New Low-Loss Surface Acoustic Wave Transducers in the UHF Range

Absfracf-Some new low-loss surface acoustic wave (SAW) transducers are described which can be used for UHF range applications. The characteristic of these new low-loss SAW transducers is that the electrode widths are about X0/4 (X,:SAW wavelength), so the frequency limitation of these devices is almost the same as that for the conventional uniform two-electrodeslwavelength transducers. The theoretical analysis includes the internal reflections within the transducers caused by the difference of the thickness of metal electrode film or both the difference of electromechanical coupling coefficients (K’) and the mass loading effect. The fundamental equations are derived and some simple results are given. In addition, the new low-loss SAW transducers are fabricated by a self-aligned angle evaporation technique which use only one photo mask, and no mask alignment is needed in the fabrication process. Experimental results show directivities of 1026 dB/transducer at about 500 MHz and 10 dB at about 2 GHz. A prototype SAW filter has shown a very encouraging passband characteristic with a minimum insertion loss of 2.8 dB at about 500 MHz on 128” Y-X LiNb03.