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) testing the first time. The same techniques used for reducing EMI will mitigate interference, suggesting a universal PCB layout philosophy. This article is presented in three sections. This second article covers several PCB interconnect examples illustrating exactly how to implement the techniques presented in Part 1. Finally, Part 3 will discuss PCB layout strategies for complex boards that will satisfy the presented solutions.
Introduction
Part 1 covered all the essential physics needed to understand why we need to use transmission lines to confine field energy in our layout to manage electromagnetic interference (EMI). It ended with a short list of common printed circuit board (PCB) layout challenges that need to be addressed, repeated below.
On the PCB, field confinement is commonly lost when
- Signals transition between layers
- Signals share the same volume over a common ground plane
- Signals cross one another over a shared ground plane
- Signals are run in parallel
- Field fringing occurs
- Signals propagate down a microstrip or other imperfect transmission line
This second article addresses these issues with two examples representing the most common sources of PCB radiation overlooked by even experienced engineers.
The Transmission Line Can Radiate
Consider a logic gate about to raise its output voltage. Assuming that the IC’s decoupling capacitor is close by, where is the stored energy that is about to be used? It is locally stored in the dielectric (the space) inside the decoupling capacitor. Now imagine the IC raising its output voltage by moving charge to the output pin previously at ground. The charge that was moved has a field that will immediately reach out at the speed of light and exert a force on any charge that it encounters. Fortunately, the ground plane will be the closest source of charge and, being a good conductor, it takes very little energy to move a canceling charge directly under the trace. The electric force ensures that this canceling charge is as close as possible to the original charge provided by the logic gate. From this point on, for distant locations outside this small dipole, the field will be nearly zero and this becomes truer the farther in time and distance a location gets from these two canceling charges. For the fields generated from the accelerating charges, there will be a displacement current in the decoupling capacitor as well as in the dielectric between the output trace and the ground plane where the voltage transition is taking place. There are accelerating charges throughout the entirety of this electrically small loop (some provided by that changing electric field, which is a current). When the size of a current loop is small compared to the distance from which it is observed, the accelerating segments of the loop cancel each other out. As a result, the net accelerating charge appears to be zero from a distant vantage point.
The relativistic field described in Faraday’s law is critical to understanding EMI. All charges possess a coulomb field but when charges are moving (relatively speaking) and accelerating, there are two additional electric fields that contribute to the total electric field. Motion and acceleration twist the coulomb field to be the sum of three components. When charges move relative to others, special relativity compresses space just enough so that normally electrically neutral circuits develop a net charge. Mathematically, this is called magnetism, but a real electric field is developed due to this relative motion. The third and last electric field is caused from the acceleration of charges and is directed transverse to the original acceleration (but opposite direction). The energy contained within these two additional fields is different from what is stored in the coulomb field. Both the magnetic field as well as the transverse E field are relativistic. This means that the stored energy involved can be situational. It is only real from a dimensionally orthogonal perspective, and this has the interesting consequence of removing a coordinate of space. Where the coulomb field energy is stored in three dimensions of space, this transverse field’s energy is present and stored in two dimensions of space. For Faraday’s law, this means that the line integral of E around any closed path will be nonzero when this transverse field is present (when charges are accelerating). It also means that the energy attenuates less aggressively with distance than the coulomb field, spreading with the surface area rather than the volume.
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