SAW transducer configurations for reversing the directivity of NSPUDT substrates

Natural single phase unidirectional transducers (NSPUDT) have attracted much attention for implementing low-loss SAW devices. Since the unidirectionality of NSPUDT is inherent to the substrate orientation, it is necessary to make the receiving transducer unidirectional in the opposite direction for obtaining a low-loss filter using two transducers with reverse directivity. In this paper, we report new transducer configurations to reverse the directivity of NSPUDT substrates. The proposed methods utilize the shift effects of centers of transduction and reflection due to floating electrodes inserted within active interdigital electrodes, and reflector banks within the ladder transducer. The configurations have the advantage in that these can be fabricated through a single level and a single metallization. The principle of operation is described. Experimental results are given for test devices on the NSPUDT orientations of quartz and Li₂B ₄O₇ substrates; which show good reversed unidirectionality without degradation of directivity. A 162 MHz low-loss filter on quartz has minimum insertion loss of 4.7 dB. A 213 MHz low-loss filter on Li₂B₄O₇ has minimum insertion of 2.9 dB. Theoretical filter response using coupling-of-modes theory is also shown