Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
1999-05-27
2001-10-02
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S193000, C310S31300R
Reexamination Certificate
active
06297713
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device which operates by generating a surface acoustic wave containing as a major component an SH (shear horizontal) type wave such as a BGS (Bleustein-Gulyaev-Shimizu) wave, a Love wave, or other such wave, and, more particularly, to an edge-reflection type surface acoustic wave device.
2. Description of the Related Art
Surface acoustic wave devices are used in a wide variety of devices such as resonators, bandpass filters, and other such electronic components. With ordinary surface acoustic wave devices, it is necessary to form reflectors on both sides of an interdigital transducer (IDT), which necessarily increases the size of surface acoustic wave devices.
In order to cope with this problem, there has been proposed an edge-reflection type resonator which uses an SH type surface acoustic wave. In the edge-reflection type resonator, an IDT is disposed on a piezoelectric substrate. The SH type surface acoustic wave excited by the IDT is reflected between the edges of the piezoelectric substrate which are located on opposite sides of the IDT. Thus, the two edges opposite to each other are used to reflect the surface acoustic waves instead of using the reflectors to reflect the waves. Accordingly, it is unnecessary to provide the reflectors.
An example of the above-described edge-reflection type resonator is disclosed in Japanese Unexamined Patent Publication No. 60-41809. In this publication, there is disclosed an edge-reflection type resonator which utilizes an SH type surface acoustic wave.
FIG. 8
is a schematic plan view of an edge-reflection type resonator which is described as a conventional example in the above prior art publication.
An edge-reflection type resonator
51
includes a piezoelectric substrate
52
having a rectangular plate shape. On the upper surface
52
a
thereof, a pair of interdigital electrodes (comb-shaped electrodes)
53
and
54
are provided so as to define one IDT. The interdigital electrodes
53
and
54
have a plurality of electrode fingers, respectively, which are interdigitated with each other. In the edge-reflection type resonator
51
, each of the electrode fingers of the interdigital electrodes
53
and
54
is composed of a split electrode pair, that is, two split electrodes, except for the electrode fingers
53
a
and
53
c
which are located at the outermost sides in the surface acoustic wave propagation direction. For example, the electrode finger
53
b
of the interdigital electrode
53
is composed of split electrodes
53
b
1
and
53
b
2
. Each of the electrode fingers
54
a
-
54
c
of the interdigital electrode
54
is composed of a pair of two split electrodes, that is,
54
a
1
and
54
a
2
,
54
b
1
and
54
b
2
, and
54
c
1
and
54
c
2
, respectively.
The surface acoustic wave propagation direction is perpendicular to the lengthwise direction of the electrode fingers
53
a
-
53
c
and
54
a
-
54
c
. The excited surface acoustic wave is reflected between the two edges
52
b
and
52
c
disposed opposite to each other, and thereby, resonant characteristics can be obtained.
As described above, by use of the electrode fingers, each composed of two split electrodes, that is, a pair of the split electrodes, the desired band characteristics, which can not be obtained if the ordinary single type electrode fingers are used, can be attained.
In the conventional edge-reflection type resonator
51
as shown in
FIG. 8
, the ratio of each electrode finger to the gap between the electrode fingers is 1:1, though it is not clearly stated in the above prior art, and ordinarily, the width of the split electrode is &lgr;/8 in which &lgr; is the wave length of an excited surface acoustic wave. In the prior art, the widths of the electrode fingers
53
a
and
53
c
at the outermost sides in the surface acoustic wave propagation direction are equal to the width of the respective split electrode fingers which constitute the remaining electrode fingers. Further, the resonator
51
is arranged so that the reflection end surfaces are located at the centers of a set of the two electrode fingers which are positioned at the outermost sides, respectively, or the distance between one of the centers and the corresponding reflection end-surfaces are an integral multiple of &lgr;/2.
However, in the case of the edge-reflection type resonator constructed according to the above-described prior art, there is a problem that the ratio of the anti-resonant resistance Ra to the resonant resistance Rr, that is, the ratio of “the top” to “the bottom” is not sufficient. In addition, there is a problem that unnecessary ripples are produced in the frequency characteristics, and good band characteristics can not be achieved.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provide an edge-reflection type surface acoustic wave device such as a resonator, a filter or other component, including electrode fingers each having two split electrodes (electrodes of a double electrode type) of which the ratio of the anti-resonant resistance to the resonant resistance, that is, the ratio of “the top” to “the bottom” is very high, and so that undesirable ripples of the frequency characteristics are prevented and so that the band is significantly narrowed.
According to one preferred embodiment of the present invention, an edge-reflection type surface acoustic wave device which is constructed to operate by generating an SH type surface acoustic wave and so that the surface acoustic wave is reflected by its two opposite edges, includes a piezoelectric substrate having first and second main surfaces and the two end surfaces opposite to each other which connect the first and second main surfaces, respectively, and an IDT including a pair of interdigital electrodes disposed on the first main surface of the piezoelectric substrate and arranged so that electrode fingers thereof are interdigitated with each other. Each of the electrode fingers of the interdigital electrodes preferably includes a plurality of split electrodes, except for the interdigital electrodes located at the opposite sides in the surface acoustic wave propagation direction. The electrode fingers located at the outermost sides have a width that is different from that of the respective split electrodes.
According to the edge-reflection type surface acoustic wave device of preferred embodiments of the present invention, narrow band characteristics which previously could not be obtained in an edge-reflection type surface acoustic wave device including the single type electrode fingers, can now be achieved in the edge-reflection type surface acoustic wave device according to preferred embodiments of the present invention. In addition, by providing the difference in width of the electrode fingers at the outermost sides, the ratio of the anti-resonant resistance to the resonant resistance, that is, the ratio of “the top” to “the bottom” is greatly increased. Further, by devising the above-described difference, undesired ripples in the frequency characteristics are prevented.
Accordingly, the edge-reflection type surface acoustic wave device according to preferred embodiments of the present invention achieve a desired band width and excellent resonant characteristics with almost no ripples. Even if a piezoelectric material having an electromechanical coupling factor corresponding to an intended band is not available, the intended band can be easily achieved by simply adjusting the structure of the above-described interdigital electrodes.
It is preferable that the widths of the electrode fingers at the outermost sides in the surface acoustic wave propagation direction are more than about &lgr;/8, so that the ratio of the anti-resonant resistance to the resonant resistance is significantly increased, and the bandwidth is greatly enlarged. Further, undesirable ripples, which may appear on both sides of the main response, are effectively prevented. Thus, the
Ago Junya
Horiuchi Hideya
Ikeura Mamoru
Kadota Michio
Keating & Bennett LLP
Murata Manufacturing Co. Ltd
Pascal Robert
Summons Barbara
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