Wideband Fully Tunable Bandpass Filter Based on Flexibly Multi-Mode Tuning

In this letter, a wideband fully tunable bandpass filter with three adjustable transmission poles is proposed. The fundamental structure consists of two half-wave length (λ/2) resonators with center-tapped open and shorted stubs. Due to the same odd-mode resonant frequency, these two dual-mode resonators can form a wideband bandpass filter with three poles. Further, the fully tunable filter is designed by utilizing varactors to control three transmission poles flexibly. As a demonstrator, the prototypical filter with six varactors is implemented to validate the design concept. The filter reports 60.5% fractional bandwidth (FBW) at 2.48 GHz. In this state, the filter also achieves 138% FBW tuning and 20.8% center frequency tuning. Meanwhile, two transmission zeros (TZs) located at both sides of the passband are introduced by the center-tapped stubs to enhance the selectivity.