Parasitic bulk waves in wide band SAW filters designed on 128° rotated Y-cut of LiNbO3

This paper presents the results of bulk wave transmission investigation in a wide band SAW filter designed on 128° YX LiNbO ₃. Bulk wave transmission in the filter was investigated in both frequency and time domain. Fourier transform and windowing techniques were used to identify the most important bulk wave modes and their contribution to the frequency response of the device. The influence of the substrate backside grooving on bulk wave propagation was examined. Application of grooving significantly reduces the level of bulk wave modes bouncing off the backside of the substrate. As a result substantial improvement in stopband rejection was obtained. It was found that in the devices with their backside grooved one of the most prominent bulk waves is the longitudinal wave travelling directly between the IDTs of the filter. The longitudinal wave limits the rejection in the upper stopband to approx. 40 dB