Low-Loss Surface Acoustic Wave Filter on Natural-Single Phase Unidirectional Transducter Orientations of a Li2B4O7 Substrate.

Natural-single phase unidirectional transducers (NSPUDT) have attracted much attention for the implementation of low-loss surface acoustic wave (SAW) devices. In this paper, we describe a low-loss SAW filter using NSPUDT orientations of a Li 2 B 4 O 7 substrate previously reportetl by the authors. Since the unidirectionality of NSPUDT is inherent to the substrate orientation, it is necessary to make the receiving transducer unidirectional in the opposite direction in order to obtain a low-loss filter using two transducers with reverse directivity. A new transducer configuration to reverse the directivity of an NSPUDT substrate is proposed, which utilizes reflector banks within the ladder transducer. The configuration has the advantage that it can be fabricated thorough a single level and a single metallization with minimum electrode width of λ/4. The principle of the operation of a reversal of directivity transducer is described. A 200-MHz-band low-loss SAW transversal filter with minimum insertion loss below 3 dB and low passband ripple are demonstrated with a theoretical filter response using coupling-of-modes theory.