Many engineers do not understand how grounding is implemented in circuit simulation. As a result, electromagnetic (EM) simulators are often misusedwith erroneous results. An 11-page white paper from AWR titled "Understanding Grounding Concepts in EM Simulators" defines ground connections in EM simulators. It also shows how to correctly choose among various grounding options. For example, many modern simulators support "local grounding," whereby different ports can use different ground definitions.
The paper begins by explaining how grounding is achieved in circuit simulation. The author, Dr. John M. Dunn, devotes a lot of this discussion to the examination of where ground is defined in EM simulators and how this is a function of the solver and port types. All EM simulators must define ports to derive the S-parameters, whether the port is considered for incident, transmitted, or reflected energy. A given S-parameter is defined as the ratio of the reflected or transmitted power to the incident power. All ports are assumed to be perfectly matched to the system's characteristic impedance, which means any wave going into the port will not be re-reflected back into the system. Yet EM simulators take different approaches to carrying out this procedure. The various types of ports used in EM simulators are thus examined in terms of how ground is defined for those ports.
To understand how simulators define ports and their associated grounds, it is essential to understand the different types of simulators that are available. The white paper classifies them according to basic two-dimensional (2D) simulators for transmission lines, through threedimensional (3D) planar simulators for planar circuits, to full-wave 3D simulators for complete 3D simulations. Surfaces to be modeled are covered by meshes formed of geometric shapes with smaller elements providing finer resolution.
For example, 3D planar simulatorswhich also are known as 2 D simulatorsuse method of moments (MoM). The currents are solved on the conductors by meshing the lines into a series of triangles and rectangles. To get the current on each mesh, a matrix equation is solved. Ports are attached to excite the currents. The S-parameters can then be calculated. Although there are geometry restrictions, they do not pose a serious problem for many geometries of interest regarding boards, packages, and chips.
To show how circuit and EM simulators work with S-parameters, specific examples at the board, package, and chip level are discussed. The paper ends by providing circuit tricks that may aid ground studies, such as balanced ports and exposed ground nodes. To gain an understanding of EM-simulator grounding assumptions, the note emphasizes that the engineer must begin by looking at the ports. All ports have some type of grounding assumption. The document centers on the firm's AXIEM 2009 3D planar EM simulator, which enables the designer to specify where the ground reference is in a design.
AWR Corp., 1960 E. Grand Ave., Suite 430, El Segundo, CA 90245; (310) 726-3000, FAX: (310) 726-3005, Internet: www.awrcorp.com.
