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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>
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