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Design >> Analog Design >> Poles and zeros in a PLL- Clarification
https://designers-guide.org/forum/YaBB.pl?num=1440492929

Message started by Rosh on Aug 25^th, 2015, 1:55am

Title: Poles and zeros in a PLL- Clarification
Post by Rosh on Aug 25^th, 2015, 1:55am

Hi,

The VCO has a pole at the origin.
The huge capacitor as part of the RC Low pass filter contributes a second pole very close to the origin.
These two poles give a 180 phase shift and that along with the feedback loop( feedback loop adds 180 phase shift) gives a total of 360 phase shift and the PLL becomes a postive feedback system.

Is my understanding correct so far?

To correct this, a zero is added to the PLL and this adds a +90 phase shift causing the total phase shift to now change to 270 which is stable.

However the zero (which is basically a resistor) can cause tiny ripples to disturb the VCO and this is solved using another capacitor (now the third pole) in parallel with the RC.

Since this capacitor is also big it will contribute a pole at the origin, won't it cancel out the effect of the resistor and bring the total phase shift back to 360?