JP3340583B2 - Surface acoustic wave convolver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave convolver used as decoding means on a receiving side, such as a communication apparatus employing a spread spectrum communication system.

[0002]

2. Description of the Related Art A surface acoustic wave convolver has been developed by utilizing the fact that an electric signal of a product of two signals is obtained by nonlinear interaction of surface waves propagating in opposite directions on a piezoelectric substrate. FIG. 2 shows a plan view of a conventionally known surface acoustic wave convolver. This element is placed on the piezoelectric substrate 1
A convolution output electrode 2 is provided which extends in a belt-like shape by an appropriate length just along its longitudinal direction. Then, a pair of interdigital electrodes 3 and 4 are arranged with the convolution output electrode 2 interposed therebetween. In the interdigital electrodes 3, 4, positive electrodes 3A, 4A and negative electrodes 3B, 4B having comb-like teeth are arranged so that their teeth mesh with each other. The surface acoustic wave propagates on the piezoelectric substrate 1 in the longitudinal direction. That is, the teeth of the interdigital transducers 3 and 4 are arranged so as to be substantially perpendicular to the signal propagation direction.

Basically, the interdigital electrodes 3 and 4 propagate surface acoustic waves in both the left and right directions in the figure. In this case, the surface acoustic waves S propagating from the left interdigital electrode 3 in the direction of arrow H1.
1 and a surface acoustic wave S2 propagating in the direction of arrow H2 from the right interdigital electrode 4 are used. In the figure, S1, S
Numeral 2 shows a waveform of a surface acoustic wave propagating on the substrate for explaining the operation of the convolver.

That is, a code (pseudo noise) modulated by a PN code is supplied from a terminal T1 and information modulated by a PN code is simultaneously supplied from a terminal T2. The terminal T
The electric signals supplied to T1 and T2 have mutually inverted time axes. When the surface acoustic waves S1 and S2 excited in this way propagate in the directions of arrows H1 and H2 in the figure, and just overlap at the convolution output electrode 2 as shown in the figure,
At that moment, an electric signal S3 having a pulse-like peak from an output terminal T3 connected to the convolution output electrode 2.
Is taken out.

Therefore, when a code modulated with a PN code is input from a terminal T1 and information modulated with a PN code is input from a terminal T2, a pulsed electric signal S3 is output only when the PN codes match. Therefore, if a set of PN codes is associated with a 1-bit digital signal, the terminal T2
A digital signal having a matching PN code can be selected from the electric signals input to the terminal T and output to the terminal T3.

[0006]

The above-mentioned surface acoustic wave convolver has conventionally had the following problems to be solved. In the spread spectrum communication system, P
By modulating with a spreading code called an N code, information including a voice signal and various data is spread over a much wider band than originally possessed and transmitted. Therefore, the surface acoustic wave convolver is required to have a wide band.

For this purpose, it is preferable to reduce the number of teeth of the IDTs 3 and 4 shown in FIG. However, this lowers the excitation intensity. Further, the efficiency is poor because the nonlinearity of the piezoelectric substrate is used. The efficiency of a surface acoustic wave convolver is roughly represented by the following equation. M = Vo / (PS1 / PS2 / W) (1) The above-mentioned M is an intrinsic amount directly corresponding to the nonlinearity of the substrate material. Vo is an output voltage. PS1, PS2
Is the power of the surface wave output from the IDTs 3 and 4 respectively. W is the width of the convolution output electrode shown in FIG.

In the above equation (1), the powers PS1 and PS2 that can be excited by the interdigital transducer and the amount M that is specific to the substrate material
Has limited adjustability. Therefore, in order to increase the output voltage Vo, the width W of the convolution output electrode 2 is required.
It is conceivable to increase the energy density of the surface wave by increasing the number of electrodes, or to increase the number of electrode pairs to make the nonlinear effect more likely to occur. However, if the width W of the convolution output electrode 2 is reduced, the effective width W of the teeth of the interdigital electrodes 3 and 4 is also reduced, and the excitation intensity of the surface wave is reduced. On the other hand, if the number of electrode pairs is increased, the frequency band of the surface acoustic wave convolver becomes narrower, and it becomes impossible to transmit the signal spread over a wide band. Further, the shape of the entire surface acoustic wave convolver becomes large, which is against the demand for miniaturization of electronic components.

