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Slit Effect of Ground Patterns on Conducted Noise Currents from Printed Circuit Board

Electromagnetic disturbances for vehicle-mounted radios can be caused mainly by conducted noise currents flowing out from vehicle-mounted multi-layer printed circuit boards (PCBs) to wire-harnesses. Using actual electronic equipment for vehicles, we previously showed that combining, not separating, the ground layer patterns of a digital circuit and an analog circuit is more advantageous to reduce the noise current outflow, while the mechanism remains unknown. In the present study, based on an idea that this mechanism should generate from a change in the amount of common return currents flowing to the ground, we measured noise current outflows from five types of simple PCBs having different ground patterns, which were validated from simulation with the method of moment. As a result, we could confirm that slits on a ground pattern allow conducted noise currents to flow out from PCBs, while the levels for the symmetric two slits ground types are smaller compared to the case for the asymmetric two slits ground types. The mechanism could be explained qualitatively from balanced conditions of an equivalent bridge circuit.