General Matrix Synthesis for Cascaded-Block Filters With Flexible Bandwidth

This article presents a general synthesis method for achieving flexible bandwidth of a bandpass filter by cascading modular blocks. The synthesis utilizes a matrix procedure in the bandpass domain, enabling the extraction, relocation, and cascading of various bandpass blocks (including bandpass doublets, triplets, quadruplets, and boxes) together. These blocks can consist of capacitive or inductive couplings and can be further reduced through specific matrix transformations that introduce resonant couplings. The resulting prototype, incorporating transmission zeros (TZs) generated by each cascaded block, facilitates the realization of sharp selectivity without limitations on filter degree or bandwidth. Additionally, an
N
th-order in-line prototype composed of bandpass doublets in series is specially discussed, which allows for independent generation and control of massive finite TZs (with at least 1 TZ dedicated to 0), even in very wideband cases. To evaluate the proposed approach, numerous synthesis examples are presented, showcasing different orders, TZ allocations, and coupling pathways. Furthermore, an experimental design is demonstrated for the validation, which features a fifth-order low-temperature superconducting (LTS) filter with a passband of 3.5–6.5 GHz and 4 TZs at 0/1.9/7.6/8.1 GHz. The simulated and measured results are well correlated, thus showing the effectiveness of the proposed approach.