U.S. patent application number 10/993299 was filed with the patent office on 2005-04-28 for surface acoustic wave device.
This patent application is currently assigned to Murata Manufacturing Co., Ltd.. Invention is credited to Kando, Hajime.
Application Number | 20050088057 10/993299 |
Document ID | / |
Family ID | 26596749 |
Filed Date | 2005-04-28 |
United States Patent
Application |
20050088057 |
Kind Code |
A1 |
Kando, Hajime |
April 28, 2005 |
Surface acoustic wave device
Abstract
A surface acoustic wave device includes an asymmetrical double
electrode which prevents a mismatch between reflected waves and
propagating surface acoustic waves on strips, and which is capable
of realizing a superior unidirectionality. This surface acoustic
wave device includes the asymmetrical double electrode in which a
half wavelength section includes first and second strips which have
mutually different widths. The half wavelength is arranged to
define a basic section. The surface acoustic wave device includes
at least two of these basic sections disposed on a piezoelectric
substrate. The absolute value of the vector angle of the reflection
center is within approximately 45.+-.10.degree. or within
approximately 135.+-.10.degree., when the center of the-basic
section is the reference position. Alternatively, the absolute
value of the phase difference between the excitation center and the
reflection center is within approximately 45.+-.10.degree. or
approximately 135.+-.10.degree..
Inventors: |
Kando, Hajime;
(Nagaokakyo-shi, JP) |
Correspondence
Address: |
Keating & Bennett LLP
Suite 312
10400 Eaton Place
Fairfax
VA
22030
US
|
Assignee: |
Murata Manufacturing Co.,
Ltd.
Nagaokakyo-shi
JP
617-8555
|
Family ID: |
26596749 |
Appl. No.: |
10/993299 |
Filed: |
November 18, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10993299 |
Nov 18, 2004 |
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09887173 |
Jun 22, 2001 |
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6841918 |
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Current U.S.
Class: |
310/313B |
Current CPC
Class: |
H03H 9/14505
20130101 |
Class at
Publication: |
310/313.00B |
International
Class: |
H03H 009/25 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 26, 2000 |
JP |
2000-226073 |
Aug 10, 2000 |
JP |
2000-242884 |
Claims
1-5. (canceled)
6. A surface acoustic wave device, comprising: a piezoelectric
substrate; and at least two basic sections disposed on said
piezoelectric substrate, each of the at least two basic sections
including an asymmetrical double electrode defining a half
wavelength section and having first and second strips with
different widths from each other; wherein an absolute value of a
phase difference between an excitation center and an reflection
center of said asymmetrical double electrode is within
approximately 45+10.degree. or approximately 135.+-.10.degree..
7. A surface acoustic wave device according to claim 6, wherein
reflection amounts of surface acoustic waves at edge positions of
said strips are substantially equal to one another.
8. A surface acoustic wave device according to claim 6, wherein
said asymmetrical double electrode is an interdigital
transducer.
9. A surface acoustic wave device according to claim 6, wherein
said asymmetrical double electrode is a reflector.
10. A surface acoustic wave device according to claim 6, wherein
said piezoelectric substrate is made of a quartz crystal
material.
11-15. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a surface acoustic wave
device for use in, for example, a resonator or a filter, and more
particularly, to a surface acoustic wave device having an
asymmetrical double electrode used as a unidirectional interdigital
transducer or a dispersive reflection type reflector.
[0003] 2. Description of the Related Art
[0004] A surface acoustic wave device such as a surface acoustic
wave filter is widely used in mobile communication equipment or
broadcasting equipment, or other such apparatuses. Particularly
because the surface acoustic wave device is compact, lightweight,
tuning-free and easy to manufacture, the surface acoustic wave
device is suitable for an electronic component for use in portable
communication equipment.
[0005] The surface acoustic wave device is broadly divided into a
transversal type filter and a resonator-type filter, based on its
structure. In general, the transversal type filter has advantages
of having (1) a small group delay deviation, (2) a superior phase
linearity, and (3) a high degree of flexibility in the pass band
design based on weighting. However, the transversal type filter has
a disadvantage of having a large insertion loss.
[0006] An interdigital transducer (hereinafter referred to as an
"IDT") used in a surface acoustic wave filter transmits and
receives surface acoustic waves with respect to both sides of an
IDT, that is, the IDT transmits and receives surface acoustic waves
bilaterally in an equal manner. For example, in a transversal type
filter in which two IDTs are spaced apart from each other by a
predetermined distance, one half of the surface acoustic waves
transmitted from one IDT is received by the other IDT, but the
surface acoustic waves propagated from the one IDT to the opposite
side of the other IDT become a loss. This loss is called a "two-way
loss", and has become a big factor in increasing insertion loss of
a transversal type filter.
[0007] In order to reduce the above-described two-way loss, various
types of unidirectional IDTs have been proposed. In such
unidirectional IDTs, surface acoustic waves are transmitted and
received at only one side alone thereof. Also, low-loss transversal
type filters which utilize these unidirectional IDTs have been
developed.
[0008] For example, Hanma et al., have proposed an asymmetrical
double electrode in "A TRIPLE TRANSIT SUPPRESSION TECHNIQUE" 1976
IEEE Ultrasonics Symposium Proceedings pp. 328-331. FIG. 14 is a
schematic partially cutaway plan view showing the asymmetrical
double electrode disclosed in this prior art.
[0009] In an asymmetrical double electrode 101, half wavelength
sections Z constituted of two strips 102 and 103 having different
widths from each other, are disposed repeatedly many times along
the propagation direction of surface acoustic waves. Such an
electrode defined by half wavelength sections Z constituted of two
strips having different widths from each other, is called an
"unbalanced double electrode" or a "asymmetrical double
electrode".
