Direct Synthesis Technique of Quasi-Canonical Filters Comprising Cascaded Frequency-Variant Blocks

This paper presents a direct synthesis process to realize quasi-canonical filters (i.e. filters contain $N$ resonators while perform $N$ - 1 finite transmission zeros) that consist of cascaded blocks. The synthesis relies on specific manipulation of a triplet, where one cross-coupling is eliminated by introducing frequency variance into the structure. Based on such idea, three kinds of quasi-canonical frequency-variant blocks (FVBs) containing two, three, and four resonators, respectively, are prepared. Complex quasi-canonical filters with high orders can thus be constructed by arranging the FVBs in a row, whose transformation sequence is also introduced. Compared with traditional CT and CQ topologies, the proposed FVBs are advantageous to achieve more transmission zeros with less coupling elements. For the validation, three illustrative examples comprising diverse FVBs are carried out, where both synthesis and circuit simulation results are given.