Microstrip Parallel-Coupled Filters With Cascade Trisection and Quadruplet Responses

Microstrip parallel-coupled filters with generalized Chebyshev responses are presented. The basic structure of the proposed filter is a conventional parallel-coupled filter of which the physical dimensions can be easily obtained by the well-known analytical method. With the aid of the equivalent circuit corresponding to a conventional parallel-coupled filter, the relative insertion phase from source or load to each open end of resonators can be easily obtained by observing the two-port admittance matrix. Applying the cross coupling from source or load to a proper nonadjacent resonator, a trisection or a quadruplet coupling scheme can be realized with prescribed transmission zeros. More importantly, the proposed trisection can be designed to have a transmission zero on the lower or upper stopband by just adjusting the length of the cross-coupling strip. Using the proposed structure, the conventional time-consuming adjusting procedure to obtain initial physical dimensions of filters is no longer required. In this paper, a fourth-order parallel-coupled filter is used as the basic structure to demonstrate various combinations of transmission zeros. Simulated and measured results are well matched.