JPH11274887A - Surface acoustic wave converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently and stably excite surface acoustic wave without any notch in the frequency characteristic in a using band by specifying the length of crossing part of positive and negative electrodes, the length of their noncrossing part and the width of bus bars, which are respectively parts of interdigital electrodes, at a prescribed ratio. SOLUTION: When the length of crossing part of electrodes 1, 2 of an interdigital electrodes is set a1, the length of noncrossing part is set a2 and the widths of the bus bars 3 and 4 to be a3, the values of a1 to a3 are respectively varied to satisfy a prescribed relation to prevent the generation of a notch frequency. A notch exists not only in a specific frequency but exists with specific distribution by setting a specific frequency to be a peak. Then, the existence of the notch with respect to frequency characteristic causes the increase of an insertion loss not only in a specific frequency but extending the whole using band. Namely, the insertion loss can be reduced by preventing the notch.

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 converter having interdigital electrodes, and more particularly to a surface acoustic wave capable of efficiently and stably exciting and receiving a surface acoustic wave in a band of use. Related to a converter.

[0002]

2. Description of the Related Art Conventionally used surface acoustic wave converters in which interdigital electrodes serving as positive and negative electrodes are formed of a metal thin film such as an aluminum thin film on a piezoelectric or electrostrictive substrate such as lithium niobate. FIG. 4A and FIG. 4B show the electrode configuration of the vessel. As shown in the figure, positive and negative electrode fingers 1 and 2 for exciting surface acoustic waves, bus bars 3 and 4,
It is composed of terminals 5 and 6 for supplying an electric signal,
The electrode width of the positive and negative electrode fingers 1 and 2 and the period in which the electrode finger pairs are arranged are constant in any part of the electrodes.

Further, in order to provide a surface acoustic wave converter with a bandwidth or to provide a directionality to a surface acoustic wave converter for use in a convolver or the like, a plane configuration of an electrode configuration shown in FIG. As shown in the figure, a dispersion type configuration in which the width of the electrode fingers and the cycle in which the electrode finger pairs are arranged is gradually changed may be employed. Further, the present inventor shown in FIG.
As in the surface acoustic wave transducer disclosed in No. 261001, a plurality of dispersed interdigital transducers having the same width of the electrode finger and the same direction of the change of the period in which the electrode finger pair is arranged and different center frequencies are used as one elastic wave electrode. By adopting a configuration in which the antenna is arranged on a surface acoustic wave propagation path, there is a case where a response to a wider band is performed.

[0004]

However, a practical fractional bandwidth of a surface acoustic wave converter having a broadband interdigital transducer is generally limited to about 20%. That is, if the center frequency is 300 MHz, the band is 270
The practical limit was about 330 MHz.

For example, in the electrode configuration of a surface acoustic wave converter, only the electrode width and the period in which the electrode pairs are arranged are used with a center frequency of 300 MHz and a specific band corresponding to a wide band of about 60%. If you configure the filter
In the actual frequency characteristic, a frequency that does not pass is considered to be caused by spurious or the like (hereinafter referred to as a notch).
Was observed, so in practical use the fractional band had to be determined in such a way as to avoid notches.

The present invention has been made in view of the above problems, and provides a surface acoustic wave converter capable of efficiently and stably exciting a surface acoustic wave without generating a notch in the frequency characteristic in a used band. The purpose is to:

[0007]

In order to achieve the above-mentioned object, a surface acoustic wave converter according to the present invention comprises a piezoelectric or electrostrictive substrate having an electrode width and a period in one direction. A surface acoustic wave transducer having at least one set of gradually changing positive and negative electrodes interleaved interdigital electrodes and a positive or negative bus bar connected to either the positive or negative electrode. The length of the part of the interdigital transducer where the positive and negative electrodes intersect, and the length of the part of the interdigital transducer where the positive and negative electrodes do not intersect And the width of the bus bar has a predetermined ratio.

[0008]

The operation of the present invention will be described below with reference to the drawings. The same components as those shown in the prior art are denoted by the same reference numerals. FIG. 1 is a plan view of an electrode for describing an electrode configuration in a surface acoustic wave converter according to the present invention.

