RSPUDT(谐振型单相单向换能器)滤波器的改进型设计

IMPROVED STRUCTURE OF RSPUDT (RESONANT SPUDT) FILTERS

RSPUDT滤波器通过引入声谐振腔 ,改善了普通SPUDT(单相单向换能器 )滤波器的形状因子和通带特性 .但由于它结构复杂 ,加权和优化很困难 .提出一种改进型RSPUDT结构 ,将原来集中的声谐振腔均一地分布在换能器中 ,成为许多相同的谐振腔单元 .这种结构使得RSPUDT可以采用基本的加权方法 ,优化过程更加简单 .34°ST切割石英基片上RSPUDT滤波器的计算和实验表明 :改进型RSPUDT较之于原型具有更好的形状因子和更平坦的通带 .改进型RSPUDT的这些优点将使其在低插损中频滤波器的设计中得到广泛应用 .

With excellent frequency characteristics, the SPUDT (Single Phase Unidirectional Transducer) has been widely used in IF passband filters. The main drawback of this technique is that normal SPUDT filter cannot achieve flat passband and good shape factor because of the distribution of reflective strips. P. Ventura first presented a new SPUDT concept called RSPUDT. Its basic principle is to create resonant cavities inside the SPUDT that can make SPUDT filters achieve better filtering characteristics. However, the original RSPUDT put forward by P. Ventura is too complicated to be weighted for high quality performance and is not flexible in changing the radiation resistance of transducers. In order to solve these practical problems, we have designed a new type of RSPUDT based filter. The improved RSPUDT developed by us is easier to be weighted and optimized and has better performance compared with the original one. In the improved RSPUDT, large acoustic resonant cavities are uniformly distributed as a lot of little resonant cavities called RSPUDTcells. Then normal weighting functions for SPUDT based filters can be used in the improved RSPUDT filters through weighting these elemental RSPUDTcells. Also we can change the global characteristics of the improved RSPUDT filters by adjusting the structure of these uniform RSPUDTcells such as the distance between the reflective center and the transduction center. Some weighting and optimized results are discussed in this paper. Both calculation and measurement results in this paper show that the improved RSPUDT filter has smaller ripple in passband, better shape factor, lower insertion loss and higher sidelobe suppression compared with those of the original RSPUDT filter with the same package size and the same metallization thickness. With these obvious advantages, the improved RSPUDT will be a very useful structure for high quality low loss IF filters in the future.