Electromagnetic modeling, simulation, and design of balanced ceramic IF SAW filters

The trend of improving the performance of surface acoustic wave (SAW) filters requires accurate software tools for the simulation of all relevant effects and interactions affecting the filter performance. Acoustic tracks are geometrically complex structures that typically feature hundreds of fingers and stubs with extreme aspect ratios. Moreover, one filter consists of potentially many acoustic tracks. Thus, electromagnetic effects of the details of the acoustic tracks are typically neglected. In this paper, it is shown that for some filters having a considerable acoustic track area, such as, for example, the intermediate frequency (IF) filter presented, acoustic tracks can have a decisive influence on electromagnetic feedthrough, affecting the filter performance. Especially the stop band rejection may suffer from uncontrolled electromagnetic feedthrough. A new model accounting for the electromagnetic interaction of acoustic tracks with other potentially relevant structures, for example neighboring structures such as pads, is presented. The approach is applied to an IF filter for Wireless Local Area Network (WLAN) applications at 462 MHz in a 5 mm × 5 mm ceramic package for Surface Mount Technology (SMT). The performance of the filter is improved by means of minor changes of the layout based on the accurate simulation of acoustic and electromagnetic effects. The influence of acoustic tracks on electromagnetic feedthrough is investigated with the help of a method for the experimental suppression of surface acoustic waves.