AWR eBooks

Best Practices for Efficient and Effective Planar EM Simulation 8 www.cadence.com/go/awr Best Practice Tip #3: Use parameters to control shapes in the drawing environment. Either shape modifiers, which work directly on the polygons, or parameterized cells (PCells), which are pre-defined shapes controlled with parameters, can be used. Designers often need to be able to modify their layout with parameters. Parameters are usually employed to set up multiple EM simulations, the results of which can be used as an EM model. There are two ways to modify shapes in layout: PCells and shape modifiers. All layout shapes in AWR Microwave Office software are polygons, which are composed of straight-line segments connected to each other at vertices. A polygon does not have any other properties other than it is on a draw layer, which is then (hopefully) mapped to the correct EM layer, as previously discussed. Shape modifiers can be used to change the shape of the polygon by changing the length of a side and moving the vertices, thereby changing the polygon shape. To use a modifier, select the polygon of interest, go to the Draw menu, and look for Parameterized Modifiers. A simple example is shown in Figure 7. The stub filter's length is changed by the shape modifier. The modifier is using the parameter named "stub_ length." Figure 7: A shape modifier is used to change the length of a stub Modifiers can also be used to move polygons and perform Boolean operations. When polygons are selected, permitted opera- tions can be selected in the Parameterized Modifiers menu; forbidden operations are grayed out. A common point of confusion is that modifiers work on entire draw layers, not individual shapes, except of course when the modifier changes the shape of an individual polygon. The advantage of using layers is that multiple shapes can be easily manipulated, for example, an array of vias. The designer moves an array of via pads on a given layer a specified distance and then subtracts them from a ground layer to create anti-pads (holes) in the ground layer. The disadvantage of using layers is that if an individual polygon is to be moved on a layer with other shapes, the designer must place it on a different layer, move it, and then put it back on the original layer. Shape modifiers are usually controlled with parameters, which are then swept over various values using swept variable blocks. The parameters can be used in equations if desired. Parameters and control blocks are placed on the project EM schematic page, which is viewed in the same way as a normal schematic, by clicking the schematic icon, selecting View Schematic from the View menu, or using a hot key. The term "EM schematic" can cause some confusion because it is not a normal schematic, where circuit simulations can be run. An EM schematic is used for parameters, control blocks, equations, and PCells. Figure 8 shows the EM schematic for the stub filter example in Figure 7. Figure 8: The EM schematic for the stub example in Figure 7, the circled region shows the parameter and swept variable block

• Cover