Estimating Maximum Radiated Emissions From Printed Circuit Boards With an Attached Cable

The common-mode current induced on cables attached to printed circuit boards can be a significant source of radiated emissions. Previous studies have shown that coupling from electric and magnetic field sources on circuit boards can be effectively modeled by placing equivalent voltage sources between the board and the cable. The amplitude of these equivalent sources can be estimated by using closed-form equations; however, estimates of the radiated emissions from these board-cable geometries have required full-wave simulations, and full-wave simulation results depend on the exact cable length and placement, which are not normally fixed during radiated emissions testing. This paper develops a closed-form equation to estimate the maximum radiated fields from a voltage source driving a board relative to an attached cable over a ground plane. This equation is evaluated for various cable and board geometries by comparing the calculated results to full-wave simulations. The maximum radiation calculated by using the closed-form expression generally predicts the peak full-wave simulation results within a few decibels for various board sizes and cable lengths.

I. Introduction

The common-mode current induced on cables attached to printed circuit boards (PCBs) can be a significant source of unintentional radiated emissions [1]–[3]. The signal traces, integrated circuits (ICs), and heatsinks on a PCB produce electric and magnetic fields that couple to these cables inducing common-mode currents. The maximum radiated emissions from a typical PCB with attached cables due to common-mode current can be numerically calculated using a full-wave simulation. However, this type of simulation requires extensive computational resources and does not calculate the maximum emissions, but rather the emissions expected from the precise configuration modeled. Alternately, expert system algorithms for estimating maximum radiated emissions from PCBs employ closed-form calculations that anticipate the maximum possible radiation [4], [5]. In this way, sources and coupling mechanisms that cannot contribute significantly to radiated emissions can be systematically eliminated, and attention can be focused on the features of a given design that may be the source of electromagnetic interference (EMI) problems.