| Filter is a kind of frequency selection device that can be set to achieve the interception or passage of certain frequencies and is widely used in electronic technology and electrical equipment.the Q value of the quartz crystal is at least two or three orders of magnitude higher than that of the capacitor-inductive element,and the crystal temperature characteristics of some cut-type crystals are good to realize very small frequency variation in the wide temperature range,so it becomes the first choice of high frequency filter.However,this block market is mainly monopolized by foreign countries,and there are only a handful of domestic manufacturers that can produce it and can't reach the capacity demand in the future.Based on this,we are preparing to develop a high-basic crystal filter to meet domestic demand.This paper analyzes the various aspects of crystal filter manufacturing,focuses on the development process of high-base wave wafers and crystals.The research on the ultra-thin wafer grinding method and the process of improving the crystal parasitic and improving the crystal index consistency are studied,and a high-basic crystal filter is developed.The main research content is divided into four parts,as follows:1.Prepare high quality crystals.The effect of OH-ions in NaOH on the Q value of the crystal is avoided mainly by replacing the existing NaOH solvent with Na2CO3.During the growth process,electric cleaning is added,and a series of means are applied to the crystal material to apply a high-voltage electric field at a high temperature to change its internal charge compensation state.The crystals with quality factor greater than 3000 K were obtained,which laid the foundation for the production of high-basic crystal resonators.2.Study the high frequency wafer fabrication process.Chemical mechanical polishing was introduced during the basic grinding stage to produce wafers with flatness that meet the design requirements and improved surface roughness.In the high-frequency production stage,the chemical etching method and the ion etching method are studied and improved in detail,and a set of process flow suitable for high-frequency wafer production is explored.3.Study the theory of resonators and design crystals with low temperature and low frequency in low parasitic and wide temperature regions.Different types of crystals have different temperature-frequency characteristics.By calculating the appropriate cut-off type,the crystal frequency is minimized from-40 to 70 °C,thus the frequency stability of the filter can be improved.And the aging of the resonator caused by the "mass load effect" and "stress relaxation effect" is studied.By analyzing the processing of the resonator,we have solved the aging problem of the crystal filter.4.Select the filter function that meets the requirements.The circuit and working principle of the crystal filter are studied and the characteristics of each response function are known,we proposed to design the filter circuit based on the Chebyshev and Bessel functions,which effectively guarantees the stopband suppression and group delay requirements.The simulation of the filter function is carried out by computer to adjust the component values,which effectively reduces the repeatability of the development process and improves the development efficiency.Through in-depth study of the above aspects,a 124.8MHz high-basis crystal filter with low insertion loss,high resistance band rejection,small group of time delay and wide temperature range was finally made. |
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