Transient PLL spur simulation setup

Hi, Officers, 

I have a question on how to accurate estimate the spur at PLL output by using transient simulation with DFT function in SpectreRF. 

For example, if the spur is located at 100 kHz offset from the PLL output with the power (or variation) of 1 ps, what would be proper 

setup for time step in transient simulation ? 

Do we need to set the time step to be smaller than 1 ps to extract this spur accurately in the simulation ? 

Sincerely, 

Ruixin