Evaluation of Imbalance Component and EM Radiation Generated by an Asymmetrical Differential-Paired Lines Structure

Recently, differential-signaling (DS) techniques such as low-voltage differential-signaling (LVDS) have been widely used in digital electronics devices in order to suppress electromagnetic interference (EMI). But in practical terms, a complete topologically and structurally symmetrical differential line is impossible. In this paper, we newly attempt to quantify the imbalance component and electromagnetic (EM) radiation when the structure and topology change from a symmetrical to an asymmetrical differential paired lines. Four different differential-paired lines structures are prepared for the comparison: PCB1 is a basic symmetrical structure taken as an “ideally balanced” case, PCB2 is an asymmetrical structure due to differences in bend and length, PCB3 is a symmetrical length structure with a bend region, and PCB4 is an asymmetrical topology with equi-distance and bend routing. Firstly, the differential voltages and mixed-mode scattering parameters are selected as a measure. The conversion parameter from differential-mode (balance component) to common-mode (imbalance component), Scd21, is dramatically increased by the difference of the length. Even if the differential paired lines have a bend-region, equi-distance routing can suppress Scd21. Secondly, spatial distributions of near magnetic fields are measured at certain resonant and out of resonant frequencies of Scd21. Although the differential paired lines are excited by the differential-mode, propagated magnetic field component at the end terminal of the differential paired lines at the resonant frequencies of Scd21 could be changed to the common-mode. Thirdly, the far-electric fields at 3 m are measured and calculated. Even if equi-distance routing is suitable for the improvement of signal integrity (SI) issues, it is not enough for the suppression of the far-field potential radiation. It is clear that Scd21 is one evaluator but it is not sufficient for predicting the EM radiation completely. The facts shown in this study suggest the basic characteristics of EM radiation from practical differential paired lines with asymmetrical structure and some significant problems in the design of a meander delay line for high-speed clock distribution.