In the interdigital electrode shown in FIG. 1, the length of the bus bars 3 and 4 is b3, the length of the intersection of the electrode fingers 1 and 2 is a1, and the intersection is the length of the bus bar. A continuous area indicated by a1 × b3 is defined as A1. Next, the lengths of the non-intersecting portions of the electrode fingers 1 and 2 are denoted by a2, and the two non-intersecting portions indicated by a2 × b3 continuous in the length direction of the bus bar are denoted by A2. , The width of the bus bars 3 and 4 is a3,
Areas on the busbars 3 and 4 are A3, respectively, and areas outside the busbars 3 and 4 on the side different from the electrode fingers 1 and 2 are A4.

[0010] In the prior art, for example, Japanese Patent Publication 6-8
As shown by 5492, a1 or a1 / (a1 + 2 × a) which is a main excitation part in the surface acoustic wave converter
We focus only on 2). However, the surface acoustic waves exist in the regions A1, A2, A3, and A4, respectively.
In addition, since the energy, velocity, and direction of the surface acoustic wave in each region are different from each other, the notch at an unintended frequency in the electrode configuration is caused by the interaction of the surface acoustic wave in each region, etc. Can act as a waveguide, and in this waveguide, the generated surface waves are weakened, or obstacles such as mutual cancellation by the interaction of the surface waves occur, and it is considered that only the main excitation portion cannot cope.

[0011] The present invention is based on the above idea, and
The value of a3 is changed, the influence of these values on the notch is grasped, and the notch is prevented by satisfying a predetermined relationship for each value.
It is considered that the hem of the surface acoustic wave also exists and influences the terminal portions 5 and 6, but the effect of the presence of the terminal portion by reducing the area of the terminal portion or narrowing the width is considered. It can be minimized.

The above description refers to the case where the electrodes are of a distributed interdigitated electrode configuration. However, for example, even when a non-dispersive interdigital electrode as shown in FIG. 4A is used, the size of the notch or the frequency of the notch observed on the measured frequency characteristics is different from that of the dispersed interdigital electrode structure. Although it may be different from the case, a1, a2, a3
The effect of the change of each value on the notch has the same optimum value or optimum range as in the case of the distributed IDT structure. Also, when the electrode structure becomes double or when there is a floating electrode, etc.
It is considered that the optimal values or optimal regions of 1, a2, and a3 exist. Furthermore, the excitation characteristics of the surface acoustic wave and the waveguide characteristics of the surface acoustic wave converter itself also depend on the piezoelectric or electrostrictive characteristics of the substrate. The electrode structure for preventing the formation of a notch in the band, that is, the preferable values of a1, a2, and a3 also change, but the effect of the change of each value on the notch has the same optimum value even when the substrate changes. Alternatively, it is considered that an optimum range exists.

The notch does not exist only at a specific frequency, but has a specific distribution with a specific frequency as a peak. For this reason, it is conceivable that the presence of the notch in the frequency characteristic causes an increase in the insertion loss not only at the specific frequency but also over the entire use band. That is, the prevention of the notch makes it possible to obtain a surface acoustic wave conversion element having a small insertion loss.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, embodiments of the present invention will be described, particularly when a surface acoustic wave filter is constructed using a set of surface acoustic wave converters having varied values of a2 and a3, and transmission and reception are performed. The effects of a2 and a3 will be described with reference to the figures showing the insertion loss characteristics. The width of the paired electrodes and the period in which the paired electrodes are arranged have a center frequency of 300 MHz and a fractional band of about 60%.
In order to set such that, two dispersed interdigital transducers having the same direction of change in the width of the electrode finger and the period in which the electrode finger pair is arranged and different center frequencies are arranged on one surface acoustic wave propagation path. As the piezoelectric substrate, Y-cut Z-propagated lithium niobate was used as the piezoelectric substrate, and aluminum thin films were used for the electrodes 1, 11 and 2, 12 and the bus bars 3, 13, 4, and 14.

Further, in the first embodiment, the value of a1 is three times the center frequency (hereinafter 3) of the center frequency in the band used by the surface acoustic wave converter.
Shown as λ. ) Fixed. Hereinafter, embodiments will be described. In the frequency characteristics in the embodiments described below, a notch occurs near 290 MHz.
This is due to the interaction between the two interdigital electrodes as described in JP-A-9-261001 and has no relevance to the effect of the present invention.