[0010] The width of a half wavelength section is set to
0.5.lambda.. The width of a strip 102 having a relatively narrow
width is set to .lambda./16. The width of a strip 103 having a
relatively wide width is set to 3.lambda./16. The width of a gap
between the strips 102 and 103 is set to 2.lambda./16. The width of
an outer gap of the strip 102 in the half wavelength section is set
to .lambda./16. The width of the outer gap of the strip 103 in the
propagation direction of surface acoustic waves in the half
wavelength section is set to .lambda./16.
[0011] Between adjacent basic sections, the electrical polarities
are opposite to each other.
[0012] In the above-described asymmetrical double electrode, a
reflection per basic section can be expressed by a resultant vector
that is generated by synthesizing reflected waves from the edges X1
to X4 of the strips 102 and 103 shown in FIG. 15. FIG. 16 shows the
reflection vectors at the edges X1 to X4 when the reference
position is set to the center of a basic section, and the resultant
vector thereof. As can be seen from FIG. 16, the resultant vector V
is located at an angle of 67.5.degree., and the reflection center
is located at an angle of 67.5.degree./2=33.75.degree..
[0013] Also, in this asymmetrical double electrode, the outer edge
X1 of the strip 102 and the outer edge X4 of the strip 103 are
disposed bilaterally symmetrically with respect to the center of
the half wavelength section. Hence, the distances between the
center of a basic section and the outer edges of the nearest strips
in the adjacent basic sections, are also equal to each other. In
the asymmetrical double electrode, therefore, an excitation center
is located at the center of the basic section Z, with a phase
difference of about 33.750 generated between an excitation center
and the reflection center. Thus, the asymmetrical double electrode
operates as a unidirectional electrode.
[0014] Table 1 below shows the inter-mode coupling coefficient
.kappa..sub.12/k.sub.0, the phase difference between the excitation
center .psi. and the reflection center .phi., and the reflection
center .phi., when forming an asymmetrical double electrode of
aluminum film having a 3% film-thickness on a ST-cut crystal quartz
substrate, as an example of the above-described asymmetrical double
electrode.
1 TABLE 1 Item Calculated value Inter-mode coupling coefficient
.kappa..sub.12/k.sub.0 0.00257 Phase difference between excitation
center .psi. 31.3.degree. and reflection center .phi. Reflection
center .phi. 33.8.degree.
[0015] Here, k.sub.0 is a wave number of surface acoustic waves
propagating through an IDT. The ratio .kappa..sub.12/k.sub.0 and
the phase difference between the excitation center .psi. and the
reflection center .phi. can be obtained from the resonant frequency
determined by the finite element method, using the technique of
Cbuchi et al., ("Evaluation of Excitation Characteristics of
Surface Acoustic Wave Interdigital Electrode Based on Mode Coupling
Theory", Institute of Electronics, Information and Communication
Engineers of Japan, Technical Report MW90-62). Also, the reflection
center .phi. is determined by the phase difference between the
excitation center .psi. and the reflection center .phi., and the
excitation center obtained from the fundamental wave component
which is acquired by Fourier-transforming the electric charge
density distribution on the electrode obtained by the finite
element method.
[0016] Japanese Unexamined Patent Application Publication No.
61-6917 discloses an electrode which has implemented
unidirectionality by disposing two strips having mutually different
widths in a half wavelength section, as in the case of the
above-described asymmetrical double electrode. The electrode
disclosed in this Japanese Unexamined Patent Application
Publication No. 61-6917 is also supposed to operate as a
unidirectional electrode due to the asymmetry of the two strips
thereof. However, in the method disclosed in the Japanese
Unexamined Patent Application Publication No. 61-6917, no means for
controlling the reflection center and the reflection amount are
disclosed. In addition, no feasible reflection center and
reflection amount are described.
[0017] The article "Direct Numeral Analysis SAW Mode Coupling
Equation and Applications Thereof", 27th EM symposium preprint, pp.
109-116, Takeuchi et al., describes the principle of a
unidirectional IDT which provides flat directivity over a wide band
in the structure wherein positive and negative reflection elements
are dispersively disposed in a unidirectional IDT. Herein, however,
no means for forming a reliably superior unidirectional IDT are
described.
[0018] In general, when surface acoustic waves are caused to be
incident on an IDT constituted only of double strips without
reflection, reflection is caused by re-excitation. As a result, in
the case of a conventional transversal type filter, waves called
"triple transit echo" or TTE, occur, and cause ripples or other
undesired wave characteristics that adversely effect filter
characteristics. The above-described literature by Hanma et al.,
discloses a method for canceling out reflection due to
re-excitation by means of acoustic reflected waves of an
asymmetrical double electrode. This method, however, has created a
problem that new ripples are caused by acoustic reflection when the
acoustic reflection is larger than the reflection caused by the
re-excitation. Therefore, such a method for canceling out the
reflection by re-excitation is subjected to the restriction of
piezoelectric substrate material or electrode material, since the
reflection vector length which represents the acoustic reflection
amount is fixed in an asymmetrical double electrode.
[0019] On the other hand, the article "About One Weighting Method
For SAW Reflector", 1999, General Convention of Institute of
Electronics, Information and Communication Engineers of Japan, p.
