Cancellation of common mode currents due to differential or counter-directed signals

In the design of high speed printed circuit boards (PCBs), differential signals are preferred to distribute clock signals across the board. Differential signals are highly resistant to other signals or external fields, and do not cause strong radiation levels, due to the field cancellation resulting from the signal´s opposite polarity, Differential signals can also reduce the generation of common mode (CM) currents. This paper examines the potential cancellation of common mode currents due to the relative phase shift between signals on two parallel transmission lines on a PCB. The main parameters of interest are the relative phase shift itself and the trace separation. This case is expanded towards the more general one of the common mode cancellation in meandered traces, which are used as delay lines. In an intermediate step, the signal direction in one of the transmission lines is reversed in order to study the excited current distribution within the reference plane. From the conclusions of the numerically obtained results, algorithmic improvements are derived for the use within EMC-Expert systems.