JP2745167B2 - 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 in which surface acoustic wave transducers having interdigital electrodes having different electrode widths and periods in the direction of surface acoustic wave propagation are combined.

[0002]

2. Description of the Related Art Conventionally, surface acoustic wave converters having interdigital electrodes comprising positive and negative electrodes provided on a piezoelectric substrate (including a piezoelectric thin film substrate) or an electrostrictive substrate generally have an equi-periodic structure. Electrode arrangement. FIG. 3A is a plan view showing a surface acoustic wave convolver using a conventional surface acoustic wave converter, and FIG. 3B is an XY cross-sectional view thereof. 3
Reference numeral 1 denotes a first surface acoustic wave converter that converts an electric signal into a surface acoustic wave, 32 denotes a second surface acoustic wave converter that converts a similar electric signal into a surface acoustic wave, and 33 denotes a surface acoustic wave converter.
An output electrode for detecting a surface acoustic wave generated and advanced by the sensors 1 and 32 and extracting a convolution output as an electric signal. Each of the interdigital transducers of the surface acoustic wave converters 31 and 32 has an electrode arrangement structure with an equal period. That is,
When the electrode width of the interdigital transducer is m and the period is p, p is constant, and m / p is a constant (often “0.5”) everywhere.

In a surface acoustic wave converter having such a periodic electrode arrangement, the generated surface acoustic wave propagates with substantially the same amplitude in both the left and right directions. Therefore, it can be said that it has the same insertion loss characteristics in both directions, so to say, has bidirectional characteristics.

On the other hand, as a technique for obtaining a unidirectional surface acoustic wave converter having a characteristic of low insertion loss only in one direction by using a surface acoustic wave converter having an electrode arrangement structure with an equal period,
Conventionally, for example, a method using a 120-degree phase shifter, a method using a 90-degree phase shifter, and an internal reflection type one-way characteristic in which one-way characteristics are obtained by asymmetrically disposing a reflective electrode between positive and negative electrodes at regular intervals. There was a method of using a converter.

[0005]

A surface acoustic wave converter having an equi-periodic electrode arrangement structure has bidirectional characteristics, and does not provide characteristics of low insertion loss in only one direction. Therefore, even if a surface acoustic wave convolver is configured using such a surface acoustic wave converter, a convolver with high convolution efficiency cannot be obtained. In the above-described technology for obtaining a unidirectional surface acoustic wave converter, a characteristic of low insertion loss in one direction can be obtained, but a surface acoustic wave converter having an electrode arrangement structure with an equal period is also used. Therefore, wideband characteristics cannot be obtained. Therefore, even when applied to a surface acoustic wave convolver, wideband characteristics cannot be obtained.

An object of the present invention is to provide a surface acoustic wave convolver having high convolution efficiency and wide band characteristics in view of the above-mentioned problems in the conventional example.

[0007]

In order to achieve the above object, the present invention provides first and second interdigital electrodes for exciting a surface acoustic wave on a piezoelectric or electrostrictive substrate. And an output electrode for detecting a surface acoustic wave of the above and extracting a convolution output as an electric signal, wherein the first and second interdigital electrodes have a predetermined thickness, and the first and second interdigital electrodes have a predetermined thickness. The interdigital transducer is formed by alternately arranging positive and negative electrodes whose electrode width and period gradually decrease toward the output electrode.
The interdigital transducer is characterized in that positive and negative electrodes whose electrode width and period gradually increase toward the output electrode are alternately arranged.

When the acoustic impedance of the metal film of the interdigital transducer is Zm and the acoustic impedance of the electrode gap is Zg, if Zm / Zg of the first and second interdigital transducers is smaller than 1, When the electrode width of the interdigital transducer is m and the period is p, the first interdigital transducer is 0.2 ≦ m / p ≦ 0.7, and the second interdigital transducer is 0.72 ≦ m / p ≦ 0.9.

