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Brooks' Bits: Electromagnetic Fields, Part 3 - How They Impact Coupling

In Part 1 and Part 2 of this series, I talked about how helpful it can be to recognize what the electromagnetic field looks like around a conductor or trace and how that field may change as we change the stackup or trace parameters. Visualizing how the electromagnetic field changes can give us insights as to how parameters like impedance and propagation speed might be influenced by the changes in the stackup or trace parameters. In Part 3, we will look at how changes in the electromagnetic field relate to changes in coupling between traces (crosstalk) or between a trace and the outside world (EMI).

(Special note: The figures in these three parts are largely taken from my new video lecture series just published by Prentice Hall, PCB Signal Integrity LiveLessons, and in particular from Lesson 1.4. Learn more about these LiveLessons at www.ultracad.com or at the Informit website.)

Let’s begin the analysis by looking at Figure 1. It shows the electromagnetic field around an 8 mil microstrip trace that is 50 mils above the plane. It is separated from other traces by 50 mils. Note how the electromagnetic field extends way out into the air and over to other traces. This poses a significant possibility of EMI problems.

Figure 1: 8 mil trace, 50 mil height, 50 mil separation.

On the other hand, look at Figure 2. This shows the same three traces, but this time they are only 5 mils above the underlying plane. Now a substantial portion of the electromagnetic field is “captured” between the trace and the plane. This significantly reduces the potential for EMI radiation off the board. Simply by looking at the electromagnetic field we can intuitively see that this configuration will perform better from an EMI standpoint.

Figure 2: 8 mil trace, 5 mil height, 50 mil separation.Read the full collumn here.Editor's Note: This column originally appeared in the April 2014 issue of The PCB Design Magazine.