AWR eBooks

Best Practices for Efficient and Effective Planar EM Simulation 13 www.cadence.com/go/awr The grid spacing is set in the Enclosure menu that resides under the EM project being analyzed. The first tab of this menu is the Enclosure tab and has the Grid_X and Grid_Y settings, the values for which can be changed for the specific EM project being simulated. The default size for the grid setting is set in the LPF file—it is not set under Options > Layout Options under the Layout tab. That setting only works for normal schematic layout and artwork cell layout. The default can be changed in the LPF file by opening the text version of the file. At the bottom is a section with the title DEFAULT_EM_ENCLOSURE. The reason the default grid setting is controlled in the LPF is historical: EMSight, the first EM simulator offered by AWR Corporation. The designer can use the grid setting to set the minimum facet size. It can also be set independently of the grid. The settings controlling facet size are under the mesh tab of EM project options for the specific project being studied. The default settings for all new AWR Microwave Office EM projects can be changed under Options > Default EM Options and the Mesh tab. Once an EM project has been created, it can only be changed with local options. The Mesh Size section enables the designer to choose the Relative to Grid or Absolute Dimensions setting (Figure 14). Figure 14: In the first case (top), the mesh is too coarse so the edge mesh can' t be created, while in the second case (bottom), the mesh is better and an edge mesh is created The Relative to Grid setting uses the grid size as the minimum mesh, while the Absolute Dimensions setting specifies the minimum mesh size in length units. The advantage of the latter is that the designer can decouple the grid settings from mesh issues and the grid is then simply used as an aid in drawing. Note that all three EM products (AWR AXIEM, AWR Analyst™, and AWR EMSight simulators),use the grid differently. The AWR EMSight simulator always uses the grid size for the minimum facet size, while the AWR AXIEM simulator can either use the grid size or absolute numbers, and the AWR Analyst simulator only uses the grid for drawing purposes and has nothing to do with the size of the mesh. The minimum edge length is the smallest length of the side of the mesh element. As mentioned earlier, a good rule of thumb is to set this to one-tenth of a typical line width. The edge mesh of a line is set to this number. The edge mesh could not be created in the first case in Figure 14 because the minimum edge length was set to too large a number. The minimum feature length is the minimum size feature of the layout that will be kept; for example, a small bump in a layout will be ignored when meshing. The minimum feature length should be kept below the size of the minimum edge length. The maximum aspect ratio is the ratio of the largest to the smallest side of a facet. The setting is used to limit how far facets can deviate from squares and equilateral triangles. Long, skinny elements lead to matrices that are difficult to solve accurately (mathematically speaking, the matrix is becoming ill-conditioned). If mesh elements become too badly behaved, AWR AXIEM software can give high-aspect ratio-facet (HARF) errors or warnings and will not run. HARF errors can be difficult to correct because usually the geometry is complicated and changing it is not a HARF viable option. There are a few things the user can try. First, try merging the shapes together on a given layer by using the union Boolean operation. Second, turn off the path detection setting. This setting is in the Mesh tab of the Options menu and is a secondary setting under Mesh Features. Path detection looks at a layout and tries to make a rectangle inside of it, which can then be nicely meshed. However, path detection can force the mesher to create some poorly defined elements at its boundary with the rest of the shape.

• Cover