[RTTY] Another method for FSK generation (long and rambling)

[Doug Hall]

Hi all,
For quite some time I've been using MMTTY to ragchew, chase DX, and contest
with and it works very well. I used a COM port TXD line to drive a
transistor to generate an FSK signal to the FSK input on my TS-850 and later
on my TS-2000. I'm a believer in FSK. It's fairly easy to wire up an FSK/PTT
circuit using the TXD and RTS lines on a COM port.

But there are situations where it's not easy to generate an FSK signal, and
in the future it's probably going to get harder. Many of the current crop of
UARTs (the 16450 and 16550-style) support 5 bit data mode (required to
generate the Baudot FSK signal), but some of the newer UARTS (including some
of the USB-to-serial adapters) do not, and probably will not. Or you may not
have a spare COM port, or you may not have a COM port at all, which is where
I found myself recently. I replaced my Win2K hamshack machine with a
non-Windows system but I wanted to continue using FSK when operating RTTY.
The available USB-to-serial converters worked for things like rig control,
rotator control, etc. but not for 5 bit FSK generation. In a conversation
with W7AY Chen suggested feeding the AFSK tones generated by the computer
into a simple demodulator and using its output to drive the FSK input on the
rig.

That's literally thinking "out of the box" and was sufficiently intriguing
that I looked through my junque box to find some Exar 2211 demod chips that
I had squirreled away years ago. I decided to wire up an Exar 2211 as a
demodulator and feed its output to the FSK input of my TS-2000. While I was
looking for my stash of 2211's I came across an old Kantronics TU that I
picked up at a hamfest for 50 cents. It's called "The Interface" and was
sold back in the early 80s for use with computers like the Atari 800 and the
Commodore 64. I didn't have a schematic for it and didn't know if it worked,
so I tore it apart and started poking around. It consists of a diode clipper
feeding an LM324 which then feeds some kind of PLL that I'm not familiar
with. The box has a demod out (open collector) as well as a current loop
out, so I knew I could probably make it work in FSK service. Unfortunately I
could find nothing that resembled a carrier detect line (for generating PTT)
so I had to fabricate one using the clipped/limited LM324 output. I
rectified and filtered it and ran it to an open drain circuit (2N7000) and
used that to drive the PTT line. Similar to what the SignalLink box does,
only mine has the FSK demod as well. There are some photos of the
(admittedly ugly) construction at:
http://k4dsp.homeip.net/~doug/misc/AFSK_FSK/
in case anyone is interested.

Got it all hooked up to the Mac Mini and split the computer audio output
between the Kantronics box and the rear panel audio input on the Kenwood (so
I could still do PSK31.) The  result is that it works beautifully. The rig
keys just fine in RTTY and PSK modes, and I can run RTTY in FSK mode, just
like Father Baudot intended. It may seem somewhat overkill, but if you don't
have a COM port and want to run FSK it's actually fairly simple. The 2211 IC
is easy to wire up if you don't have an old terminal unit to press into
service.

Thanks to Chen, W7AY for the suggestion, and especially for cocoaModem,
which was the missing link allowing me to complete the migration to the Mac
OS X operating system in my hamshack. If you've been eyeing one of the $499
Mac Minis then cocoaModem is yet another reason to get one.
73,
Doug, K4DSP