A Compact Wideband Bandpass Filter With Tunable Notched Band

A compact wideband BPF (bandpass filter) with tunable notched band is presented in this paper. The presented filter consists of three quarter-wavelength short-circuited stubs, which are connected by half-wavelength microstrip line. To improve the selectivity of filter, the input and output microstrip lines of filter are folded to the middle of filter to realize multiple TZs (transmission zeros) at the left and right side of the passband. The H-shaped resonator loaded in the filter can generate odd-mode and even-mode resonant frequencies in the passband and upper stopband respectively. The odd-mode resonant frequency lied in the passband form a notched band and the even-mode resonant frequency lied in the upper stopband can expand bandwidth of the stopband. The tunable notched band is realized by loading a varactor diode in H-shaped resonator. The filter is optimized with HFSS, simulation results show that the IL (insertion loss) is less than 1.2 dB and RL (return loss) is greater than 10dB within 3.7-9.9 GHz band. The upper stopband frequency range is 10.4-15.9 GHz; the adjustable notch frequency range is 5.63-7.25 GHz, the tuning rate is 25.2%, and the depth of notched band is greater than 20dB in the whole tuning range.