| Summary: | 博士 === 義守大學 === 電機工程學系博士班 === 93 === Surface acoustic wave (SAW) devices have the advantages of miniature size, rugged structure, ease of mass production and wide frequency ranges (10M~10G Hz), etc. The needs of SAW devices for intermediate frequency (IF) and radio frequency (RF) filters are increasing because of the above advantages and the fast growth of mobile communication. Hence, it induces a challenge of designing SAW devices with low insertion loss, flat passband ripple, high sibelobe suppression and great shape factor to meet the specification of modern communication systems. For these reasons, in this dissertation, an improved transmission matrix method is proposed to analyze the electrical and acoustic characteristics of SAW devices.
When comparing our approach with conventional transmission matrix method, we found its advantages are that we can easily cascade the elements of SAW devices and calculate the overall frequency responses of SAW devices which are constructed in parallel or series connection. Besides, in our approach, the effects of self-coupling and mutual-coupling are taken into account, thus the proposed method can be used to analyze IDT with different types of electrodes easily, e.g., double-electrode and single-electrode.
By using this approach, different structures of SAW devices can be designed easily such as wireless identification card, liquid sensor, resonator and duplexer. In this dissertation, two-track SAW filter and SAW filter with two-track slanted interdigital transducers are illustrated. The width-controlled reflectors are proposed to improve the performance of the two-track SAW filter. Several window functions and scaling factors are applied to design the width-controlled reflectors. Different stepwise weighting schemes are then employed to recalculate the line width distribution for the realization of the modulated reflector. The designed IF filters are applicable to the code-division multiple access (CDMA) system.
The SAW filter with two-track slanted finger interdigital transducers (SFITs) is the combination of SAW SFIT filter and two-track SAW filter. Thus the novel filter has both advantages of SFIT filter and two-track SAW filter. Curve fitting technique is used to flatten the passband amplitude of the filter and the gap lengths of each track are designed to eliminate the passband ripples. Besides, Chebyshev polynomial and two-step weighting technique are employed to increase the sidelobe suppression level. Simulation results show that a broadband IF filters with high sidelobe suppression, flat passband and small passband ripple can be obtained.
To confirm the feasibility of the improved transmission matrix method, the two-track SAW filters are realized and measured. According to the simulation and measurement results, it can be found that the simulation results correspond to the measurement results. Thus the feasibility of the proposed approach for SAW devices design is proved.
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