The New Coupling Structure In Microwave Filters Applied Research

Microwave filter is a very important part in communication and radar systems. The cross-coupled filter technology is developing rapidly from the 80s of the 20th century. Flexible topology and coupling structures are used to realize transmission zeros, thereby enhancing the stop band rejection or making more flat group delay.Coupling structure is a very important part of the cross coupling filter. This paper studies deeply on the coupling structure, introduces its definition, equal-circuit, coupling coefficient, how to classify and determine the nature of coupling, microwave structure of coupling and its application in filters. Especially, a new so called resonant coupling structure has been studied in detail. This new structure uses resonant structure as coupling structure, provides nice response by appropriately setting its parameters. It can realize different coupling nature (capacity or inductive) at different frequency bands. Thus, a single CT element in cross coupling filter can realize 2 transmission zeros (Only one by using traditional coupling structure) .This paper theoretically studies the characteristics of the resonant circuit and describes what condition should be met for using the resonant circuit as coupling structure. Several real microwave structures have been proposed, including shorting-circuit stub type, dual-iris type and resonant post type. Studying has been done to reveal the rules of the coupling nature, amount of coupling and its adjustable range. Using these types of coupling structures, completed 4 of microwave filters design, 3 of which were fabricated and tested. Theoretical analysis, simulation and tested results agree well with each other, validating the feasibility of the resonant coupling structure and demonstrating its superior features and value.