Design And Implementation Of High Frequency And Small Size Crystal Filter

The filters are very important in the wireless communication equipments.The crystal filters are widely used because of their narrow relative bandwidth and high stopband attenuation.With the fast development of the communication technology,the crystal filters with higher frequency and smaller volume are demanded by the communication equipments.In this paper,the development course of the crystal filters is introduced,the research status of the high frequency and miniature crystal filters at home and abroad is analyzed,and the design principle of the high frequency and miniature crystal filters is studied.The typical 70 MHz crystal filter is taken for example to research the design method and processing technology in detail.The research contents include:1 The design of an individual crystal resonator.By means of the comparison of various piezoelectric single crystal materials,the quartz crystal is chosen as the 70 MHz filter material.By means of the study of different crystal cut types,the proper cut type is determined.By means of the research of the relationship between the thickness and the frequency in the thickness-shear mode,the crystal thickness is calculated.By means of the theory of equivalent circuit model,the crystal area is calculated,and the effect of the crystal size on the parasitic vibration is analyzed.By means of the combination of the Chebyshev function and energy trapping effect,the optimized electrode size is determined.2 The design of the filter structure.Because the 70 MHz crystal filter requires high technical performance,only the three-resonator structure filter design can satisfy the requirements of the rectangle factor and the stopband attenuation.In this part,The filter structure is studied and the coupling capacitance is calculated.3 The study of the filter processing technology.In this part,the whole processing of the 70 MHz filter is studied in detail,including: the crystal orientation,the crystal scribing,the crystal grinding,the crystal etching,the crystal cleaning,the crystal coating,the frequency trimming,the crystal packaging,the device assembling,and the device commissioning.The processing difficulties(like the crystal scribing,the crystal grinding,and the crystal etching)are deeply researched,and some optimization and improvement are presented based on the traditional fabrication process.4 The design of the filter reliability.In this part,the thorough research on the design method to improve the temperature-shock strength and mechanical strength of the 70 MHz crystal filter is presented.The reliability studies of the filter structure design,processing technology and process control are presented.All of the technical indexes of the fabricated 70 MHz crystal filter reach the expected target.The achieved methods and results will be widely applied to the design and fabrication of various high frequency and miniature crystal filters.