279, Tajima et al., discloses a method for performing weighting
with respect to the reflection coefficient of a reflector. This
method uses a plurality of strips having mutually different widths
and makes use of the change of the reflection coefficient of a
strip based on the strip width. However, when the strip width is
changed, the sonic speed is also changed. As a result, when
attempting to perform weighting based on the strip width, a testing
method and apparatus is needed to find a correct sonic speed and to
change the arrangement pitch of the strip in accordance with this
corrected sonic speed. This poses a problem that the design
requires an extremely high degree of technique.
[0020] As described above, various IDTs or resonators each
operating as a unidirectional electrode by asymmetry of two strips
have been proposed, but conventional asymmetrical double electrodes
have not yet achieved sufficient unidirectionality. In addition,
the reflection center and the reflection amount of the conventional
asymmetrical double electrodes have been very difficult to
control.
SUMMARY OF THE INVENTION
[0021] In order to overcome the problems described above, preferred
embodiments of the present invention provide a surface acoustic
wave device using an asymmetrical double electrode which achieves
more superior unidirectionality of surface acoustic wave
propagation while effectively and easily controlling the reflection
amount per basic section.
[0022] In accordance with a preferred embodiment of the present
invention, a surface acoustic wave device includes a piezoelectric
substrate, and including at least two basic sections including an
asymmetrical double electrode in which a half wavelength section
includes first and second strips having different widths from each
other, the at least two basic sections being disposed along the
propagation direction of surface acoustic waves. In this surface
acoustic wave device, the absolute value of the vector angle of the
reflection center obtained from the resultant vector generated by
synthesizing the reflection vectors at the edges of the first and
second strips is preferably within approximately 45.+-.10.degree.
or approximately 135.+-.10.degree., when the center of the each of
the at least two basic sections is the reference position.
[0023] In accordance with another preferred embodiment of the
present invention, a surface acoustic wave device includes a
piezoelectric substrate, and including at least two basic sections
including an asymmetrical double electrode in which a half
wavelength section includes first and second strips having
different widths from each other, the at least two basic sections
being disposed along the propagation direction of surface acoustic
waves. In this surface acoustic wave device, the absolute value of
the phase difference between the excitation center and the
reflection center of the asymmetrical double electrode, is
preferably within approximately 45.+-.10.degree. or approximately
135.+-.10.degree..
[0024] In accordance with a still another preferred embodiment of
the present invention, a surface acoustic wave device includes a
piezoelectric substrate, and including at least two basic sections
including an asymmetrical double electrode in which a half
wavelength section includes first and second strips having
different widths from each other, the at least two basic sections
being disposed along the propagation direction of surface acoustic
waves. In this surface acoustic wave device, when the edge
positions of the first and second strips are X1 to X4, each of
which is a value corrected using the sonic speed difference between
a free surface and a metallic surface, and when the resultant
vector length of normalized reflected waves from the strip edges is
.vertline..GAMMA..vertline., and the center position of the basic
section is 0(.lambda.), and X1.congruent.X4, each of the positions
of X2 and X3 is a value substantially satisfying the following
equations (1) and (2).
[0025] Mathematical Expression 4
X2[.lambda.]=A.times.X1[.lambda.].sup.2+B.times.X1[.lambda.]+C.+-.0.1[.lam-
bda.] (1)
[0026] Mathematical Expression 5
X3[.lambda.]=D.times.X1[.lambda.].sup.2+E.times.X1[.lambda.]+F.+-.0.05[.la-
mbda.] (2)
[0027] Mathematical Expression 6
A=-34.546.times..vertline..GAMMA..vertline..sup.6+176.36.times..vertline..-
GAMMA..vertline..sup.5-354.19.times..vertline..GAMMA..vertline..sup.4+354.-
94.times..sym..GAMMA..vertline..sup.3-160.44.times..vertline..GAMMA..vertl-
ine..sup.2+10.095.times..vertline..GAMMA..vertline.-1.7558
B=-15.464.times..vertline..GAMMA..vertline..sup.6+77.741.times..vertline..-
GAMMA..vertline..sup.5-153.44.times..vertline..GAMMA..vertline..sup.4+147.-
20.times..vertline..GAMMA..vertline..sup.3-68.363.times..vertline..GAMMA..-
vertline..sup.2+6.3925.times..vertline..GAMMA..vertline.-1.7498
C=-1.772.times..vertline..GAMMA..vertline..sup.6+8.7879.times..vertline..G-
AMMA..vertline..sup.5-17.07.times..vertline..GAMMA..vertline..sup.4+16.092-
.times..vertline..GAMMA..vertline..sup.3-7.4655.times..vertline..GAMMA..ve-
rtline..sup.2+0.8379.times..vertline..GAMMA..vertline.-0.3318
D=12.064.times..vertline..GAMMA..vertline..sup.6-45.501.times..vertline..G-
AMMA..vertline..sup.5+57.344.times..vertline..GAMMA..vertline..sup.4-22.68-
3.times..vertline..GAMMA..vertline..sup.3+12.933.times..vertline..GAMMA..v-
ertline..sup.2-15.938.times..vertline..GAMMA..vertline.-0.1815
E=7.2106.times..vertline..GAMMA..vertline..sup.6-30.023.times..vertline..G-
AMMA..vertline..sup.5+45.792.times..vertline..GAMMA..vertline..sup.4-29.78-
4.times..vertline..GAMMA..vertline..sup.3+13.125.times..vertline..GAMMA..v-
ertline..sup.2-6.3973.times..vertline..GAMMA..vertline.+1.0203
F=1.0138.times..vertline..GAMMA..vertline..sup.6-4.4422.times..vertline..G-
AMMA..vertline..sup.5+7.3402.times..vertline..GAMMA..vertline..sup.4-5.474-
.times..vertline..GAMMA..vertline..sup.3+2.3366.times..vertline..GAMMA..ve-
rtline..sup.2-0.7540.times..vertline..GAMMA..vertline.+0.2637
[0028] In the surface acoustic wave device in accordance with
another preferred embodiment of the present invention, it is
preferable that the reflection amounts of the surface acoustic
waves at the edge positions X1 to X4 of the above-described strips
be substantially equal to one another.
