Study On Multi-cavity Filter Based On Coupling Double-gap Cavity Unit

The information technology is developing rapidly today, microwave communication frequency resources are becoming more and more precious and crowded, and the demands on microwave filter index are also getting higher and higher. Due to its advantage of good electromagnetic shielding, compact structure, lower pass-band insertion loss, small in size and big power capacity, metallic cavity filter has long been a preferred variety of mobile communication base station transmitting filter.The metallic cavity filter based on multi-coaxial cavity with cross-coupling has good filter characteristics and is easy to tuning, so it is applied widely in the current mobile communication system, but modern wireless communication base station is pursuing smaller volume and lower cost as well as better performance index, and multi-coaxial cavity filter has technology bottleneck on narrowing volume and improving1dB bandwidth. With our fully research and analysis, we proposed the innovative ideas firstly that coupling double-gap cavity replacing double-coaxial cavity as a fundamental unit of cavity filter. And research has shown that filter consisting of a single double gap cavity filter has a clear advantage over coaxial cavity on1dB bandwidth and volume.For confirming the feasibility that coupling double-gap cavity replacing double-coaxial cavity as a fundamental unit of cavity filter which is brought up for the first time, using the method of equivalent circuit, the transmission function of coupling double-gap cavity loaded by coaxial on both ports is presented, researching filter characteristics of coupling double-gap cavity, and confirming the analysis results by3D simulation in this paper. For better understanding advantages of filter characteristics of coupling double-gap cavity, compared transmission characteristic and volume between coupling double-gap cavity filter and double-coaxial cavity filter by the3D simulation method. The results of this research, on the same gap width of both cavities, demonstrate clearly that coupling double-gap cavity filter has wider1dB width and smaller size than double-coaxial cavity filter, but has flatter pass-band descend edges. This research conclusion will provide the powerful theoretic foundation for developing smaller and big-power metal cavity filter based on coupling double-gap cavity filter in future.On the basis of this theory, we designed a four cavity filter based on coupling double-gap cavity unit, implemented larger1dB bandwidth with less cavity units, provided a big power, wide band, small size and high power capacity. During the design process, we originally made the concept of T-type coupling holes, and controlled the amount of coupling strength by changing the location, size of the hole to achieve the needed filter characteristics, and solved some problems, such as insufficient bandwidth and big pass-band insertion loss when two coupling double-gap cavity are in the mutual coupling. In addition, as computer performance improves and advances in microwave CAD technology, the simulation accuracy and efficiency of high frequency electromagnetic simulation software are increasing, we also improved design accuracy and shortened the development cycle by combining traditional design method of equivalent circuit with high-frequency electromagnetic simulation software.Finally, we also manufactured and tested the designed filter, compared and analyzed simulation results with test; verified the correctness of the derivation of theoretical and design, completed the development of the filter.