Effect of skew and rise time-fall time asymmetry on PCB emissions

Common mode current is a well-known source of PCB emissions. One source of common mode current in PCBs is asymmetries in the ground return path. Integrated circuits employed in PCBs can also cause common mode currents. Asymmetries in the IC design can lead to mismatches between the rise time and fall time of the signals. Additionally intra differential pair skew can also exist thereby adding to common mode current. In this paper, contribution to common mode voltage from rise time-fall time asymmetry and skew is discussed with focus on the first four harmonic frequencies. Ideal sources such as pulse and PRBS15 signals operating at 10.3125Gbps are used. The study is then extended to a PCB operating at 10.3125Gbps to understand the effect of skew and rise time-fall time asymmetry on PCB 3m emissions.