PLL极点

确定环路的稳定性，看看信号与系统的书吧，书上讲得很明白

closed-loop stability issue, phase error, application-specified considerations, ... so on and so forth ...

PLL是离散系统，其pole zero跟普通的连续系统有一定区别

PLL is a discrete system
but when fref>=10*loop-bandwidth
It can be approximated as a continuous time system
All the stability criteriaused in continuous-time can be applied

But if you design a system fref~ 2 or 3 *loop-bandwidth , you should use discrete-time system analysis method

the saying "pll is discrete time system" is not quite accurate.only when a digital PD is used, it is a discrete time system.

typeII PLL原点有2极点而不振荡，所以这样看来还是跟连续系统有区别。