Compact Reconfigurable Dual-band bandpass Filter with Constant Absolute Bandwidth

A compact dual-band bandpass filter (BPF) based on stepped impedance resonator (SIR) and defected-feeding structure is investigated in this paper. The SIR with T-stub centrally loaded provides more degrees of freedom to adjust the dual-band resonant frequencies. By combining the SIR with the defected-feeding structure and adjusting their dimensions, the absolute bandwidth can be easily controlled. Transmission zero is derived through simulation due to the combination of SIR and defected-feeding structure. Finally, simulation results show the second passband switched among 1.55 GHz, 1.75 GHz and 2.05 GHz with a constant absolute bandwidth of 230 MHz +/- 5.6%, and the first passband fixed at 0.90 GHz with a constant absolute bandwidth of 32 MHz. The measurement of the fabricated example shows good agreement with the simulation.