[GreenKeys] TU replacement

[Ralph Mowery]

The PLL circuits are fine for signals that are in the clear and have no
noise such as the itty.
They do not work very well at all if there are other signals near by or
noise.

If you want a simple circuit to build that works fairly well on a radio
receiver, look at the ST-5.  It 
Uses 2 ICs, 2 of the 88 mh loading coils and a power supply.  It will work
better if you put the 
Filter of the st-6 in front of the first IC.  It is basically a solid state
unit of the simple tube circuits 
Of the past.  

A step up that works very well is the ST-6 but it is a lot more complicated
to build.  Its main advantage is that if
The mark or space tone fades but not the other , it will still copy.   It
was interesting to watch
A scope hooked to it and watch one tone fade for a second or two and then
come back and the other tone fade in and out.

Ralph ku4pt






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