Recent Progress in Synthesis Techniques of Microwave Ultra-Wideband (UWB) Filters

This paper describes our recent research works on the synthesis theory for designing novel microwave ultra-wideband (UWB) bandpass filters (BPFs). Based on Z-transformation and analytical method of variable separation, we have implemented two different synthesis techniques, in terms of which three types of microstrip UWB BPFs were developed by comparing the newly-derived ideal and real transfer functions to obtain the structural parameters. The filter prototypes include the one using periodically loaded short-/open-ended stubs, the ones consisting symmetrical/quasisymmetrical/asymmetrical multi-mode stepped-impedance resonators (SIRs) and the other ones with two short-/open-circuited stubs positioned at both sides of SIRs. Several design examples of wideband BPFs are provided. All of them are verified well by experiments and exhibit good performances, such as compact sizes, low return loss in passband, flat group delay and good stopband property etc.