> Does anyone have a link to a schematic of the first modems based
> apparently on
> LM555s ? Or anything similar ? Old RTTY modems, including valved ones, are
> not
> quite in this class although they come close.
>
> thanks,
>
>   Peter

Well, for the modulate side, as others have pointed out, you can FM an
astable by varying the voltage on pin 5. You could generate FSK that way.
You could also do it with the 566 VCO, but I like the XR2206 best. It's
similar to the 566 but has a sine output and two timing resistors. You
select between them with a logic level on one pin, so you key that to get
FSK. The XR2206 can also do linear FM and has a balanced modulator, so you
can also do AM and DSBSC.

For demodulation, I imagine you could use the 555 as a monostable. On an
edge of the incoming FSK, trip the monostable. Connect it to the clock
input of a D flipflop. Connect the D input to the FSK. On the trailing
edge of the monostable, the D will sample the FSK. If the period (or half
period, depending on timing) is shorter than the monostable timeout, the Q
output will be in one state. If longer, it will be in the other.

My first FSK demod was vacuum tube based with 88mH telephone loading
coils. I then moved to the NE565. I ended up on the XR2211, which is like
an NE567, but has an FM output and an FSK comparator. Works nice!

Harold



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