[GreenKeys] 170-Hz discriminator?

[jhhaynes at earthlink.net]

I can assure you that 88mh toroid filters work just fine for 170 Hz
shift.  Lots of TUs were built with them.

For the application you have, a very simple TU should work OK since you
have clean signals.  Instead of toroids you might want to use a PLL.
PLL chip TUs don't work very well at HF with all the noise and QRM but
are good enough for VHF and in your case locally generated tones.

Do you have access to a library with old QST magazines?  There were
several designs in there over the years.

July 1958...A Filterless Terminal Unit for FSK
Dec 1965..."Starter" RTTY Converter
Sep 1984...Radio Modem with FSK RTTY for HF Rigs
Jun 1971...First Steps in RTTY
May 1971...RTL-1 RTTY Converter
and there's a lot more stuff in QST and still more if you have Ham Radio.
Also there are the various old RTTY handbooks, many with TU designs
Also there are often cheap RTTY modems at swap meets


-- 

jhhaynes at earthlink dot net