Design of Wideband Filtering Balanced-to-Unbalanced In-Phase Power Divider with High Common-Mode Suppression

In this paper, a new design of wideband filtering balanced-to-unbalanced (BTU) in-phase power divider (PD) is proposed based on two different three-line coupling structures and a pair of stub-loaded resonators. Herein, the balanced signal with high common-mode suppression is generated by two half-wavelength microstrip lines symmetrically coupled with a central quarter-wavelength conductor, where the differential ports are set at the opposite terminals of two side lines. To obtain a pair of in-phase signals, the output section is constructed by a symmetrical three-line coupled structure. Then, two open circuited stub resonators are adopted to broaden the passband and improve filtering selectivity. Good in-band isolation is realized with a lumped resistor bridged between two output lines. By theoretical calculations, relevant parameters of the filtering PD can be predicted with given specifications. For demonstration, one prototype is designed and simulated. Results indicate that the proposed filtering BTU PD operates at the center frequency of 2.4 GHz with a fractional bandwidth of 30.7%. On the other hand, common-mode signals input from the balanced ports can be suppressed up to 30 dB over a wide frequency band and the in-band isolation is better than 20 dB.