I tried to figure out how to create a mod/ dmod FSK but came up with this while playing around. The second 555 chip (on the right) acts as a carrier and frequency modulator. Transformer seems be acting as... a filter.. with the help of the resistor connected to it and inverting input of the op amp. Behold the power of the op amp (at the top) configured into a schmitt trigger! This component can really take a chaotic signal and make sense of it... interesting.
Blue= the original signal before the modulating stage
Orange= frequency modulated signal
Red= filtered signal in the demodulating stage
Green= end result signal after schmitt trigger
