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Best Practices for Efficient and Effective Planar EM Simulation 9 www.cadence.com/go/awr The parameter "stub_length" is being swept over five values with the sweep block, and the S-parameter file will contain all five simulations. Technically, a collection of S-parameter results in one file is called a MDIF file; when used in a schematic, the designer can choose one of the values. There are two problems that can make shape modifiers difficult to use for complicated layouts. First, there is no concept of two polygons are connected. It is therefore easy for shapes to be disconnected from one another. For example, the stub in Figure 7 can become detached from the line if the wrong end is held fixed. Second, modifiers are executed in a certain order. If the order of operations is changed, a different layout can result. Moving shapes and then subtracting them from another layer will result in a different layout than subtracting them from each other first and then moving the resulting layer. The order of operations can be changed if necessary. Normally, the operations that work on a polygon shape are executed first and then the layers with shapes are moved. Finally, Boolean operations are carried out. The designer can also use PCells to modify a layout with parameters. PCells have two advantages over shape modifiers. First, parameters are used for a PCell layout: For example, Figure 9 shows PCells being used to make the stub filter shown in Figure 7. The cells for the straight lines are MLIN elements, which are used for microstrip lines above a ground plane. Notice that only the layout is being used here: Fundamentally, the layout of an MLIN is a rectangle. The EM schematic is not using the MLIN model in a circuit simulation, rather, it is using the PCell layout to draw a rectangle. The length and width of the rectangle are controlled by two parameters in the EM schematic. Notice the stub uses the same MLIN PCell, which would be incorrect if the designer was running a circuit simulation. The stub should use a MLEF model, which includes the capacitance of the open end of the stub. But, for layout purposes, the MLIN has the same layout as a rectangle. The length of the stub is being controlled by a parameter "stub_length," which is being swept. The second advantage of using PCells is that if they are connected to other PCells, they will stay connected as the layout is changed. Figure 9 shows connectivity lines between various parts of the filter in the EM layout. The layout will snap back together before it is run in the AWR AXIEM simulator. The elements need to be connected in the EM schematic for the layout to be connected. Figure 9: PCells are used in the EM schematic to make the same stub filter layout as in Figure 8 The use of PCells is similar in concept to EM extraction in AWR Microwave Office software. In EM extraction, elements are selected in a normal schematic with the use of an extraction block. The layout of these elements is then sent to the AWR AXIEM simulator. There are differences, however: in extraction, after the simulator finishes, the results are placed back into the schematic and a circuit simulation is run using the new S-parameter results for the selected layout. The graphs are then updated automatically. There is no concept of models and circuit simulation when using an EM schematic and PCells. In extraction, the AWR AXIEM simulator runs synchronously and the AWR Microwave Office project is locked during the simulation. This is to protect designers from using an out-of-date EM result when running their circuit simulation. When the AWR AXIEM simulator is run in standalone mode, as in the case of using an EM schematic, it runs asynchronously, and the software is not locked. The EM simulation can be carried out on another machine using remote simulation if so desired. (See Best Practice Tip #9 for a more detailed discussion of remote simulation.)

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