A Wideband Filtering Balun Using CPW-to-Slotline Transitions

In this paper, a wideband filtering balun with good passband selectivity is proposed, which consists of a triple-mode slotline resonator, two CPW-to-slotline transitions, and a CPW short-stub. Three transmission poles are allocated by the triple-mode slotline resonator to achieve the wideband operation. To further improve the magnitude balance and out-of-phase properties, two CPW-to-slotline transitions are used. Besides, the CPW short-stub is introduced to enhance the selectivity performance. To verify the mechanisms above, a wideband filtering balun operating at 2.1 GHz with 3-dB fractional bandwidth (FBW) of 101.9% is fabricated. The measurement exhibits the in-band amplitude-imbalance lower than ±0.31 dB and phase-imbalance lower than ±1.3°.