A Tunable Balanced Coupler With Improved Phase Balance and Extended Bandwidth

This paper proposes a balanced coupler with improved phase balance and extended bandwidth for a wide range of tunable power-dividing ratios. This differential coupler uses only two varactors and can demonstrate tunable power-dividing ratio and quadrature phase difference between outputs with theoretically perfect differential-mode return loss and port isolation, and complete common-mode suppression. The coupler analysis, design equations/graph, and parametric study will be given in detail. As a proof-of-concept, a microstrip prototype operating at 1 GHz was fabricated. Measured results show that the power-dividing ratio can be tuned from -5.5 dB to 11.9 dB at the center frequency where the insertion loss is less than 0.84 dB, the phase imbalance and isolation between differential outputs are less than 3.9° and greater than 23.2 dB, respectively. Moreover, the differential-mode return loss and the common-mode rejection are greater than 27.5 dB and 35.2 dB, respectively. Measured and simulated results are in good agreement.