Estimation of common mode current on coaxial cable with twisted wire pair

In this study, common mode current, a dominant factor of unintentional radiation, on coaxial cable with twisted wire pair (TWP) is estimated. This cable model is a simplified model of a popular combination of balanced and unbalanced circuit, such as printed circuit board (PCB) with TWP. For the convenience of calculation and measurement the coaxial cable is employed as an unbalanced circuit instead of PCB. The estimation method is based on the imbalance theory which focuses on the difference of balance factors between connected circuits, and some effects of TWP are considered. According to the theory the common mode current is proportional to a differential mode voltage at the connecting point. Thus the differential mode voltage is calculated by cable transmission theory with accounting on TWP electrical characteristic and radiation impedance of cables. The estimated result is compared with measurement, and good agreement is obtained.