[moved] Explaination of Loop Filter for PLL

Hi Everyone,

I need help regarding loop filter in this PLL. Can anyone explain the importance of r1, c1, r2 and c2 in detail. Thanks.

Hi niteshtripathi,

If you apply an input current to the loop filter (current because the output of charge pump is current) and assume the voltage on the C2 as the output of the filter, then the filter will have a pole at the origin, a zero at z1=1/(r1 * c1) and another pole at p2 = 1/( (r1 + r2) * (c1 in parallel with c2)). I think based on the values you choose z1 must be smaller than p2. In this case, z1 can improve the phase margin of the closed-loop system and improve the stability. In addition to these poles and zeros, we will have another pole at origin from the VCO. To have a stable system while having two poles at origin (for the open loop system), it is necessary to add a zero to the system to prevent the open loop transfer function from having a phase shift of 180 degrees at the unity gain (this is achieved by adding r1, r2, and c1 in the above circuit).

Intuitively, the current source of the charge pump and a capacitor is an integrator (a pole at origin), but, by adding the resistors we destroy this integrator as the frequency rises and the impedance of the capacitor becomes comparable with that of the resistors.

This is what I can remember from years ago, but please verify them you yourself.