In order to ensure the normal operation of mobile devices in the Wi-Fi band without interference from adjacent frequency bands, a BAW filter for the Wi-Fi 802.11b band (2402-2482 MHz) is designed. An initial structure ladder filter based on a one-dimensional Mason equivalent circuit model of thin-film bulk acoustic resonator (FBAR) is designed. The resonance area value of series FBARs and the ratio of resonance area value of parallel FBARs to series FBARs are made into two types of optimization parameters reasonably. According to the required insertion loss and out of band rejection of filter as the optimization objective, the optimized values are obtained by the algorithm based on gradient and genetic in ADS software. In order to make the simulation results more accurate, the combined acoustic-electromagnetic method is used to simulate and compare with the simulation results of the Mason equivalent circuit model in the filter design process. The results show that the performance of the filter is decreased, insertion loss increased 1.6 dB, ripple increased 1.1 dB, out of band rejection is basically the same. The design of Wi-Fi band BAW filter has low insertion loss (less than 3 dB) and high out of band rejection (more than 40 dB) performance.
In order to reduce the volume of the filter and increase the number of chips on the wafer, while ensure the filter performance, a design method of the bulk acoustic wave (BAW) ladder filter is proposed. This layout design method consists of 11 design criteria and a 6-step flow. The 11 design criteria limit the shape and position of the BAW resonators (BAWRs), the distance between the BAWRs, the distance between the BAWRs and the pads and the interconnecting wire. The layout design flow has 6 steps. 1) Preset the shape of each BAWR (square/pentagon) according to its active area values. 2) Add an auxiliary circumcircle for each BAWR, tightly align all the series resonator circumcircles along a central line in order, and mate the corresponding electronically neighboring parallel resonator circumcircles one by one at a position above/below the center line. This makes an initial 3-row and n-column 2D arrangement, and the column number N is determined by the filter order. 3) Fix the very first series resonator circumcircle position and incrementally “compress” the initially self-assembled 3-row structure along the row width direction until the row height for row width bargain is no more cost effective. 4) Apodize the square series resonators and fine-tune each resonator’s shape and rotation according to above-mentioned related design criteria. 5) Wiring BAWRs and pads together. 6) A combined acoustic-electromagnetic BAW filter simulation method is used to validate the layout result. In a 5-order BAW ladder filter layout demo case, a layout fill ratio over 44% is obtained. An auto-layout program “BAW-filter Auto-layout Tool (BAT®)” based on the presented method is also presented.
In order to obtain the high-fidelity model of latching failure threshold power of the capacitive RF MEMS switch, it is necessary to find out the rough dielectric layer effect on its down-state capacitance degradation. The comparative modeling method between the 3-D electromagnetic simulation and the equivalent circuit simulation is proposed. First, the simulation curve of the switch isolation (S21) is attained at different roughness levels with the HFSS 3-D electromagnetic model. And then the simulation curve of the S21 of the ADS equivalent circuit model is consistent with the simulation result of the 3-D electromagnetic as far as possible by tuning the down-state capacitance in the equivalent circuit. Hence, the relationship between the dielectric layer roughness and the down-state capacitance is identified. By changing the roughness level of dielectric layer and repeating the above steps, the relationship between the dielectric layer roughness and the down-state capacitance degradation is identified. Rationality and feasibility of the method is verified by comparing the calculated values of the down-state capacitance with the measured values in a relevant literature. And analytical equation of the latching failure threshold power of the capacitive RF MEMS switch with perfect smooth dielectric layer is modified, according to the relationship between the dielectric layer roughness and the down-state capacitance degradation, which is also suitable for predicting the power handling capacity of the switch with rough dielectric layer.
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