Sorry I bring this topic back. I finally got IC615+MMSIM11.1 which is the latest version my server manager can get from his current license. Now I have the "lorentzian" option to use, and got the phase noise and jitter plot as shown in the attachment. The red curve, related to the left Y axis, is plotted from "directly plot" -> "pnoise" -> "phase noise", which flattens the "non physical" data in low offset frequency. The green curve, related to the right Y axis, is plotted from "directly plot" -> "pnoise jitter" -> "phase noise", and it seems the "lorentzian" method is disabled here. Then I get the same issue on the jitter calculation (jc). For only one cycle, the jitter results from different start integration frequency is not much. But with a long term, the jitter calculation gives very different results.
Please let me know if I need to submit a support request. It takes time since I have to go through my server manager for the account. Thanks a lot for your time.