As a first embodiment of the present invention, when two dispersed interdigital electrodes are used, the value of a3 is fixed at 12λ, and the value of a2 is λ, 2λ, 3λ. FIG. 2A, FIG. 2C, and FIG. 2C show the frequency characteristics of the respective surface acoustic wave converters in the case of performing the above. In FIG. 2A, a notch is observed at a frequency of 274 MHz, and in FIG. 2B, a notch is observed at a frequency of 248 MHz.
In C, a slight attenuation tendency was observed in the low frequency region, but no large notch was observed in the used band. That is, it is understood that the notch can be shifted to the lower frequency side by increasing the value of a2.

From this, the value of a3 in this embodiment is 12
In the case of λ, it can be seen that by setting the value of a2 to 3λ or more, a surface acoustic wave converter that does not generate a notch in a wide band can be obtained. However, there is a concern that a notch due to a higher-order transverse mode spurious or the like generated on the higher frequency side in accordance with the shift of the notch toward the lower frequency side due to the increase of the value of a2 may enter the used band. , A2
Increase in the value of a1 results in a relative decrease in the value of a1, resulting in an increase in the size of the surface acoustic wave converter itself. It is preferable to set an appropriate value that satisfies the condition.

Next, FIGS. 3A, 3B, and 3C show the frequency characteristics of the respective surface acoustic wave converters when the value of a2 is fixed at 2λ and the value of a3 is 3λ, 6λ, and 12λ. . In FIG. 3A, a notch was observed at a frequency of 225 MHz, which is near the lowest frequency of the used band, in FIG. 3B, a notch was observed at a frequency of 236 MHz, and in FIG. 3C, a notch was observed at a frequency of 248 MHz. That is, it is understood that the notch can be shifted to the lower frequency side by reducing the value of a3.

From this, the value of a2 in this embodiment is 2λ
In this case, it can be understood that by setting the value of a3 to 3λ or less, a surface acoustic wave converter that does not generate a notch in a wide band can be obtained. However, there is a concern that a notch due to a higher-order transverse mode spurious or the like generated on the higher frequency side in accordance with the shift of the notch toward the lower frequency side due to the decrease of the value of a3 may enter the used band. Since the decrease in the value of a3 causes the generation of Joule heat by increasing the DC resistance component of the bus bar, the excitation efficiency with respect to the input power is reduced, and the bus bar may be worn due to this. It is preferable to set an appropriate value that satisfies the above-described conditions in consideration of the band to be used and the design restrictions of the surface acoustic wave converter.

Further, in the frequency characteristics shown in FIGS. 2A to 2C and 3A to 3C, as the notch is closer to the center frequency, the insertion loss in the entire use band increases, and in FIG. At -20 dB or less. That is, it is also possible to obtain a surface acoustic wave converter having a low insertion loss by setting the notch out of the used band.

Although the present embodiment has described the individual effects of the values of a2 and a3 when the fractional band is set to 60%, the above effects are mutually related. Further, for example, when the surface acoustic wave converter in the present embodiment is used with a fractional band of 70%, the value of a2 is reduced, the value of a3 is increased, or a combination of the two prevents the occurrence of a notch in the used band. Needless to say, this is also possible. Further, in a surface acoustic wave converter for a narrow band such as a surface acoustic wave resonator, the notch generation in the used band is achieved by optimizing the values of a2 and a3 according to the present invention. Needless to say, an increase in insertion loss can be prevented.

As a second embodiment of the present invention, when the fixed value of a1 is 2λ in the first embodiment, the change of the notch frequency when a2 and a3 are changed is as follows. It is shown in Table 1.

[Table 1] As shown in the above table, the same relationship between a2, a3 and the notch was confirmed in the second embodiment as in the first embodiment. Further, the insertion loss is about 10% in the case where the notch exists, as compared with the case where the notch exists.
A dB improvement was made.

As a third embodiment of the present invention, the following table shows changes in the notch frequency when a2 and a3 are changed in a non-dispersion type interdigital electrode as shown in FIG. 4A.
The evaluation in this embodiment is different from those of the first and second embodiments in that a set of surface acoustic wave converters for a narrow band having a center frequency of 265 MHz and a relative band of about 0.5% is used. The evaluation was performed by measuring the insertion loss.
The value of a1 is fixed at 2.5λ.

[Table 2] As shown in the above table, in the comparison results of the insertion loss in the third embodiment, the same a2, a3,
And the improvement of insertion loss was confirmed, and improvement up to 15 dB was confirmed.

Since the surface acoustic wave converter according to the present invention has frequency characteristics as shown in FIG. 2C, it is also possible to configure a notch filter or the like that does not pass only a specific frequency within a certain frequency band. It is possible. In addition, from the relationship obtained by the embodiments and the like, it is also possible to know the frequency at which a notch is generated by calculation using the values of a1, a2, a3, etc., for a surface acoustic wave converter already used, for example. It is.

