Dual-Bandstop Substrate-Integrated-Coaxial Tunable and Static RF Filters

This letter discusses the RF design and practical realization of dual-bandstop filters (BSFs) with compact size and two independently controlled rejection bands. They are based on miniaturized substrate-integrated-coaxial (SIC) resonant stages that are made from two capacitively loaded posts and a thru-line. Each of the stages creates two transmission zeros (TZs) that can be exploited for the realization of dual-bandstop RF filters with approximately half the size of conventional single-post series-cascaded SIC filter configurations. Further miniaturization is achieved by tuning the center frequency of the bands. For proof-of-concept validation purposes, three prototypes were designed, manufactured, and measured. They include a static (rejection bands centered at 4 and 4.5 GHz) and a tunable (between 3.6-4.6 GHz) dual-bandstop second-order RF filters and a third-order dual-bandstop RF filter with bands centered at 4 and 4.4 GHz.