Thank you, Ken! My thinking is that if the CF is lower you would have to
tune less far away from zero beat to get a sig into the center of the
filter. So with the reduction spectrum you might encounter fewer
interfering sigs, on average. In this case 750-500=250Hz closer to zero
beat on either side. Diminishing returns perhaps (if I am even correct)!
My main objective is to lower the CF to a value pleasant to my ears! Nice
to know there are others out there who like a low CW frequency!
73,
Garry
On Tue, Aug 22, 2023 at 1:27 PM Ken Brown <kenradiobrown@gmail.com> wrote:
> Hi Gary,
>
> It is clear that you already know a lot more about those filters than
> I do. So I can't help you with your decision about changing R and C for
> filter center frequency, bandwidth and Q.
>
> I just wanted to say that two bandpasses on opposite sides of zero
> beat give you a total receiving bandwidth of 2X the audio filter bandwidth,
> regardless of the filter center frequency. Selectivity would be the same as
> long as the bandwidth of the filter is the same at the two center
> frequencies you are comparing. Whether you will experience interference
> from signals on the opposite side of zero beat (in the other sideband) just
> depends on whether they happen to be on that frequency.
> Maybe fewer operators will start up a QSO or call CQ only 1 kHz away
> than they will 1.5 kHz away, so perhaps you would experience a bit less QRM
> with the 500 Hz center frequency. Really though I think the deciding factor
> is the center frequency you like to listen to. I prefer about 450 to 500
> Hz. I'm glad my radio lets me change it at will, but I find myself keeping
> it at 450 Hz almost all of the time.
>
> Ken N6KB
>
> On Tue, Aug 22, 2023 at 4:51 AM Garry Nichols <garrnich@gmail.com> wrote:
>
> > Hi Folks;
> >
> > I have a C-22 (S/N: 666) and have verified that the AF filter has the
> .0022
> > uF caps in it and that the three resistors that set the filter
> > characteristics are the same as in the Ten-Tec circuit diagram. This
> > supposedly gives about a 750 Hz center frequency and approximate testing
> by
> > me agrees with this number.
> >
> > Please Note: the Ten-Tec circuit diagram indicates .0033 uF caps which
> are
> > the ones that gave a center frequency of 500 Hz. I have heard that the
> > initial run of these radios were set up for 500 Hz.
> >
> > Does anyone know if the 500 Hz radios were intentional and that 500 Hz
> was
> > not received well by hams prompting a change to .0022 uF and 750 Hz? Or
> > was it considered a goof up by Ten-Tec?
> >
> > My old ears do not like frequencies around 750 Hz. So I am considering
> > converting the filter to 500 Hz for that reason.
> >
> > Also, it would seem that 500 Hz would give somewhat better selectivity
> > considering the "signals on both sides of zero beat" architecture of this
> > rig. I reason that tuning out from zero beat 500 Hz on either side
> covers
> > a narrower spectrum than tuning out 750 Hz to a filter peak. Is this
> sound
> > thinking???
> >
> > I have a print from somewhere on the internet of a mod that takes the
> > filter from 510 Hz center frequency to 600 Hz. I don't know where I
> found
> > it but it is apparently by a Chinese ham. He gives a link to an active
> > filter calculator which I have been playing around with:
> >
> > https://www.changpuak.ch/electronics/Narrow_Bandpass_2.php
> >
> > I've noticed that with a change to .0033 uF (the 500 Hz CF) and no change
> > to resistors the bandwidth reduces to 208 Hz. Using the calculator with
> > .0022 uF the characteristics are likely the same as my current
> performance
> > and the calculator gives a bandwidth of 313 Hz. I find the filter to be
> > fairly "peaky" at 313 Hz BW and am concerned that if narrowed to 208 Hz
> it
> > might have touchy tuning. (I do realize this filter does not have the
> > steep sides of a crystal filter, but that is still pretty tight, I
> think.)
> >
> >
> > So I changed the Q of the filter from 2.4 (which is what gives my
> apparent
> > current performance with caps and resistors) down to 1.6 in steps and
> > continued to use the .0022 uF caps that are in it and wound up with a
> > change needed only to R3, the frequency determining resistor. So that
> > would be 4 R component changes instead of 8 C changes and a calculated
> > bandwidth of 313 Hz which is what I have now with the current components.
> > The rolloff will be more gradual with the reduced Q, but being a newbie
> to
> > all this I'm not sure if it would be a detrimental rolloff.
> >
> > Does anyone have any experience with active filters or with C-22 filter
> > mods who can give any insight into my ideas? This is a four section
> > filter.
> >
> > TU ES 73,
> >
> > Garry WA1GWH, near Syracuse, NY
> > _______________________________________________
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> > TenTec@contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
> >
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