Differential Lines for Common-Mode Suppression Based in Hybrid Microstrip/CPW Technology

A symmetrical pair of differential microstrip lines implemented in hybrid microstrip/coplanar waveguide (CPW) technology is proposed. Transmission-line models are used to analyze differential- and common-mode responses, allowing efficient design with minimal optimization effort. The structure behaves as a conventional transmission line pair under differential-mode excitation, whereas asymmetrical coupled transmission line theory has to be applied to characterize common-mode operation. The common mode is strongly suppressed thanks to the introduction of a controllable transmission zero. A two-stage version of the structure is used to increase the common-mode rejection bandwidth. All the electrical parameters of the transmission lines have been obtained using an in-house fast quasi-TEM code. The good agreement between transmission-line models, full-wave simulations and measurements confirms the benefits of the structure and the design procedure.