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In order to determine the order of BAW ladder filter, the relationship among out-of-band rejection, area ratio of parallel and series FBAR, and the filter order N are studied by simulating. Based on FBAR Mason model, firstly, one-order to six-order BAW ladder filters are constructed; next, the area ratio of parallel and series FBAR (Cps) is set from 1 to 6, and the six BAW ladder filters are simulated in ADS; finally, the left out-of-band rejections are extracted from the simulation result, and plotted as a graph. In addition, as the filter orders can be set to a half order, the filters with order N (N=1.5… 5.5) are simulated in the same way. Simulation results show that the out-of-band rejection has an equal increase with the filter order in number when Cps remains constant, and that the out-of-band rejection increases as Cps increases when filter order keeps constant. When optimizing the design, the area ratio of parallel and series FBAR (Cps) is usually set from1 to 4, within which the out-of-band rejection will increase by about 10 dB when the filter increases by one order. And when the filter increases half order, the out-of-band rejection will increase by 5 dB, which is about half of the value by increasing an integer order. In addition, the structure of ladder filter is discussed, and the influence of the filter order and capacitance ratio on the passband performance is studied.
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