A Tunable Balanced Coupler With Improved Phase Balance and Extended Bandwidth

Abstract

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 degrees 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.