[0009]

When a large number of positive and negative electrodes whose electrode width m and period p gradually change in the propagation direction are arranged, and the thickness of the electrodes is large enough to cause reflection of the excited wave, the phase of the wave caused by the direct excitation and its phase The phase of the wave generated by reflection of the wave on the electrode can be the same in one propagation direction and the opposite phase in the other propagation direction. Thus, a surface acoustic wave converter having a unidirectional characteristic in which the surface acoustic wave is strongly excited in a direction in which the phase of the generated wave is the same as the phase is obtained. A surface acoustic wave converter having high convolution efficiency can be obtained by facing the directionality of the surface acoustic wave converter having such unidirectional and broadband characteristics and forming an output electrode therebetween. However, when the positive and negative electrodes whose electrode width and period gradually change are arranged as described above, a so-called dispersive property in which the delay characteristic depending on the frequency changes appears. In order to eliminate this dispersibility, the surface acoustic wave converters (interdigital electrodes) on both sides of the output electrode reverse the width m and the period p in the propagation direction (direction toward the output electrode). There is a need. In such a configuration, when the first and second interdigital electrodes are manufactured in the same process, one of the interdigital electrodes has a direction opposite to that of the output electrode. If the direction of the first interdigital electrode is directed to the output electrode, the direction of the second interdigital electrode is directed away from the output electrode, so that improvement in convolution efficiency cannot be expected. On the other hand, as a result of measuring the directionality of such an interdigital electrode, as shown in FIG.
The directionality is relatively strong in the range of ≦ m / p ≦ 0.7, and m / p
Is relatively weak in the range of 0.72 or more.
However, it is difficult to manufacture interdigital electrodes having m / p exceeding 0.9. Therefore, the m / p of the interdigital transducer having directivity toward the output electrode is set to 0.2 ≦ m /
p ≦ 0.7, and m / p of the interdigital transducer having a directionality away from the output electrode is set to 0.72 ≦
If m / p ≦ 0.9, a convolver having a relatively high convolution efficiency can be obtained without impairing productivity. The directionality appears in the down-chirp direction when Zm / Zg is smaller than 1, and appears in the up-chirp direction when Zm / Zg is larger than 1. Here, the value of Zm / Zg is determined by the electrode material, the electrode film thickness, the electrode width, and the substrate material. In addition, as described later, a floating electrode is provided between the positive and negative electrodes of the IDT,
Zm / Z can also be obtained by short-circuiting between these floating electrodes.
g can be smaller or larger than 1.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a plan view of a surface acoustic wave convolver according to an embodiment of the present invention. Reference numeral 2 denotes an excitation-side first surface acoustic wave converter disposed on the piezoelectric substrate, and reference numeral 3 denotes an excitation-side second surface acoustic wave converter disposed on the piezoelectric substrate. The first surface acoustic wave converter 2 has a positive electrode 4 and a negative electrode 5 (first interdigital electrode). The second surface acoustic wave converter 3 has a positive electrode 6 and a negative electrode 7 (second interdigital electrode). Reference numeral 8 denotes an output electrode. The positive electrode 4 and the negative electrode 5 are alternately arranged so that the electrode width m and the period p gradually become shorter toward the output electrode. The positive electrode 6 and the negative electrode 7 are alternately arranged so that the electrode width m and the period p gradually increase toward the output electrode. Here, Y-cut Z-propagating lithium niobate was used as the piezoelectric substrate 1, and aluminum films were used as the electrodes 4, 5, 6, and 7.

A graph 21 in FIG. 2 shows a relationship between m / p and insertion loss in the surface acoustic wave converter. Graph 21 shows the insertion loss characteristics in the forward direction of the surface acoustic wave converter (the direction in which a surface acoustic wave having a larger amplitude is obtained). Graph 22
Conversely, shows the insertion loss characteristics in the backward direction of the surface acoustic wave converter (the direction in which a surface acoustic wave of smaller amplitude is obtained). From FIG. 2, the insertion loss in the backward direction is larger than the insertion loss in the forward direction, and the magnitude of the insertion loss is m
It can be seen that it differs depending on / p. The forward direction may be reversed depending on the cut and propagation of the crystal of the piezoelectric body and the type of the deposited film.

It is preferable to use one of the surface acoustic wave converters on the excitation side having a strong one-way characteristic and the other surface acoustic wave converter having a weak one-way characteristic and almost bidirectional. Therefore, from FIG. 2, one surface acoustic wave converter satisfies 0.2 ≦ m / p ≦ 0.7,
The other is preferably about 0.72 ≦ m / p ≦ 0.9. In the convolver of FIG. 1, the positive electrode 4 and the negative electrode 5
Is m / p = 0.5, and the positive electrode 6 and the negative electrode 7 are m / p = 0.5.
0.8. As described above, a surface acoustic wave convolver having high convolution efficiency and wide band characteristics can be obtained. In fact, the surface acoustic wave convolver of FIG. 1 has a bandwidth of 30% or more and a convolution efficiency of -60 dBm or less.

