Compact Stepped Impedance Resonator Bandpass Filter with Tunable Transmission Zeros

This paper proposes a compact microstrip bandpass filter (BPF) with tunable transmission zero, narrow bandwidth and low insertion loss. Transmission zeros are the key to improve the band rejection and filter frequency selectivity. A λ/4 stepped impedance resonator (SIR) with two additional via holes has been adopted to obtain a compact size and a pair of transmission zero (TZ). The BPF is designed to operate at 3.5 GHz with fractional bandwidth (FBW) of 7.2%.Furthermore, three techniques have been developed to create a pair of controllable transmission zeros on both side of each passband. The TZ can be controlled by adjusting either magnetic or electric coupling. The measured return losses and insertion lossis larger than 18 dB and 2.2 dB respectively. The overall size of the proposed design filter is 5.3mm x 5.5mm without considering the feeding lines.