Wideband bandpass filter with tunable passband

In this paper, a tunable wideband bandpass filter based on stub-loaded stepped-impedance ring resonator is proposed. By changing the characteristic impedances of the loaded stubs, the bandwidth of the passband can be controlled with the center frequency unchanged. The even-and odd-mode method is applied to obtain the expression of S11 and S21. To validate the proposed strategy, a filter operate at 2 GHz is designed. The simulated results of the BPF show that the fractional bandwidth can be tuned from 149% to 65% with return loss better than 12.5 dB.