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Electromagnetic Fields, Part 2: How They Impact Propagation Speed

In my August 2013 column, I suggested that thinking in terms of what the electromagnetic field looks like around our traces might offer significant insight into how our circuits might be performing. In that column, I pointed out that the electromagnetic field had more to do with trace impedance than the specific trace dimensions did. That is, a trace can be “scaled” without changing the impedance (or the shape of the field.) But if the field distribution changes, then the impedance will change.

In this column, I am going to make similar observations about signal propagation speed. Recall that electronic signals travel at the speed of light, or 186,282 miles per second. This equates to 11.8 inches/ns (or what we sometimes round off to a foot per nanosecond.) In any other material, the speed of light slows down. It slows down by the square root of the relative dielectric coefficient, Equation 1.

Equation 1

Consider the situation shown in Figure 1. This is derived from a HyperLynx simulation. Here we have a trace in a stripline environment, surrounded by a dielectric. If we assume the relative dielectric coefficient of the dielectric is 4.0, then the propagation speed of the signal will be 11.8/2 = 5.9 in/ns (we sometimes round this off to 6”/ns.) Note the electromagnetic field in this figure. It is completely contained within the dielectric between the two planes on either side of the trace.

Read the full column here.


Editor's Note: This column originally appear