确定BAW梯形滤波器阶数的方法

为确定体声波(BAW)梯形滤波器的阶数，使用仿真法研究了滤波器的带外抑制与并、串联薄膜体声波谐振器(FBAR)的电容比Cps和滤波器阶数N的关系。基于FBAR的Mason 解析电路模型构建了1~6阶BAW梯形滤波器，Cps取值为1~6，对1~6阶BAW梯形滤波器进行仿真，取滤波器的左带外抑制进行统计并绘制曲线图。此外，BAW梯形滤波器的阶数可包括半阶数，用同样仿真法改变阶数(N=1.5,…,5.5)对滤波器进行仿真。仿真结果表明，当Cps一定时，带外抑制基本随滤波器的阶数等量增加。当滤波器的阶数一定时，带外抑制随Cps的增加而增加。在优化设计时，并、串联FBAR谐振器区面积比(即Cps)一般设置为1~4，在此范围内滤波器阶数每增加一阶，带外抑制平均增加约10 dB；滤波器的阶数增加半阶时，带外抑制约增加整阶数的一半，即5 dB，而此时通带内的插损基本保持不变。故设计滤波器时，根据设计指标中的带外抑制可初步确定滤波器的阶数。

A Method of Determining the Order of BAW Ladder Filter

In order to determine the order of the BAW ladder filter, the relationships among the out of band rejection of the filter and the capacitance ratio of the parallel/series FBAR as well as the order N of the filter are studied by using simulation method. The BAW ladder filters with one to six orders has been built based on the Mason model of FBAR. The value of capacitance ratio Cps of parallel/series FBAR is from 1 to 6. The BAW ladder filters with 1~6 orders are simulated and the left out of band rejection of the filter is taken for statistics and drawing curves. In addition, we use the same simulation method to simulate the filter by changing the orders (N = 1.5, ... 5.5) when the orders of the BAW ladder filter are including the half order. The simulation results show that the out of band rejection the basically equal to the order of the filter when Cps is constant, while the out of band rejection will increase with the increase of Cps when the orders of the filter are constant. When optimizing the design, the area ratio of the parallel/series FBAR (ie, Cps) is generally set at 1 to 4. In this range, the out of band rejection of will increase by about 10 dB when the order of the filter increases one. When the order of the filter is increased by half order, the out of band rejection is increased by about half of the whole order, that is 5 dB, while the insertion loss in the passband remains substantially unchanged. Therefore, when designing the filter, the order of the filter can be initially determined according to the out of band rejection in the design specifications.