Electronically tunable lumped element 90 hybrid coupler

Introduction: With the increasing variety of commercial wireless applications, there is now a requirement to incorporate several transceivers into a single hardware device [1]. To achieve high integration and minimise costs, it is an advantage to design transceivers that can be reconfigured for different frequency bands or share common blocks, rather than implementing the transceivers separately. Electronically tunable circuits based on varactors can be used to realise frequency agile components suitable for reconfigurable designs. In this Letter, the varactor-based tuning technique is applied to a lumped element quadrature hybrid coupler suitable for RF integrated circuits (RFICs). Varactors have previously been used to design tunable couplers based on parallel coupled microstrip lines. In [2], the coupling level of a directional coupler was adjusted between 4 and 20 dB by changing the junction capacitance of varactor diodes. More recently, an LC resonator was added to a microstrip coupler to allow tuning of the centre frequency of the device [3]. In this case, the coupling level remained constant. The concept of varactor tuning of the centre frequency can also be applied to the hybrid coupler.