Extension of the asymptotic waveform evaluation technique with the method of characteristics

Methods for simulating distributed element~uch as systems of coupled, lossy transmission lines-with the Asymptotic Waveform Evaluation (AWE) technique have been described in [2/ , [I3/ . One problem with these methods is that they use complex exponentials to approximate pure delays in the time responses of transmission lines. The approximation can lead to spurious ringing in the simulation results. In this paper, we describe how this problem can be overcome within the context of a SPICE-based simulator. The generalized method of characteristics [4] is used to derive an equivalent circuit model for a system of coupled lines. This allows the pure delay to befactored out; AWE can then be used to approximate the characteristic admittance, as well as the propagation functions of the lines. The AWE approximation can be simulated efficiently within a modijied version of SPICE. Examples are presented to demonstrate that this technique is both accurate, while being faster than other existing methods.