Hairpin Tunable Bandpass Filter with Improved Selectivity and Tunability

In this paper, a hairpin tunable bandpass filter which has enhanced selectivity and tunability is proposed. The varactors loaded open stubs between quarter-wavelength resonators are introduced to exhibit transmission zeros near the passband and thus better selectivity in the tuning range. In addition, symmetrical filter shape can easily obtained by each of two open stubs which are useful to control notch frequencies. The optimal design of two-stage tunable bandpass filter of varactor loaded hairpin resonator with open stub which works as K- inverter makes possible to meet the wideband coupling relationships in overall frequency tuning range. The measurement results show the proposed hairpin tunable bandpass filter not only has the enhanced tuning range of 23.5%, but also has the symmetrical filter shape in overall tuning range in comparison with conventional hairpin tunable bandpass filter. The hairpin tunable bandpass filter is designed and tested at 1.7 GHz.