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, 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.
______________________________________________________________
GreenKeys mailing list
Home: http://mailman.qth.net/mailman/listinfo/greenkeys
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:GreenKeys@mailman.qth.net
>>> Jordan Spencer Cunningham's GreenKeys Search Tool: https://teletype.net/gksearch
>>> 2002-to-present greenkeys archive: http://mailman.qth.net/pipermail/greenkeys/
>>> 1998-to-2001 greenkeys archive: http://mailman.qth.net/archive/greenkeys/greenkeys.html
>>> Randy Guttery's 2001-to-2009 GreenKeys Search Tool: http://comcents.com/tty/greenkeyssearch.html
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to navy.radio@gmail.com