Bulk acoustic wave filters for GPS with extreme stopband attenuation

RF filters for GPS receivers inside phones require extreme stopband attenuation in order to protect the weak GPS signals from interference by the cellular phone bands and ISM bands. In contrast to RF filters for cellular phone bands which typically require a relative bandwidth up to 4.5% a GPS filter needs less than 0.5% relative bandwidth. The reduced bandwidth requirement can be traded in for low insertion loss and high stopband attenuation. The impact of this tradeoff on the design of the bulk acoustic wave (BAW) filters is discussed in this paper. The choice of the filter topology and acoustic layer stack forming the resonators as well as the spurious mode suppression need to be re-optimized under the new constraints. As a result of this optimization, a filter for GPS signals with a minimum insertion loss of 1.2 dB and a stopband attenuation of 50 dB is presented.