Stub-loaded-based bandpass filters with dynamically-controlled in-band notches

The RF design of stub-loaded-based wide-band bandpass filters (BPFs) with multiple tunable rejection bands in their transmission range is presented. The proposed concept exploits the replacement of the short-circuited resonant stubs of the stub-loaded-based BPF with multi-band bandstop (BS) sections. Each multi-frequency BS section is formed by several in-parallel-cascaded 90°-long stubs that are terminated by frequency-agile resonators so that each of them results in an independently-controlled transmission zero. In this manner, a plurality of high-order reflective-type rejection bands can be created and arbitrarily controlled. Further operational characteristics include: (i) scalability to any number of rejection bands and degree of selectivity, (ii) independent tuning of each notch without any influence on the remaining ones, and (iii) spectral-merging of multiple stopbands to increase their bandwidth (BW) and enable controllability of the number of active notches in the passband. The aforementioned principles are experimentally validated through a prototype that exhibits a wide-band (fractional BW: 68%), low-loss ( 60 dB) notches that can be independently tuned within the 870–1150 MHz range.