SPICE doesn't inherently understand the differences between and impacts of differential and common modes. If a pair of edge-coupled traces turns a corner, the trace on the outside edge will travel farther than the one on the inside edge. This causes a conversion of some of the differential energy into common-mode energy. This can be partially modeled by adding a section to the transmission line at the point in the model where the bend takes place, but this is only an approximation. In the real physical world, the subsequent trace will radiate energy much worse than did the section where the signals were matched. SPICE will not model this effect. On the other hand, the procedure of adding a section of transmission line for the corner is valid in that the SPICE simulation will likely accurately model the increased crosstalk caused by the added common mode. So it is recommended that this procedure is used.

Recall that a short segment of transmission line often has a disproportionate impact on simulation time. Where segments are short, as this would be, it is much preferable to model the segment as an equivalent L-C section. The equations to do this were presented earlier and will be again later. In this short segment, you can ignore loss.

With any instance where an edge-coupled differential pair is to make a bend, add a small segment of trace to the inside trace to account for the phase shift caused by the bend.