In PLL loop Filter Design , What is the role of VCOcap ?
This capacitor may influence the frequency of the last pole of the loop filter. So if the design recommends 560 pF capacitance in the final stage, then if the VCO capacitance is 120 pF, this value should be subtracted from the final capacitance, to give a recommended cap value of 440 pF.
For ADI VCO's, this tuning port capacitance is subtracted automatically from the final cap value.
That is ok. Somewhere , I have read , capacitor that is in shunt with Tuning pin should be at least 3 to 4 times higher than VCO input capacitance. Otherwise it may deteriorate phase noise performance of PLL.
Because VCO input capacitance varies across the frequency . So , keeping high value of shunt capacitor in loop filter , equivalent capacitance will be less affected.
I have read somewhere , shunt capacitor ( loop filter ) should be 3 to 4 times higher than the VCO input capacitance. Otherwise , phase noise performance will be deteriorated.
VCO input capacitance is varied as frequency changes . So keeping high shunt capacitance, equivalent capacitance will be less affected.