Dan Dvorscak
Ansys Employee

Typically yes. By using the defualt of 50 Ohms, you have a huge mismatch between the low impedance of the PDN (at lower frequencies where it is close to lossless) and the port impedance. One of the issues here is that doing so causes a loss of numerical precision in the resultant S-parameters So instead of magnitudes varying betwen values of 0 to 1, they will only go between 0 to ~0.001 (potentially). This is especailly traumatic when converting the S parameters to Z. Small errors in S can be made much worse by the lack of precisions. Then at low frequencies where the stucture is very low loss, those small errors in S can produce a huge variation in Z.