As a prior art for solving such a problem, Japanese Unexamined Patent Publication No. Hei 6-334478 discloses a technique in which a surface acoustic wave excited by an interdigital electrode is effectively concentrated on a convolution output electrode. An example in which arc-shaped interdigital electrodes are selected to have a length within a predetermined opening angle range is introduced. In Japanese Patent Application Laid-Open No. 6-314946, the shape of the interdigital transducer is adjusted in consideration of the speed dependence of the direction of propagation of the surface acoustic wave on the anisotropic piezoelectric substrate. Examples are introduced. Further, Japanese Patent Application Laid-Open No. 6-260881 discloses that one of a pair of interdigital electrodes sandwiching a convolution output electrode has a signal output characteristic having a directionality toward the convolution output electrode, and the other has a bidirectional characteristic. Are introduced to improve the efficiency. SUMMARY OF THE INVENTION An object of the present invention is to provide a surface acoustic wave convolver in which, when forming the arc-shaped interdigital transducer, the excitation efficiency is increased while the productivity is improved, and the characteristics are improved and the size is reduced.

[0010]

A surface acoustic wave convolver according to the present invention comprises at least a pair of interdigital transducers opposing each other with a convolution output electrode interposed therebetween on a piezoelectric substrate. The positive electrode and the negative electrode having teeth are arranged so that the teeth mesh with each other, and the teeth of one of the interdigital transducers at one end of the convolution output electrode face the focal point near the one end. Draw a polygonal line that is selected so as to be gradually shorter and interconnects the ends of line segments that are almost perpendicular to the direction toward the focal point, and any of the interdigital electrodes gradually move toward the focal point.
Arrange the line segments that become shorter at a predetermined pitch, and
It is formed by connecting multiple sets, and each line segment of the IDT
These are also selected to be less than the wavelength λ of the surface acoustic wave to be excited.
And the one interdigital electrode is connected to the positive electrode adjacent to each other.
The pitch between the electrode teeth and the negative electrode teeth is convoluted.
Gradually narrows when viewed in the direction toward one end of the output electrode
And the other interdigital electrodes are adjacent to each other.
The arrangement pitch of the positive electrode teeth and the negative electrode teeth that are in contact with each other is
Looking away from the other end of the convolution output electrode
It is selected to be gradually narrower, and one of the interdigital electrodes
When the pitch of the teeth of the other interdigital transducer is projected,
The feature is that they are matched so as to overlap with each other.

It is preferable that each line segment of the IDT which draws a polygonal line is selected to have a length equal to or shorter than the wavelength λ of the surface acoustic wave to be excited. Further, it is preferable that each line segment of the interdigital transducer which draws a polygonal line is selected to be not more than the wavelength λ of the surface acoustic wave to be excited and not less than 0.1 μm. When the arrangement pitch of the teeth of one interdigital electrode and the other interdigital electrode is projected, they can be matched so as to overlap as they are.

[0012]

The surface acoustic wave convolver has a wide band characteristic because the arrangement pitch of the teeth of the positive electrode and the negative electrode of the IDT is gradually reduced in a certain direction. In addition, since the teeth of each interdigital transducer are selected so as to draw a polygonal line connecting lines that are almost orthogonal to the direction toward the focal point near the end of the convolution output electrode, surface acoustic waves are efficiently concentrated at one point. I do.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the embodiments shown in the drawings. FIG. 1 is a plan view showing an embodiment of a surface acoustic wave convolver of the present invention. In the drawing, a piezoelectric substrate 1 uses, for example, Y-cut Z-propagation lithium niobate, which is often used for this type of device. On this surface, a convolution output electrode 2 as shown in the figure and a pair of IDTs 3 and 4 which face each other with the convolution output electrode 2 interposed therebetween are arranged. These are formed by an aluminum thin film or the like. IDT 3
Is arranged such that the positive electrodes 3A and 3B having comb-like teeth are meshed with each other. The IDT 4 also has a positive electrode 4A and a negative electrode 4B having the same configuration. The positive electrodes 3A and 4A of the IDTs 3 and 4 and the negative electrode 3
The arrangement pitch of the teeth B and 4B is selected so as to become gradually narrower when viewed from the one end 2A of the convolution output electrode 2 toward the other end 2B.

