Behavior Model for RFPA

Hi Guys

Im curious to learn how does behavior model for RFPA is extracted from the PA. Currently im having tough time getting the simulation models for RFPA. Would appreciate if any one could suggest a way to extract the behavior model for RFPA

Thanks

X-parameters?

I ever heard X-parameters from X-VNA provided by Agilent, but I have no chance to use that equipment yet so far.