Analysis of Induced Common-Mode Voltage for Four-Wire Shielded Cable Considering Cable Imbalance

Four-wire shielded cables are used in USB (Universal Serial Bus) systems. According to the increase of their transmission speed, the communication signals become the source of radiated disturbance in the VHF (Very High Frequency) band. This paper investigates the analytical model considering the imbalance of four-wire shielded cables to study the radiation mechanism. The CM (Common Mode) voltage generated at the signal line was calculated using a chain parameter matrix and compared with the measured value. As a result, the difference in conductor radius between the signal line and the power line and the twist of the wire did not significantly affect the CM voltage for the cables with twisted quad type. On the other hand, the cable with twisted pair type needed the model considering the twist of wires. Finally, we found that the unexpected imbalance of the cable that does not appear in the cross-sectional structure can be approximately modeled by adding a capacitance between one wire of the signal lines and the shield. These results indicate that the cable model for the CM voltage can be improved by considering wire twists and unexpected imbalances.