[0029] Also, in the surface acoustic wave device in accordance with
other preferred embodiments of the present invention, the
above-described asymmetrical double electrode may be an
interdigital transducer, or may instead be a reflector.
[0030] Furthermore, in accordance with another preferred embodiment
of the present invention, preferably, quartz crystal is preferably
used as the above-described piezoelectric substrate. Alternatively,
however, in other preferred embodiments of the present invention,
the piezoelectric substrate may be constituted of another
piezoelectric single crystal such as LiTaO.sub.3, or a
piezoelectric ceramic such as lead titanate zirconate-based
ceramic. Also, a piezoelectric substrate constructed by forming a
piezoelectric thin-film such as a ZnO thin-film on an insulative
substrate such as a piezoelectric substrate or alumina substrate,
may be used.
[0031] The above and other elements, characteristics, features, and
advantages of the present invention will be clear from the
following detailed description of preferred embodiments of the
present invention in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1A is a plan view of an asymmetrical double electrode
in accordance with a preferred embodiment of the present
invention;
[0033] FIG. 1B is a partially cutaway sectional view of an
asymmetrical double electrode in accordance with a preferred
embodiment of the present invention;
[0034] FIG. 2 is a diagram showing the edge-position dependence of
the excitation center of the asymmetrical double electrode in a
preferred embodiment of the present invention;
[0035] FIG. 3 is a diagram showing the relationship between the
edge position X1=-X4 and each of the edge positions X2 and X3, when
a resultant vector length .GAMMA. is 0.20.lambda..
[0036] FIG. 4 is a diagram showing the relationship between the
edge position X1=-X4 and each of the edge positions X2 and X3, when
a resultant vector length .GAMMA. is 0.50.lambda..
[0037] FIG. 5 is a diagram showing the relationship between the
edge position X1=-X4 and each of the edge positions X2 and X3, when
a resultant vector length .GAMMA. is 1.00.lambda..
[0038] FIG. 6 is a diagram showing the relationship between the
edge position X1=-X4 and each of the edge positions X2 and X3, when
a resultant vector length .GAMMA. is 1.25.lambda..
[0039] FIG. 7 is a diagram showing the relationship between the
edge position X1=-X4 and each of the edge positions X2 and X3, when
a resultant vector length .GAMMA. is 1.50.lambda..
[0040] FIG. 8 is a diagram showing the relationship between the
edge position X1=-X4 and each of the edge positions X2 and X3, when
a resultant vector length .GAMMA. is 1.70.lambda..
[0041] FIG. 9 is a diagram showing the change in the reflection
center .phi. when the edge position X2 obtained by the equation (1)
changes, in preferred embodiments of the present invention.
[0042] FIG. 10 is a diagram showing the change in the reflection
center .phi. when the edge position X3 changes in preferred
embodiments of the present invention.
[0043] FIG. 11 is a schematic plan view showing the electrode
structure, for evaluating directivity of an IDT in accordance with
another preferred embodiment of the present invention.
[0044] FIG. 12 is a diagram showing the relationship between the
number of basic sections and the directivity, which relationship
has been obtained in a further preferred embodiment of the present
invention, and the relationship between the number of the basic
sections and the directivity when using a conventional asymmetrical
double electrode prepared for comparison.
[0045] FIG. 13 is an explanatory plan view of the electrode
structure of an IDT having a reflector in accordance with yet
another preferred embodiment of the present invention.
[0046] FIG. 14 is a schematic partially cutaway plan view showing a
conventional asymmetrical double electrode.
[0047] FIG. 15 is a partially cutaway sectional view for explaining
the edge positions of the strips in the asymmetrical double
electrode shown in FIG. 14.
[0048] FIG. 16 is a diagram showing the relationship between the
reflection vectors in the edges X1 to X4 shown in FIG. 15 and the
resultant vector V thereof.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0049] In order to realize the unidirectionality using an
asymmetrical double electrode, the inventors of the present
application have conducted extensive research and have discovered
that, when the reflection amount of surface acoustic waves per
basic section is small, the frequency unidirectionality
characteristics of the unidirectional electrode can be estimated by
forming reflection elements using a unidirectional electrode
wherein the phase difference between the excitation center and the
reflection center is approximately +45.degree. (-135.degree.) or
approximately -45.degree. (+135.degree.), and by disposing these
positive and negative reflection elements, regarding them as
positive and negative impulses, respectively. Furthermore, the
present inventors have discovered that, when the phase difference
between the excitation center and the reflection center largely
deviates from approximately .+-.45.degree. (.+-.135.degree.) in the
positive and negative elements, it becomes difficult to regard as
the positive and negative elements as simple positive and negative
impulses, respectively, because of the phase mismatching of surface
acoustic waves.
[0050] Moreover, the present inventors have discovered that, when a
weighting method in a unidirectional IDT using an asymmetrical
double electrode is used, it is possible to perform weighting with
respect to reflection coefficients, when positive and negative
reflection elements wherein the reflection centers thereof are
located at angles of approximately +45.degree. (-135.degree.) and
-45.degree. (+1350), respectively, with respect to the center of a
half wavelength section, are formed and are utilized as a
reflector. When attempting to perform weighting to the strip width,
it has been necessary to change the electrode pitch. However, this
weighting method by reflection coefficient allows a reflector to be
easily produced, since sonic speeds of the positive and negative
elements are identical with each other.
