Hi Clark, I'll attempt to explain the best that I can;
The 6.3 MHz. IF filters are adjusted to the peak desired frequency with the
Pass Band Tuning (PBT), so the center frequency of those filters have a
range of adjustment when cascading with the 9 MHz. filter/s. The 9 MHz.
filters are not adjustable for center audio frequency peak, but determined
totally by all the crystals frequencies in the filter itself. The typical
center frequency for the standard 9 MHz. CW filters has been 750 Hz. until
the model #221 filter became available. This filter is centered at 500 Hz.
(audio pitch frequency). It was specially made for us CW folks who prefer to
listen to a lower pitch CW note. When cascaded with a 6.3MHz. IF filter, the
PBT controls provides plenty of range to allow you to shift the 6.3 MHz.
filter center and peak the desired signal to the 500hz. audio note where the
#221 filter is centered. The new 600hz.bandwidth CW roofing filter is
cascaded with the 9 MHz. CW filter/s and because it is in the front end 9
MHz. IF, where it needs to be for maximum effectiveness, it should have the
same center frequency as the center frequency of the other 9 MHz.CW filter
which it will be cascaded when inline. If that filter is a #221 for the
special lower 500hz.offset and the lower CW pitch, matching the roofing
filter center frequency to this 500 Hz. will provide for the maximum
throughput at the 500 Hz. offset. Remember the VBT takes care of centering
the 6.3 MHz. filter and when the VBT is peaked and the other two 9 MHz.
filters are in line, everything is centered at the desired lower CW pitch
frequency of 500 Hz.
If you use the standard 600 Hz.wide (don't get bandwidth confused with
center frequency offset) CW roofing filter centered at 750 Hz., cascades
with the #221, your PBT will require being set at some compromise setting
and optimal pass through of the 500 Hz. tones you want to hear will be
somewhat attenuated. This is why the request was made for this special INRAD
roofing filter made to match the center frequency of the special model #221
CW filter at 500 Hz. rather than the higher pitched 750 Hz. that the
standard CW 9 MHz. filters are centered. I hope this is clearer than mud to
you. If you are using the model #221 TenTec CW filter, and wish to add the
CW 600hz. bandwidth INRAD roofing filter kit, to optimize performance, You
would want it centered at 500hz. also. If it isn't, no two cascaded filters
are matched and PBT is somewhat compromised, making for some attenuation and
undesired double or even possibly triple, (with the PBT offset between the
roofing filter and the #221 filter?) tone peaking frequencies. This would
certainly produce undesired results as well as some minor attenuation due to
the mismatches in the center frequencies throughout the IF chain.
If I confused you and you wish to discuss this further, I'd be happy to do
it in private to K9RB@arrl.net or you can probably even get a better
description from George at INRAD. If you've already got the CW 600 Hz.
bandwidth standard roofing filter kit installed, as you say you do, I'm
certain all you would need to do is swap out the crystal filter on the added
INRAD board to accomplish the change and make everything right. By having
different center frequencies in the cascaded 9 MHz. filters now, I can't
tell you how much loss you are experiencing with a difference of 250 Hz. on
center frequencies, but there certainly is a compromise being made, not only
in the difference in the fixed 9 MHz. filter frequencies, but also in the
need to split the difference in the PBT to get a signal peak and essentially
then nothing is perfectly centered at the desired center frequency, which
for optimum pass through, should be all at the same center frequency.
Now to simply answer your questions, see * lines below, added to your
original posted message.
vy 73, -=Rog-K9RB=-
----- Original Message -----
From: "Clark Savage Turner" <csturner@kcbx.net>
To: <tentec@contesting.com>
Sent: Saturday, January 29, 2005 10:28 PM
Subject: [TenTec] Inrad taking orders for Omni VI roofing filter***500hz.
> I may want to get just the crystal filter since I already have the
> board in my OMNI VI,
* Most definitely.
> but how big a deal is this?
* If you installed the kit, you can change out the filter on the board.
Should be four solder connections....
>
> I use the model 221 filter (my favorite)
* which has a design center frequency 500 Hz. offset.
>
> and the 600 Hz roofing filter centered wherever it is
* that would be at 750 Hz.offset on the standard CW filter kit.
>
> works just fine, not much loss.
* I'm sure it appears to, but the mismatch is causing more loss than you
think.
>
> I suppose it would be optimized by a few db with the other filter ... and
I can't
> resist finding out ... so I'm sure to swap filters.
* A few dB. is big as even 1 dB is considerable. Remember 3dB is equal to
50%
>
> But is there any analysis to say this will make a big difference?
* Just simple electronic theory at work
>
>(Excuse me if I missed the answer, I have been on/off for a while here).
* Not a problem, Clark, I'm happy to help with understanding wheneverr I
can. -=RB=-
>
> Clark
> WA3JPG
>
>
> On Jan 29, 2005, at 5:13 PM, Chuck Guenther wrote:
>
> > Ron and Roger,
> >
> > I sent an e-mail to INRAD asking if I should place an order for the
> > special 500 Hz offset CW roofing filter. Georgia replied and said go
> > ahead, just be sure to specify the 500 Hz offset in the comments
> > section of the web order page.
> >
> > I'm on that list too. I really need that unit. The more I
> > operate with the 221 filter, the less I like high tones. Come on guys,
> > get your orders in.
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
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