its look like your loop is unstable.
PFD frequency is 20MHz? C-R-C filter is used? If yes, which value of R?
Really VCO gain is 1.7GHz/V ?? Its so much.
Ayyanar M ,
Try to check your cadence results whith a program for calculating the loop parameters (ADSimpll from Analog devices), or EasyPLL Loop Filter Design Tool from national semiconductor.
AlexVD
---------- Post added at 13:04 ---------- Previous post was at 12:28 ----------
Ayyanar M , you can use simpll program from **broken link removed**