Filters in Generalized Box Configuration With Redundant Resonances and Redundant Couplings

This article provides a comprehensive discussion of the block named generalized SymBox (GSB). The analytical synthesis approach and a set of design examples are presented. Redundant couplings or redundant resonances are introduced into conventional box topology to avoid negative couplings, forming the GSB block, regardless of the position of transmission zeros (TZs). The working principle of GSB is analyzed in detail from the circuit level for the first time. Furthermore, to generate more TZs within compact structure, frequency-dependent couplings (FDCs) are incorporated with GSB, constituting a new topology. With the proposed direct synthesis method and coupling matrix (CM) rotation sequence, intricate circuit transforms and additional optimization can be avoided in synthesis stage. Also, flexible synthesis solutions can be achieved, bringing more degrees of freedom in filter designs. For the realization with smaller size and higher level of integration, the designs of multilayer sheet metal filters are elucidated. Finally, two five-pole filters are designed and fabricated, followed by the measured results.