The Design Of Interdigital Bandpass Filters For 5G Communication

With the development of the fifth generation mobile communication(5G),the requirements of communication system for filters are getting higher and higher.Miniaturization,integration,high performance and easy processing have become the mainstream development directions of filters.Interdigital filter has many advantages,such as wide application range,small insertion loss,small size and far second passband.It can meet the design requirements of millimeter wave filter.In this thesis,two interdigital bandpass filters are designed for two working bands of5 G communication,namely 4.8GHz-5GHz and 24.75-27.5GHz,based on the fabrication technology of micro-electro-mechanical system(MEMS).Meanwhile,the miniaturization of interdigital filter is realized.The specific work is as follows:(1)Miniaturization design of interdigital MEMS filters.For working band24.25GHz-27.5GHz,a K-band interdigital microwave filter is designed,which is composed of five layers: upper metal layer,upper dielectric layer,metal electrode layer,lower dielectric layer and lower metal layer.The input and output microstrip lines of traditional interdigital filters are located in the metal electrode layer.In order to feed in and out signals,the lower dielectric layer needs to be designed slightly longer than the upper dielectric layer.In this thesis,the input and output structure of the filter is changed by setting the input and output microstrip lines in the upper metal layer.The signal transmission channel between the upper dielectric layer and the metal electrode layer is constructed through a metal through-hole in the upper dielectric layer,thus the length of the lower dielectric layer can be reduced and the miniaturization design of the filter is realized.Then the K-band interdigital filter with traditional structure and the one with miniaturized structure are respectively modeled and simulated by circuit and electromagnetic simulation software.The simulation results show that the size of the filter with miniaturized structure is reduced by about 24% compared with the one with traditional structur,while the performance of the filter is almost unchanged.Further analysis shows that the position and shape of the through hole mainly affect the rectangular coefficient of the filter,and meanwhile the depth of the air chamber has an effect on bandwidth,center frequency and rectangular coefficient of the filter.(2)Design of interdigital MEMS filter based on step impedance Resonator(SIR)structure.For working frequency band 4.2GHz-5.6GHz,a C-band interdigital MEMS filter is designed with SIR structure.Traditional interdigital filters are usually designed with Uniformity Impedance Resonator(UIR)structures,in which each resonator has only one linewidth.Therefore,there are few design parameters in UIR structure.In this thesis,the structure of resonator is changed.SIR resonator is formed by connecting two transmission lines with different characteristic impedance in series,and therefore the number of design parameters has increased.The simulation results show that SIR structure can effectively shorten the filter length,while a part of the filter width is widened and the performance of the filter is almost unchanged.What's more,the influence of transmission line width and line length on central frequency and bandwidth in SIR structure is simulated and analyzed.(3)Manufacturing and testing of interdigital MEMS filters.The optimization model of K-band interdigital filter with miniaturized input-output structure is mapped by AutoCAD,and after that,the manufacturing process of this filter is completed based on the technology of MEMS.Moreover,the validity of the proposed miniaturized input-output structure is verified by testing the performance of K-band interdigital filter and comparing it with the simulation results.