That is, the arrangement pitch projection straight lines 7A and 7B shown in FIG.
As can be seen from FIG. 5, the interdigital transducer 3 on the left has a longer tooth pitch P on the left and shorter on the right. The arrangement pitch P of the teeth of the other interdigital transducer 4 on the right side is longer on the left side and shorter on the right side. The reason why the teeth are arranged as described above is to widen the band so as to be able to be excited by electric signals in a wide frequency range and to improve the efficiency by giving directionality to the surface acoustic wave. The interdigital transducer 3 having such an arrangement is called a down-chirp type electrode, and the interdigital transducer 4 is called an up-chirp type electrode.

FIG. 3 is an explanatory diagram for widening the band. As shown in FIG. 3A, when the surface acoustic waves S1 and S2 are transmitted from the IDTs 3 and 4, respectively, the convolution output electrode 2 has a predetermined phase as shown in FIG. Match at the timing. At this time, a surface acoustic wave having a long wavelength is excited in a portion where the pitch of the interdigital electrodes 3 and 4 is wide, and a surface acoustic wave having a short wavelength is excited in a portion where the pitch is small. As shown in the figure, the surface acoustic waves propagate in directions that collide with each other and meet at the convolution output electrode 2. In order for the respective frequency components of the surface acoustic waves S1 and S2 output from the interdigital transducers 3 and 4 and coming out at the convolution output electrode 2 to overlap their phases at the same time, it is necessary to use the right-hand interdigital transducer. When the pitch of the arrangement of the teeth of the electrode 3 and the interdigital electrode 4 on the right side is projected, it is preferable that they coincide so as to overlap as shown in FIG.

For this reason, as shown in FIG. 1, in the surface acoustic wave convolver of the present invention, the arrangement pitch of the teeth is selected as shown by the arrangement pitch projection straight lines 7A and 7B in the figure.
In addition, by setting the tooth arrangement pitch as described above, the output surface acoustic wave has directivity. For example,
The surface acoustic wave output by the excitation of the interdigital transducer 3 shown in FIG. 3A is sent out more in the right direction than in the left direction in the figure, and has higher efficiency than in the case where it is sent out with the same power in both the left and right directions. Is shown. This function is described in, for example, JP-A-4-331505 and JP-A-6-260.
No. 881 is also introduced.

When the structure of the left interdigital electrode 3 is applied to the right interdigital electrode 4 as it is, more surface acoustic waves output from the right interdigital electrode 4 are sent out to the right, and the efficiency is reduced. Therefore, as shown in FIG. 3B, the right interdigital electrode 4 may have a double electrode structure in which, for example, two electrodes having the same polarity are arranged in parallel and combined. This electrode structure is also described in JP-A-4-331505.

Referring back to FIG. 1, the surface acoustic wave convolver according to the present invention has a configuration in which the tooth length of one of the interdigital transducers 3 gradually increases toward the focal point 5A near one end 2A of the convolution output electrode 2 in this embodiment. The teeth are selected so as to be shorter, and each tooth describes a polygonal line connecting the ends of line segments substantially orthogonal to the direction toward the focal point 5A. Further, the tooth length of the other interdigital electrode 4 is selected so as to gradually decrease toward the focal point 5B near the other end 2B of the convolution output electrode 2, and the teeth of the interdigital electrode 4 are further reduced in the focal point 5B. A polygonal line is drawn in which the ends of line segments that are substantially perpendicular to the direction toward are connected to each other.

