Simulation study on a 3-dB quadrature coupled structure

The circuit presented here is a variation of the classic unfolded Lange coupler. The structure is to provide equal power output, while maintaining a 90° phase difference, for as wide a frequency range as possible, between the coupled and through ports. This coupler has the advantage of using coplanar waveguide and microstrip transmission lines, while avoiding the use of air bridges as the traditional Lange coupler does. The proposed coupler instead uses a defected ground plane, vias, and short PCB traces to connect the interdigitated structure. The circuit is designed on a single-layer substrate and can easily be manufactured with PCB processes. For low-cost fabrication, FR-4 substrate, with one-ounce copper, is proposed. The Sonnet™ simulation results show that the structure can provide a tight coupling factor of 3.6 dB when dielectric and conductor losses are included. The circuit is able to maintain a phase difference between 86° and 100° from 0.1-2 GHz.