Precautions in the calculation of impedance in FDTD computations

The finite-difference time-domain (FDTD) method has been widely used in the modeling of microwave integrated circuits and interconnects in electronics packaging. One of the most important properties computed for a transmission line is its characteristic impedance, which is often used in the extraction of the equivalent circuit model of an interconnection discontinuity. The calculation of the characteristic impedance from FDTD numerical results seems to be trivial it can simply be obtained by the ratio of the Fourier transformed voltage and the Fourier transformed current. However, it is shown that, if the characteristic impedance is not calculated properly, significant amount of error can be introduced in its numerical solution.