[GreenKeys] Simple PLL TU

[Harold Hallikainen]

On Tue, October 1, 2024 12:53 pm, Ken LoCasale wrote:
> Gang,
>
> I found a simple two IC terminal unit for anyone that might want to try
> mechanical rtty on a budget. I think I'll layout a board and give it a
> try. Has anyone else seen it before and given it a try?
>
> I put the .pdf here. https://qrpbuilder.com/simple-pll-terminal-unit
>
> ken - wa4mnt

I built a demodulator like that (based on the NE565) back then. I don't
think the 741 limiter gains you anything. It's pretty easy to just
saturate the NE565 input as a limiter. The datasheet (which includes a
very similar demodulator https://www.fabian.com.mt/viewer/44073/pdf.pdf )
says only 10 mV is required for the PLL to lock.

A nicer (obsolete) chip is the XR2211 (
https://assets.maxlinear.com/web/documents/xr2211av104.pdf ). I designed
several products around this chip. It includes the PLL of the NE555 along
with the data slicer (comparator typically wired as a schmitt trigger) and
a lock detector (mark hold). As shown at figure 11
https://assets.maxlinear.com/web/documents/xr2211av104.pdf#page=14 , the
lock detect and FSK demodulator outputs are open collector, so they can
just be tied together to get mark hold.

Harold
https://w6iwi.org


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