A low-loss SAW filter using two-finger per wavelength electrodes on the NSPUDT orientation of quartz

The invention of the Natural Single-Phase Unidirectional Transducer (NSPUDT) in 1986 by P.V. Wright opened up a possible new class of low-loss Surface Acoustic Wave (SAW) devices. The unidirectionality of the NSPUDT was obtained by using the crystal asymmetry of the substrate to shift the transduction centers to the edges of the electrodes of the 2-finger/wavelength radiating transducer. Since the unidirectional property is inherent to the substrate orientation, all reported working low-loss SAW transversal filters using NSPUDTs have 4-finger/λ bidirectional transducers (BDTs) to receive the acoustic energy. These filters still have an inherent loss of 3 dB. This paper reports a new method to reverse the unidirectionality and the successful fabrication of a low-loss SAW transversal filter using two 2-finger/λ transducers of the same metallization and metallization thickness on the NSPUDT orientation of quartz