Investigation of novel floating-electrode unidirectional SAW transducers (FEUDT's)

For low-loss SAW device applications, a variety of single-phase unidirectional transducers (SPUDT's) have been developed. In particular, the DART, using electrodes of different widths, is well established in practical devices. Alternatively, the FEUDT, introduced by Yamanouchi in 1984, employs floating electrodes to introduce directivity. This paper is concerned with FEUDT's in which the electrodes have constant width and pitch. For these cases the transducer reflection coefficient is governed by electrical effects, and consequently the FEUDT is much less critical of film thickness than the DART. Also, unlike the DART, the FEUDT's are effective on strongly-piezoelectric substrates such as lithium niobate. Recently a new analysis of FEUDT's was reported. This is a simplified method using the quasi-static approximation, which is applicable because reflections would be negligible if all the electrodes were shorted. The FEUDT's obey this condition because they have more than two electrodes per wavelength. In this paper, further investigation of FEUDT's is reported using the same analysis, and a variety of novel FEUDT's with 4,5 and 6 electrodes per wavelength are described. Data are given for the transduction, reflection and capacitance parameters per period, and the relative phases between reflection and transduction which is needed to calculate directivity. In many cases the reflection and transduction centres are easily identified from geometrical considerations. The relative performance of the new FEUDT's is reviewed.