Research On Surface Acoustic Wave Filter Based On Cu/15°yx-linbo₃

Surface and bulk acoustic wave (SAW/BAW) filters are now widely used in themobile communication systems for their excellent characteristics, small size and lowinsert loss. Recently, SAW/BAW resonators in combination with variable capacitors(VCs) are applied in tunable RF-filters to relax the complex design of the radiofrequency (RF) front-end of current mobile terminals, which have to support multi-bandand multi-mode operations.One of the key techniques to realize tunable SAW filter is piezoelectric materialswith extremely large electromechanical coupling factor applied for SAW resonators.Cu-grating/15oYX-LiNbO3structure brings extremely large piezoelectricity. Based onthe material, we study the character of SAW propagation, how the performance of filtercan be got and the tunable range and nonlinearity character of tunable SAW filter. Someimportant and valuable results which bring forth some new ideas are accomplished andlisted as followings,1. Cu/LiNbO3structure has so strong confinement effect in the aperture that thedominant mode decays under the apodizing aperture for the suppression of transversalmodes. By applying scalar potential theory to the IDT expressed as waveguide modelwhich has multiple tracks, characteristic of the transversal modes can be derived. Theresult revealed that setting proper length of dummy electrode makes two transversalmodes coupled or higher transversal modes leaky. Suppression mechanism of transversemodes is assumed that the modes trapped into aperture are coupled with ones intodummy electrodes and they become leaky. Based on these assumptions, these couplingand leaking of transversal modes can be theoretically modeled using coupling-of-model(COM) theory. This model allows simulating energy transfer of each transversal mode.By fabrication of resonators in which shape of its dummy electrodes is changed, thisanalytical method shows efficacy.2. A new SAW filter structure with constant group delay in passband is proposed.The bandpass Bessel LC filter is designed, and LC resonators in the filter were replacedby SAW/BAW resonators. It is shown ripples in the group delay could be suppressed by optimal design of SAW/BAW resonators. Norton s first transform was also applied toreduce variation of resonator shunt capacitances. It is shown that flat group delay withthe deviation of3ns is realizable over the frequency range of98015MHz.3. A new design technology to flexibly control the passband is proposed with highpiezoelectric materials. A bandpass LC filter is designed based on the traditional ellipticfilter design. We replace the LC resonators with SAW/BAW resonators in this filter andoptimize the design of the resonators to suppress passband ripples generated by finitecapacitance ratio γ. It revealed that filters with different relative bandwidth are easy tobe designed using one piezoelectric substrate. A SAW filter with relative bandwidth of5%is realizable at the center frequency of about1GHz on Cu/15oYX-LiNbO3substrate.The tunable range and IMD2characteristic of two types of tunable filters usingSAW/BAW Resonators and VCs are discussed.4. A wide band DMS SAW filter is designed and farbricated withCu/15oYX-LiNbO3substrate. The design-simulator that is supposed the using ofCu-grating/15YX-LiNbO3-substrate structure was built. For the accuracy enhancementabout that design-simulator, parameters needed in that are determined by probativelyfabricated one-port SAW resonators on the Cu-grating/15YXLiNbO3-substratestructure. DMS filters were optimally-designed by using that simulator. As a result, it isshown that the relative-3dB bandwidth more than12%is achievable. However, toolong aperture is needed in that design. In this case, because electrical resistance andresistance loss at interdigital transducer (IDT) become extremely large, that is notsuitable for fabrication. Therefore, the pitch modulation technique was applied for thedesign of IDTs. The pitch modulation technique allows us increasing the number ofdesign variables, and much shorter aperture is achieved by using this technique. Finally,the designed DMS filter was fabricated on Cu-grating/15YX-LiNbO3-substratestructure. The measured result agrees well with the design, and the passband width of14.2%and the minimum insertion loss of1.3dB were obtained at900MHz range.