A new type of surface wave transducer is described : the edge bonded transducer. This excitation technique uses a piezoelectric plate, bonded on a face perpendicular to the propagation direction, and whose back electrode is positioned near the edge in the skin thickness of the surface wave. The main geometrical parameter which controls the efficiency is the width of the back electrode. When this electrode is about one wavelength wide, the Rayleigh wave conversion loss is as low as 6 dB without any electrical tuning and the relative bandwidth is the same as that obtained with a bulk wave transducer.These transducers have been used,(l)to make a Love wave dispersive delay line with a compression ratio of 240,(2)to excite a Rayleigh wave on a PIEZOMOS transversal filter with 5 dB conversion loss(3)to make a reflective array compressor on a fused silica substrate with a compression ratio of 1800 and 24 dB total insertion loss.Surface wave generation on non-piezoelectric substrate has been the object of considerable work, since for some applications, it is interesting to get low insertion loss combined, either with the advantages of an isotropic medium, or of large or low propagation velocities, or of very low temperature coefficients. Various excitation means were described which involve interdigital transducers on ZnO overlay!, or on a "buttjoint" structure2. A new type of transducer is presented iu this paper, the edge bonded transducer, whose geometry approaches the one of a bulk wave transducer.
PSN transduFigure 2 : Rayleigh wave insertion loss for different back-electrode height 6 Figure 1 : Delay line structure with surface wave edge bonded transducersThis structure is shown in Figure I . A piezoelectric plate, thickness e, is bonded on a side perpendicular to the free surface of propagation substrate and is provided with an electrode, height h, placed near the edge within the skin thickness of the surface wave. The transducer is directed so as to generate a shear displacement, either parallel to the free surface for a Love wave ,or perpendicular to the free surface for a Rayleigh wave. In that case, the Rayleigh wave is more easily excited since the normal displacement is greater and decreases more slowly in the subs tra t e3. Geometrical parameter s controlling performance of this kind of transducer are thickness e, length L, and height h of electrode. Thickness e determines resonant frequency (although it depends slightly on h, as shown further on). Length L is involved in impedance matching between transducer and external circuit, as well as in diffraction losses. Influence of h dimension is relatively complex to analyze. Hereafter is presented an experimental study of the Rayleigh wave excitation, which gives the optimum value of ratio hfh where h is the working wavelength. frequency (MHZ) , Figure 3 : Transducer input conductance for different back-electrod?height Work sponsored by