Ultra-Wideband Bandpass Filter Using Hybrid Microstrip-Defected-Ground Structure

A novel ultra-wideband (UWB) bandpass filter (BPF) with a fractional bandwidth (FBW) of about % 110 is presented using the hybrid microstrip-defected-ground structure (DGS). The proposed filter was designed using a combination of hybrid microstrip-defected-ground structure lowpass filter (LPF) with typical quarter-wavelength short-circuited stubs highpass filter (HPF), followed by an optimization for tuning in-band performance. The high selectivity LPF using hybrid microstrip and four backside slots on ground plane is equivalent to a typical 9-ploe stepped-impedance LPF. The proposed LPF is employed to attenuate the upper stopband and etching on the ground plane with four rectangular slots provides attenuation pole to suppress the second spurious harmonic. Four shunt quarter-wavelength short-circuited stubs are used to realize the lower stopband for an optimum distributed HPF. 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 a good performance, including an ultra-wideband bandpass (3.1-10.6 GHz), low insertion loss, sharp rejection, and a group delay of about 0.12 ns over the most center band. The filter also demonstrated an UWB reject band from 11.4 GHz to more than 19 GHz at -30 dB.