Common-mode-current generation caused by difference of unbalance of transmission lines on a printed circuit board with narrow ground pattern

This paper explains a mechanism of common-mode generation on a printed circuit board with a narrow ground pattern. A transmission line has its value of degree of unbalance. At a connection point of two transmission lines having different degrees of unbalance, common mode voltage is generated proportional to the difference, and it drives common mode current. The authors propose a method to evaluate common mode current distribution and verify it by measurement. Although calculated common mode current is larger than measured one by a few dBs, both of them are proportional to the degree of unbalance. An EMI reduction technique, 'unbalance matching,' is also proposed.