A Wideband Forward Coupler with Tunable Coupling Ratio Using Varactor Based Tuning Network

This paper proposed a wideband 90-degree forward coupler with tunable coupling ratio. The proposed coupler is designed based on two 90°hybrids and varactor based tuning networks. The varactor based tuning network changes the reflection phase difference between the even- and odd-mode circuits of the proposed coupler and leads to the variable coupling ratio. The varactor diodes in the tuning network with two different bias voltages can achieve wide tunable coupling ratio and operation bandwidth. Example circuit covers 1-2GHz. The input return loss and isolation can be kept in good performance across the designed band while the coupling ratio is tuning from +9dB to -20dB. The prototype circuit is fabricated on a 0.5mm-thick RO4003 substrate to demonstrate the proposed wide band tunable coupler.