Hello,

I have this circuit that I got from a book which I simulated and works great. UIA creates a 1KHz signal and UIB creates a 455 KHz approx. U2A is the modulator which the output is an open drain (ground) when ON but I don't get how this modulates.

Thanks

 

The 1kHz signal is your data signal and the other is your carrier. The open drain outputs allow each of the signals to affect the output. So the output will be pulled low whenever either the carrier or data signal are logic 1. It is an OR operation of sorts, selecting between allowing the output to float or pulling the output to ground.

 

I don't get it either. In fact, I can't even see the circuit diagram.