With such a configuration, the surface acoustic waves excited by the broken lines of the teeth of the IDTs 3 and 4 are all focused on the focal points 5A and 5B. Therefore, the surface acoustic wave can be excited by the teeth sufficiently longer than the width W of the convolution output electrode 2. Thereby, the efficiency of the surface acoustic wave convolver can be increased. Also, the shorter the tooth length is nearer to the convolution output electrode 2 is that each tooth
This is because the power of the surface acoustic wave reaching A and 5B is made uniform to reduce the frequency dependency.

Incidentally, Japanese Unexamined Patent Publication No.
JP-A-334478 and JP-A-6-314946 disclose an arc-shaped IDT. In the latter, it is described that the velocity dependence due to the propagation direction when surface acoustic waves propagate is considered, and based on this theory, the shape of the electrode should be formed as an elliptical arc or parabola. May also be preferred. However, it is not always easy to form an electrode pattern while drawing a minute and precise curve on the piezoelectric substrate, which causes an increase in cost. Therefore, in the present invention, the teeth of the interdigital electrode are formed by broken lines. This facilitates control when drawing an electrode pattern formed by a photolithographic etching process or the like.

FIG. 4 shows a specific example of such an electrode pattern. As shown in FIG. 3A, if the line segments 11 that gradually become shorter toward the focal point 5A are arranged at the above-described pitch, and are connected in the direction of the arc, a set of the blinds according to the present invention can be obtained. The electrode is completed. Therefore, for example, the design in the case of considering the speed dependency due to the propagation direction is much easier than drawing a smooth and complicated carp.
As shown in FIG. 4 (b), a negative electrode 3C and a positive electrode 3D are added to form a plurality of interdigital electrodes independently in the direction of the arc, and each of the interdigital electrodes according to the present invention. It may be a broken line. In consideration of the propagation characteristics of the surface acoustic wave excited by the electrode, it is preferable that the length of each segment forming the polygonal line is shorter than the wavelength λ of the surface acoustic wave excited. This is because the difference between the center and the focal point of each line segment and the distance between the end and the focal point cannot be ignored.

Incidentally, in order to improve the efficiency, it is preferable that the excitation power has a directivity for effectively propagating the excitation power toward the convolution output electrode. FIG.
A combination example of a directional electrode as shown in (a) and a non-directional electrode as shown in (b) will be described. FIG. 5 (a)
Shows that both interdigital electrodes are directional electrodes 10A.
It consisted of. In this case, the number of teeth can be reduced as compared with the case where the non-directional electrode is included, so that the manufacture is easy. The power loss of one of the electrodes may be adjusted according to the level of the input signal.

In FIG. 5 (b), both interdigital electrodes are constituted by non-directional electrodes 10B. In this case, the power loss is slightly larger than that of (a). FIG. 5 (c),
(D) shows one of the directional electrodes 10
A, in which the non-directional electrode 10B is provided on the other side, and the positional relationship is free. The configurations of the directional electrode 10A and the non-directional electrode 10B are disclosed in, for example, Japanese Patent Application Laid-Open No. 4-331505, in addition to the above-described embodiment. However, the direction can be adjusted freely by selecting the electrode material and the like. With such a configuration, the efficiency can be further improved.

[0024]