The dispersed IDT according to the present invention can be made unidirectional as described in JP-A-4-331505. That is, assuming that 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 is smaller than 1, a direction in which the electrode width and the period are strongly excited in a direction in which the electrode width and the period gradually decrease. Zm /
When Zg is set to be larger than 1, it is possible to provide a direction in which excitation is strongly performed in a direction in which the electrode width and the period gradually increase. It is also possible to set Zm / Zg = 1, or to make the interdigital electrodes have a double-electrode structure, etc., so as to have bidirectionality.

Further, as described in JP-A-7-212184, a floating electrode is provided in the gap between the electrodes to reverse the unidirectionality. The directionality can be controlled by providing a groove in the gap between the electrodes as proposed. However, the optimum values of a1, a2, and a3 in this case are considered to be different from the above values.

[0027]

According to the present invention, in the interdigital transducer, when the length of the intersecting portion of the electrode fingers is a1, the length of the non-intersecting portion is a2, and the width of the bus bar is a3, a1, a2,
By changing the value of a3 to satisfy a predetermined relationship, it is possible to prevent the occurrence of a notch frequency. As a result, even in the used band, the notch does not occur in the frequency characteristics, and the efficiency and stability are improved. Thus, a surface acoustic wave converter capable of exciting a surface acoustic wave can be provided.

[Brief description of the drawings]

FIG. 1 is a plan view of an electrode for describing an electrode configuration in a surface acoustic wave converter according to the present invention.

FIG. 2A is a view showing a frequency characteristic in the surface acoustic wave converter according to the present invention.

FIG. 2B is a diagram showing a frequency characteristic in the surface acoustic wave converter according to the present invention.

FIG. 2C is a view showing a frequency characteristic in the surface acoustic wave converter according to the present invention.

FIG. 3A is a view showing a frequency characteristic in the surface acoustic wave converter according to the present invention.

FIG. 3B is a view showing a frequency characteristic in the surface acoustic wave converter according to the present invention.

FIG. 3C is a diagram showing a frequency characteristic in the surface acoustic wave converter according to the present invention.

FIG. 4A is a plan view of a conventional surface acoustic wave converter.

FIG. 4B is a sectional view taken along line YY in FIG. 4A.

FIG. 5 is a diagram showing a conventional example of a surface acoustic wave converter.

FIG. 6 is a diagram showing a conventional example of a surface acoustic wave converter.

[Explanation of symbols]

 1, 2, 11, 12 electrode 3, 4, 13, 14 bus bar 5, 6 terminal

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masao Takeuchi 1810-1 Takagasaka, Machida-shi, Tokyo (72) Inventor Kazuhiko Yamanouchi 37-13 Matsugaoka, Taishiro-ku, Sendai, Miyagi Prefecture

Claims (3)

[Claims] An interdigitated electrode in which positive and negative electrodes whose electrode width and period gradually change in one direction are alternately arranged on a piezoelectric or electrostrictive substrate; A surface acoustic wave transducer having positive and negative bus bars respectively connected to the electrodes, wherein the length of a part of the interdigital transducer and at which the positive and negative electrodes intersect, A surface acoustic wave converter, wherein a length of a part of the interdigital electrodes, where the positive and negative electrodes do not intersect, and a width of the bus bar have a predetermined ratio. 2. An interdigital transducer in which positive and negative electrodes having a constant electrode width and period are alternately arranged on a piezoelectric or electrostrictive substrate, and connected to the positive and negative electrodes, respectively. A positive and a negative bus bar, wherein the length of a part of the interdigital electrode, at which the positive and negative electrodes intersect, a part of the interdigital electrode Wherein the length of the non-intersecting portion of the positive and negative electrodes and the width of the bus bar have a predetermined ratio. 3. An interdigital transducer in which positive and negative electrodes are alternately arranged on a piezoelectric or electrostrictive substrate; and positive and negative bus bars connected to the positive and negative electrodes, respectively. In a surface acoustic wave transducer having: a length of a part of the interdigital electrode, at which a positive and a negative electrode intersect, a part of the interdigital electrode and a positive and a negative A surface acoustic wave converter, wherein a length of a portion of the electrode that does not intersect and a width of the bus bar have a predetermined ratio.