Research And Design Of Wideband Filters For Mobile Communications

The wide bands defined in 5G new radio not only require RF filters used for mobile communications possess miniaturization and high selectivity performance,but also bring severe challenges to the design of wideband filters.Bulk acoustic wave(BAW)filter with chip-level size and superior selectivity performance based on MEMS technology is the preferred filter choice for mobile terminals.However,the traditional narrowband BAW filter cannot meet the maximum 24.2%fractional bandwidth(FBW)of 5G wide bands.The low temperature co-fired ceramic(LTCC)technology can be adopted for wideband filters,and its multi-layer structure can also meet the miniaturization requirements of mobile communications.However,its roll-off and out-of-band suppression performances still need to be improved.To break the bandwidth limit of the BAW filters and obtain wideband LTCC filters with high performance,these two types of filters are studied in this thesis.The main innovations are as follows:1.Aiming at the problem that the traditional Mason model cannot predict the loss of BAW filter accurately in a wide frequency range,the modified Mason model with frequency-adaptive loss mechanism is proposed.Intrinsic loss factors of materials are introduced to the Mason model,and frequencyadaptive expressions of elements in the Mason model are obtained by solving the acoustic wave equations.By comparison,the same simulation results can be obtained based on the modified Mason model and the Finite Element Method.2.Aiming at the narrowband problem of the traditional ladder typology BAW filter,two methods of introducing external inductors and negative capacitance circuits are proposed to expand the filter bandwidth.By separately using LTCC inductors and 130 nm-CMOS negative capacitance circuits,N41 band BAW filters are achieved,which improves the FBW of the ladder topology BAW filter from 4%to 7.5%.3.A wideband BAW filter based on the multi-mode coupled resonator filter(CRF)is proposed.The coupling layers of CRF are regarded as a stepped impedance resonator that has multiple resonant modes.External inductors are introduced for port match and a 12.8%～35.5%FBW tuning range of the CRF is obtained by changing the thicknesses of coupling layers.Based on the triplemode CRF,an N77 band filter with FBW of 24.2%is designed,which achieves a 1.5 dB insertion loss and a better than 18 dB return loss.4.Synthesis methods of the wideband CRF with single/three coupling layer(s)are proposed based on the modified Butterworth-Van Dyke(BVD)model.Inductors and capacitors are introduced to increase the filter bandwidth and improve the FBW tuning flexibility.The BVD model is modified by taking electrodes into account to enhance the simulation accuracy of the CRF.By reducing the number of coupling layers,approximation errors are reduced when transforming the multiple coupling layers into an impedance inverter.Based on the CRF synthesis method,a fourth-order N77 band filter with a single coupling layer and a fifth-order N78 band filter with three coupling layers are designed.5.Wideband LTCC filter with miniaturization,high stopband rejection,and steep skirts performance,and its synthesis method are proposed.Lumped resonators with transmission zeros(TZs)are cascaded to generate high-order LTCC filter with multiple TZs.The lumped filter typology is modified for proper elements values,which is also beneficial to improving the filter performance.To obtain generalized Chebyshev response,a synthesis method is proposed to design the lumped filter based on the lowpass prototype with nonresonant nodes(NRN).For verification,four N77 and N78 band LTCC filters are designed and fabricated based on the proposed method.