Quantifying EMI resulting from finite-impedance reference planes

Abstract

Parasitic inductance in printed circuit board (PCB) geometries can detrimentally impact the electromagnetic interference (EMI) performance and signal integrity of high-speed digital designs. This paper identifies and quantifies the parameters that affect the inductance of some typical PCB geometries. Closedform expressions are provided for estimating the inductances of simple trace and ground plane configurations. Index Terms-Electromagnetic coupling, electromagnetic interference, inductance I. INTRODUCTION T HE parasitic inductance, capacitance, and resistance of traces, vias, and planes on a printed circuit board (PCB) are important at high frequencies for modeling electromagnetic interference (EMI) and susceptibility processes. These parasitics comprise the effective noise source mechanism and coupling path of an integrated circuit (IC) source to an unintentional "antenna," which can result in an EMI or susceptibility problem. Parasitic inductance in PCB geometries is often the most difficult parameter to quantify. The concepts of inductance and partial inductance play a key role in PCB modeling. The inductance of the signal path is an important parameter in high-speed signal integrity calculations. Delta-I noise modeling, crosstalk calculations, and common-mode noise-source identification all rely on good estimates of the inductance associated with traces, vias, and signal return paths on PCB's. Equivalent circuit models of EMI processes at the board level for geometries known to lead to problems that exceed regulatory limits are desirable at the design stage for estimating radiated emissions. A useful model includes an effective noise source, and the parasitics (inductance, capacitance, and resistance) that comprise the coupling path of the noise-source to the EMI antenna. This equivalent circuit can then be used together with a known or suspected EMI antenna and a fullwave solution of Maxwell's equations to estimate radiation.

Performance-Based EMC Design Using a Maximum Radiated Emissions Calculator

“…Common-mode currents as low as 5 μA induced on a 1-m cable can result in radiated fields that exceed the FCC Class B limit. The physics of this coupling was first described and quantified in [9,10], and refinements to the original model I/O coupling EMI calculator have been developed over the years [14,16,17,19,21]. Electric-field (or voltage-driven) coupling is illustrated in Fig.…”

Method of Edge Currents for Calculating Mutual External Inductance in a Microstrip Structure

“…Assume that the fringing magnetic flux that wraps the ground plane in a microstrip geometry is produced by current filaments ∆I 1 and ∆I 2 placed at the points y = ±w g /2, parallel to the signal current. These edge currents correspond to the total current in the "tails" distributed along |y| > w g /2 for the infinitely wide case [19]. They are found by integrating the corresponding surface current density as…”

“…Microstrip structures with comparatively narrow ground planes (GP) have various applications in modern high-speed electronic equipment, for example, to increase product assembly density, for interboard connector design, or for microstrip on-chip interconnects on silicon [3,14,15]. A major problem in most of the structures with narrow ground planes, or when signal traces come close to the edge of a printed circuit board is so-called "ground plane noise", which is actually a common-mode voltage, that appears on the reference plane due to fringing magnetic fields wrapping the plane [16][17][18][19][20][21]. This voltage drives unintentional "antennas" formed by parts of the electronic equipment, such as PCB reference planes, cables, and the conducting chassis that are connected to the reference plane of the microstrip structures.…”

“…At frequencies where the PCB structures must be modeled using distributed parameters, quantifying the inductive coupling mechanism is an important, but a difficult problem. Recently, there have been many publications aimed at this problem [16][17][18][19][20][21][22][23][24].…”

“…Van Horck considers the mutual inductance between two signal traces shifted from the center and placed on different sides of a ground plane [16], using Carson's approach for calculating the return current distribution over a wide ground plane [38]. For a ground plane of finite size, closed-form expressions are available only for the microstrip case with either a non-shifted strip [16][17][18][19][20], or a strip shifted to the very edge of the ground plane [16]. Some papers contain numerical or simplified analytical evaluations of the ground plane internal impedance of microstrip lines using the quasistatic current density distribution in the ground plane produced by a signal trace [16,18,22,39,40].…”