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

[Barry N1EU]

 FWIW, I measured the spectrum and the passband of my Inrad 600hz 4-pole roofing filter in the Orion measures 650hz at the -6dB points and about 900hz at the -20dB points, and agrees well with Inrad's plot at their Web site.  The 0dB passband is 500hz wide. If the 221 center freq is 250hz lower than the roofing filter, that would put the center point of your 221's passband right at the left (low freq) knee of the roofing filter's curve.  The left knee of your 221's passband (-125hz) is going to be -10dB on the roofing filter.  I don't think that would be acceptable. 73,Barry N1EU--- On Sun 01/30, Clark Savage Turner < csturner at kcbx.net > wrote:From: Clark Savage Turner [mailto: csturner at kcbx.net]To: tentec at contesting.comDate: Sun, 30 Jan 2005 10:42:01 -0800Subject: Re: [TenTec] Inrad taking orders for Omni VI roofing filter***500hzRoger 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 that1) 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 cons
 iderations 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.ClarkWA3JPGOn 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 r
 esponse > 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 y
 ou some flexibility for > listening to slightly lower CW pitches, say 400 to 450 Hz.>> I've chosen option B for an interim solutio
 n 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>>>> _______________________________________________> TenTec mailing list> TenTec at contesting.com> http://lists.contesting.com/mailman/listinfo/tentec>>Clark Savage Turner, J.D., Ph.D.Associate Professor of Computer ScienceCal Poly State UniversitySan L
 uis Obispo, CA. 93407_______________________________________________TenTec mailing listTenTec at contesting.comhttp://lists.contesting.com/mailman/listinfo/tentec

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