Varactor-tuned quarter-wavelength dual-bandpass filter

A tunable dual-band bandpass microstrip filter is proposed and presented. It is composed of two pairs of quarter-wavelength transmission line resonators and two bandstop structures made of open stubs. Each operation bandpass frequency, denoted by f1 and f2, is generated by a pair of coupled resonators. The first coupled resonator is fed via a tap-coupled structure, while the second is fed through a coupling structure. The tunability of each bandpass and harmonic suppression is achieved by modifying the electrical length of the resonators and the electrical length of the open stubs by using varactor-diode capacitors. Finally, a 2.5 and 4.25 GHz dual band bandpass filter is designed, fabricated and measured as one example and a tunability of 22.2% and 19.6%, respectively, is obtained for the first and second bands. These excellent experimental results validate the proposed structure and design technique for dual-band bandpass filters.