Highly -Selective Dual -band Bandpass Filter with Extremely Wide Tuning Range for Passbands

In this paper, a second-order dual passband filter with an extremely wide tuning range of 0.7-1.5 GHz and three out-of-band transmission zeros is proposed. Good tunability is achieved by placing two varactors in back-to-back connection at each open-circuited end of this resonator. Two out-of-band transmission zeros are obtained through the combination of electric coupling and magnetic coupling. By properly adjusting the electric coupling, the upper and lower transmission zeros can be located at the desired frequencies to provide good selectivity. A middle common via simultaneously provide shorted point for upper and lower quarter-wavelength resonators, which reduces the size of the proposed filter. The tuning range of the lower passband is from 0.7 to 1.2 GHz (52.6%), whereas that of the upper passband ranges from 0.9 to 1.5 GHz (50.0%). At last, a good agreement is obtained between the simulated and measured results.