To measure psrr of ring vco, I've simulated the ring vco with pss and pxf.(the configuration of pss and pxf follows the guide in PSRR_Osc_AN.pdf). But there's error message such as
*Error * Evaluating expression (db(_drplModePxfVgain('SSB 0 'PM 1 '("/V0" "/VDDH" "/gnd!"))))
*Error* ("difference" 0 t nil ("*Error* difference: can't handle (nil - 0.0)"))
when I plot the result graph after the simulation.
How can I solve the problem?
That suggests that it is not able to access one of the simulation outputs. Without seeing the simulation setup, it's going to be rather hard to debug - it would probably be easiest for you to log a service request via Cadence Online Support so that an AE can take a look at this with you.