the going gets
rough or I need to dig into the noise.
So, the bottom line seems to be that
1) I lose a bit of PBT range (the intersection of the response curves
is narrower than it ought to be) that is due to extra losses where
either filter curve drops off.
I had thought that a 4 pole filter with 600 Hz 6 db points would not
have much of a shape factor (I ought to go look at the curve) and it
would be a very small effect. I certainly don't notice much loss at
the center of the 221 filter when I use it (with the roofing filter
engaged). I understand completely how the edges of the response will
drop off faster than the might otherwise.
Heck, I use the INRAD 400 Hz filter in both IF's, too, I ought to check
the center frequencies for those. I recall that the 9 MHz version of
the 400 Hz CW filter from INRAD had a compromise center frequency per
the considerations above, is it 600 Hz? Heck, I should draw myself a
block diagram of my IF/PBT chain in my OMNI VI and think about it.
Again, thanks for helping me think about this.
Clark
WA3JPG
On Jan 30, 2005, at 5:47 AM, Chuck Guenther wrote:
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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Clark Savage Turner, J.D., Ph.D.
Associate Professor of Computer Science
Cal Poly State University
San Luis Obispo, CA. 93407
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