A study on differential mode to common mode conversion due to asymmetric structure in differential transmission line

Recently, differential transmission lines which have low electromagnetic radiation performance are widely used for high speed digital interconnections. Though high electrical balance is required to obtain that performance, degradation of the balance may often occur from asymmetry of the structure, such as additional patterns and electrical components connected to one or both of single lines of differential transmission line. In that case, some part of differential-mode signal is converted to common-mode signal, which may often increase electromagnetic radiation from the differential transmission line. It is therefore important to reduce the differential mode to common mode conversion ratio (S_cd21) by keeping the electrical balance in the design stage in order to provide low EMI equipments. In this paper, we present an equivalent circuit model to evaluate the mode conversion due to the asymmetry of structure. The additionally connected patterns and components are modeled with a shunt capacitor connected between the single line and the ground. Then, S_cd21 can be expressed as a simple function of the frequency and the capacitance difference between each single line of the differential transmission line, and is independent on the position of the capacitors. The proposed model is verified by the comparison of calculated S_cd21 with the experimental results, and is applicable in low EMI design of differential transmission lines.