Reduction effect of ground patterns on conductive noise currents from printed circuit board

Conductive noise currents flowing out from vehicle-mounted electronic equipment composed of multi-layer printed circuit boards (PCBs) to wire-harnesses form a major disturbance source for vehicle-mounted radios. Using actual vehicle-mounted electronic equipment, we previously showed that, to reduce the noise current outflow of this type, combining, not separating, the ground layer patterns of a digital circuit and an analog circuit is more advantageous, while the mechanism remains unknown. In the present study, based on an idea that this mechanism generates from a change in the amount of common-mode return currents flowing to the ground, we simulated noise current outflows with the method of moment from three types of simple PCBs having different ground patterns, which were validated from measurement of scattering parameters. Furthermore, we could confirm that slits on a ground pattern allow conductive noise currents to flow out from PCBs.