Analytical solution of the electromagnetic radiation from coupled differential microstrip pairs

Nowadays differential traces as interconnects are extensively used since the electromagnetic radiation can be ideally cancelled by the out-of-phase and same amplitude signals on the two tightly coupled differential traces. However, the ability of differential pair in mitigating EM interference (EMI) may be seriously jeopardized by possible unbalance of the excitation, since both the common mode and the differential mode current may introduced to the differential pair. In this work, based on the mode decomposition technique and the far-field Green´s function, an analytical solution of the radiated electric field from coupled traces is formulated by taking both the common and differential-mode currents into consideration. Thus the radiated electric field from any arbitrary excitation can be easily calculated. The validity of the analytical formulation is verified by comparing the calculated results with the full-wave simulation results.