A SAW basestation filter on Langasite

A SAW, wideband CDMA basestation filter has been designed using SPUDT tapered transducers on Langasite (LGS). The filter has a bandwidth of 4 MHz centered at around 400 MHz. The rejection of the filter is over 60 dB so one filter can replace two cascaded ST-quartz filters. The total insertion loss of 38 dB from the cascaded pair of ST-quartz filters has been replaced by the single filter insertion loss of 14 dB of the LGS filter. This results in improvements in system level performance in noise figure, gain and overall size (elimination of a gain stage and a filter). The properties of the optimal cut of LGS that make this design possible are minimal diffraction (γ = -1) and a coupling coefficient 3 times higher than ST-quartz. In addition, this material has a zero first order TCF and has a velocity that is 14% lower than ST-quartz, resulting in a smaller die size. The diffraction property allows us to use tapered transducers which generally can not be used on ST-quartz. The propagation characteristics are analyzed by using a laser probe. The data shows good diffraction characteristics with a good acoustic beam profile