A Coupling Matrix and Admittance Function Synthesis for Mixed Topology Filters

In this paper, a direct coupling matrix technique is presented for narrowband mixed topology filters that possess a series of extracted-pole sections. With respect to previous works, this technique exhibits distinct benefits due to its simplicity as all the coupling elements, of both cross-coupled and extracted-pole structures, can be obtained as an entity at one time. As the basis, a unique process is introduced to rebuild the couplings and admittance functions for an Nth-order cross-coupled low-pass prototype, from which a series of extracted-pole sections can be naturally generated with no more operations, thus deriving a mixed topology transversal array directly. To show the flexibility of this paper, a couple of examples, including fully canonical cases and fully extracted-pole cases (considered as a particular kind of mixed topology filters here), are demonstrated. Moreover, a miniaturized coupled line structure is proposed to properly construct filters of fully extracted poles, which provides a reasonable possibility to design compact planar mixed topology filters, in comparison with others in the literature. The simulated and measured results are in good agreement.