Recent development of ultra-wideband (UWB) bandpass filters using distributed stub transmission line structures

A brief review and recent progress in developing ultra-wideband (UWB) filters using distributed stub transmission line structures is reported in this paper. Several advanced UWB filters are demonstrated with electromagnetic (EM) simulated and experimental results. The filters are based on a circuit model for an optimum distributed highpass filter consisting of short-circuited stubs separated by non-redundant connecting lines. It is shown that a cross-coupling between non-adjacent resonators or between the input and output (I/O) feed lines can be implemented to generate new transmission zeros which improve the performance of the stub filters. It is also demonstrated that the uniform connecting lines of the stub filter can be replaced by stepped-impedance lines to suppress the spurious harmonic passbands. In addition, stepped-impedance resonators (SIRs) are implemented to generate a narrow notched band inside the UWB passband to avoid interference with existing radio signals.