Re: [TenTec] Inrad taking orders for Omni VI roofing filter***500hz

[Clark Savage Turner <csturner@kcbx.net>]

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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