[SOLVED] BPSK Design with transistors

[Minchuu]

Hello! 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] ). The Vmessage is 50kHz, by the way.

I have previously used the MOSFET 2N7002 in lieu of the JFET, but sadly the result was more distorted than the signal above. By using the JFET ( and reversing the diode), the phase shifts are more clearer during the rise and fall of the Vmessage. Bu weirdly enough, the BPSK signal is still distorted when the message is high. I have tried varying the resistor values and it didn't change much.

Do you guys have any idea? I really have no intuition in choosing the kinds of components to use since I am still a new student at this so any help will be much appreciated. Thank you!

 

[FvM]

The circuit topology is o.k., component values are not. Unclear how you arrived at e.g. 200 ohm for R10. It will probably cause OP1 current limiting.

You have all necessary informations at your fingertips, just look at the node waveforms.

--- Updated May 14, 2021 ---

Just a quick try with LTspice

TL07x/08x has only 4 MHz GBW and is a bit slow to process 500 kHz carrier exactly.