Modeling radiated emissions from cables attached to printed circuit boards driven by power bus noise

Abstract

This paper describes a two-step technique for modeling the radiated electromagnetic emissions from a printed circuit board with attached cables when the source of the emissions is noise between the board's power and ground planes. The first step calculates the electric fields at the edges of the power-ground plane pair. The second step replaces the power plane with equivalent sources embedded within the ground plane structure. Examples are provided that demonstrate the accuracy of this approach. Printed circuit board geometries present a significant challenge to 3-D numerical modeling tools. The size discrepancy between the small detailed noise source configurations on a typical circuit board and the relatively large enclosures or cables attached to these boards makes it difficult to model these structures efficiently. Among the most difficult source geometries to model are closely spaced power plane structures. 3-D modeling of the volume between a power plane pair requires small model elements with a maximum dimension comparable to the distance between the planes. If the same 3-D model is used to analyze the entire structure including an attached cable, much larger elements must be used to model the cable. Unfortunately, employing elements that vary in size by several orders of magnitude in the same model, can generate significant numerical error making it difficult or impossible to get useful results. This paper presents a two-step approach to modeling radiated emissions from printed circuit boards with cables or enclosures driven by power bus noise. The approach presented can greatly reduce the computational resources required to model boards with power planes and attached cables or enclosures. It also provides helpful insight regarding the physics of the power-bus-noise-to-cable coupling that can be useful when attempting to minimize this coupling. II. DESCRIPTION OF THE METHOD Typically, active components connected to the power planes induce voltages between the planes that may result in unacceptable levels of radiated emissions. Although the sources of this noise are on the board, the antennas are often larger metallic objects connected to the board such as cables or enclosures. The goal of the method described here is to model the electromagnetic coupling between the noise on the planes and these attached structures.