Experimental and numerical investigations of fundamental radiation mechanisms in PCB designs with attached cables

Stacked-card and modules-on-backplane printed circuit board geometries are advantageous for conserving real-estate in many designs. Unfortunately, at high frequencies, current-driven noise sources may develop at the connector. The connector may effectively drive the daughter-card against the motherboard and attached cables, resulting in common-mode radiation. The connector geometry can be modified to reduce the level of the effective noise-source when high frequencies are routed between the mother-board and daughter-card. Current speeds and PCB board sizes result in geometries that are of significant dimensions in terms of wavelength at the upper frequency end of the signal spectrum. Geometries are then of sufficient electrical extent to be effective EMI antennas. The resonant lengths of the EMI antennas may, however, be quite removed from the typical dipole resonances of half-wavelength intervals. The finite-difference time-domain method can be used to numerically analyze the printed circuit-board geometries, determine antenna resonances, and investigate EMI noise source mechanisms.