Planar ultra-wideband bandpass filter using edge coupled microstrip lines and stepped impedance open stub

The design and implementation of a planar ultra-wideband (UWB) bandpass filter are presented. Three interdigital edge coupled microstrip lines are used for coupling enhancement. A stepped impedance open stub is used for realizing transmission zeros simultaneously in upper and lower stop bands as well as impedance matching in ultra-wide pass band. A pass band from 3.1-10.6 GHz is achieved with an insertion loss of 0.5 dB, a return loss of about 18 dB, a sharp out-of-band-rejection, and a low group delay of only 0.21 ns. Single- and double-section filters are realized to meet the UWB mask requirement. The design of the filter is simple, and it shows good frequency response.