[TenTec] TenTec/ Harley

[Dr. Gerald N. Johnson, P.E.]

They are so impressed by the price tag, they don't understand why they
don't hear like other better radios.

Years ago when I was using my 75S3B a great deal I noticed such things
as I hung a scope on the IF of it. For its first couple years it spent
more time on its side out of the cabinet than in the cabinet. So much
that I had to relearn control position when I put it in the cabinet
after making my modifications. I talked to the engineer at Cedar Rapids
(I was working for Collins in the broadcast transmitter department as an
engineer) who was the support for the 75S3B about my mods. And mentioned
the filter ringing. He said he didn't have one, but wondered by the
locals on 75 meters shut down just because of lightning cracks saying
they couldn't copy anymore while he was doing just fine with his HQ-129X
(that used multiple IF transformers to get selectivity). Its all in the
time response. For a decade or more I had my S-line set up with a patch
panel so I could patch in a separate receiver or run two receiver
simultaneously and for 75 meters in the summertime, my best alternate
receiver was a crystal controlled converter in front of a BC-453. All
the selectivity by undercoupled 85 KHZ IF transformers, it doesn't ring
on lighting static either. When the static turned to signal obliterating
crashes in the S3B, I'd switch to the Q-5er and continue with the QSO.
Worked far better.

The mechanical and conventional crystal lattice filter designs are based
on Tchebychev frequency responses that give the steepest skirt responses
for the fewest elements but at the cost of a terribly ringing time
response. Their rapidly changing phase response near the pass band edges
give rise to a great deal of data damage in the digital modes.

A few years ago I demonstrated the effect of static on a couple
receivers for the local radio club. I compared a TS-120 with crystal
filter to that Q-5er using a pulse generator for the noise source. The
Q-5er would take a pulse repetition rate something like 5 times faster
than the TS-120 before loosing the received signal and that's in the
SAME bandwidth 2+ Khz.

73, Jerry, K0CQ

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