| The demand of high-speed and accuratetransmission of information in 5G era puts forward higher requirements for communication system.Radio frequency(RF)front-end devices and modules in the physical layer play an increasingly important role in ensuring high speed,accurate and low loss information transmission.The filter plays a core role in the communication system,among which the thin film bulk acoustic resonator filter(FBAR)gradually occupies the RF front-end market with high quality factor,low loss,high electromechanical coupling coefficient,high rectangle coefficient,good out-of-band suppression and 5G band applicability.However,there are some shortcomings in the application of thin film bulk acoustic filter in high frequency and broadband.Therefore,this paper starts with FBAR,and first puts forward a high accuracy material parameters fitting model.Subsequently,a series of wideband and highly selective multi-function hybrid filters are designed and fabricated based on accurate model and a variety of laminate technologies.The work of this paper includes the following aspects.(1)The extraction of material parameters of FBAR in longitudinal mode is studied.Specifically,a simplified stepped transmission line resonator model is proposed to accurately fit the parallel resonant frequency(fp)of the measured FBAR resonator.The proposed simplified model removes the effects of loss resistors,series resonant frequency(fs)and effect of static capacitance on the fp in the original Mason model,so that the designer can use the longitudinal acousitic velocity and material density to fit the fp more accurately.In this work,the resonance condition of the simplified model is analytically derived and the error of the material density fitted with Mason is controlled within 0.13%.At the same time,the fp of the measured FBAR in five different regions of the wafer is fitted simultaneously by using the simplified model,and the results show that the maximum error is less than 6 MHz,and a good frequency fitting is achieved.The proposed model maximizes the accuracy of fitting while simplifying the cost of parameter extraction.The relevant research results have been published in the international academic journal IEEE Transactions on Circuits and Systems Ⅱ.(2)The performance of thin film bulk acoustic resonator based on different microstructure is studied.Different resonator performance can be obtained by controlling the size of many kinds of frame structures.The performance of the relevant resonators is calculated by data processing,and the most appropriate microstructure combination is selected.The above accurate model is used to fit the shunt frequency(fp)to obtain the Mason model which can accurately predict the fundamental frequency of the filter.Finally,using the accurate Mason model,a LTE Band-3(B3)duplexer is designed.The duplexer is designed by traditional ladder type network,and the port impedance of the duplexer is adjusted by the grounding inductors respectively.Finally,the B3 duplexer with low loss and high isolation is realized.The meaured results verify the accuracy of the circuit design and the model.The isolation exceeds 55dB,the insertion loss is 2.5 dB and the return loss is more than 11 dB.At present,more than 8 million chips of this B3 duplexer have been shipped through Huntersun Company.Relevant research results have been published in international academic journal IEEE Transactions on Circuits and Systems Ⅱ.(3)Two hybrid filters with planar structure are proposed.Firstly,a hybrid wideband filter with high frequency selectivity is proposed.The analytical of the hybrid filter combines the traditional Mason model and the theory of microwave network.The conditions for generating transmission zeros and transmission poles are obtained.The simulation results inversely verify the proposed theory.The error is controlled within 1.69%.In order to verify the proposed theory,the hybrid filter with center frequency in 2.14 GHz is fabricated and measured,and the test results are in good agreement with the simulation results.The test results of the hybrid filter proposed in this work show the advantages of 24.9%3dB relative bandwidth,minimum insertion loss of 1.87 dB,and 91%shape factor.Then,a planar centrosymmetric hybrid filter is proposed.A hybrid filter with 22.4%relative bandwidth is creatively proposed.By taking the electrical equivalent impedance of FBAR as an element in the network,the corresponding conditions for the generation of transmission poles and zeros are obtained by using the odd-even mode analysis method,and the above conditions are verified by simulation parameters to form a closed loop.The final measured results and simulation results also show a high degree of consistency.The relevant research results have been published in the international academic journal IEEE Transactions on Microwave Theory and Techniques and IEEE Transactions on Circuits and Systems Ⅱ.(4)Two high frequency film bulk acoustic wave filter chips are proposed.First of all,based on experience,various splits of the microstructure of the high frequency FBARs are given and taped out to obtain the accurate model.Then,the N79 subband filter is designed with frequency band 4.8-5 GHz,and the suppression of the upper and lower sideband is balanced in a special way.Under the condition that the low frequency suppression is more than 40 dB,the suppression of the 5G Wi-Fi band is more than 50 dB.Finally,the hybrid filter of FBAR is designed by combining integrated passive device technology(IPD).Through the passband compensation of lumped devices of IPD and the addition of out-of-band zeros,a full-band N79 integrated hybrid filter chip with upper sideband suppression effect is successfully developed.Its bandwidth is 4.4-5 GHz,and the upper sideband suppression is more than 45 dB.The bandwidth of FBAR filter chip is successfully expanded by hybrid technology,which has a great application prospect.A number of Chinese patents have been granted. |
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