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

[Richard Knoppow]

     A clear explanation. Its obvious that the discriminator type 
detector is exactly identical to the FM detector used for voice or music 
modulation. It should be optimized for the deviation of the transmitter, 
which is the same as the amount of mark to space shift for teleprinter. 
It seem to me that a universal converter should have two or three 
discriminator widths for various shifts.
On 8/3/2017 6:59 PM, Jim Haynes wrote:
> 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.
> 
> 
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL