SURFACE ACOUSTIC WAVE TRANSDUCER

Abstract: The surface acoustic wave transducer in accordance with the present invention comprises a piezo-electric subs­ trate, and a first type electrode-strips set (A) which generates an electric reflection alone with no acoustic reflection and a second type electrode-strips set (B) which generates both of an electric reflection and an acoustic reflection, both types being formed on the piezo-electric substrate. The first type electrode-strips set (A) is formed by electrode strips with approximate width of 1/8 of the wavelength of the surface acoustic wave, while the second type electrode-strips set (B) is formed by electrode strips with approximate width of 1/8 and approximate width of 3/8 of the wavelength of the surface acoustic wave.

Description: Field of the Invention The present invention relates to a low-loss acoustic surface wave transducer with improved spurious reflection components. Description of the Prior Art As a surface acoustic wave transducer, a type with a construction in which a first and a second comb-shaped electrodes are interdigitated with a certain gap on the surface of a piezoelectric substrate (I.D.T.) has been practically used. In such a device it is possible to realize a variety of frequency characterstics by varying the pitch, interdigitated length of the electrode strips of an IDT. As spurious components which invariably as troublesome in various surface acoustic wave devices constructed by utilizing such an IDT, there are spurious reflection components of the surface acoustic wave due to the IDT. When a filter or a delay line is actually constructed by using such an IDT, these spurious reflection components appear in the form of the triple transit echo (TTE), acting as a cause for deteriorating the characteristic of such a device markedly. The factors of the spurious reflection components are the reflection components due to the difference in the acoustic impedances of the electrode portion and the gap portion of the IDT (referred to as R A hereafter) and the reflection components due to the electric re-excitation of the IDT(Referred to as R E hereafter). Therefore, it will be seen that in order to remove the spurious reflection components it is necessary only to cancel the reflection components R A and R E each other by giving them opposite phases. In the past, there has been disclosed in Japanese Patent Publication No.58-84517 a transducer which is arranged to have the reflection components R A due to the acoustic impedance difference and the reflection components R E due to the electric re-excitation cancel each other based on the above technique. The transducer includes a piezo-electric substrate on which a first comb-shaped electrode and a second comb-shaped electrode are formed. The electrode-strips sets of the transducer comprise a first type electrode-strips set A which generates the electric reflection R E but no acoustic reflection R A , and a second type electrode-strips set B which generates both of the electric reflection R E and the acoustic reflection R A . The strip widths of the first type electrode-strips set A are set at λ/8 and 5 λ/8, which the strip widths of the second type electrode-strips set B are set as a combination of λ/8 and 3 λ/8, where λ is the wavelength of the surface acoustic wave. The region covered by the electrode type B forms the shape of a triangle. That is, since the electric reflection is generated at both regions A and B, the wave form on the time axis of the resultant electric reflection characterristic of the IDT becomes the shape of a triangle which represents the result of the self-convolution of a quadrangle. Therefore, to cancel out the electic reflection R E at all the points on the time axis it is sufficient to choose the wave form of the acoustic reflection R A as a triangle with the shape as the triangular region of the electrode type B. Translated in terms of event on the frequency axis, it means to arrange the frequency-amplitude characterstic of the acoustic reflection Ra and the frequency-amplitude characteristic of the electric reflection R E to be identical over a wide range of frequency. In other words, when a filter is constructed by an IDT with the above range of frequency, it should be possible to restrain TTE over a wide range of frequency. However, when the overall reflection characteristic is examined by an actually constructed IDT with the above structure, it was observed that the overall reflection charcteristic did not disappear on the entire range of the time axis but left spurious components over small time ranges corresponding to the vicinity of the extremities of the IDT. The Japanese Patent JP-A-56-10724 describes a transducer with two sets of electrode strips, the first set having a width of 1/8 λ and the second set having a width of 1/8 λ and 3/8 λ . An object of the present invention is to provide a surface acoustic wave transducer with an overall characteristic that has no spurious components over a wide range of frequency. Another object of the present invention is to provide a surface acoustic wave transducer which is capable of realizing a low-loss filter with extremely few ripples within the frequency band. Still another object of the present invention is to provide a surface acoustic wave transducer which is capable of reducing the spurious reflections from the portions of extremities of the electrode. These and other objects of the present invention are solved by the provision of an improved surface acoustic wave transducer, a piezo-electric substrate a first interdigitated electrode-strips set formed on said piezo-electric substrate, said first interdigitated electrode-strips set being formed from a plurality of electrode strips with approximate width of 1/8 of the wavelength ( λ ) of the surface acoustic wave propagating on said piezo-electric substrate; and a second interdigitated electrode-strips set formed on said piezo-electric substrate, said second interdigitated electrode-strips set comprising a first part of electrode strips with an approximate width of 1/8 ( λ ) and a second part of electrode strips with a width greater than 1/8 ( λ ), said electrode strips being spaced apart by intervals of 1/8 ( λ ); characterized in that an end portion of said second part electrode strips has a width of 1/8 ( λ ). These and other objects, features and advantages of the present invention will be more apparent from the following description of a preferred embodiment, taken in conjunction with the accompanying drawings, in which: Fig.1a and 1b are front elevational view of a prior art surface acoustic wave transducer; Fig.2 is the diagram illustrating the reflection wave from for the surface acoustic wave transducers shown in Fig.1; Fig.3 is the front elevational view of a surface acoustic wave transducer embodying the present invention; Fig. 4a to 4c are the diagrams illustratiang the acoustic reflection wave forms of the surface acoustic wave transducers of the present invention and of the prior art; and Figs. 5,6,8 and 9, are the front elevational view illustrating the electrode construction of each of other embodiments in accordance with the present invention. Fig. 7 shows the construction of a surface acoustic wave transducer. To facilitate the understanding of the present invention, a brief reference will be made to prior art surface acoustic wave transducer illustrated in Figs.1a and 1b. Referring to Fig.1a, this prior art surface acoustic wave transducer includes a substrate 10 made of a piezo-electric material, and a first comb-shaped electrode 12 and a second comb-shaped electrode 14 formed mutually interdigitated on the piezo-electric substrate 10. The electrode-strips pairs of the transducer comprises a first type electrode-strips set A which generates the electric reflection R E alone with no acoustic reflection R A , and a triangular second type electrode-strips set B which generates both of the electric and acoustic reflections. The strip widths of the first type electrode-strips pair A are arranged to be λ/8 and 5 λ/8, and the strip widths of the second type electrode-strips pair B are arranged to be a combination of λ/8 and 3 λ/8, where λ is the wavelength of the surface acoustic wave. Figure 1b show a modification of the device in Fig. 1a in which the region of electrode type B is distributed in the direction of interdigitation width, or the direction perpendicular to the propagation direction of the surface acoustic wave. In Fig.1a, the reason for arranging a triangular shape for the region of electrode type B may be explained in the following manner. Namely, in the case of the figure, due to the fact that the electric reflection is contributed from both regions of the electrode types A and B, the wave form on the time axis of the resultant electric reflection characterstic becomes triangular in shape which represents the result of the self-convolution of a quadrangle. Therefore, in order to cancel out the electric reflection at all points on the entire time axis, it is sufficient to choose the wave form of the acoustic reflection in the form of an identical triangle. Translated in terms of event on the frequency axis, it means to arrange the frequency-amplitude characteristic of the acoustic reflection R A and the frequency-amplitude characteristic of the electric reflection to be identical over a wide range of frequency. In other words, it suggests that it is possible to restrain TTE over a wide range of frequency when a filter is made by an IDT with such a construction. However, when the overall reflection characterstic is examined by actually constructing an IDT as shown in Fig.1a, it was learned, as shown in Fig.2, that the overall reflection characteristic did not vanish over the entire range of the time axis, with spurious components over small time ranges corresponding to the vicinity of the extremities of the IDT. The present invention is aimed at resolving the problems as in the above existing in the prior art devices, and it will be described in what follows by referring to Figs 3 to 9. Referring to Fig.3, the electrode-strips pairs of this transducer are constructed by mutually interdigitating the electrode strips of a first comb-shaped electrode 16 and a second comb-shaped electrode 18. As shown in the figure, the electrode-strips pairs comprise the region of a first type electrode-strips set A which generates the electric reflection components R E alone with no acoustic reflection components R A , and the triangular region of a second type electrode-strips set B which generates both of the electric and acoustic reflection components R E , R A. The region of the first type electrode-strips set A is construced by strips with width of λ/8 and the region of the second type electrode-strips set B is constructed by strips of widths λ/8 and 3 λ/8. The result of measurement of the acoustic reflection characteristic of an actual transducer constructed with the above structure is the wave form as shown in Fig.4a. It should be mentioned that the measurement of the acoustic reflection characteristic was made under the state of no electric reflection by short-circuiting the electric terminals 16 and 18. For comparison, the wave form of the acoustic reflection in the case of the prior art construction shown in Fig.1a is illustrated in Fig.4b. As seen from these figures, in the prior art device there is produced conspicuous spurious reflections in the vicinity of the extremities of the electrode, whereas according to the present invention it is obtained a reflection wave form with almost ideal triangular shape. The reason for this may be explained as the following. Namely, according to the prior art construction, a considerable perturbation effect on the surface acoustic wave is created due to the electrode strips with large width of 5 λ/8 for the regtion of the electrode type A. Then, because of a large difference in the displacement distribution of the surface acoustic wave occurring between the region where no IDT exists and the region of the IDT, a mode mismatch arisesat the extremity portions of the IDT. It may be considered that the spurious reflection components increase as a result. By contract, according to the present invention, it becomes possible to reduce markedly the spurious reflection components at the extremities of the electrode because of the reduction in perturbation due to the construction of the region of electrode type A with electrode strips of narrower width λ/8. Next, description will be made of another embodiment according to the present invention. The transducer shown in Fig.5 is basically similar to the embodiment given in Fig.3, with a difference in that one electrode strip of width λ/8 is added to one end of the IDT up to the length of about one half of the interdigitation width. The result of measurement on the acoustic reflection characteristic of the transducer with this additional electrode strip 20 is shown by Fig.4c. As seen from the figure, there is obtained a characteristic with smaller spurious components at the electrode extremities compared with Fig.4a. The reason for this may be understood in the following way. Namely, a total path difference of λ/2 in back and forth is created between the waves that propagate in the upper and lower halves of the IDT due to the arrangement of the added electrode strip 20. Then, the spurious reflection which is still remaining in the embodiment shown in Fig.3 is made by the new arrangement to be given opposite phases to the reflected waves in the upper and lower halves of the IDT to produce a mutual cancellation. The transducer shown in Fig.6 is another embodiment according to the present invention. The transducer is arranged such that the first type electrode-strips set A and the second type electrode-stripe set B in Fig.5 are distributed in the interdigitation width of the electrode strips, resulting in an effect of uniforming the amplitude in the direction of the interdigitation length. As a result, the cancellation effect of the R E and R A reflections in the direction of the surface wave beam can also be realized, leading to an advantage of increasing the freedom of choice for the counterpart IDT in the case of constructing a filter or the like. Referring to Figure 7, the construction of a transducer is shown wherein the electrode type A and the electrode type B are distributed in the direction of propagation of the surface acoustic wave. Using a transducer with such a construction, it is possible to control the weighting of the excitation efficiency by the so-called electrode-strip withdrawal technique. In addition, it will be possible, corresponding to the wave form of the electric reflection in the above case, to control the pattern of the acoustic reflection region by the density distribution in the direction of propagation of the surface acoustic wave. With this construction, uniform power distributions along the direction of width of the surface wave beam is obtained for both of the electric reflection R E and the acoustic reflection RA so that the reflection cancellation between R E and R A is accomplished nearly perfectly. At the same time, it is possible to achieve a reduction in loss due to absence of the so-called apodization loss. When the above IDT is applied to filters, the effects are found to be particularly conspicuous for narrow-band filters with specific band with less than 3%. In the embodiments given in the foregoing, description was made for the case in which all the inter digitation widths of the electrode strips have a constant value. However, it is possible to apply the present invention to the case in which the interdigitation width varies along the direction of propagation of the surface wave, as shown by Fig.8. In such a case, in contract to the electric reflection which is generated from the region of interdigitation of the electrode strips, the acoustic reflection can be made to be generated from the region where there is no interdigitation of the electrode strips (the region of dummy electrode) also. In this case, the pattern for the region of acoustic reflection (the region of electrode strips with width λ/8 and 3 λ/8) may be determined roughly in the following manner. Namely, after determining the impulse response corresponding to the interdigitation region 22, the self-convolution is evaluated, and then it is sufficient to arrange the acoustic reflection range so as to correspond to the wave form with the time interval compressed to one half. In this case, together with the ordinary region (the region C of the figure) in which the electric reflection and the acoustic reflection have opposite phases, there may arise a region (region D of the