【摘要】 传统的一、三模纵向耦合谐振滤波器在频率响应的高端部分有一个固有的隆起,严重地影响了器件的性能,限制了该器件的应用范围.为了改进器件的性能,利用单端对谐振器的反谐振峰将其作为陷波器用以抑制传统的一、三模纵向耦合谐振滤波器的高端边瓣.首先,利用COM理论,在42°Y-XLiTaO3基片上,分析了单端对谐振器以及一、三模纵向耦合谐振滤波器与之串联的频率特性.然后,优化了单端对谐振器以及一、三模纵向耦合谐振滤波器的结构,并在42°Y-XLiTaO3基片上对改进前后的结构进行了实验.理论和实验结果一致表明,通过改进,传统的一、三模纵向耦合谐振滤波器的高端边瓣得到了有效的抑制.进一步在实验上将两个相同器件级联,获得了接近40dB的高频边瓣抑制,比改进前的器件提高了8~10dB的高端边瓣抑制量.更多还原

【Abstract】 By the development of communication industry, the longitudinally coupled resonator filter with first-third modes becomes increasingly important in the wireless communication system, because of its characteristics of small volume, large bandwidth, low insertion loss and high frequency. However, its significant drawback is that the out-of-band rejection is relatively poor; especially there is a shoulder at the range of higher frequencies in its frequency response, which will limit the application field of the device seriously. For improving the performance of the longitudinally coupled resonator filter with first-third modes, this paper utilizes the anti-resonant peak of a one-port resonator as a notch filter to suppress the sidelobes caused by the shoulder at higher frequencies. The examples of the experiment are realized on the substrate of 42° Y-X LiTaO₃. Firstly, the paper uses Coupling-of-modes （COM） theory to analyze one-port resonators and the device elements by cascaded of the one-port resonator and common longitudinally coupled resonator filter with first-third modes. Both the one-port resonator and common longitudinally coupled resonator filter with first-third modes are devided into several building blocks which consist of gratings, IDTs （interdigital transducer） and acoustic transmission lines. Every building blocks is described by its transmission matrix which gives the relation between the waves at their twos ends. The Y-parameters of the device can be calculated by cascading the individual transmission matrixs. Secondly, the paper optimizes the structure of the one-port resonator and longitudinally coupled resonator filter, such as the number of finger of reflective array and interdigital transducer, the distance between the reflective array and IDT and the aperture of resonator etc. Both simulation and experimental results of device element show that the sidelobes at the shoulder of the longitudinally coupled resonator filter is improved about 5 dB. In order to improve the side-lobe rejection further, we cascaded two identical device elements together in the experiment, and got side-lobe rejection at the range of higher frequencies to nearly 40 dB, which is about 8 to 10 dB improvement comparing with the cascaded structure without notch filter. It is obviously to see that the improvement of side-lobe rejection using notch filter is significant, and experimental results agree well with the simulation results. It should be pointed out that the insertion loss should be increased after using notch filter and cascaded two device elements, so the optimizing of the structure to make an impedance match between notch filter and longitudinally coupled resonator filter is important to avoid great increase in insertion loss. This paper provided a way of saw resonant filters with improved structure, and got initiatory success. But we still have to do more study on improvement of pass- band characteristic and insertion loss and so on.更多还原