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, electro-magnetic interference (EMI) resulting from the no nnegligible impedance of the signal return at the connector may occur. This effective EMI coupling path results in the daughtercard being driven again st the motherboard and attached cables, resulting in common-mode radiation, The connector geometry can be modified to minimize the EMI coupling path w hen high frequencies are routed between the motherboard and daughtercard. C urrent speeds and printed circuit hoard (PCB) sizes result in geometries th at are of significant dimensions in terms of a wavelength at the upper freq uency end of the signal spectrum, The PCB geometries are then of sufficient electrical extent to be effective EMI antennas, The resonant lengths of th e EMI antennas may, however, be quite removed from the typical half-wavelen gth dipole resonances. The finite-difference time domain method can be used to numerically analyze the printed circuit-board geometries, determine ant enna resonances, and investigate EMI coupling paths. EMI resulting from the stacked-card configuration has been investigated experimentally and numeri cally to ascertain the EMI coupling path at the bus connector, and EMI ante nnas.