Microstrip dual-band bandpass filter with independently tunable passbands using varactor-tuned stub loaded resonators

In this paper, a dualband bandpass filter with independently tunable passband is proposed. Two half-wavelength resonators with shunt stub have been placed side by side, fed with a common input-output microstrip line to achieve the individual tunability without affecting other passband. For tuning resonance frequency, varactor diodes are used at the ends of the half wavelength resonators and also at the end of the shunt stubs. Proper shunt stub length and width are derived numerically in such a way that only one control voltage is required in each passband. Measured results show that lower passband can be tuned in a frequency range from 1.78 to 1.96 GHz, whereas the upper passband varies from 2.27 to 2.39 GHz individually. H shaped DGS is integrated below the input-output feed lines to suppress higher order harmonics up to 21 GHz with more than 19 dB attenuation.