[0051] Next, the principles of various preferred embodiments of the
present invention will be described in more detail with reference
to the drawings.
[0052] An asymmetrical double electrode 1 shown in FIGS. 1A and 1B
is taken as an example. In this asymmetrical double electrode 1,
basic sections Z each of which is constituted of strips 2 and 3
having mutually different widths, are repeatedly arranged in the
propagation direction of surface acoustic waves. Now, let one basic
section be disposed at the positions from -0.25.lambda. to
+0.25.lambda.. Here, .lambda. denotes the wavelength of a surface
acoustic wave.
[0053] Letting the positions of the edges of the first and second
strips 2 and 3 be disposed within this basic section, that is, this
half wavelength section be X1' to X4', and the sonic speed of
surface acoustic waves propagating through a free surface be
V.sub.f, and the sonic speed of surface acoustic waves propagating
through a metallic surface be V.sub.m, the edge positions X1 to X4
corrected based on the sonic speeds of the free surface and the
metallic surface are expressed by the following equation:
[0054] Mathematical Expression 7
X1 to
X4=(V.sub.fL.sub.m+V.sub.mL.sub.f)/(V.sub.fL.sub.m0+V.sub.mL.sub.f0)
(3)
[0055] In the above equation (3), L.sub.m denotes the sum of the
distance on the metallic surface from the center of the half
wavelength section, that is, 0.lambda. to X1 to X4 in the
propagation direction of surface acoustic waves, and L.sub.f
denotes the sum of the distance on the free surface from the center
of the half wavelength section, 0.lambda. to X1 to X4. L.sub.m0
denotes the sum of the distance of the metallic surface in the
entire half wavelength section, and L.sub.f0 denotes the sum of the
distance of the free surface in the entire half wavelength
section.
[0056] Next, the reflection in a single electrode in which only a
single strip is disposed within the half wavelength section, will
be discussed. Suppose that the single strip is arranged so that the
center thereof is located at the reference position 0.lambda. of
the half wavelength section Z. Letting the reflection vector at the
one edge position .GAMMA.Xs of the single strip be .GAMMA.s1, and
the reflection vector at the other edge position +Xs thereof be
.GAMMA.s2, the resultant reflection vector .GAMMA.s at the
reference position is expressed by the equation (4) below. Here, j
in the equation (4) denotes an imaginary number, and k denotes the
wave number.
[0057] Mathematical Expression 8
.GAMMA.s=.GAMMA.s1.times.e.sup.-2.multidot.j.multidot.k.multidot.(-Xs)+.GA-
MMA.s2.times.e.sup.-2.multidot.j.multidot.k.multidot.Xs (4)
[0058] The length .vertline..GAMMA.s.vertline. of the
above-described resultant vector .GAMMA.s denotes the reflection
amount of a single strip.
[0059] Here, when conducting a normalization such as
.vertline..GAMMA.s1.vertline.=.vertline..GAMMA.s2.vertline.=1, we
can express .GAMMA.s1=-.GAMMA.s2=-1 under the condition that the
acoustic impedance on a free surface is larger than that on a
metallic surface. Therefore, when defining the reflection center
.phi.s as the center of the single strip, the reflection center
.phi.s can be determined by the following equation (S), using the
angle .angle. F of the resultant reflection vector .GAMMA..
[0060] Mathematical Expression 9
.phi.s=-0.5.times..angle.(j.times..GAMMA.s) (5)
[0061] Next, discussion will be made of an asymmetrical double
electrode wherein two strips having mutually different widths are
disposed in the half wavelength section, as in the case of the
single strip. Letting the reflection vectors of surface acoustic
waves at the edge positions X1 to X4 in FIGS. 1A and 1B be .GAMMA.1
to .GAMMA.4, the resultant reflection vector .GAMMA. at the
reference position 0.lambda. is expressed by the equation (6)
below.
[0062] Mathematical Expression 10
.GAMMA.=.GAMMA.1.times.e.sup.-2.multidot.j.multidot.k.multidot.X1+.GAMMA.2-
.times.e.sup.-2.multidot.j.multidot.k.multidot.X2+.GAMMA.3.times.e.sup.-2.-
multidot.j.multidot.k.multidot.X3+.GAMMA.4.times.e.sup.-2.multidot.j.multi-
dot.k.multidot.X4 (6)
[0063] The length .vertline..GAMMA..vertline. of the
above-described resultant vector .GAMMA. denotes the reflection
amount of a unidirectional electrode. The reflection center of the
unidirectional electrode is defined in the same way as the single
strip, and is expressed by the equation (7) below.
[0064] Mathematical Expression 11
.phi.+=-0.5.times..angle.(j.times..GAMMA.) (7)
[0065] In the case where, in the asymmetrical double electrode, a
unidirectional IDT is constructed such that the electric polarities
of adjacent basic sections are alternately inverted, when the width
of the inter-strip gap between a basic section and the adjacent
basic section on one side in the propagation direction of surface
acoustic waves, and the width of the inter-strip gap between the
basic section and the adjacent basic section on the other side in
the propagation direction of surface acoustic waves, are equal to
each other, and simultaneously when these inter-strip gaps are
disposed symmetrically with respect to the center of the center
basic section, the excitation center of the asymmetrical double
electrode is located at the substantially central portion of the
half wavelength section.
