Imbalance control by open stub for reduction of common-mode conversion at differential transmission line bend

At a differential transmission line bend, a differential mode propagated in a differential transmission line is converted to a common mode due to asymmetry of the transmission line. The common mode conversion leads to problems with signal integrity. We propose a method to improve electric symmetry of the transmission line bend and thus reduce the common mode conversion. The self-capacitance and inductance of one line are different from those of another. In order to fit those parameters, the narrowing signal line for increasing self-inductance and placing a conductor shape which is like a stub are used to reduce the imbalance degree of the transmission line bend. We obtained a 20-dB reduction of common mode excitation by optimizing the signal line width and stub length. In addition, an equivalent circuit model for the transmission line bend is proposed to evaluate the reduction. The simulation results using the proposed equivalent circuit model are in good agreement with the results obtained by the 3D electromagnetic solver.