Targeting EM Simulators Study Field Patterns

Electromagnetic simulators are powerful analysis tools that can be used as stand-alone programs or fully integrated with higher-level circuit and system software suites.

Increased reliance on electromagnetic (EM) simulation by high-frequency circuit designers was apparent by the number of EM simulator booths at the recent MTT-S Exhibition in Boston, MA. With the Sonnet Software (www.sonnetusa. com) booth visible from the entranceway, and through the scattered collection of EM simulation booths on the show flow, from dedicated shops like Zeland Software to full-service suppliers like Applied Wave Research and Agilent Technologies, it was clear that few designers using computer-aidedengineering (CAE) tools go far without their EM simulator of choice.

And the choice in EM simulators is currently greater than ever, with a staggering array of stand-alone programs and tools that seamlessly integrate into larger design suites. Basically, an EM simulator determines the way that current is distributed in the metal traces and surfaces for a given layout and how much EM coupling will occur and what energy will be radiated.

The software tools solve Maxwell's equations to find the electric and magnetic field patterns generated by current flow through a conductor or conductive structure, such as a waveguide or antenna, but do so using different algorithms and with different requirements in terms of computer power and processing time. When using an EM simulator, a substrate or a package is considered a three-dimensional (3D) object while a metal trace on a substrate is considered a two-dimensional (2D) object. In addition to providing libraries of circuit structures, and allowing operators to define their own structures, most EM simulators can import data from mechanical drawing programs.

The classic trade off in the choice of EM simulator is analysis power for processing speed. For example, a true 2D planar EM simulator based on the method of moments (MoM) can provide insight into the 2D behavior of current flow through a conductor with only moderate demands on computing power. For a more detailed study, a planar 3D EM simulator can show the current flow in three dimensions, albeit with increased demand for computer processing power and time.

The different approaches used in EM simulators are aimed at finding solutions for integral equations and partial differential equation. In a simulator based on the MoM approach, often known as the boundary element method (BEM), partial differential equations are approximated by integral equations and boundary conditions are applied to the integral equations to find solutions. In the finite-element method (FEM), the partial differential equations are approximated through numerical methods or eliminated as steady-state problems.

In the finite-difference-time-domain (FDTD) approach, time and space are divided into discrete segments. When many FDTD cells are combined together to form a three-dimensional shape, the result is an FDTD grid or mesh. In the FDTD approach, finite-difference equations representing the mesh are tackled by solving for the electric-field vector components in a volume of space for a given instant of time, and then for the magnetic-field vector components in a volume of space for a given instant of time, and then repeating the process.

In Sonnet Software's EM Suite of planar 3D EM software, analysis of a planar structure is assumed to take place within a sixsided box, with ideal ground reference and sidewalls composed of lossless metal. The approach helps solve for S-parameter data with dynamic range exceeding 100 dB. In addition, the use of the shielded box allows circuits to be studied outside of an enclosure, for example, as part of a system.

Momentum G2 is a second-generation 3D planar EM simulator from Agilent Technologies designed to work within the firm's Advanced Design System (ADS) suite of CAE software tools. Momentum, which is based on the MoM approach, works with a function called the Advanced Model Composer to create planar 3D models not found in simulation libraries. The EM simulator is written to provide multi-threading simulation on 64-b computers with as many as 16 processing cores. It can perform adaptive frequency sweeps to quickly find resonant frequencies.

As with the Sonnet approach AWR Corp. offers an EM simulator, EMSight, that assumes a circuit is enclosed within a rectangular conducting box with closed-boundary conditions. The simulator is part of the company's Microwave Office suite of design tools. The company also provides the AXIEM full-wave planar 3D EM simulator, which is designed to handle arbitrary shapes and assumes a circuit under analysis is in free space.

CST's Microwave Studio () is an integrated software suite with EM simulation capabilities. Using perfect boundary approximations, the 3D EM simulator is ideal for signal-integrity (SI) studies and analysis of electromagneticinterference (EMI) and radio-frequencyinterference (RFI) issues.

The full-wave MoM EM simulator IE3D from Zeland Software solves for current distribution on 3D and multilayer circuits, including RF integrated circuits (RFICs) and low-temperature-cofired-ceramic (LTCC) circuits. Multiphysics (Version 3.5a) from COMSOL is a suite of software modules for RF, acoustics, structural analysis, chemical engineering, and other areas. One of the longest-running EM simulators, HFSS from Ansoft, is now available in Version 11, which is designed for efficient utilization of computer memory. Widely used for 3D EM field simulation based on FEM analysis, HFSS provides accurate extraction of full-wave SPICE and S-parameter data for use in other circuit modeling software tools. Vector Fields offers a number of different versions of its finite-difference time-domain (FDTD) EM simulator Concerto.

Yet another 3D EM simulator based on the FDTD method, EMPIRE XCcel, employs adaptive onthe- fly code generation for the fast processing speed to analyze large structures, such as antennas. The software features an intuitive graphical user interface and support for a wide range of import and export data formats. Finally, XFdtd Version 7 from Remcom is a 3D EM simulator based on the FDTD approach that supports most operating systems, including Windows, Mac OS X, and Linux.

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