I'm not sure either.
But i do remember that Ali Niknejad, a Professor at berkeley, said something about this during one of his
lectures last semester. He said sometimes , people think they are looking at the phase
noise, but its not acutally the phase noise, its the distortion due to the windowing effect of running the simulation for only a finite amount of time.
This effect probably is lessened if you run your simulation long enough?
You can watch his lectures here. The Label on the lectures and the actual content
may be slightly off.
https://webcast.berkeley.edu/courses/archive.php?seriesid=1906978242
We also did homeworks on calculating/simulation the Phase noise. We used Leeson's equation to calculate the phase noise.
**broken link removed**
look at HW#9 , problem 1. BONUS.
The solutions are also posted.
I think the TA used Spetre RF to do the phase noise simulation.
If you have lots of time on your hands, you can try to simulate the phase noise using your method and see if its close to what he gets?
Good Luck