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Brooks' Bits: 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 appeared in the September 2013 issue of The PCB Design Magazine.