Balun diplexer design in hybrid structure of microstrip line and slot-line

In this paper, a new balun diplexer consists of hybrid microstrip and slot-line resonator structures is designed with broad-band common-mode (CM) suppression. The balun diplexer was formed by combining two balun bandpass filters (BPFs) operating at 1.92 GHz and 2.45 GHz, respectively, through a T-junction. Cross-coupling between the feed-lines and the SISLRs at a proper position is adopted to obtain an appropriate coupling strength between the above two resonators. In addition, the incompleteness of the slot-line resonator results from the open-boundary condition along the symmetry plane in CM operation is employed to effectively suppress the CM signals. A prototype of the proposed balun diplexer was implemented and measured with a differential-mode isolation of higher than 25.4 dB and a CM suppression of larger than 27.5 dB in the frequency range of 1-6 GHz. Good agreement was observed between the simulated and the measured results.