PLL中关于环路滤波器的问题

djallonchina

如上图所示PLL中的LP，在忽略PFD和CP的非理想因素下，电容C2的作用是什么？对CP的功能有什么影响？因为前段时间被问到这个问题，感觉没有什么思路，请大家多多指导！

wfcawy

It is second order of lpf with C2,and C2 can significantly reduce the high freq ripples on the control,usually 1/10 of C1.
Be noted ,C2 here is not for stability reason of the whole pll.

djallonchina

回复 2# wfcawy


    ripple考虑了CP的非理想因素。如果不考虑这些非理想因素，单纯从CP的功能出发，这个C2起什么作用呢？

wfcawy

Ripple comes from harmonics of current pulse, no matter how ideal the cp is, it always has ripple.

djallonchina

回复 4# wfcawy


   除了可以减小ripple，对CP的功能有什么影响？

hezudao

你算算pole zero还有transient

djallonchina

回复 6# hezudao


   不是很明白，是算CP的零极点还是LP的？麻烦详细说明一下。

wfcawy

回复 5# djallonchina


    I see nothing effect on the behavior of CP, but you should know the  discharging / charging current pulse is discrete, and your loop transfer function is based on the average current, so the higher order harmonics of reference is neglected, but in real case it always there, so you need another capacitor to reduce the corresponding time domain ripples.

from the transfer function, if there is no C2, only a zero(1/R1C1) ,2 poles (0Hz) in the open loop gain which means the amplitude frequency response firstly goes down at a slope of -40dB/dec, and becomes -20dB/dec @ zero , and keep the same all the times. -20dB/dec is usually not enough for filtering.

djallonchina

我明白你的意思。C2是否对CP过载、VCO过载有影响呢？

buckaroo

这是最基本的问题
看看清华李宇根的PLL讲义
CP电流充到R和C1上，电压会突变！所以需要加一个C2来防止大的电压突变。因为周期性的突变电压会产生reference spur.