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How Electromagnetic Fields Determine Impedance, Part 1

We talk a lot about trace dimensions and relationships when discussing signal integrity issues. In particular, we like to talk about how close traces are to underlying planes and how close traces are to each other. But there is another way to think about things that is sometimes much more practical and intuitive, and that is by thinking about the electromagnetic field around the trace(s) we are considering. The position and shape of the electromagnetic field can tell us a lot about trace impedance, EMI and crosstalk coupling, and signal propagation speed.

When a current flows down a conductor an electric field and a magnetic field radiates away from that conductor. Collectively, this is called the electromagnetic field. What is important to note is that this field always exists. Furthermore, the electromagnetic field and the current are inseparable. That is: (a) the electric field can’t move ahead of the magnetic field; (b) the magnetic field can’t move ahead of the electric field; and (c) neither field can get ahead of or fall behind the current itself. They all have to move together along the conductor.

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Editor's Note: This column originally appeared in the August 2013 issue of The PCB Design Magazine.