Low-loss SAW filters using thickness difference type of IDT on the NSPUDT orientation substrate

The invention of the Natural Single-Phase Unidirectional directional transducer (NSPUDT) in 1985 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. Therefore, the reverse directivity transducers for low-loss SAW transversal filters using NSPUDT orientation of substrates are required. The reverse directivity of 2-finger//spl lambda/ transducer is developed by using a different metallization on a transducer by P.V. Wright and grooves on quartz with the same metallization by C.S. Lam. This paper reports a new method to forward and reverse the unidirectionality with the electrode thickness difference (ETD) type of UDT on the NPSPUDT orientation of substrates. The electrode widths of new IDT are about /spl lambda//4 for the narrow gap IDT and /spl lambda//8 for the /spl lambda//8 gap IDT with the short or open floating electrodes. The two transducers for forward and reverse directivities have almost the same mass loading effects. Therefore the center frequency shifts between two transducers due to IDT's electrode difference don't occur such as Wright's and Lam's methods. The calculation result of ETD-RDT (RDT: Reversal of Directivity IDT) for forward and reverse directivities about 10 dB is obtained. The ETD-RDT is fabricated by the methods of Electro-Chemical Effects or Narrow Gap Anodic Oxidation Process. The experimental results show the large directivity and the low insertion loss filters.