A stopband-enhanced UWB bandpass filter using short-/open- stubs embedded ring resonator

A class of ultra-wideband bandpass filters using short-/open-stubs embedded ring resonator is discussed. The square ring resonator is first excited by two interdigital coupled-lines at two sides to achieve a triple-mode ultra-wideband bandpass filter performance. Two short-circuited stubs are then added to the upper and bottom sides of this square ring so as to constitute a quadruplet-mode bandpass filter, thus widening the desired passband while sharpening the rejection skirts around the cutoff frequencies. Next, the four open-circuited stubs are symmetrically attached at four corners of this ring, thus further improving the upper-stopband behaviors of this filter. The predicted results of the final filter shows good in-band and out-of-band filtering performances and it was well confirmed in experiment.