[GreenKeys] ITTY comments and demodulator

[Harold Hallikainen]

Thanks for posting the schematic! Lots of hardware!

The UART section remnds me of a design I did back in 1980 (
https://bh.hallikainen.org/uploads/HaroldHallikainen/20130416125339_Part1.pdf).
As noted on that schematic, the design originally used the General
Instrument AY5-1013 UART which required +5V and -12V, so there is a charge
pump to generate the -12V using an NE555. Later production used the
Intersil  6402 UART which only required +5V. Note that U5 generates the
Reset Data Available pulse based on the incoming Data Available signal.
This also provides a strobe to latches that hold the data out of the UART
on the display board (
https://bh.hallikainen.org/uploads/HaroldHallikainen/Harold_009.pdf ).

Note that I used the XR2211 as the PLL FSK demodulator, while your circuit
uses the NE565. The XR2211 has the data comparator within it and also has
a quadrature detector that is used for carrier sense for mark hold. The
data comparator typically has some positive feedback (making it a Shcmitt
trigger). I did make a demodulator using the NE565 that drove a Teletype
model 15. I had no hysteresis on the data comparator and could hear the
selector magnets sing as tone leaded through (actully two times tone out
of the phase detector). On the transmit side, the XR2206 was a great chip.
It could generate a square wave, triangle wave, or sine wave. It directly
supported FSK. It also supported linear FM and double sideband suppressed
carrier AM. The DSBSC mixer (analog multiplier) was used in a subcarrier
demodulator (subcarriers between 26 kHz and 110 kHz) at
https://bh.hallikainen.org/uploads/HfDrc190.pdf#page=298 . The XR2206
oscillator acted as a local oscillator that was sent to the analog
multiplier along with the incoming subcarrier. The incoming subcarrier was
upconverted to 455 khz where it went through an IF amplifier with a
ceramic filter, then a ceramic discriminator to demodulate the audio.

By the way, those schematics (which are pretty hard to read) were
generated by the Dasoft CAD software that ran under CP/M. It did schematic
capture and PCB layout.

The NE565, XR2206, and XR2211 were all amazing chips, but are now no
longer manufactured.

Today, I'd be tempted to do this all in one chip (a microcontroller) like
in the https://w6iwi.org/rtty/DspTU3/ , but that may be cheating.

Harold
https://w6iwi.org



On Thu, September 24, 2026 1:19 am, Dale Hughes via GreenKeys wrote:
> Hello folk,
>
>
> Greetings from VK1 (Canberra, Australia). I've been printing some of the
> ITTY streaming news which has been fun and interesting. Many thanks to
> those people who provide the excellent and useful service.
>
> Previously I printed the 45 baud version of the news using my old RTTY
> modem with a Kleinschmidt TT4C machine, however I have now updated the
> system so I can use my Teletype Model 28RO which otherwise wasn't getting
> a lot of use.
>
> The new demodulator uses a PLL device and includes a UART to regenerate
> and upconvert the baud rate of the demodulated characters, so I can now
> easily print the 45, 50 and 75 baud services to a single machine running
> at 75 baud. The PLL can be easily tuned to the various tones that are used
> using a front panel control. Attached is the schematic of the new unit, it
> is easy to build and it works very well. I have written up the design
> which includes the various source documents like application notes and
> datasheets and I am happy to send the file to anyone who is interested.
> It's about 12
> Megabytes so I haven't attached it to this post.
>
>
> Regards
> Dale VK1DSH
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