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The Importance of EM Analysis for RF/ Microwave Design

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The EXTRACT block itself controls how hierarchy is handled. This is especially important if the designer has separate EXTRACT blocks at lower levels of the design, which might be used to develop a self-contained model for a particularly complex subcircuit, such as a filter or coupler. Early in the design flow, it may not be desirable to consider the filter or coupler in an EXTRACT block of the metal connecting the filter to other parts of the circuit. However, in later stages of the design, it may be essential to do just that. By using the hierarchy parameter on a lower level block, the EXTRACT flow can be instructed to include the block in higher-level EXTRACT EM documents or to generate its own EM document. There is no problem with having more than one EXTRACT block on the schematic for distinct EM solvers that do not couple. If input and output matching networks are electromagnetically isolated, they can be partitioned into two EXTRACT blocks. The circuit elements and layout structures in the input matching network would be enabled to go to one EXTRACT block and the output matching network circuit elements and layout structures would go to another EXTRACT block. It is also possible to freeze an EM document created by an EXTRACT flow and continue to use the result, which is a powerful feature that allows EM structures to be "frozen" and worked on separately from the design as a whole. The EM solver can be optimized accordingly to the need for greater speed or accuracy from the simulation, or different EM solvers can be explored, all as a separate problem from the overall circuit and then reintegrate without having to re-enter data, move DXF or GDSII files, or cut-and-paste. This freeze option associated with a EXTRACT block is also useful for performing a separate study of the EM structure created by the EXTRACT flow— like creating a reusable, standalone part. Real-World MMIC Example Perhaps the best way to understand and appreciate the elegance of the EXTRACT flow is through a real-world design. Designing a MMIC distributed amplifier requires a delicate balance between the cascaded input and output transmission lines for each section of the amplifier. Isolated, it is a simple task to obtain the correct input and output for each stage. When laid out within the confines of a space-constrained, cost-driven MMIC, any assumptions of the stages being electromagnetically independent must be revisited. The EXTRACT flow within an optimization loop is an ideal way to get closure on this critical aspect of the distributed amplifier design. This EXTRACT flow is one of the standard AWR Microwave Office ® examples and takes advantage of several key features in the software. Figure 7 shows the initial distributed amplifier design. The extracted EM document is seen in the upper left pane and the ACE analysis engine was chosen in lieu of traditional EM solvers so that the couplings considered in the analysis can be easily controlled to optimize the design. The arms of the input and output stages are modeled by separate instances of the same hierarchical, parameterized subcircuit. The subcircuits along with additional feed lines comprised of MLINs, routable MTRACEs, and iNets, can be combined together in a single EXTRACT flow to gather all of the interconnect couplings critical to this distributed amplifier's performance. Figure 7: The EXTRACT flow creates an EM document for ACE analysis The Importance of EM Analysis for RF/Microwave Design 5 www.cadence.com/go/awr

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