[0066] FIG. 2 is a diagram showing the edge-position dependence of
the excitation center in the above-described asymmetrical double
electrode. Herein, an asymmetrical double electrode formed of an
aluminum film having a thickness of, for example, approximately
0.02.lambda., is disposed on a ST-cut quartz substrate. In this
figure, there is shown the edge position dependence of the
excitation center obtained from the fundamental wave component
which is acquired by Fourier-transforming the electric charge
density distribution on the electrode obtained by the finite
element method, when X1=-X4=-0.1875.lambda., and
X3-X2=0.125.lambda., and when X2 is used as a parameter.
[0067] It can be confirmed that even at a position wherein the
degree of asymmetry of the asymmetrical double electrode is very
high, that is, at X2=0.172.lambda., the vector angle of the
excitation center is located at about +4.6.degree., that is,
substantially at the central portion. The strip width and the gap
width of an IDT constituting a surface acoustic wave device is
restricted by the electrical resistance of a strip and/or the
patterning process.
[0068] The edge positions X and X3 can be uniquely determined with
respect to the .vertline..GAMMA..vertline. and the edge position
X1, by letting X2-X1>0.02.lambda., X3-X2>0.02.lambda.,
X4-X3>0.02.lambda., and X4=-1, assuming that the vector lengths
of .GAMMA.1 to .GAMMA.4 are equal to one another, performing a
normalization such that .GAMMA.1=.GAMMA.4=-1, .GAMMA.2=.GAMMA.3=+1,
and finding the conditions such that the equations (6) and (7)
satisfies .phi.=45.degree., by the Monte Carlo method. The
approximate equations expressing X2 and X3 are given by the
following expressions (8) and (9), using
.vertline..GAMMA..vertline. and X1 as independent variables.
[0069] Mathematical Expression 12
X2[.lambda.].congruent.A.times.X1[.lambda.].sup.2+B.times.X1[.lambda.]+C
(8)
[0070] Mathematical Expression 13
X3[.lambda.].congruent.D.times.X1[.lambda.].sup.2+E.times.X1[.lambda.]+F
(9)
[0071] In the equations (8) and (9), A to F are obtained by the
following equations.
[0072] Mathematical Expression 14
A=-34.546.times..vertline..GAMMA..vertline..sup.6+176.36.times..vertline..-
GAMMA..vertline..sup.5-354.19.times..vertline..GAMMA..vertline..sup.4+354.-
94.times..sym..GAMMA..vertline..sup.3-160.44.times..vertline..GAMMA..vertl-
ine..sup.2+10.095.times..vertline..GAMMA..vertline.-1.7558
B=-15.464.times..vertline..GAMMA..vertline..sup.6+77.741.times..vertline..-
GAMMA..vertline..sup.5-153.44.times..vertline..GAMMA..vertline..sup.4+147.-
20.times..vertline..GAMMA..vertline..sup.3-68.363.times..vertline..GAMMA..-
vertline..sup.2+6.3925.times..vertline..GAMMA..vertline.-1.7498
C=-1.772.times..vertline..GAMMA..vertline..sup.6+8.7879.times..vertline..G-
AMMA..vertline..sup.5-17.07.times..vertline..GAMMA..vertline..sup.4+16.092-
.times..vertline..GAMMA..vertline..sup.3-7.4655.times..vertline..GAMMA..ve-
rtline..sup.2+0.8379.times..vertline..GAMMA..vertline.-0.3318
D=12.064.times..vertline..GAMMA..vertline..sup.6-45.501.times..vertline..G-
AMMA..vertline..sup.5+57.344.times..vertline..GAMMA..vertline..sup.4-22.68-
3.times..vertline..GAMMA..vertline..sup.3+12.933.times..vertline..GAMMA..v-
ertline..sup.2-15.938.times..vertline..GAMMA..vertline.-0.1815
E=7.2106.times..vertline..GAMMA..vertline..sup.6-30.023.times..vertline..G-
AMMA..vertline..sup.5+45.792.times..vertline..GAMMA..vertline..sup.4-29.78-
4.times..vertline..GAMMA..vertline..sup.3+13.125.times..vertline..GAMMA..v-
ertline..sup.2-6.3973.times..vertline..GAMMA..vertline.+1.0203
F=1.0138.times..vertline..GAMMA..vertline..sup.6-4.4422.times..vertline..G-
AMMA..vertline..sup.5+7.3402.times..vertline..GAMMA..vertline..sup.4-5.474-
.times..vertline..GAMMA..vertline..sup.3+2.3366.times..vertline..GAMMA..ve-
rtline..sup.2-0.7540.times..vertline..GAMMA..vertline.+0.2637
[0073] From the above results, it can be recognized that an
asymmetrical double electrode which corresponds to a desired
reflection amount, and having the reflection center at an angle of
about 45.degree. can be obtained. As can further be recognized, in
an asymmetrical double electrode which is constructed in accordance
with the equations described above, the excitation center is
located at the center of a half wavelength section. As a result,
when this asymmetrical double electrode is used as a unidirectional
electrode, the phase difference between the excitation and the
reflection center becomes substantially 45.degree., allowing this
asymmetrical double electrode to operate as a unidirectional
electrode having very superior characteristics.
[0074] As examples, FIGS. 3 to 8 show the results of X2 and X3
obtained by equations (8) and (9), for
.vertline..GAMMA..vertline.=0.20.lambda., 0.50.lambda.,
1.00.lambda., 1.25.lambda., 1.50.lambda., and 1.70.lambda..
Meanwhile, in the above description, the reflection coefficient has
been treated based on the premise that the acoustic impedance on a
free surface is larger that that on a metallic surface. Conversely,
under the condition that the acoustic impedance on a free surface
is smaller that that on a metallic surface, only the sign of
.vertline..GAMMA..vertline. is reversed, or in other words, that
.phi. is shifted by 90.degree..
