Design of a Differential Coupled-Line Directional Coupler Using a Double-Side Coplanar Waveguide Structure With Common-Signal Suppression

In this work, a rigorous approach for the design of a differential coupled-line directional coupler (DCLDC) implemented in double-side coplanar-waveguide (CPW) technology is presented. Due to the double symmetry of the structure, it is possible to obtain the coupling level and port characteristic impedances of the coupler in a simple and efficient way. In addition, due to the broadside coupling of the involved differential CPWlines, a wide range of coupling levels can be covered. Design curves relating the geometrical parameters of the structure to the electrical parameters of the DCLDC are provided, and a 3-dB coupler with an impedance of 50 Omega at the terminal ports has been designed, fabricated, and measured. Common-mode (CM) transmission to both through and coupled output ports has been reduced by adding a simple and compact CM filter at the input port. The good agreement between the results obtained with EM simulation, transmission line models, and measurements confirms the benefits of the proposed structure and the suitability of the design procedure.