An Optimum Ultra-Wideband (UWB) Bandpass Filter with Spurious Response Suppression

A high selectivity ultra-wideband (UWB) bandpass filter (BPF) with a fractional bandwidth (FBW) of about 110% is presented using a microstrip structure. The filter is based on a circuit model for an optimum distributed highpass filter consisting of only 5 short-circuited stubs separated by non-redundant connecting lines. As a result, the filter in a compact structure exhibited a selective filtering characteristic equivalent to a 9-pole Chebyshev filter. In addition, an optimum bandstop filter was added in order to suppress the second spurious harmonic. The design was successfully realized in theory and verified by full-wave EM simulation of the full layout and the experiment. The proposed filter had low insertion loss, sharp rejection, and a group delay of about 0.2 ns over the most center band. The filter also demonstrated an UWB reject band from 11.4 GHz to more than 20 GHz at -30 dB