[GreenKeys] Fwd: [MMRCG] RTTY Articles

[Harold Hallikainen]

Thanks for posting! It reminds me of how much hardware it took to do
anything in those days. There also seems to be a fair amount of "brute
force engineering" where parts are thrown at the design. For example,
there's a demodulator
(https://ia601603.us.archive.org/34/items/73-magazine-1977-09/09_September_1977.pdf#page=55
) with voltage followers after the active filters. Since the active
filters already have a low impedance output, I see no reason for the
followers. There are separate "limiter amps" in front of each filter, but
they are, once again, voltage followers and not limiters (unless the
output is limited by the supply voltage). A single voltage follower could
be used to drive both filters. On the inputs to these amplifiers, there's
a 100k, a 200k, and a 1uF capacitor in series. A single 300k could be
used. 1uF seems a bit large since this gives a cutoff frequency of .063
Hz. There are a lot of DC blocking capacitors that could be removed since
the entire circuit is biased to Vc.

The use of the LM3900
(https://www.ti.com/lit/ds/symlink/lm3900.pdf?ts=1602780585063 )
complicates things since it's not really an op amp. It has current inputs
instead of voltage inputs. It "looks for" balanced input currents instead
of balanced input voltages, so the bias resistors to Vc are required but
would not be if a true op amp were used.

Anyway, thanks for posting! Though a Teletype is complicated, I doubt
there are any extra parts in there!

Harold
"The ideal design has zero parts."
https://w6iwi.org




> ---------- Forwarded message ---------
> From: Ray Fantini <RAFANTINI at salisbury.edu>
> Date: Thu, Oct 15, 2020 at 10:11 AM
> Subject: [MMRCG] RTTY Articles
> To: MMRCG at groups.io <MMRCG at groups.io>, Military Radio Collectors
> Association <mrca at mailman.qth.net>
>
>
> Stumbled across the 1977 September issue of 73 Magazine while looking for
> ideas on a Baudot to ASCII converter project. Just thought I would mention
> it to the group being it was their RTTY issue and it has lots of useful
> information on running RTTY before everyone went the desktop software
> route.
>
> Stuff about building your own filters, demods and test generators and the
> like. Never did much reading of 73 back in the day but just looking at
> that
> one issue have to say it was way better then QST or CQ, at least from the
> project standpoint.
>
>
>
> https://archive.org/details/73-magazine-1977-09/mode/2up
>
>
>
> Ray F/KA3EKH
>
>
>
>
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