Narrow band bulk acoustic wave filters

Bulk acoustic wave (BAW) filters are competing successfully in the range of 800 MHz to 10 GHz with SAW filters. USPCS and UMTS filters require a bandwidth of 60 MHz at approximately 2 GHz (fractional BW Δf/f≈3%). This can be realized by optimum design and high quality piezoelectric AlN which allows figure of merit, k²Q, values of up to 100. On the other hand, relatively narrow band solidly mounted BAW filters can also be made. We present a narrow band filter with approximately 8 MHz bandwidth at a center frequency of approximately 1.2 GHz (fractional BW Δf/f≈0.5%). The resonators required for narrow band filters can be designed to have extremely good temperature characteristics (down to -2 ppm/K) and high Q (>1000), which makes such devices interesting for new applications.