The effect of the intrinsic impedances of logic families on the predicted radiated emissions from printed circuit boards

Analysis results on modeling the effect that source and load impedances have upon the predicted emissions from printed circuit boards are presented. It is shown that, while controlling circuit geometries is important, knowing what effect the impedances of the signal source and the load will have upon the radiated emissions and being able to control those effects are also important. It is demonstrated that these two factors can change the predicted emissions quite dramatically. In the case of short tracelengths, emissions can be controlled through circuit geometry quite acceptably, the distance to the ground plane being the circuit board geometric variable that gives the most control. Unfortunately, when the tracelengths approach a quarter wavelength of one of the higher harmonics of the fundamental frequency, circuit board geometry does not have as much of an effect. At longer tracelengths, one must consider controlling the source and load impedance in order to reduce unwanted emissions. In addition, when only real resistive loads and sources are considered, the emissions from the circuit may actually be 10-15 dB higher than was presumed