figure) in which the electric reflection and the acoustic reflection are in phase. In an IDT with construction as shown in Fig.8, it is possible to distribute unevenly the acoustically reflecting portion in the direction of the interdigitation width, in order to accomplish also the cancellation of R E and R A along the direction of width of the surface wave beam. It should be mentioned in the above description that it becomes necessary to take into account of the change in the wave form of the electric reflection due to load conditions and the like of the exterior curcuit, and also the effects on the acoustically reflected waves due to multiple reflections among the electrode strips. However, these effects may be analyzed by an equivalaent circuit model for the IDT so that the final pattern determination could be made based on the result of such an analysis. In still another embodiment of the present invention as shown by Fig.9, the first type electrode-strips set A is formed by electrode strips with approximate width of λ/8, while the second type electrode-strips set B is formed by a combination of electrode strips with widths of approximately λ/8 and approximately λ/4. The spurious reflection is reduced in this embodiment by arranging to give opposite phase to the resulteant electric reflection and to the resultant vector of the acoustic reflections originating from the first type electrode-strips set A and the second type electrode-strips set B. Namely, the first comb-shaped electrode 30 is constructed to have an electrode strip 30-a with approximate width λ/8 and an electrode strip 30-b with approximate width λ/8, spaced apart each other for a distance of λ/8. Further, an electrode strip 30-d with approximate width λ/4 is arranged at a position about λ/16 away from the left-hand extremity of the electrode strip 30-b. These electrode strips 30-b, 30-c, and 30-d are constructed in a form which is short-circuited electrically. The second comb-shaped electrode 32 has a construction which is a repetition of the electrode strip 32-a with approximate width λ/8. In particular, the electrode strip 32-a with width λ/8 is arranged adjacent to the first comb-shaped electrode 30 at a distance λ/8 away from it. In addition to the effects applicable to the embodiments described earlier, it is possible in this embodiment to obtain a transducer with large maximum acoustic reflection. Since it becomes possible then to increase also the amount of the electric reflection in comparison to the prior art cases, there will become such advantage available as to realizing a surface acoustic wave transducer with still lower losses. Although several embodiments of the surface acoustic wave transducers in accordance with the present invention have been described, the present invention will not at all be limited to these embodiments. Various modifications, such as joint use with multi-strip couplers (MSC), joint use with thin film dielectrics, application to piezo-electric films, use of grooves for the portion of the acoustic reflection and others will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.

Claims: A surface acoustic wave transducer for interconverting surface acoustic waves and electrical signals, including: a piezo-electric substrate; a first interdigitated electrode-strips set formed on said piezo-electric substrate, said first interdigitated electrode-strips set being formed from a plurality of electrode strips with approximate width of 1/8 of the wavelength λ of the surface acoustic wave propagating on said piezo-electric substrate; and a second interdigitated electrode-strips set formed on said piezo-electric substrate, said second interdigitated electrode-strips set comprising a first part of electrode strips with an approximate width of 1/8 λ and a second part of electrode strips with a width greater than 1/8 λ, said electrode strips being spaced apart by intervals of 1/8 λ ; characterised in that an end portion of said second part electrode strips has a width of 1/8 λ. A surface acoustic wave transducer as claimed in claim 1, in which said width greater than 1/8 λ is approximately 3/8 λ. A surface acoustic wave transducer as claimed in claim 1, in which said end portion of said second part electrode strips has a length of about 1/2 of the interdigitation width of the electrode strips. A surface acoustic wave transducer as claimed in claim 1, in which said first interdigitated electrode-strips set and said second interdigitated electrode-strips set are arranged distributed in the direction of interdigitation width. A surface acoustic wave transducer as claimed in claim 1, in which said first interdigitated electrode-strips set and said second interdigitated electrode-strips set are arranged distributed in the direction of propagation of said surface acoustic wave. A surface acoustic wave transducer as claimed in claim 1, further comprising: a third interdigitated electrode-strips set with vanishing contribution to the electric reflection. A surface acoustic wave transducer as claimed in claim 6, in which said third interdigitated electrode-strips set is a dummy electrode portion. A surface acoustic wave transducer as claimed in claim 1, in which said width greater than 1/8 λ is approximately 1/4 λ. A surface acoustic wave transducer as claimed in claim 8, in which said width greater than 1/8 λ is approximately 1/4 ± 1/8 of the wavelength of the surface acoustic wave.

Inventors: KODAMA, TOSHIKAZU, SATO, KOJI
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1984
Main classification: H03H9
IPC codes: H03H9, H03H3