Compact Constant Bandwidth Tunable Wideband BPF With Second Harmonic Suppression

A compact tunable wideband bandpass filter (BPF) having constant absolute bandwidth (CABW) with tunable selectivity and second harmonic suppression is presented in this letter. The BPF is designed using non-uniform quarter-wave spiral resonators loaded with varactor diodes and a capacitively coupled stub. The BPF consists of combline coupled section also. It is shown that the inherent transmission zeros (TZs) of a combline coupled section and coupling-coefficient are controlled using varactor diodes which results a CABW response with improved upper stopband suppression level near the passband. The tunability has been controlled by varactors loaded at the end of combline coupled section. A tunable BPF having tunable band of 0.9-1.5 GHz is designed and fabricated with a CABW of 290 ± 10 MHz. Along with tunable passband, tunable selectivity also has been achieved by controlling the TZs. The results of a compact BPF having size of 0.065λg ×0.065λg, show a wideband filtering responses having 3.4% bandwidth (BW) variations.