iip3 simulation in cadence

Normally I can get IP3 from PSS or PSS/PAC simulation in IC4.46, QPSS or QPSS/QPAC simulation in IC5.0. When I designed LNA before I can easily get a clear 3:1 slope. But when I use cadence to run the IP3 simulation over one single device, I can not get the same 3:1 slope no matter how I changed the tolerance parameters. I did not use matching network in my simulation as in LNA design, only for the device itself. Anybody met the similar problem before on IP3 simulation? Any suggestions? Thanks!

Did you set the small-signal power level (or amplitude level) of PAC? It's located in PORT instance's characteristic menu.

You said you did not get a 3:1 relation, so what kind of the curve did you see?

Yes, I already set PAC magnitude(dBm).

Not a smooth curve, the slope is less than 3:1.

Maybe you can try a lower power level. For example, set input power level to -40 dbm if the estimated IIP3 = -10 dbm.

I already tried that, still did not work.