In the present invention, it has been found for the first time that the surface acoustic wave transducer is made directional by positively utilizing the reflection at the electrodes arranged on the piezoelectric substrate. A surface acoustic wave convolver having high convolution efficiency over a wide bandwidth by utilizing the characteristics. In this case, the electrode needs to have a certain thickness so that the surface acoustic wave is reflected at the electrode. Assuming that the thickness of the electrode is H and the wavelength of the surface acoustic wave is λ, it is preferable that about 0.01 ≦ H / λ ≦ 0.10.

Further, when Zm is the acoustic impedance of the electrode metal and Zg is the acoustic impedance of the electrode gap, the directionality of the surface acoustic wave converter is determined according to the value of Zm / Zg.

FIG. 5 shows that the thickness of the electrode made of Al is 2
FIG. 9 is a diagram showing frequency characteristics and electrode arrangement for explaining the directionality of the chirp transducer by equivalent circuit analysis when Zm / Zg = 0.98 when 000 Å is set. As shown in FIG. 5B, a down-chirped IDT 7 in which the density of the arrangement of the positive and negative electrodes gradually increases toward the IDT (interdigital transducer, interdigital transducer) 74 having a pair of positive and negative electrodes.
3 has a frequency characteristic as shown by a solid line 71 in FIG. Conversely, as shown in FIG.
In a configuration in which an up-chirped IDT 75 in which the density of the arrangement of the positive and negative electrodes gradually increases toward the IDT 76 having a pair of positive and negative electrodes, the frequency characteristic as shown by a broken line 72 in FIG. Become. As the frequency characteristic, the solid line 71 is better than the broken line 72. Therefore, the IDT 73 has the direction shown by the arrow 73D,
It can be seen that T75 has the direction shown by arrow 75D.

FIG. 6 is a diagram showing frequency characteristics and electrode arrangement for explaining the directionality of the chirp transducer by equivalent circuit analysis when Zm / Zg = 1.00. As shown in FIG. 6B, in a configuration in which a down-chirped IDT 83 in which the density of the arrangement of the positive and negative electrodes is gradually increased is arranged adjacent to the IDT 84 having a pair of positive and negative electrodes, as shown in FIG. A frequency characteristic as shown by a solid line 81 is obtained. Further, as shown in FIG. 6C, the up-chirp I is such that the density of the arrangement of the positive and negative electrodes gradually decreases toward the IDT 86 having the pair of positive and negative electrodes.
Even in a configuration in which the DTs 85 are adjacent to each other, the frequency characteristics are similarly indicated by a solid line 81 in FIG. Therefore, IDT8
It can be seen that both No. 3 and IDT 85 have no directionality.

FIG. 7 is a diagram showing frequency characteristics and electrode arrangements for explaining the directionality of the chirp transducer by equivalent circuit analysis when Zm / Zg = 1.02. As shown in FIG. 7B, in a configuration in which a down-chirped IDT 93 in which the density of the arrangement of the positive and negative electrodes is gradually increased is arranged adjacent to the IDT 94 having a pair of positive and negative electrodes, as shown in FIG. A frequency characteristic as shown by a solid line 91 is obtained. Conversely, as shown in FIG. 7C, the up-chirp I where the density of the arrangement of the positive and negative electrodes gradually decreases toward the IDT 96 having the pair of positive and negative electrodes.
In the configuration in which the DT 95 is adjacent, the broken line 92 in FIG.
The frequency characteristics shown in FIG. As a frequency characteristic, the broken line 92 is better than the solid line 91. Therefore,
The IDT 93 has a direction indicated by an arrow 93D, and the IDT 9
5 has the direction shown by arrow 95D.

As a result of investigating the directionality of the chirped transducer under the various conditions as described above, it was found that, when Zm / Zg <1, the arrangement density of the positive / negative electrodes was changed from coarse to dense. It has directionality, and when Zm / Zg = 1, it has no directionality and Zm / Z
When g> 1, it was found that the arrangement density of the positive and negative electrodes had a direction from the dense to the coarse direction. Therefore, if a convolver is configured in consideration of the directionality of such a transducer, it is possible to have a low insertion loss and a wide band characteristic.

