Issue link: https://iconnect007.uberflip.com/i/1071356
78 DESIGN007 MAGAZINE I JANUARY 2019 two dimensions, but for the wave propagating along the trace, it is essentially a one- dimensional periodic structure (it becomes 2D for leaky-mode investigation). To extract the per-unit-length and modal parameters of such a structure, 3D electromagnetic analysis is needed due to the non-TEM structure of the waves propagating along the traces. Most of the transmission lines in PCB or packaging interconnects have so-called quasi- TEM waves with the electric and magnetic fields mostly perpendicular or transverse to the propagation direction. Parameters of such transmission lines can be accurately approximated with the analysis of a single cross-section in a 2D field solver. With the cutouts in the reference plane, the waves become non-TEM and not even quasi-TEM due to the presence of the longitudinal components in the electric and magnetic fields. The electric field above the cutouts is shown in Figure 2. You can clearly see the non-transverse nature of the electric field in the areas of angles in the reference conductors. Each cutout is a discontinuity that distorts the electric and magnetic fields. As a result, Figure 1: A trace over a meshed plane as a periodic structure. The geometry of the two cells is required to extract modal and per-unit-length parameters. Figure 2: Peak values of electric field intensity in the plane with a Z-coordinate of 35 µm (right above the cutouts).