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

[Harold Hallikainen]

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

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