[GreenKeys] German WX service via SDR & Websdr...

[Jim Haynes]

I can understand why the Boehme has trouble with narrow shifts, because
it is designed for high-speed Morse operation with shifts as wide as
15 KHz.

The W2PAT converter that Kenneth has can probably work at those very
narrow shifts, perhaps by retuning the discriminator coils.

It's all a rather long story, but I'll try to keep it short.

Originally FSK RTTY was demodulated using a limiter-discriminator type
of converter (and shift of 850 Hz).  With narrower shifts the 
discriminator simply has less voltage swing in the output, so you may
need some amplification of the discriminator output to get adequate
keying of the converter output stage.  Ideally you have a narrow filter
in the receiver, or in front of the converter, so you don't let any 
signals into the limiter that are very far from the frequencies you
want to receive.  Ideally the discriminator peaks are quite a bit further
apart than the shift you expect to receive, and you have enough post-
discriminator gain to control the TTY selector magnet or whatever.

So far as I know all the amateur converter designs of the 
limiter-discriminator type are defective in that the discriminator peaks
are too close together, typically right at the mark and space frequencies,
and there is inadequate filtering ahead of the limiter to suppress
interfering signals.

Then in the early 1960s we got into the limiterless or two-tone detection
schemes, and these require fairly narrow filters at the mark and space
frequencies.  So these don't work at all well with shifts other than the
one(s) they are built for because the narrow filters don't detect both
mark and space frequencies.

So the ST-6 and other two-tone converters are not likely to work well
with odd shifts.  The Dovetron and similar converters can work well 
because the mark and space frequencies can be tuned to whatever the
incoming frequencies are.  But they may not; I'm not sure how low a
shift they can tolerate and still work.

The CV-89 and some other military converters are limiter-discriminator
types.  The only reason they might not work well with very narrow
shifts is that there may not be a large enough discriminator output
to key the output signal correctly.

Some of the early converters such as W2PAT use neon bulbs to intentionally
reject signals from the discriminator that are small.