Common mode current characteristics in transmission line with common mode choke represented by two-port chain matrix

Abstract

For common mode noise current reduction, a common mode choke (CMC) is widely used in a transmission line. However, the characteristics of common mode noise current reduction have not been clearly analyzed yet in the case where a CMC(s) is inserted in a balanced transmission line. In this paper, a common mode current calculation method using a simplified equivalent circuit of a CMC, of which two-port constants are known, is proposed. We examined common mode current characteristics of a transmission line with CMC(s) and losses of the CMC in differential mode and in common mode with the method. It was proved by the experiments that the proposed calculation method can be applied for the calculations of (1) the common mode currents in a pair cable, and the effect on the common mode currents by insertion of a CMC, and (2) losses of the CMC in differential and common modes up to around 30 MHz.