FIG. 4 shows that the thickness of the electrode made of Al is 2
FIG. 9 is a diagram showing frequency characteristics and electrode arrangement for explaining the directionality of the chirp filter by equivalent circuit analysis when Zm / Zg = 0.98 when 000 angstroms is set. In other words, it shows the frequency characteristics and the like in the case of forming a filter in which the transducers used in the example of FIG. 5 are arranged. As shown in FIG. 4B, the IDT of the down-chirp
In a configuration in which the IDT 63 and the down-chirp IDT 64 are adjacent to each other, the frequency characteristic is as shown by a solid line 61 in FIG. Conversely, as shown in FIG.
In a configuration in which the T65 and the up-chirp IDT 66 are adjacent to each other, the frequency characteristic is as shown by a broken line 62 in FIG. The difference in the frequency characteristics is caused by the fact that the IDTs 63, 64, 65, and 66 have the directions indicated by arrows 63D, 64D, 65D, and 66D, respectively, as described with reference to FIG.

In particular, as one method, the value of Zm / Zg can be adjusted by providing a floating electrode as shown in FIG. 8, thereby giving directionality to the transducer. In FIG. 8, a transducer 102, which is a surface acoustic wave converter, has a positive or negative electrode 10
4,105. Furthermore, these positive and negative electrodes 10
The floating electrode 106 is provided in a gap between the first and second electrodes 106 and 106. Every other floating electrode 106 is short-circuited. By providing such a short-circuited floating electrode 106, Zm / Zg of the transducer 102 became smaller than 1. Also, although not shown, by providing the floating electrode 106 that is open without short-circuiting the floating electrode 106,
Zm / Zg of the transducer 102 became larger than 1.

[0022]

As described above, according to the present invention,
A surface acoustic wave convolver with a thick interdigital electrode provided on a piezoelectric or electrostrictive substrate is placed in consideration of the directionality of the interdigital electrode, so that convolution efficiency is high and broadband characteristics are improved. The surface acoustic wave convolver having the above is obtained.

[Brief description of the drawings]

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

FIG. 2 is a graph showing the relationship between m / p and insertion loss.

FIG. 3 is a plan view and a cross-sectional view of a filter using a conventional surface acoustic wave converter.

FIG. 4 is a diagram showing frequency characteristics and electrode arrangement for explaining the directionality of the chirp filter when Zm / Zg = 0.98.

FIG. 5 is a diagram showing frequency characteristics and electrode arrangement for explaining the directionality of a chirp transducer of Zm / Zg = 0.98.

FIG. 6 is a diagram showing frequency characteristics and electrode arrangement for explaining the directionality of a chirp transducer of Zm / Zg = 1.00.

FIG. 7 is a diagram showing frequency characteristics and electrode arrangement for explaining the directionality of a chirp transducer of Zm / Zg = 1.02.

FIG. 8 is a plan view showing a transducer using a floating electrode.

[Explanation of symbols]

2, 31 ... first surface acoustic wave converter, 3, 32 ... second surface acoustic wave converter, 4, 6 ... positive electrode, 5, 7 ... negative electrode,
8, 33 ... output electrodes.

Continuation of front page (56) References JP-A-4-331505 (JP, A) JP-A-3-74921 (JP, A)

Claims (1)

(57) [Claims] 1. A first or second interdigital electrode for exciting a surface acoustic wave on a piezoelectric or electrostrictive substrate, and detecting these surface acoustic waves to extract a convolution output as an electric signal. A surface acoustic wave convolver provided with an output electrode, wherein the first and second interdigital electrodes have a predetermined thickness, and the first interdigital electrode gradually has an electrode width and a period toward the output electrode. The second interdigital transducer is configured by alternately arranging positive and negative electrodes whose electrode width and period gradually increase toward the output electrode. The acoustic impedance of the metal film
m, when the acoustic impedance of the electrode gap is Zg, Zm / Zg of the first and second interdigital electrodes is smaller than 1, and the electrode width of the interdigital electrode is m, and the period is p. The first IDT is 0.2 ≦
m / p ≦ 0.7, and the second interdigital transducer has a diameter of 0.1 μm.
A surface acoustic wave convolver, wherein 72 ≦ m / p ≦ 0.9.