Abstract
Any product designed today that requires high speed clocks can be troubled with electromagnetic compatibility (EMC) compliance issues. This article outlines an electromagnetic (EM) field-oriented perspective for printed circuit board (PCB) design intended to help the reader pass electromagnetic interference (EMI) compliance testing the first time. The same techniques used for reducing EMI will also mitigate interference, suggesting a universal PCB layout philosophy. This article is presented in three parts. The first article covers the essential physics needed to understand why the techniques described within this article will mitigate EMI on a PCB layout. Part 2 will cover how the material in Part 1 plays into a practical board layout described through examples. Finally, Part 3 will discuss PCB layout strategies for complex boards that will satisfy the presented solutions.
Introduction
Why is it that compliance is often the last thing addressed during the design process? This is often the case because the entire system needs to be available before any of the actual testing can take place. Problems uncovered late in the design may require printed circuit board (PCB) changes, which are always expensive when many people are involved late in the product design cycle. This is unfortunate because electromagnetic compatibility (EMC) compliance (radiated, conducted, or susceptibility) can be confidently designed to pass simply by following the best practices described within this article. The methods described will help keep electromagnetic interference (EMI) compliance in mind during the architecture, design, and layout of PCBs. This article aims to do much more than demonstrate how to design a board to pass EMC testing. The goal is understanding the underlying principles and why they work so that the principles can be leveraged elsewhere.
The fundamentals behind EMI, interference, and susceptibility really are quite simple. Mitigating EMI is all about controlling and confining electric and magnetic fields. Simply put, if electromagnetic (EM) fields are exposed to free space and allowed to expand, interference and EMI will result. How is this field confinement accomplished? All that is needed is to design for zero NET accelerating charge everywhere on the circuit board (and in any connected cables). This means for each charge that is accelerating, there is another charge very close by that is permitted to accelerate in the opposite direction to the original charge. When these events occur together in the far field, the effects of each charge can and will cancel. Without field energy, there can be no EMI or interference. This will leave all EM field energy confined inside components or the PCB dielectric directly between the signal trace and the ground plane. This is the idea for traces on the boards but in truth, the electrically long antennas with unconfined fields encountered on isolated systems is the bigger problem. This topic will be discussed in a future article.
Physics of Static and Dynamic Forces on Charge
Charges move only when they are exposed to an electric field. The electric field at a given location in space can be static (such as the field from a stationary charge). The electric field at a location can also change with respect to time (such as the field from a moving charge). Once charges are moving, the magnetic field comes into existence. The magnetic field, like the electric field, can be static or can change with respect to time. We know from the Lorentz force, F = q (E + vxB) that a moving charge in a magnetic field will experience a force either toward or away from the moving charges generating the magnetic field. Since only an electric field can exert a force on charge, the magnetic field is really a relativistically generated electric field. It is a manifestation of the original coulomb field of the moving charges. The electric and magnetic fields are really one thing that can look different depending on your frame of reference.
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