| Summary: | 博士 === 義守大學 === 電機工程學系博士班 === 98 === The analytic approach and the finite element method (FEM) are employed to evaluate propagation characteristics of surface acoustic waves (SAWs) in layered piezoelectric media. The extracted coupling-of-mode (COM) parameters are then applied in the transmission matrix method for the design of SAW filters in the high frequency band. In the analytic approach, we analyze SAW propagation in AlN/128°Y-X LiNbO3 and (100) AlN/64°Y-X LiNbO3 structures. The phase velocity and coupling coefficient of the multi-layer structures with interdigital transducers (IDTs) and /or thin metal films deposited at various interfaces are calculated numerically. The effects of the polarity of the substrates on SAW characteristics are illustrated. Simulation results show that the multi-layered structures with an AlN film can improve the SAW velocity and coupling coefficient. For the IDT/AlN/128°Y-X LiNbO3 structure, the effects of the polarity were apparent in the coupling coefficient. In the Rayleigh wave region, the AlN/IDT/128°Y-X LiNbO3 structure shows that the effects of the polarity of the substrate can be neglected, and this structure exhibits the highest coupling coefficient. The (100) AlN/IDT/64°Y-X LiNbO3 structure exhibits the largest coupling coefficient. In the leaky wave region, the (100) AlN/64°Y-X LiNbO3 structures shows that the effects of the polarity of the 64°Y-X LiNbO3 substrate can be ignored.
Propagation characteristic of SAWs of electrodes in (100) AlN and (100) ZnO films on diamond structures were analyzed using the FEM. These extracted COM parameters are then applied in the COM theory for the design of SAW devices. However, we found that the elements of SAW devices were difficult to cascade and constructed in parallel or series connection by using the COM matrix. In this dissertation, the transmission matrix method are then applied to design SAW devices, this is because its advantage that we can easily cascade the elements of SAW devices and calculate frequency responses of SAW devices which are constructed in parallel or series connection. Therefore, the transmission matrix method are employed to calculate the IF two-track SAW filter in the IDTs/(100) AlN/diamond structure applicable for the code-division multiple access (CDMA) system and the RF dual mode SAW (DMS) filter, ladder filter, and duplexer in the IDTs/(100) ZnO/diamond structure applicable for the wideband code division multiple access (WCDMA) system.
In this dissertation, the two-track SAW filter, DMS filter, ladder filter, and duplexer are illustrated in the multi-layer structures. The width-modulated reflectors are proposed to improve the performance of the two-track filter. To enhance the sidelobe suppression, the Chebyshev function and scaling factors are adopted to modify the reflectivity distribution of width-modulated reflectors. Simulation results show that the value of the maximum reflectivity will affect the bandwidth as well as the sidelobe suppression level of the width-modulated reflectors and the transmission characteristics of the two-track filter. And results indicate that the maximum reflectivity of width-modulated reflectors is around 0.014 the tailored SAW filters can meet the specification of a current CDMA IF filter with high sidelobe suppression, low insertion loss, and good shape factor. As the (metallization ratio) MR of IDTs for the design of the DMS filter decreases, the 3 dB bandwidth increases, the insertion loss increases, and the shape factor decreases, respectively. Results show that if the maximum reflectivity of electrode is around 0.15, the tailored receiving DMS filter can meet the specification of a current WCDMA RF filter with high sidelobe suppression, low insertion loss, and good shape factor. The transmission matrix method is also employed to calculate the characteristic impedances of one-port resonators in the series arm and one in the parallel arm, while the frequency responses of the transmitting ladder filter cascaded from one-port resonators are constructed via the image parameter method. In this dissertation, the duplexer which consists of the receiving DMS filter and the transmitting ladder filter in parallel connection is illustrated. When the duplexer operates in the transmitting band and the transmission line for the receiving DMS filter is employed for impedance matching, the transmitting signal should not go through the receiving DMS filter. When the duplexer operates in the receiving band and the transmission line for the transmitting ladder filter is employed for impedance matching, the signal should not go through the transmitting ladder filter. This indicates that the transmitting signal and the receiving signal will not interfere. Results show that with a proper design of the transmission line for impedance matching a switchless duplexer can be tailored applicable for the WCDMA wireless communication system.
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