Aigneryu
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There could be some issues which depend on how the simulation is performed.
If your open-loop result is derived, the close-in phase noise can be very low. For example, the integer numbers are first obtained from a DSM, then use phase=2*pi*cumsum(inst_divisor-ave_divisor)/ave_divisor.
If a behavioral VCO is used to drive the DSM, and then the periods are extracted based on edge (or zero-crossing) detection, and the phases are then derived from those periods, the close-in phase noise will not have the ideal shape since numerical error due to the transient timing can introduce a 1/f^2 roll-off to the VCO phases which again drives the DSM.
Are you using spectre to run the simulation?
Sincerely, Shih-an
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