SURFACE ACOUSTIC WAVE DEVICE

Abstract: A surface acoustic wave device consists of a substrate of a piezo-electric material provided with an input transducer T′ I and ten output transducers T′₀₁ .... T′₀₁₀ spaced apart along the substrate. The output transducers are arranged in two tracks (20,30) and are spaced apart so that a surface acoustic wave, launched in a propogation direction P and received at the output transducers will be subject to respective delays. These delays exclude one or more delays which would have arisen if the output transducers had been arranged on the substrate with the same relative spacings but in a single track only.

Description: This invention relates to a surface acoustic wave device and it relates especially to such a device suitable for subjecting a surface acoustic wave (SAW) to a number of different delays. A surface acoustic wave device typically comprises a substrate of a piezoelectric material, e.g. quartz lead zirconate ceramic (PZT) or lithium niobate (LiNbO₃), which is provided, at one of its major surfaces, with an input transducer for launching a SAW along a suitable cystallographic direction and an output transducer for receiving the wave, after a delay due to the propagation time. Usually, the input and output transducers are in the form of interdigital arrays of strip electrodes applied to the substrate by a suitable photolithographic technique. The value of a device of this and lies in the fact that the velocity of a SAW propagating at the surface of a piezoelectric substrate is much smaller than that of an electromagnetic wave used to excite the input transducer; thus relatively large delays can be achieved. It may be desirable in practice to subject a SAW to different delays and to this end a plurality of output transducers is arranged on the substrate, individual transducers being spaced apart along the propogation direction. A device of this kind is shown, by way of example, in Figure 1; the input transducer is represented at T I and six output transducers are represented at T₀₁, T₀₂ ... T₀₆. A wave launched by the input transducer is received at the output transducers T₀₁, T₀₂ .... T₀₆ after respective delays t s ; t s +t₁₂; .... t s +t₁₂ + t₂₃ +t₃₄ +t₄₅ +t₅₆ where t s is the elapsed time due to propagation of the wave over a distance d s separating the input transducer and the nearest ouput transducer T₀₁, and t₁₂; t₂₃ .... t₅₆ are the elapsed times due to propagation of the wave over distances d₁₂, d₂₃ .... d₅₆ separating respective pairs of adjacent output transducers. A wave may suffer a partial reflection at an output transducer and, in these circumstances, transducer T₀₁, for example, may receive a wave after a delay t s +2t₁₂ due to a reflection occurring at transducer T₀₂. If, as is commonly the case, the output transducers are spaced apart at regular intervals on the substrate such that t₁₂ = t₂₃ = .... t₅₆ a wave may be received simultaneously at different output transducers. Far example, a wave received at transducers T₀₁ and T₀₃ may be subject to the same delay t' = t s +2t₁₂ = t s +t₁₂+t₂₃. In some signal processing applications it is desirable to arrange that a surface acoustic wave device be capable of rejecting a wave which has been subjected to a specific (undesirable) delay. Hitherto, it has been customary to omit one of the output transducers. As shown in Figure 2 of the accompanying drawings, an output transducer which would otherwise occupy a position referenced at T OX , intermediate transducers T₀₅ and T₀₆, has been omitted - thus a wave delayed by an amount t x , the elapsed time due to propagation of the wave directly over a distance d x on the substrate, as shown, remains undetected. In practice, however, a signal delayed by t x may still be received, albeit of reduced amplitude, on account of reflections occurring at the output transducers and, in general, this will happen whenever the total path length due to reflections corresponds to the elapsed time t x . Thus, for example, a wave received at the output transducer T₀₄ may be subject to a delay t x on account of a reflection occurring at output transducer T₀₅. It is an object of the present invention to provide a surface accoustic wave device wherein the above-mentioned problem is substantially alleviated. Accordingly there is provided a surface acoustic wave device wherein a substrate (10) of a piezoelectric material is provided with input transducer means (T' I ) for launching a surface acoustic wave in a fixed, propagation direction (P) and a plurality of output transducers, (T'₀₁...T'₀₁₀) spaced apart along said fixed, propagation direction (P) to receive surface acoustic waves subject to respective delays so launched, characterised in that some of said output transducers are arranged on a first track (20) on the substrate and others of said output tranducers are arranged on a different track (30) on the substrate, whereby the distribution of output tranducers on the first and second tracks is such that surface acoustic waves, of delay values which would have been received only by virtue of reflections at output transducers if the output transducers had been arranged at the same spacing but along a single track only, are avoided. In a preferred embodiment the first and different tracks are on the same side of said input transducer means. In order that the invention may be carried into effect an embodiment thereof is now described, by way of example only, by reference to Figure 3 of the accompanying drawings which shows a perspective view of a surface acoustic wave device. Referring now to Figure 3, a substrate 10 is provided, at a major surface, with an input transducer arrangement T' I arranged to launch a relative broad wave along a fixed, propagation direction P. The wavefront thus presented is shown at AA'. In this example the output transducers T'₀₁ , T'₀₂ ... T₀'₁₀ are arranged at regular intervals along the propagation direction, except for an interruption at the position referenced T Y where a transducer has been omitted. With this arrangement, a wave delayed by an amount t y the elapsed time due to propagation of a wave directly from the input transducer to position T Y remains undetected. Moreover, the output transducers are arranged on the substrate on two distinct tracks, in side-by-side relationship. Transducers T'₀₁, T'₀₄, T'₀₇, T'₀₈ and T'₀₁₀ are arranged on a first track, indicated generally at 20, and transducers T'₀₂, T'₀₃, T'₀₅, T'₀₆ and T'₀₉ are arranged on a second track, parallel to the first, indicated generally at 30. As will be apparent from the drawing, the relative spacings of the transducers in each track is such that it is not possible to construct a path length, from the input transducer to any one of the output transducers, corresponding to the delay t y -even taking into account reflections occurring at the output transducer. Thus, in constrast to the device described hereinbefore by reference to Figure 2 wherein the transducers are arranged on a single track only, any significant wave, delayed by an amount t y , remains undetected. It will be appreciated that the output transducers need not necessarily be arranged along parallel tracks, in side-by-side relationship, as shown in Figure 3. In an alternative embodiment the input transducer may be positioned centrally on the substrate, the output transducers being arranged in their respective tracks 20, 30 on opposite sides of the input transducer, but with the same relative spacings.

Claims: A surface acoustic wave device wherein a substrate (10) of a piezoelectric material is provided with input transducer means (T' I ) for launching a surface acoustic wave in a fixed, propagation direction (P) and a plurality of output transducers, (T'₀₁...T'₀₁₀) spaced apart along said fixed, propagation direction (P) to receive surface acoustic waves subject to respective delays so launched, characterised in that some of said output transducers are arranged on a first track (20) on the substrate and others of said output tranducers are arranged on a different track (30) on the substrate, whereby the distribution of output tranducers on the first and second tracks is such that surface acoustic waves, of delay values which would have been received only by virtue of reflections at output transducers if the output transducers had been arranged at the same spacing but along a single track only, are avoided. A surface acoustic wave device according to Claim 1 wherein said first and second tracks (20, 30) are arranged in side-by-side relationship on the same side of said input transducer means (T' I ) A surface acoustic wave device according to Claim 1 wherein said first and second tracks (20, 30) are arranged on opposite sides of the said input transducer means (T'₁).

Inventors: BLUNDELL, STEPHEN JAMES
Applicants: EMI LIMITED
Application year: 1985
Main classification: H03H9
IPC codes: H03H9