hi,
  can any body please tell me how to simulate dead zone. I used to find like by sweeping phase difference between feedback and ref clock near to zero and see how different from its ideal current vs phase plot? But how to find exactly?

Thanks,
Rajasekhar.

If the dead zone applies to a phase/frequency detector plus a charge-pump:

Use for UP and DOWN pulse a slight different frequency. Start the faster pulse a litle later. So you can simulate with one transient all phase differences in discrete steps. For more steps you need a smaller frequency difference.

Connect the charge-pump to a fixed voltage in mid of the target output voltage range. In the postprocessing integrate the output current and measure the charge increase for every phase difference. Now you have the charge over phase curve. The dead zone is the area where the gain is less than in other ranges. A pure gain of zero (true deadzone) is rare.