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Best Practices for Efficient and Effective Planar EM Simulation 10 www.cadence.com/go/awr Best Practice Tip #4: Use geometry simplification rules to simplify the layout before it is meshed. Complicated polygons are simplified, and unnecessary layout details are removed, resulting in a faster simulation. Geometry simplification rules can be used to simplify a layout before it is meshed by the AWR AXIEM simulator. A smaller mesh count results in faster simulations and less memory use. Additionally, a simpler geometry can result in a better-behaved mesh, in which there are fewer high-aspect ratio elements. Long, skinny triangles and rectangles can create a poorly behaved matrix, which becomes difficult to solve accurately. (Mathematically speaking, the matrix becomes ill-conditioned.) Simplification rules are written down in the Rules tab of the STACKUP block and these rules are then applied to any EM project creating using the STACKUP block. Most PDKs have several rules defined, which can be turned off by checking the Enable Rules box at the bottom of the menu. Simplification rules are commonly used to "clean up" a geometry, for example, removing small shapes and features and snapping shapes together. Figure 10 shows an example of a small notch being removed. The rule being used is DECIMATE_ MIN_EDGE. Figure 10: Original layout with small notches exaggerated for clarity (left) and small notches removed after shape simplification (right) Rules are frequently used to reduce the number of sides of a circle, most often for vias. A circle in AWR Microwave Office software is typically drawn as 36 sides, as set in the layout options. This can result in a large number of facets, because of the large number of sides of the polygon. The simplification rules can reduce the number of sides, as shown in Figure 11. The S-parameters are usually not affected by the simplified vias, and the simulation runs much faster with the smaller number of facets. It is possible to have different rules for ground-power plane vias and signal vias. The accurate meshing of the signal vias can be more critical to the simulation results than that of the grounding vias. In Figure 11, the signal via is simplified to eight sides and the ground vias are simplified to four sides. The rules determine if a shape is a ground plane by the setting MAX_TRACE_ WIDTH. If the width of a polygon is greater than this distance, it is assumed to be a power net. (Width is defined by looking at the longest side and measuring the distance to the opposite side.) Any via on a layer defined by RESHAPE_GROUND_VIA_ CIRCLES_DIVS will be simplified using the ground via rules. Vias on signal nets will be simplified by RESHAPE_TRACE_VIA_ CIRCLES_DIVS. Figure 11: (from left to right) 2D layout view, 3D layout view from the bottom of the board of a via, simplified 2D and 3D layouts

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