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46 The PCB Design Magazine • April 2014 In the Part 1 and Part 2 of this series [1] , I talked about how helpful it can be to recog- nize 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 electromagnet- ic 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 trac- es (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 se- ries just published by Prentice Hall, PCB Signal Integrity LiveLessons, and in particular from Les- son 1.4. Learn more about these LiveLessons at 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. Douglas Brooks ulTrACAD Design BROOkS' BITS Electromagnetic Fields, Part 3: How They Impact Coupling column Figure 1: 8 mil trace, 50 mil height, 50 mil separation.

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