声表面波谐振滤波器中纵横向耦合模式的研究及其复合应用

在通常的声表面波谐振滤波器中,由于采用的模式不同,纵向耦合谐振器具带宽大和插损低,带外抑制较差的特点;而横向耦合谐振器则具有带外抑制高,低插损及通带较窄的特点．为了综合两种滤波器的优点,采用了一种改进的滤波器结构,可同时利用纵向与横向耦合模式,得到既有较大带宽又有高带外抑制的声表面波谐振滤波器．运用耦合膜理论对纵横向模式的复合进行了分析,并在计算机上对这种结构的滤波器特性作了模拟,实验结果与模拟结果得到了较好的吻合．

Longitudinally Coupled Modes and Transversely Coupled Modes in SAW Resonator Filters and their Compound Applications

In traditional SAW coupled resonators, there are longitudinally coupled resonators and transversely coupled resonators. Because they apply different coupled modes, both kinds of resonators have different characters. The longitudinally coupled resonator can provide a large bandwidth and very low insertion loss, but its significant drawback is that the out-of-band rejection is relatively poor. The transversely coupled resonator offers possible enhancement of out-of-band rejection and low insertion loss while the bandwidth can" t be very large. To combine the advantages of both kinds of coupled resonators, we apply an improved resonator structure to excite both longitudinally coupled modes and transversely coupled modes to achieve not only a wide bandwidth but also a good out-of-band rejection.Couplingof-modes (COM) theory is used to analyze this kind of SAW resonator filters combining longitudinal and transverse modes. The filter is divided into several building blocks vhich consist of gratings, IDTs and acoustic transmission lines. Every building block is described by its transmission matrix which gives the relation between the waves at their two ends. The Y-parameters and S-parameters of the total SAW filter can be calculated by cascading the individual transmission matrixes. But because the structures of IDTs of SAW resonator filters combining longitudinal and transverse modes are not uniformly distributed, their transmission matrixes can not be derived from the COM. models directly. To solve this problem, we divide IDTs into several different building blocks, including dummy fingers, uniformly distributed IDTs parts and acoustic transmission lines. From these building blocks' respective COM models their transmission matrixes can be derived. Then the total transmission matrixes of IDTs can be calculated out. Several filters are designed, simulated, fabricated and measured. Simulation results are shown to agree well with experiments.