I will reply how to do in spectre.
First, you need to model PFD+CP. You can use VCCS source.
The gain of PFD+CP is Icp/2/pi
Second the VCO is modeled by Kvco/S. You can use a capacitor to model them.
Third, you can model divider. It is only a gain with 1/N
Fourth, you can model the loop filter using real R and C.
Then, you can connect them in an open-loop mode and apply frequency sweep.
You can get what you want.
If you connect them in a closed loop mode, you can know the overshoot when you apply a step input.
Yibin.