Novel Compact Ultra-wideband Bandpass Filter with Steep Skirt Selectivity

A UWB filter with improved skirt selectivity and compact size was implemented by a new efficient design scheme. Firstly, based on synthesis theory, short-circuited stubs are utilized together with a 5lambda₀/4 stepped-impedance resonator (SIR). Furthermore, keeping other parameters unchanged, two-section stepped-impedance open-circuited stubs are theoretically designed to replace the short-circuited ones. As a result, a pair of controllable attenuation poles is introduced to effectively improve selectivity of the filter. To validate the proposed techniques, the designed UWB filter was simulated, fabricated and measured. The measured frequency responses of the UWB filter agree well with the theoretical ones and successfully satisfy outdoor limit of FCC. In addition, the fabricated filter also exhibits compact size, low return loss in passband, flat group delay and good stopband property as well. Moreover, to be highlighted here, the application of the stubs successfully enhances the coupling coefficient of parallel-coupled structure, and the coupling gap size in this filter is enlarged to 0.16 mm from 0.06 mm in conventional one without the stubs.