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The Importance of EM Analysis for RF/ Microwave Design

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Overview Modern RF/microwave design flows make extensive use of EM analysis in many ways, and its co-existence and concurrency with circuit design and analysis cannot be undervalued. Prior to the circuit design and especially in larger designs, EM tools are used to create model library parts such as inductors, transitions, and antennas. While these parts are fairly self-contained, they must ultimately be integrated into the overall design, where, at the very least, they must be connected to the rest of the circuit or, in a more complex design, be coupled to it. During both the early and later stages of a design, engineers will switch from circuit-based models to EM analysis of critical interconnects to better understand couplings and achieve greater accuracy. EM analysis is used again before the design goes to manufacturing, so that the metal in the design can be analyzed one more time to verify circuit performance alongside design rule check (DRC), layout versus schematic (LVS), and even design for manufacturability (DFM). An overriding issue common to all these applications is that at some level the EM solver must interact with other software tools. The raw materials that are input to the solver are geometries or structures but are no more than a layout with electrical material properties for all the layers. At the other end of the process, the EM solver produces S-parameters or some other linear model representation. Some solvers can output SPICE netlists directly, which must be integrated with the rest of the design in a circuit simulator. Achieving all of this successfully has historically been a less-than-seamless chore of duplicating geometries or exporting and importing structures from the design or layout tool to the solver. The results must then be imported into the circuit simulator. While much effort has gone into automation of moving GDSII or DXF files from layout tools to the solver, most of these tools lack real-time integration and have a "batch mode" feel, as though things were "auto- magically" going on behind the scenes. There has also been little help in integrating the results back into the schematic, especially when using the solver in a verification mode. Dozens if not hundreds of ports are involved in final verification, and it is often left to the designer to put together a schematic that properly reattaches each port in the solver's results to the proper pins of the remaining components. This must be performed without error, and quickly, in order to get the design to market quickly. How the EXTRACT Flow Works The EXTRACT flow starts and ends with the schematic (Figure 1). When a simulation is requested by any means, even through tuning and optimization, the EXTRACT flow finds the elements in the schematic and on the layout that has been assigned to a particular EXTRACT block, and then copies the layout representation of those elements to the EM document specified on the EXTRACT block. The EM document is set up according to the data on the EXTRACT block: type of ports, frequencies for the enclosure, dimensions and gridding, which solver to use, and any other options required of the EM solver. After the EM solver runs, the circuit simulator is notified and automatically uses the solver results in place of the models associated with the schematic elements tied to the EXTRACT block. If the simulation was initiated from within an optimization loop or by sliding a tuner control, the entire EXTRACT flow will be re-executed while any changes to the elements associated with the EXTRACT flow are also under the control of the optimization or tuning process. If the elements associated with the EXTRACT block are not changed by tuning or optimization, then no changes to the EM document will be made and the EM solver will not be rerun. Figure 1: Matching network schematic and layout with the EXTRACT flow disabled The Importance of EM Analysis for RF/Microwave Design 2

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