[0075] As described above, by selecting the edge positions X2 and
X3 so as to satisfy the equations (8) and (9), the phase difference
between the excitation center and the reflection center can be made
substantially 45.degree.. As a result, a very superior
unidirectional electrode can be achieved. However, the present
inventors have confirmed that this asymmetrical double electrode
has a very excellent unidirectionality, if X2 and X3 are located
not only at the positions satisfying the equations (8) and (9), but
also at the positions within a certain range from the positions
satisfying the equations (8) and (9). This fact will be described
with reference to FIGS. 9 and 10.
[0076] FIGS. 9 and 10 are diagrams each showing the changes in the
reflection center when X2 and X3, each obtained by substituting
.vertline..GAMMA..vertline.=1.5 and X1=-0.2188.lambda. into the
equations (8) and (9), within the range from -0.1.lambda. to
+0.1.lambda..
[0077] As described above, it is desirable that the reflection
center be located at an angle of approximately 45.degree., or the
phase difference between the reflection center and the excitation
center be approximately 45.degree., but the present inventors have
confirmed that the range within approximately 45.+-.10.degree.
would allow the phase mismatching to be greatly improved as
compared to the above-described prior art asymmetrical double
electrode. It can be seen from FIGS. 9 and 10 that the range such
that the position of the reflection center is at an angle of
approximately 45.+-.10.degree., corresponds to the range of about
.+-.0.10.lambda. with respect to the value obtained by the equation
(8) for the position of X2, and corresponds to the range of about
.+-.0.05.lambda. with respect to the value obtained by the equation
(9) for the position of X3.
[0078] In preferred embodiments of the present invention,
therefore, the positions of X2 and X3 are preferably within the
range shown in the above-described equations (1) and (2). It will
be understood that a superior unidirectionality can be realized as
a result of this unique arrangement.
[0079] A surface acoustic wave device using an asymmetrical double
electrode in accordance with preferred embodiments of the present
invention was constructed as illustrated in FIG. 1. An IDT was
constructed by forming an aluminum film having a thickness of, for
example, approximately 0.02.lambda. on a ST-cut quartz substrate,
and then performing patterning.
[0080] The IDT defining an asymmetrical double electrode was
constructed in accordance with the edge positions X2 and X3 which
were determined by substituting the values of
.vertline..GAMMA..vertline. and X1 shown in Table 2 below into the
equations (8) and (9). Table 2 shows the inter-mode coupling
coefficients .kappa..sub.12/k.sub.0 and the reflection centers
.phi. in this case.
[0081] In the asymmetrical double electrode, shown in FIG. 2, which
is constructed based on the equations (8) and (9), since the angle
of the reflection center is close to 45.degree., the phase
mismatching of the reflected waves with respect to the propagating
waves is significantly less than that of the conventional
asymmetrical double electrode. Therefore, the use of the
asymmetrical double electrode constructed based on the equations
(8) and (9), allows a surface acoustic wave device which performs
much better than the conventional surface acoustic wave devices to
be achieved, and is particularly effective when positively making
use of the reflection of strips.
2 TABLE 2 Reflection .vertline..GAMMA..vertline. X1 [.lambda.]
.kappa..sub.12/k.sub.0 center .phi. [.lambda.] 0.20 -0.19 0.0005
43.8 0.50 -0.20 0.0015 42.3 0.75 -0.20 0.0021 40.5 1.00 -0.21
0.0029 39.6 1.25 -0.21 0.0031 39.0 1.50 -0.22 0.0036 39.2 1.60
-0.22 0.0035 40.2 1.70 -0.23 0.0038 41.5 1.73 -0.23 0.0038 42.2
[0082] Next, description will be made of specific experimental
examples of the directivity when an IDT including an asymmetrical
double electrode is provided on a ST-cut quartz substrate, in
accordance with a preferred embodiment of the present
invention.
[0083] As shown in FIG. 11, IDT 11, IDT 12, and IDT 13 were formed
on a ST-cut quartz substrate (not shown) using an aluminum film
having a thickness of, for example, approximately 0.02.lambda.. The
middle IDT 11 is constituted of an asymmetrical double electrode in
accordance with preferred embodiments of the present invention, and
IDT 12 and IDT 13 disposed on the opposite sides of IDT 11 are
ordinary double electrode type IDTs.
[0084] In IDT 11 constituted of an asymmetrical double electrode,
when the edge portions of the first and second strips 2 and 3
having different widths are made asymmetric, the excitation center
deviates from the center of the half wavelength section, so that
the phase difference between the excitation and the reflection
center also deviates from approximately 45.degree.. Therefore, the
edge positions X2 and X3 obtained by substituting
.vertline..GAMMA..vertline.=1.5, and X1=-0.2188.lambda. into the
equations (8) and (9), were adjusted by about 0.05.lambda. and
corrected so that the phase difference between the excitation and
the reflection center approaches approximately 45.degree..
[0085] As a result, when X1=-0.2188.lambda., X2=-0.1185.lambda.,
X3=+0.0050.lambda., and X4=+0.2188.lambda., the phase difference
between the excitation center and the reflection center became
about 41.degree..
[0086] FIG. 12 shows the comparison between the directivity of IDT
11 which uses the electrode structure shown in FIG. 11 and which
includes the asymmetrical double electrode having the
above-described construction, and the directivity when the
conventional asymmetrical double electrode is disposed in place of
IDT 11. The solid line in the figure shows the result of IDT 11,
and the broken line shows that of the conventional example. With
regard to the directivity, an input voltage is applied to IDT 11,
then the output thereof received by IDT 12 and IDT 13 was sought,
and the directivity was evaluated from the value of this output
(dB).
