If you are doing the VCC noise transient simulation, you'd better set your time step as small as possible. For example, set option DELMAX=1ps in HSPICE, you could get a much better accuracy on jitter number. The bad thing is you got long CPU time.
If you integrate the phase noise from very low frequency, the jitter number would be big. Most often, the PLL will suppress the low frequency phase noise and then the jitter won't be large.