A tunable broadband bandpass filter based on quarter wavelength resonators

In this letter, a tunable broadband bandpass filter (BPF) based on quarter wavelength resonators was proposed. The resonators and coupling structures were mainly consisted of quarter wavelength transmission lines and tunable devices loaded at the two ends of the transmission lines. The resonant frequency of the BPF can be adjusted through changing the total capacitance at the both ends of the resonators. Especially, the coupling strength between resonators can be adjusted by adjusting the value ratio of the capacitor between the two ends. The tunable BPF in this scheme had the advantages of simply design. The bandwidth and center frequency of the tunable BPF can be tuned independently at the same time. As a demonstrator, an eight-orders filter was designed and fabricated on Rogers3006 substrate. The center frequency can be tuned in the range of 0.81 to 1.28 GHz, the relative tuning range was 45%. The relative bandwidth of the tunable filter can be tuned in the range of 14% to 38%. The simulated results were agreement well with the measured ones.