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High frequency signals on printed circuit board (PCB) traces can couple to I/O nets that carry the coupled energy away from the board resulting in significant radiated emissions. Automated tools to detect unacceptable levels of coupling during the board layout rely on fast methods for estimating the amount of coupling and the resulting radiated emissions. A modeling technique is proposed to speed up the analysis of PCBs with coupled traces that induce common-mode currents on attached cables. Based on the concept of imbalance difference, differential-mode sources are converted to equivalent common-mode sources that drive the attached cable and the PCB reference plane. A closed-form expression based on the imbalance difference model is developed to estimate the maximum radiated emissions due to I/O line coupling in PCBs.