Thermal behavior of BAW filters at high RF power levels.

The temperature increase of bulk acoustic wave filters at high RF power levels has been investigated. Self-heating due to power dissipation in the filter leads to a nonuniform frequency shift of the insertion loss. At the right filter skirt, self-heating is amplified by the negative temperature coefficient of frequency. We demonstrate that at high RF power levels, this can cause thermal instabilities resulting in an abrupt step in the insertion loss. A novel frequency transformation is introduced to describe the nonuniform frequency shift of the insertion loss as well as the thermal instabilities. A condition for the occurrence of thermal instabilities is derived. It is argued that because of this nonuniform frequency shift at high power levels, accelerated lifetime tests can overestimate the lifetime, if the stress frequency is not compensated for self-heating. Here, the frequency transformation is used to determine the stress frequencies at high RF power levels from low-power S-parameter measurements.