Advanced Synthesis and Precise Design of Inline Fully Canonical Filters With High Selectivity

This article presents an advanced synthesis and novel designs of inline fully canonical filters with $N$ cascaded extracted pole (EP) sections, providing higher band-edge selectivity and suppressing interference from adjacent bands. The conventional fully canonical prototype is reconfigured to significantly facilitate filter design and fabrication. Moreover, to synthesize realizable circuits, a general node scaling method based on admittance matrix is implemented. All associated elements, especially the junction susceptances, can be scaled directly without the aid of inverters, avoiding intricate equivalent circuit transforms. Furthermore, the importance of precise control over the junction susceptances in filter design is discussed for the first time. Accurate junction susceptance enables the exact mapping from synthesized circuits to physical structures, preventing brute-force optimization. In particular, a new and compact EP structure in coaxial technique is proposed with flexibly tuned loading phase and accurately regulated junction susceptance. The good measurement results of a coaxial EP filter with four reflection zeros (RZs) and four transmission zeros (TZs) validate the effectiveness of the proposed theories. Also, this is the first reported coaxial fully canonical filter with only build-in capacitive couplings to avoid passive intermodulation (PIM).