Abstract: This paper proposes a novel varactor-tuned microstrip bandpass filter consisting of two dual-mode open-loop resonators. No coupling exists between the two operating modes, odd-mode and even-mode. Based on this feature, a simple DC bias circuit has been used to tune the resonator’s odd and even mode frequency respectively to realize tunable passband frequency. Analysis of the coupling matrix of the resonator model shows that a transmission zero is produced at the edge of the passband, which greatly improves frequency selectivity of the filter. This filter is tuned from 0.87 to 1.03 GHz (GSM 900)  with return loss of more than 20 dB and nearly constant frequency response. Performance of a fabricated filter shows good agreement with the simulation results.

Key words: bandpass filter, dual-mode resonators, even-odd mode, tunable microstrip filter