I'm designing a BPSK modulator. Don't mind the upper left circuit of the circuit, it just generates the [Oscillator] which will serve as the 500kHz sine wave carrier signal. It's already successful. The problem I have is the BPSK circuit highlighted in red.

Especially the lower half connected to OP1. It's an inverting comparator and the transistor is mostly used as a switch that toggles the inverter circuit by grounding OP2. As mentioned, the output of the whole thing is supposed to be a BPSK modulated signal ( [Vmessage] + [Oscillator] ).
It does bounce when the binary signal of the [Vmessage] at 50kHz goes up and down, but the signal is wonky. The resistor values may be the problem, though I have tried increasing and decreasing every single one and it still won't come out properly. Do you guys have any idea?

 

When the MOSFET is switched off, the drain voltage should be free to swing positive and negative but there is an intrinsic diode between drain and source.
You might get this to work using a JFET instaed of the MOSFET but the gate voltage should vary between zero and negative.

 

When the MOSFET is switched off, the drain voltage should be free to swing positive and negative but there is an intrinsic diode between drain and source.
You might get this to work using a JFET instaed of the MOSFET but the gate voltage should vary between zero and negative.

Hello! Thank you so much for the reply. I apologize if sound ignorant, but I am really quite new at this. The signal seems clearer, and the phase shifts at the rise and fall did become more defined after I replaced the MOSFET with a JFET, and I reversed the diode to vary between zero and negative. But it is still distorted at the high logic of the message. Might it perhaps be due to the op-amp? I am still a newbie student, you see, and I have no intuition how to choose the right components.



Any help will be much appreciated! Thanks.

 

I use bipolar transistors for this, with the emitter grounded and the base drive swinging both above and below ground. Audio muting transistors are available that have large emitter-base breakdown voltages and very low on resistance. See attachment.

 

Thank you very much! My circuit is working now, I really cannot stress how much this has helped me. Much appreciated!

 

But it is still distorted at the high logic of the message.

At that time the FET should be switched off so try this with the drain disconnected and make sure that you get a clean output signal.
[Edit] I see you have it solved so ignore this.