Hi,

I am trying to simulate a VCO's PSRR.
In the past, I used to run pxf to get the results.
But recently I found in the Cadence's AN "PSRR Characterization Using SpectreRF Autonomous Circuits",
it states the "pxf modulated" should be used, and in the direct plot form SSB to PM voltage gain is selected to plot the PSRR.

I ran the simulation and compared both results of pxf and pxf modulated.
The simulation results are shown below.

The green line is pxf modulated (SSB to PM) and the purple line is pxf.
I don't understand why the two results has so large difference  (more than 200dB)?
Could anybody tell me why there exists so huge difference and which one should be used?

Thanks.
YCY