Tunable bandpass filter using varactor-loaded quarter-wavelength resonator

In this work, a tunable bandpass filter using quarter-wavelength resonator is studied. The resonant frequency of each resonator can be tuned by the mid-loaded varactors. The two resonators are connected by a metalized via which behaves as a lumped inductor. The via also provides DC bias for the varactors. In addition, source-load coupling is introduced to create two transmission zeros on each side of the passband to enhance stopband rejection performance. The proposed tunable filter exhibits wide tuning range with low loss and good stopband rejection.