Reduction of printed circuit board radiated emission

Common mode radiation caused by common mode currents has been analysed using transmission line and circuit theory. An important aspect is the derivation of the most important designable parameter, the asymmetry of a transmission line. This asymmetry has a direct impact on the common mode current amplitude. It is shown that common mode radiation can be reduced by creating a mechanically viewed, asymmetric, transmission line. This reduction can be predicted very accurately. Radiated emission measurements on several practical configurations, such as PCB transmission lines with large ground planes, shunt- and guard traces, striplines, gridded ground planes and ground planes with a slit, support the theoretically developed statements: the measurement results are in accordance with the theoretically obtained data