[GreenKeys] 850 shift history - AN/FGC-1

[Nick England]

Late in the war, and just post-war there were several adapters to convert
existing large military CW or CW/AM transmitters to RTTY operation. These
typically mixed a 200 kc FSK signal with a VFO or crystal oscillator. The
200 kc signal was symmetrically shifted by up to +/- 425 cps. There was a
multiplication control that reduced the shift proportionally if the unit’s
output was multiplied later in the transmitter chain.
This scheme was followed in new Class C transmitter designs into the
mid-late 1950s, when SSB rigs with linear amps and AFSK took over.

Frequency-synthesized military systems are channelized. The FSK signal
center frequency had to be on 100, 500, or 1000 cps steps depending on
equipment. Not so for ham gear.


On Fri, Oct 8, 2021 at 6:10 PM Harold Hallikainen <harold at w6iwi.org> wrote:

>
>
> On Fri, October 8, 2021 2:53 pm, Nick England wrote:
> > Well, I found my FGC-1 manual but it is later than the 1943 Western
> > Electric typewritten ones abstracted on my web page at
> > https://www.navy-radio.com/rtty-demod.htm
> > These came from Duncan Brown some years ago
> >
> > The one I have is August 1952 TM 11-356 which supersedes the 1943 edition
> > and is typeset in typical TM fashion. One interesting addition is that
> > when
> > radio frequencies in the 70-kc to 500-kc range are used, a frequency
> shift
> > of 170 cps is used which necessitates replacement of the M and S filters
> > to
> > 2465(M) and 2635(S).
> > You learn something new every day, huh?
> >
> > Cheers,
> > Nick England K4NYW
> > www.navy-radio.com
>
> Interesting that they changed BOTH filters when changing to 170 Hz shift
> instead of just the space filter as hams did. I wonder if it was an
> attempt to keep it centered on a bandpass filter.
>
> The geometric mean of 2125 and 2975 is 2514
>
> The geometric mean of 2465 and 2635 is 2548
>
> The arithmetic mean of 2125 and 2975 is 2550
>
> The arithmetic mean of 2465 and 2635 is 2550.
>
> So, it looks like they were trying to keep the center frequency the same,
> but the center of a bandpass filter is at the geometric mean (sqrt
> (Fl*Fh)).
>
> Anyway, I guess there is a front end bandpass filter that they did not
> want to change, so they changed the mark and space filters.
>
> Harold
>
> --
> FCC Rules Updated Daily at http://www.hallikainen.com
> Not sent from an iPhone.
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-- 
Nick England K4NYW
www.navy-radio.com
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