I'm new in telecommunications and I want to understand how this Phase Shift Keying circuit work, I don't have a clear idea of how the opamp is helping to get the phase shift keying. for your info the first signal generator is giving a sine signal (5kHz/5V) and the second a square signal (1kHz/5V). I done this circuit and knew it works, but I want to know how and why, thanks.

 

I done this circuit and knew it works, but I want to know how and why, thanks.

When the transistor is off, the opamp has the same signal applied to both inputs, thus it operates as a gain-of-one follower.
When the transistor is on, shorting the plus input to ground, the opamp acts as an inverter with a gain of -1, thus changing the phase by 180 degrees.

 

Normally one would do this with a JFET or MOSFET as switch, because the
Bipolar Transistor, when on, has a Vcesat value being input to the NI input
of OpAmp, producing a DC error/offset. The jfet/mosfet, if sufficiently low Rdson,
will help mitigate this. Also the Vcesat implies a non zero Rsat so signal still being
input into OpAmp from XFG1 pin.

Regards, Dana.