Temperature-compensated SAW Filter With Low Temperature Drift And Large Bandwidth

With the arrival of the Fifth Generation Mobile Communication System（5G）,the number of frequency bands to be supported by end products is increasing and spectrum resources are getting tighter.The demand for filters not only requires more quantity,but also better insertion loss,steeper rectangularity,better temperature stability and larger bandwidth.Temperature-compensated SAW filters（TC-SAW）are widely used in mobile communication systems such as cellular phones,and are characterized by good temperature stability,small size,and high interference immunity.In this thesis we focus on the optimization of the electromechanical coupling coefficient（k²）of the resonator,the suppression of transverse modes,the simulation model and the circuit structure of the temperature-compensated SAW based on SiO₂/Cu/Y-X°LiNbO₃ structure.1.The SiO₂/Cu/Y-Xθ°LiNb O3 model was established using the finite element simulation software COMSOL,and the propagation and temperature characteristics of the SH mode based on this structure were obtained.The effect of the rotation angleθon the resonator k² was then investigated,and the relationship between k²and the Euler angle was obtained through simulation,and it was determined that the structure has the maximum k² whenθis 5°.2.The introduction of Temperature-Compensated SiO₂ layer inevitably leads to the generation of transverse modes.The previous solution is to create a piston structure by thickening or widening the finger strips at the end of the finger strips to form a"slow-block-slow"sound velocity difference,thus limiting the energy to the IDT region to suppress the transverse modes.The effect of transverse modes is achieved.In this thesis,both wider and thicker pistons are investigated,and it is found that thicker pistons have better design stability.In addition,a new structure of transverse mode suppression is proposed,which has a better transverse mode suppression effect compared with the traditional piston structure.The related results have been patented.3.The small rotation angle will lead to the appearance of Rayleigh spurious,and the presence of parasitic spurious will lead to a large ripple in the passband.In this thesis,the resonator admittance curves of SH and Rayleigh modes with different structural parameters are analyzed by COMSOL finite element software,and the Rayleigh spurious is suppressed by adjusting the film thickness ratio（h/λ）and Duty Factor（DF）.4.The MBVD model and COM model of the commonly used SAW resonators were derived and summarized,and the resonator conduction curves were combined with COMSOL simulations to derive the parameters of the model and complete the preliminary dereference work.5.Based on the above study,the optimal structural parameters of the resonator were obtained,and the COM model was used to extract the parameters of the basic unit of the resonator according to the simulation results of COMSOL.The final result is a large bandwidth filter with a center frequency of 785.5MHz and a-3dB relative bandwidth of 8.27%.The filter size is 11mm*0.9mm,which is suitable for the current mainstream RF module chip size.