The surface acoustic wave convolver of the present invention described above has at least a pair of interdigital transducers opposing each other with a convolution output electrode interposed therebetween on a piezoelectric substrate. The positive electrode and the negative electrode having teeth are arranged so that the teeth mesh with each other, and the teeth of one of the interdigital transducers at one end of the convolution output electrode face the focal point near the one end. Draw a polygonal line that is selected to be gradually shorter and interconnects the ends of line segments that are substantially perpendicular to the direction toward the focal point, and the teeth of the other interdigital electrode at the other end of the convolution output electrode are , Draw a polygonal line which is selected so as to become gradually shorter toward the focal point near the other end, and which connects the ends of line segments substantially orthogonal to the direction toward the focal point, and The interdigital electrodes are selected such that the arrangement pitch of the teeth of the positive electrode and the negative electrode adjacent to each other becomes gradually narrower when viewed in the direction toward one end of the convolution output electrode, and the other interdigital electrodes are adjacent to each other. The arrangement pitch of the teeth of the positive electrode and the teeth of the negative electrode is selected so as to be gradually narrower when viewed in a direction away from the other end of the convolution output electrode, so that a signal is sent to the convolution output electrode side in a wide band, Energy can be concentrated by an electrode having teeth having a width wider than the width of the convolution output electrode. Therefore, an element having excellent characteristics that is easy to manufacture, has high efficiency, and can be downsized.

[Brief description of the drawings]

FIG. 1 is a plan view showing a surface acoustic wave convolver according to an embodiment of the present invention.

FIG. 2 is a plan view showing an example of a conventional surface acoustic wave convolver.

FIG. 3 is an explanatory view showing the operation of a chirped IDT;

FIG. 4 is an explanatory view showing a modified example of the interdigital transducer of the surface acoustic wave convolver of the present invention.

FIG. 5 is an explanatory diagram showing a combination example of interdigital electrodes of the surface acoustic wave convolver of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Piezoelectric substrate 2 Convolution output electrode 3, 4 Interdigital electrode 3A, 4A Positive electrode 3B, 4B Negative electrode 5A Focus near one end 5B Focus near the other end P Arrangement pitch of teeth

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kenji Noguchi 2-1-1, Sakae Oda, Kawasaki-ku, Kawasaki City, Kanagawa Prefecture Inside Showa Electric Wire & Cable Co., Ltd. (72) Inventor Takeo Yokoyama 2 Ei Oda, Kawasaki-ku, Kawasaki City, Kanagawa Prefecture Chome 1-1 Showa Electric Wire & Cable Co., Ltd. (56) References JP-A-6-334478 (JP, A) JP-A-6-314946 (JP, A) JP-A-4-331505 (JP, A) JP-A-4-337915 (JP, A) JP-A-6-260881 (JP, A) JP-A-3-74921 (JP, A) JP-A-63-36485 (JP, A) JP-A-4-249910 (JP JP, A) IEICE Technical Report US 91-52, (1991.10.18) 37-44

Claims (1)

(57) [Claims] 1. A piezoelectric substrate comprising at least a pair of interdigital transducers opposed to each other with a convolution output electrode interposed therebetween, wherein the interdigital transducer comprises a positive electrode having comb teeth and a negative electrode. The teeth of one of the interdigital transducers at one end of the convolution output electrode are selected so as to become gradually shorter toward a focal point near the one end, and Draw a polygonal line that interconnects the ends of line segments that are substantially orthogonal to the direction toward the focal point, and the teeth of the other interdigital transducer on the other end side of the convolution output electrode move toward the focal point near the other end. Draw a polygonal line that connects the ends of line segments that are selected to be progressively shorter and that are almost perpendicular to the direction of the focal point, and that any of the interdigital electrodes gradually move toward the focal point. Arrange the line segments that become shorter at a predetermined pitch, and
A plurality of sets are connected in the direction, and each line segment of the interdigital transducer is selected to have a length equal to or less than the wavelength λ of the surface acoustic wave to be excited. The arrangement pitch of the teeth of the electrode and the teeth of the negative electrode is selected so as to be gradually narrower when viewed in a direction toward one end of the convolution output electrode, and the other interdigital transducer is provided with the teeth of the positive electrode adjacent to each other. The arrangement pitch of the teeth of the negative electrode is selected so as to be gradually narrower in a direction away from the other end of the convolution output electrode, and the arrangement pitch of the teeth of one of the interdigital electrodes and the other of the interdigital electrodes is projected. A surface acoustic wave convolver characterized by matching so as to overlap as it is.