Electromagnetic (EM) simulation has long been an essential modeling tool for RF /microwave design. Before selecting a simulator, however, it helps to understand the differences among EM solvers.

On the simplest level, an EM simulator can be as basic as a cross-sectional analysis tool to derive parameters for a transmission line. For more complex structures, EM simulators can be differentiated by programs designed to analyze planar structures versus those designed for three-dimensional (3D) structures. The former offers an advantage of processing speed for such planar structures as microstrip and stripline circuits, while the latter is more accurate for 3D structures such as plated through holes (PTHs) in multilayer circuits or 3D circuits such as antennas and microelectromechanical systems (MEMS) switches.

All EM simulators solve Maxwell's equations for the electric and magnetic fields around a structure or surface, although they solve these equations in different ways. Among the various methods for solving Maxwell's equations are the finite-element (FE) method, the method of moments (MoM), the finite difference time domain (FDTD) method, and the finite integration technique (FIT) method. EM simulators based on MoM engines solve linear partial differential equations formulated as integral equations using boundary elements to define a surface. They solve only for the boundary values, rather than for the full space defined by the partial differential equations. The solvers typically form a mesh around the surface, breaking it into boundary elements that are generally represented by matrices. For large surfaces, these matrices are large, and require a great deal of processing power and/or processing time. MoM solvers generally perform calculations on fields in a homogeneous medium, such as a circuit board with uniform dielectric constant, rather than a multilayer assembly formed of different dielectric-constant materials.

Simulators using FDTD methods also solve Maxwell's equations in partial differential form, but modified to central-difference equations. These functions are solved in an iterative stepand- repeat method of calculating the electric field at one instant, then the magnetic field for the following instant, and then the electric field, and so forth. FE solvers transform a problem equivalent to the partial differential equations and integral equations representing Maxwell's equations by transforming it to a steady-state case, where the partial differential equations can be eliminated, or by transforming the partial differential equations into ordinary differential equations that can be solved with standard techniques. FIT solvers are based on a spatial discretization scheme to solve EM field problems numerically in the time and frequency domains. They apply Maxwell's equations in integral form to a set of staggered grids, with the capability of including arbitrary material properties such as changes in dielectric constant as part of a model.

The Sonnet Suites of software tools from Sonnet Software are planar EM simulators based on the MoM solution of Maxwell's equations, including parasitic, cross-coupling, enclosure and package resonance effects. The solutions can be used to develop high-frequency models based on S-, Y-, and Z-parameters as well as SPICE models for a wide range of antennas and planar circuits, including microstrip, stripline, coplanarwaveguide, and waveguide structures. A user provides a physical description of the circuit, including arbitrary layout and material properties for metal and dielectrics, and the software solves for the electric and magnetic fields for those circuits and structures.

Sonnet works closely with Computer Simulation Technology to support that firm's CST Microwave Studio suite of 3D EM simulation tools. CST's time-domain and frequency-domain EM solvers are based on an FIT approach for analysis of 3D structures at RF through optical wavelengths. The company's CST Design Studio allows co-simulation of designs using both EM and circuit simulators.

Long an industry standard for fullwave 3D EM simulation, HFSS is a FE-based solver with high accuracy in modeling on-chip components, PCB interconnects, IC packages, and other high-frequency structures. The latest version of the software is 12.1, with support for a variety of operating systems. It features new robust meshing technology and domain decomposition solver technology.

The IE3D 3D EM simulation software developed by Zeland Software was acquired by and is now supported by Mentor Graphic. The firm now offers several versions of the MoM-based simulator, including IE3D SI for engineers concerned with signal integrity (SI) issues and IE3D SSD, which is ideally suited for modeling RFICs and monolithic microwave integrated circuits (MMICs). As is the practice of many simulation software suppliers, Mentor is now offering a free evaluation version of the IE3D EM simulator on its web site.

Another change within the industry occurred when Cobham Technical Services acquired Vector Fields and the firm's FEbased Opera EM simulator. Designed for use on 32- and 64-b Windows and Linux computers, the simulator can model static EM fields, low-frequency time-varying fields, and high-frequency time-varying fields.

Agilent made major improvements in its Momentum MoM 3D planar EM simulator with its second-generation "G2" version of the software. The EM simulator is integrated with the firm's Advanced Design System (ADS) suite of simulation tools for codesign and co-simulation of advanced structures, such as integrated circuits (ICs) within a package and mounted on a PCB. Momentum G2 features parameterized passive model generation capability called Advanced Model Composer (AMC) for creating custom libraries of planar 3D models such as transitions, discontinuities or passive components not available in the standard simulation libraries. Momentum G2 supports multithreading simulation on 64-b computers with as many as 16 microprocessor cores.

Similarly, the AXIEM EM simulator from AWR is a planar 3D solver that provides openboundary solutions that can be readily integrated into the company's Microwave Office and Analog Office suites of simulation programs for performing co-simulations of circuits, ICs, and other high-frequency structures. AXIEM features a hybrid meshing technology that automatically selects triangular and rectangular elements for creating the most practical and accurate mesh around a surface to be studied. For those interested in a video comparing different EM simulators, the firm offers a six-part tutorial video lesson by Dr. John Dunn, "Introduction To EM" on its site at * www.awr.tv*.

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The XFdtd 3D EM simulation software package from Remcom is based on the FDTD method. The latest version of the software, Version 7 (XF7), includes XACT Accurate Cell Technology to resolve intricate structures with the fewest possible computational resources. In addition, the company recently announced a new version of its XGtd ray-based EM simulation software for analyzing the far-field effects and electromagneticinterference (EMI) and electromagnetic compatibility (EMC) of large structures. The software employs a new optimized ray engine (ORE) and 64-b processing to achieve high accuracy with reduced processing time.

In some cases, more than one solver is used in an EM simulator. The Wave Wizard software from Mician employs the mode-matching (MM) technique for achieving high accuracy with fast processing speed. Although the software also contains a 3D FE solver, it approaches most problems as structures that can be analyzed by means of the MM method. The EM simulator is ideal for analyzing passive waveguide components and antennas.

The COMSOL Multiphysics EM simulator from COMSOL is a FE-based solver that is available in various modules, for example, for working on RF circuits, on MEMS designs, for AC/DC circuit design, and for heat-transfer studies, as might be conducted in high-power design evaluations. It provides results as S-parameters and in other formats for creation of circuit and electrical structural models.

Some EM simulators are designed to do only one or two things, but do them well. The VeloceWired software from Helic, for example, was developed to model bond wires. It calculates the parasitic inductances for a wide range of bondwire profiles.