A Reconfigurable Fourth-order Filter With Constant Absolute Bandwidth And Four Transmission Zeros

In this letter, a reconfigurable fourth-order bandpass filter with constant absolute bandwidth (ABW) and four transmission zeros is presented. The proposed filter is composed of a pair of varactor-loaded half-wavelength resonators and a pair of quarter-wavelength resonators with varactors loading at specific position. By adjusting the loading position of varactors in quarter-wavelength resonators and coupling region in half-wavelength resonators, the required coupling coefficient and constant ABW can be achieved. Four transmission zeros are introduced by the cross coupling, which improve the stopband rejection performance and selectivity. For demonstration, the proposed filter is simulated and optimized. The simulated result shows a tuning range of 1.95-2.58GHz with 230±7MHz constant ABW, also demonstrates high selectivity and good stopband rejection.