sigma delta modulator in fractional-n pll,need help

[jerryhuang]

sigma delta fractional n pll

I just learn how to use simulink to simulate a simple pll, I want to know how to model a fractional -N sigma delta pll with simulink,i think the point is to modeling
the sigma-delta modulator,may i use the s function to model it?how can i get some useful sample, thanks

Added after 3 hours 45 minutes:

i need to learn sigma-delta fractional-n pll,i have a question when I use the matlab to model a fourth-order sigma-delta modulator ,what spec should i konw
in the behaveal modeling,
i think it is not only the nosie,
can the archtecture reduce the lock time of the pll,
is it a very import spec ?