Research Of The Miniaturization Of Cavity Filters

Filters are indispensable electronic equipments in microwave communicationsystem. Its performance directly affects the quality of communication system. With theconstant update of communication technology, the requirements of the performance ofthe filter become more and more demanding, including: small size, low insertion loss,high selectivity and parasitic passband. Among various types of filter, cavity filter hasbeen widely used because of its characteristics of low insertion loss and high power, butit also has the limitation of large size and can’t be used in some special communicationsystem, especially in the low-frequency case. This paper focuses on the size problems ofcavity filter, and is committed to the design of a new miniaturized cavity filter.This thesis firstly conducts an in-depth study on the network synthesis method andthe transmission line capacitor loaded shortening principle, makes many explorations onthe structure and implementation of load capacitance filter and helical cavity filter.Cavity filters are microwave passive devices with pure mechanical structure. Theirperformance and stability entirely depends on their characteristics of the internalstructure of the body. Therefore this thesis, focusing on the structure, discusses theresonant cavity and the coupling structure of cavity filters. Based on the networksynthesis method and load capacitance theory, an interdigital cavity filter whose centerfrequency is in S band is designed in this paper. It has realized the strong capacitanceloading and achieved the purpose of miniaturization. The size of the filter is only68cm*31cm*10cm. And then this thesis proposes a new structure of capacitive loadedresonant cavity. Based on this structure a new cavity filter whose index is identical withthe previous filter is designed. Its test curve meets the design requirements and itsvolume is39cm*16cm*7cm, only1/4of that of the previous cavity filter. In addition,using the characteristics of helical line cavity for reference, a kind of helical cavity filteris designed. Its simulation curve has the characteristics of low passband frequency andwide relatively bandwidth. And its volume is only75%of that of common cavity filters.