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Design007-Jan2019

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JANUARY 2019 I DESIGN007 MAGAZINE 83 power flow through the holes and currents flow on the opposite side of the plane (Figures 3 and 4). It can cause crosstalk if another trace gets into that area; this is yet another example of a possible violation of electromagnetic field localization [5] . However, unlike with the via holes, this loss of localization effect can be easily simulated. For example, two traces are simulated with the results shown in Figure 8. The traces are located exactly on top of each other. The worst- case scenario is with the traces right over the cutouts. It gives the maximal exposure through the holes, and as a result, maximal coupling. One can observe very significant far-end crosstalk (FEXT) (blue line on the left graph in Figure 8). This crosstalk can kill the signal with the main spectral harmonic around 8–10 GHz in this particular case. The insertion loss goes sharply down as shown on the right graph in Figure 8 (red curve), which means that the signal harmonics will not get through 10 cm of this interconnect! Shifting the traces off of the cutouts helps, but not much. With the losses and geometry of interconnects used in this example, the insertion loss null is shifted to about 17 GHz and became wider too. The near-end crosstalk (NEXT) is not significant in this case. This example shows how important to model all possible signal degradation factors. Looking at just attenuation (Figure 6) or the insertion loss in a line segment (Figure 7) may be way too optimistic in this case if the meshed reference plane is used as the common reference for traces on opposite sides. That scenario would be a disaster for signals with 8 Gbps and higher data rates! Conclusion In this article, a 3D electromagnetic analysis of traces over a meshed plane is used to illustrate the following: • Traces with meshed reference planes are periodic structures and have to be simulated as the periodic structure with a 3D EM solver • Meshed reference planes have a significant effect on the electromagnetic fields, power flow, and return current distribution • Meshed planes significantly change the characteristic impedance as well as the reflection and insertion losses • Cutouts may cause significant crosstalk as well as EMI/EMC issues Similar observations are valid for a differential case [4] . In addition to the effects observed in the single-ended case, one can expect differential- to-common-mode conversion if two traces of a differential pair are not aligned with the cutout in exactly the same way (can be considered as a type of crosstalk). All of that must be simulated to design predictable interconnects. The Simbeor 3DTF solver based on Trefftz Figure 7: Insertion loss (left) and reflection loss (right) for 10-cm traces over a solid and meshed plane.

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