Research On Ultra Wideband Saw Filter Based On PIMNT

Surface Acoustic Wave(SAW)filters are widely used in mobile communication equipment terminals due to the advantages of low insertion loss,miniaturization,low cost,no need for external circuit matching,and high consistency.In recent years,with the rapid and large-capacity development of communication systems,higher requirements have been placed on the passband width of the filter.At present,the bandwidth of commercial SAW filters does not exceed 13%,which cannot meet the needs of some emerging frequency bands.The most effective way to solve the problem of small filter bandwidth is to choose piezoelectric materials with high K~2.Therefore,based on the resonator structure ofpiezoelectricsingle crystal Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-Pb Ti O₃(PIMNT),Thesis studies whether this material can provide a higher K~2,To achieve a wider bandwidth SAW filter.The main work is divided into the following three parts.Firstly,a three-dimensional model of the resonator is first established using the simulation software COMSOL,and the propagation characteristics of the surface acoustic wave of the resonator model under different tangential directions,metallization ratios,substrate thicknesses,electrode materials and film thicknesses are studied.Among them,the SH₀mode exhibits superior characteristics,the K~2is greater than 80%,and the spectral curve is very clean,which is very suitable for the design of ultra-wideband filters.However,compared with conventional piezoelectric materials such as Li Nb O₃and Li Ta O₃,the SH₀modal velocity based on PIMNT substrate is lower,which is not suitable for designing SAW devices operating in high frequency bands.So we focused our attention on the Lamb wave with higher velocity,and studied the propagation characteristics of the six modes in front of the Lamb wave on the PIMNT substrate.Among them,the A₁mode has excellent performance.When the PIMNT thickness is less than 0.1?,the K~2of the A₁mode is greater than 40%,but it also introduces high-order harmonic parasitics,which are difficult to eliminate,which will has some impact on the performance of the resonator.Because the dispersion effect of the A₁mode is relatively strong,its velocity and K~2decrease sharply with the increase of PIMNT thickness.Increasing a certain PIMNT thickness can remove the high-order harmonic parasitics in the passband,at the same time,the K~2will decrease,but it still remains greater than 30%,and the speed is much higher than the SH₀mode,which is very suitable for the design of high-frequency broadband filters.Finally,the loss of the resonator is analyzed theoretically,and a method of external circuit is provided.The interdigital electrode is equivalent to an ideal electrode and an equivalent resistance is connected in series to simulate the electrode loss.Secondly,the finite element model simulation parameters of the resonator are extracted by the MBVD model,and a surface acoustic wave filter based on110°YX-PIMNT piezoelectric material is designed using the combined acoustic-electromagnetic method.The center frequency is 2.59GHz and the bandwidth up to 594MHz.Finally,as a comparison with PIMNT material,a piezoelectric material of42°YX-Li Ta O3 is used to design an RX SAW filter with the same frequency for communication equipment.The electrical performance test of filter samples at room temperature,the test results meet the index requirements,and the bandwidth is137MHz.