The Miniaturization Technology Of Capacity-loaded Cavity Filter

Microwave filters，as one of the most important passive components, play anirreplaceable roles in the modern microwave communication systems, such as wirelessbase station communication systems, satellite communication systems, radar systemsand electronic countermeasures systems. The quality of these filters has great effect onthe whole microwave system. Therefore, with the increasing requirements of microwavesystems being proposed, more strict standards for the filters are required such as smallsize, high-quality, low-loss, narrow transition, high power capacity, far spurious bandand low cost. Such filters are hot in academia and industry field. However, it is a hugechallenge and seems impossible for RF/microwave scholars and engineers to designfilters meet all these high requirements at the same time. But we usually make acompromise according to the engineering request. Thus, innovations both in designtheory and structure of filters can be achieved.By the research into the design of filter as well as its current development, thispaper is committed to the miniaturization technology of filters applying for thecapacitor-loading method considering the needs of practical engineering applications.The theories in this paper are verified by the wonderful performances of both coaxiallow-pass filters and single-layer capacity-loaded band-pass comb filter. This paper isorganized as follows:Firstly, some fundamental theories including the transfer matrix and its equivalentcircuit in relation to the design of microwave filters are illustrated, followed by thein-depth analysis that capacitor-loaded technology contributes to reducing the length oftransmission line. The working principle of cavity with capacitor loaded is studied indetail and verified utilizing software simulation.Secondly, the miniaturization technology of coaxial low pass filters with capacityloaded is studied and a novel structure based on the method is proposed whichsuccessfully attained spurious band suppressed less than-50dB at the cutoff frequencyfive times the center frequency, leading to the first effective spurious band appearing upto ten times the center frequency. Meanwhile, the size of filter is reduced. A sample filter has been designed and fabricated with test results satisfying all the indexes.Thirdly, the realization of filter miniaturization technology based oncapacity-loaded technology in comb filters is researched by introducing single layerdielectric capacitor into filters. Then, a novel single layer ceramic dielectriccapacitor-loaded comb filter is designed combining the theory calculation with thesoftware simulation and tested with the results analyzed and compared with that oftraditional capacity loaded filters.Finally, a summarization has been made according to the research in this paper andsome questions referred to the context have been discussed, followed by a prospect offurther research.