[GreenKeys] TU replacement

[Harold Hallikainen]

> I agree, the one i did using the 2211 is fine for itty but not really for
> weak signals on the air. Though given the amount of rfi in my urban
> neighborhood,  nothing really works very well for that.  :(
>
> On August 21, 2019 2:44:36 PM PDT, Jim Haynes <jhhaynes at earthlink.net>
> wrote:
>>On Wed, 21 Aug 2019, Harold Hallikainen wrote:
>>> Not existing right now, but I plan on building a demodulator using an
>>> XR2211. I've used this in lots of projects over the years, including
>>> driving a model 15 in the early 1970s.
>>>
>>Well there's the Volpe demodulator that uses that principle.  I'm not
>>authoritative, but have the opinion that phase locked loop demodulators
>>are only good if you have good signals, perform poorly with FSK in
>>the presence of noise and interference.  Which of course is not a
>>problem
>>with ITTY, but for on-the-air use you need something better.
>>
>>Jim W6JVE

Thanks for the comments! I just looked at several old demodulator
schematics, and they look similar to what I did with vacuum tubes in the
1960s using, of course, 22 mH loading coils. These are largely two
envelope detectors, one for mark and one for space followed by a
comparator. The bigger signal wins. That's pretty easy to do with analog
circuitry or DSP. Is that currently thought to be the best method to
demodulate FSK? Is it the PLL that is not liked or the use of an FM
demodulator in general?

THANKS!

Harold



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