[TenTec] Purpose of 250 Hz 8 pole filters
Dr. Gerald N. Johnson
geraldj at storm.weather.net
Sun Oct 21 21:52:28 EDT 2007
On Sun, 2007-10-21 at 20:58 -0400, k8vf_mark at centurytel.net wrote:
> I cant do it. I envy you. I would think most can't, hence the filters.
> I would imagine you were the exception, in my opinion. What a great talent!
> You are really lucky! If I could do that I would be thrilled!
> Lots of times in a contest there are several in the passband(500) and they
> are very close in frequency, so I really need the stacked filters in those
> Yes noisy, not crowded band. Lots of noise on 160.
> The other reason for the narrow filter is to get EXACTLY on a guy....in many
> contests, fellows are running very narrow filters and you have to "fish"
> around in the (even 250 Hz) passband to get a guy to answer you. I have run
> into that many times...it is not the signal strength, either. The guys are
> just that narrow.
> Great thread!
That skill comes with lots of practice with receivers having 2, 3, or 5
KHz bandwidth. Helps to have an ear for musical pitch so as to be able
to pick out the desired signal out of the crowd. One tool that is said
to help is a binaural amplifier that shifts the phase to the two ears
according to frequency so there's direction selectivity also. Sometimes
we use that direction ear processing to listen to one conversation while
in the midst of several (like at a party).
Matching the received signal pitch to the transmitted has been a problem
with narrow receivers for a loooong time. I had a Hint & Kink published
in QST back about 1981 after a frustrating FD with few answers because
the rig didn't transmit where I listened. After I borrowed a second
receiver to adjust RIT so the two signals matched I made many contacts.
My FT-857 when in CW mode (where it can use the narrow mechanical filter
and the narrow sets of DSP filters if I allow it) flashes an indicator
in blue when the received signal matches the chosen transmitter offset
(or tone center frequency, which is adjustable by menu from 400 to 900
Hz or more). That matches signals well and gives one a chance to learn
to read CW by light.
One year at FD, the CW station was a TS-430 with a 270 Hz filter. In
that particular 430 it was not possible to select the receive filter
independent of mode and that particular filter shaped the noise into a
continuous tone. After 4 hours of copying, I was so worn out from
copying signals against that tone that I couldn't copy anymore. If it
had only the SSB filter, I'd probably have lasted 7 or 8 hours. I've
blamed it on the Tchebychev filter passband which isn't achievable with
the ladder filter circuit that Tentec uses, but I see in this thread
some of the same complaints. I have a Collins 75S-3B with a 200 Hz
crystal filter but I've not really exercised it much with that filter to
hear what the noise gets transformed too. I'd expect it to ring like a
bell though, the SSB filter of that era does.
I had a TS-130 with a 400 Hz CW filter. It began to ring and to distort
CW signals at 30 to 35 wpm, some narrower CW filters should be even
worse with high speed CW. Of course with today's standard of 5 wpm or
lower that won't be a problem, except I can't send below 12 or so with a
straight key. I'm most comfortable at 18 to 20... Course, when I took
the extra test, I had to send to the FCC examiner too.
73, Jerry, K0CQ
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