Gonzalo J. CarracedoMurphy's law strikes again. As I was preparing the evidence for my post at @kicad 's forums, I finally got the simulation of my simple parametric amplifier **working**. This is not a final design, but a proof of concept of the phenomenon of parametric amplification happening inside a tank circuit. I consider this a huge success! Now the tricky part: turn this circuit into something I can actually use (also I got a circulator!)
Now, how is this supposed to work: if I leave this circuit with the pump disabled (PUMP = 2V), this behaves as a simple tank circuit with resonance frequency at 1.42 GHz. Delivering a pulse to the tank makes it go back an forth a few times at 1.42 GHz until it gets dissipated mostly by R1.
This configuration is called a "degenerate parametric amplifier" because the pump frequency is twice the signal frequency. The interesting aspect of this circuit is that the gain is strongly dependent of the phase of the pump frequency. In fact, at 0º phase we do not see much amplification. Neither at 180º nor at 270º
Nonetheless, at 90º phase, the peaks of the pump match the peaks and valleys of the resonant signal. This little kick increases the voltage of the varactor. In general, increasing the reverse voltage of the varactor reduces its capacitance. Since V = Q/C, and at the peak and valleys I = 0 (making Q a conserved quantity), reducing C means increasing V, making this resonant back-and-forth stay for way longer before fading away.
Note that, in all these time plots, I am windowing and filtering via FFT the contribution of the pump signal to make the effect in the 1.42 GHz signal clearer. This also explains the weird artifacts you may see in the beginning and the end of the time plots.