FDTD and experimental investigation of EMI from stacked-card PCB configurations

Stacked-card and modules-on-backplane printed circuit-board geometries are advantageous for conserving real-estate in many designs. Unfortunately, at high frequencies, electromagnetic magnetic interference (EMI) resulting from the nonnegligible impedance of the signal return at the connector may occur. This effective EMI coupling path results in the daughtercard being driven against the motherboard and attached cables, resulting in common-mode radiation. The connector geometry can be modified to minimize the EMI coupling path when high frequencies are routed between the motherboard and daughtercard. Current speeds and printed circuit board (PCB) sizes result in geometries that are of significant dimensions in terms of a wavelength at the upper frequency end of the signal spectrum. The PCB 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 half-wavelength dipole resonances. The finite difference time-domain method can be used to numerically analyze the printed circuit board geometries, determine antenna resonances, and investigate EMI coupling paths. EMI resulting from the stacked-card configuration has been investigated experimentally and numerically to ascertain the EMI coupling path at the bus connector, and EMI antennas.