Compact Ultra-wideband Bandpass Filter with Wide Fractional Bandwidth and High Skirt Selectivity

A compact ultra-wideband bandpass filter having highly selectivity and wide bandwidth simultaneously is proposed, which adopts a modified multiple-mode resonator (MMR) and annular structures.The modified MMR is constructed by multiple uniform-impedance stubs and a dumbbell-based stepped-impedance stub.Two transmission zeros are created by the dumbbell-based stepped-impedance stub at the lower and upper cutoff frequencies to improve the selectivity.Additionally, two annular defected ground structures in the aperture ground are adopted to enhance the coupling factor and reduce the etching size.This bandpass filter has a fractional bandwidth of 127% with selectivity of up to 0.939.The measured results are in good agreement with the simulation results.