[0087] For an IDT using an asymmetrical double electrode prepared
for comparison, the film thickness of the electrode was preferably
set to about 0.02.lambda., and the edge positions were preferably
set so as to be X1=-0.1875.lambda., X2=-0.1250.lambda.,
X3=0.lambda., and X4=+0.1875.lambda.. The crossing width of an
electrode finger was preferably set to about 20.lambda. in each of
the preferred embodiments and the conventional example.
[0088] For IDT 12 and IDT 13 on the opposite sides of IDT 11, the
crossing width of an electrode finger were preferably set to about
20.lambda., and the edge positions were preferably set so as to be
X1=-0.1875.lambda., X2=-0.0625.lambda., X3=+0.0625.lambda., and
X4=+0.1875.lambda..
[0089] It can be recognized from FIG. 12 that the asymmetrical
double electrode in this preferred embodiment has a better
unidirectionality than that of the conventional asymmetrical double
electrode. In addition, the present inventors have confirmed that
the phase difference between the excitation center and the
reflection center can be corrected so as to approach 45.degree. by
adjusting the edge positions X2 and X3 obtained by the equations
(8) and (9) by about .+-.0.1.lambda., or by adjusting X4 so as to
slightly depart from -X1.
[0090] FIG. 13 is a plan view showing the electrode structure of an
IDT having an reflector 21 according to yet another preferred
embodiment of the present invention. Herein, the reflector 21
constructed in accordance with this preferred embodiment of the
present invention is preferably disposed within IDT 22. In this
case, by performing weighting with respect to the reflection
coefficient of the reflector 21, it is possible to control the
frequency characteristics of the entire IDT 22 having the reflector
21.
[0091] The present invention is not limited to the above-described
preferred embodiments, but can be variously modified. For example,
in the above-described preferred embodiments, it is recognized that
a better directivity than that of the conventional example is
achieved. However, there may be a case, depending on the use, where
it is more important that the phase difference between the
excitation center and the reflection center is close to 45.degree.,
or that the reflection center when X1=-X4, is 45.degree. with
respect to the center of the half wavelength section, rather than
achieving better directivity. Although it is desirable that the
phase difference between the excitation center and the reflection
center be about 45.degree., there may be a case where, when the
reflection by a strip is positively utilized, for example, when it
is used as a reflector, priority is given to the feature that the
reflection center is located at an angle of 45.degree., over the
feature that the excitation center is located at the center of the
half wavelength section, even if the excitation center deviates
therefrom. Particularly when a strip is utilized as a reflector,
only the reflection center can be taken into consideration.
[0092] As is evident from the foregoing, in the surface acoustic
wave device using an asymmetrical double electrode in accordance
with various preferred embodiments of the present invention, the
absolute value of the vector angle of the reflection center
obtained from the resultant vector formed by synthesizing the
reflection vectors at the edges X1 to X4 of the first and second
strips when the center of the above-described basic section is set
to be the reference position, is preferably within approximately
45.+-.10.degree. or approximately 135.+-.10.degree.. Thereby, the
phase mismatching of surface acoustic waves is minimized, and the
unidirectionality of the above-described asymmetrical double
electrode is greatly improved.
[0093] Likewise, in various preferred embodiments of the present
invention, when the absolute value of the phase difference between
the excitation center and the reflection center of the asymmetrical
double electrode, is within approximately 45.+-.10.degree. or
approximately 135.+-.10.degree., the phase mismatching of surface
acoustic waves is minimized, and superior unidirectionality can be
realized.
[0094] In preferred embodiments of the present invention, in the
edge positions X1 to X4 of the first and second strips, which
constitute basic sections and which have mutually different widths,
when the center position of the basic section is 0(.lambda.), and
X1.congruent.-X4, if the positions of X2 and X3 satisfy the
equations (1) and (2), it is ensured that the absolute value of the
vector angle of the reflection center is within approximately
45.+-.10.degree. or approximately 135.+-.10.degree. when the center
of the basic section is the reference position, or that the
absolute value of the phase difference between the excitation
center and the reflection center is within approximately
45.+-.10.degree. or approximately 135.+-.10.degree.. It is,
therefore, possible to easily and reliably provide, in accordance
with preferred embodiments of the present invention, an
asymmetrical double electrode which prevents the phase mismatching
of surface acoustic waves, and which has a superior
unidirectionality.
[0095] When the reflection amounts of surface acoustic waves at the
edge positions X1 to X4 are substantially equal to one another, the
phase mismatching between reflected surface acoustic waves and
propagating surface acoustic waves is very effectively reduced.
[0096] When an IDT is constructed to include the asymmetrical
double electrode, in accordance with various preferred embodiments
of the present invention, the phase mismatching between reflected
surface acoustic waves and propagating surface acoustic waves is
prevented, thereby allowing an IDT having a superior
unidirectionality to be provided, and enabling, for example, a
low-loss transversal type surface acoustic wave device to be
provided.
[0097] When the asymmetrical double electrode in accordance with
preferred embodiments of the present invention is used as a
reflector, since weighting can be easily performed with respect to
the reflection coefficient, it is possible to provide a surface
acoustic wave device which is capable of controlling the frequency
characteristics of the overall reflection coefficient of
reflectors.
[0098] While the present invention has been described with
reference to what are at present considered to be preferred
embodiments, it is to be understood that various changes and
modifications may be made thereto without departing from the
invention in its broader aspects and therefore, it is intended that
the appended claims cover all such changes and modifications as
fall within the true spirit and scope of the invention.
* * * * *