A Planar Balanced Quadrature Coupler with Tunable Power-Dividing Ratio

This paper reports on, for the first time, planar balanced quadrature couplers with wide tunable power-dividing tuning ratio using two tuning capacitors. The differential-mode operation, common-mode rejection, and cross-mode conversion suppression are obtained for all power-dividing ratios. Closed-form design equations are derived. For verification, a 2-GHz tunable balanced quadrature coupler is designed and measured. The measured power-dividing ratio is in the range of -11.3-10.2 dB with better than 15-dB return loss and isolation. Furthermore, the insertion loss and phase difference resulting from the varactors and transmission lines are compensated using an active capacitance circuit with negative resistance. The worst case of phase imbalance is improved from 25° to 2°. The insertion loss is improved by ∼1.2 dB and nearly close to the theory value.