Modeling the imperfect ground of printed circuit boards based on TEM assumption

The radiation emission in far zones from printed circuit boards (PCBs) is o btained by treating lines on PCBs as transmission lines and calculating the far-field emission due to current distribution on lines. In this paper, we present a more precise circuit model, based on TEM assumption, to decompos e the total current into differential-mode current and common-mode current. This circuit model is based on transmission line model, but it considers t he effect of ground trace. The finite size ground trade can be viewed as an inductive reactance. A knowledge of the net inductance of the ground trace can aid in the analysis and investigation of PCBs emission. We show the de rived equations of the modified transmission lines for the geometrics of pr actical interest. As time-varying current passes through such ground trace, a voltage drop due to the inductance of the trace will act as a source of the common-mode current. Furthermore, charge stored in capacitance between signal and ground traces will cause the current pulses returning to their s ource. The magnitudes of currents are slightly unequal in the signal and gr ound traces, which can cause common-mode current to flow. An unbalanced cir cuit on a PCB constructed with signal and ground trace pairs will radiate a s an asymmetric folded-dipole. By antenna theory, the contribution of diffe rential-mode and common-mode currents to radiated emission of PCBs can be c alculated. In addition, comparisons between experimental results and calcul ation results are also given.