How much phase margin is needed for a fractional-N pll?

45 degrees is the bare minimum, and 60 degrees is the goal.

Thanks. Then in gerenal, how much the reality will be worser than the siulated result?

Actually , The frantional-N designis the same as general PLL. Just You must care spurious tone that you can accepted . Nornal the Loop- bandwith is lower for filter the spurious tone.