Thanks, i see what you mean , you are thinking of coupling the signal to the powerline via a blocking capacitor. I have seen that before too. Also, AYK, you get transformer coupling. However, the schem i have in fornt of me has none of that...it just has a BCX56 SOT89 NPN shorting the rail.....(24v rail)...well, its not shorting as its got a 75R in series with it.
But i guess due to the powerline L, that will give a bit of a dip in the rail which will be detected by the detector?
Have you seen that type of FSK system before?
I cannot see any blocking cap or signal transformer here.
The schem i have is on multiple pages with cross references going off all over the place but i deffo dont see any C or Transformer.