A Dual-band Bandpass Filter with a Tunable Passband

Abstract—In this paper, a novel dual-band bandpass filter with a tunable passband is proposed. Based on two quarter-wavelength resonators and one half-wavelength resonator, dual-band character is designed by introducing two independent coupling paths. The filter structure is designed and can be divided into two parts. The Transmission Zeros (TZs) are derived through simulation. By varying the reverse bias voltage applied to the varactor diodes which are connected to the resonators, the first passband can be independently tuned. The second passband can be easily controlled by the width of the half-resonator. Finally, simulation results show a first tunable passband with a constant fractional-bandwidth of 3.9 ± 0.3%, a center frequency of 1.472–1.886 GHz, and the second passband almost remains constant with a fractional-bandwidth 2.4% at frequency range 2.24GHz. The measurement of the fabricated example shows good agreement with the simulation.