Investigation of the effectiveness of DC power bus interplane capacitance in reducing radiated EMI from multi-layer PCBs

Much investigation has been carried out into the area of intrinsic board capacitance of power distribution systems on multi-layer printed circuit boards (PCBs). While its benefits are broadly understood, very little tangible evidence exists as to whether or not it provides an effective solution for combatting radiated EMI in complex state-of-the-art computer systems. Therefore, the EMI characteristics of a densely packed high speed computer motherboard involving clock and bus speeds up to 200 MHz with signal rise times in the order of 400 pico-seconds was analyzed extensively. Actual measured comparisons of radiated emissions and power bus transfer impedance are presented for the original motherboard and for the case of the motherboard with additional power and ground planes included.