Tunable differential-mode bandpass filters with wide tuning range and constant bandwidth

A tunable differential-mode filter with constant absolute bandwidth (ABW) is proposed. By introducing negative coupling, the bandwidth control is more flexible. Capacitors are introduced to adjust Qe. A tuning range of 74.5% can be achieved by controlling the coupling coefficient and Qe and the suitable choice of the varactor. A capacitor is loaded on the symmetry plane to separate the resonant frequencies of common modes, causing high common-mode suppression. Step impedance resonators (SIR) are utilized to shift the harmonic up to 3 GHz.