Balanced-to-unbalanced Wideband Filtering Power Divider with High Common-mode Suppression Based on Coupled Lines

Balanced circuits have higher immunity to environmental noises, lower electromagnetic interference and better dynamic range in wireless communication system. Balanced to unbalanced circuits not only have the excellent performance of balanced circuits, but also can be used to connect balanced and unbalanced devices. The multifunctional power divider has found a wide application in the modern communication systems due to its small size, low cost, lightweight, and flexible design without degrading the performance. A novel wideband balanced-to-unbalanced filtering power divider with high common-mode suppression based on coupled lines is presented in this paper. Considering its broadband response, coupled lines are selected as the main structure to carry on the design. The isolation circuit is connected through the coupled lines with a resistor. The coupled lines can also introduce two transmission zeros on both sides of the desired passband, respectively, to further improve the frequency selectivity. The H-shaped structure inside is used to increase the common mode rejection. To model and analyze this equivalent circuit, the closed form equations of the circuit for equal power division are derived through odd-even mode method and transmission line theory. This makes the design process achieve good frequency selectivity, broadband differential mode bandwidth and good common mode suppression bandwidth, simultaneously. To verify the proposed concept, one wideband balanced-to-unbalanced filtering power divider operating at 2GHz is designed, with 4dB differential-mode bandwidth of 72%, 18-dB common-mode suppression bandwidth of 112.5%. There is a good agreement between the simulation results and the proposed circuit design principle. The proposed structure demonstrates the high frequency selectivity and high common-mode suppression in the whole passband, which can also be extended to other devices such as antennas, power amplifiers, low-noise amplifiers, and so on.