Digital wave simulation of lossy lines for multi-gigabit applications

Frequency domain Vector Fitting (VF) is a well-known technique to generate circuital models of a spatially discretized lossy transmission lines from theoretical formulation of losses. The subpicosecond time steps required by multi-gigahertz bandwidths and short transmission lines included in the models, determine long Spice simulation times. A 100X speedup can be gained using the Digital Wave Simulator (DWS) instead of Spice. DWS processes the waves of a Digital Network built up connecting together scattering blocks (circuit elements, nodes and S-parameter multi-ports) coming from a Spice-like description. Being a DSP wave processor instead of a classical nodal equations solver, DWS is computationally very fast and numerically stable. Comparisons with commercial simulators like Microcap11 and CST Cable Studio show a good matching of results. A further 10-100X simulation speedup is obtained if Piecewise-Linear Fitting (PWLF) is used to describe the time-domain behaviors of Scattering Parameters. Single or multiple cell Behavioral Time Models (BTM) can be extracted by PWLF from TDR/TDT measurements and processed by DWS fast convolution algorithms. A setup de-embedding can be performed by PWL breakpoints optimization to fit actual measurements. A RG58 coaxial cable is analyzed and its VF-derived eye-diagrams are compared to PWLF measurement-derived results. At multi-gigabit rates significant differences, due to cable physical implementation effects, are observed. The modeling/simulation alternatives (VF/Spice, VF/DWS and PWLF/DWS) are compared together and the advantages of PWLF/ DWS in term of simplicity, stability and speed are highlighted.