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Claims  |
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What is claimed is:
1. A surface acoustic wave device comprising:
a plurality of surface acoustic wave resonators placed on a piezoelectric
plate proximate to one another, each of the surface acoustic wave
resonators comprising at least one interdigital transducer and at least
one pair of reflectors, each of the surface acoustic wave resonators
having different frequency-temperature characteristics, and
the surface acoustic wave resonators being transversely elastically coupled
to each other such that the plurality of surface acoustic wave resonators
compose a single coupling resonator;
wherein a condition of the transverse elastic coupling is determined
according to the following equation:
a.omega..sub.0.sup.2 (.differential..sup.2
V(Y)/.differential.Y.sup.2)+(.omega..sup.2 -.omega..sub.0.sup.2)V(Y)=0
where .omega. designates an angular frequency; .omega..sub.2 an element
angular frequency of a concerned region; V(Y) an amplitude of a surface
acoustic wave displacement in a direction of width; Y a Y-coordinate of
the surface acoustic wave device, which is normalized in terms of a
wavelength of a surface acoustic wave; and a a constant.
2. The surface acoustic wave device according to claim 1, wherein an
interdigital transducer of a first one of the surface acoustic wave
resonators is a first interdigital sub-transducer and an interdigital
transducer of a second one of the surface acoustic wave resonators is a
second interdigital sub-transducer, wherein the first and second
interdigital sub-transducers are connected in series with each other and
are excited in such a manner as to be of opposite phases.
3. The surface acoustic wave device according to claim 1, wherein the
surface acoustic wave resonators are in an oblique symmetry mode in which
a transverse elastic coupling of respective displacements is performed and
displacements oscillate in such a way as to be of opposite phase.
4. The surface acoustic wave device according to claim 3, wherein the
oblique symmetry mode is a fundamental wave mode (A0).
5. The surface acoustic wave device according to claim 1, wherein the
surface acoustic wave resonators are of the one-port type.
6. The surface acoustic wave device according to claim 1, wherein the
surface acoustic wave resonators are of the two-port type.
7. The surface acoustic wave device according to claim 1, wherein the
frequency-temperature characteristics have different peak temperatures and
are represented by nearly-quadratic functions, each of which is upwardly
convex.
8. The surface acoustic wave device according to claim 1, wherein a
difference (.DELTA..theta.) between peak temperatures of the
frequency-temperature characteristics of each pair of the surface acoustic
wave resonators is within a range of 30 to 80.degree. C.
9. The surface acoustic wave device according to claim 1, wherein a
difference (.DELTA..theta.) between the peak temperatures of the
frequency-temperature characteristics of each pair of the surface acoustic
wave resonators is realized by making film thicknesses of the interdigital
transducers differ from one another.
10. The surface acoustic wave device according to claim 1, wherein a
difference .DELTA..theta. between peak temperatures of the
frequency-temperature characteristics of each pair of the surface acoustic
wave resonators is realized by making line widths of the interdigital
transducers differ from one another.
11. The surface acoustic wave device according to claim 1, wherein an
interdigital transducer of a first one of the surface acoustic wave
resonators is a first interdigital sub-transducer and an interdigital
transducer of a second one of the surface acoustic wave resonators is a
second interdigital sub-transducer, wherein the first and second
interdigital sub-transducers are connected in parallel with each other and
are excited in such a way as to be of opposite phases.
12. The surface acoustic wave device according to claim 1, wherein the
piezoelectric plate is a K-cut quartz crystal.
13. The surface acoustic wave device according to claim 12, wherein a
finger overlap width of each of the surface acoustic wave devices is 10 to
30 times a wavelength of a surface acoustic wave.
14. The surface acoustic wave device according to claim 12, wherein the
constant a, by which a transverse mode frequency of each of the surface
acoustic wave resonators is determined, is in a range of 0.01 to 0.02.
15. The surface acoustic wave device according to claim 12, wherein a
distance between each pair of the surface acoustic wave resonators is 1 to
5 times the wavelength of a surface acoustic wave.
16. The surface acoustic wave device according to claim 1, wherein the
piezoelectric plate is an ST-cut quartz crystal.
17. The surface acoustic wave device according to claim 16, wherein a
finger overlap width of each of the surface acoustic wave devices is 10 to
30 times the wavelength of a surface acoustic wave.
18. The surface acoustic wave device according to claim 16, wherein the
constant a, by which a transverse mode frequency of each of the surface
acoustic wave resonators is determined, is in a range of 0.03 to 0.04.
19. The surface acoustic wave device according to claim 16, wherein a
distance between each pair of the surface acoustic wave devices is 1 to 5
times the wavelength of a surface acoustic wave.
20. A method for designing a surface acoustic wave device having first and
second surface acoustic wave resonators placed on a piezoelectric plate in
such a way as to be proximate to one another, each of the first and second
surface acoustic wave resonators comprising at least one interdigital
transducer and at least one pair of reflectors, the surface acoustic wave
resonators being adapted to have different frequency-temperature
characteristics, a transverse elastic coupling of the surface acoustic
wave resonators being performed in such a way that the surface acoustic
wave resonators compose a single coupling resonator, the method
comprising:
a first step of determining a resonance frequency (f) of the surface
acoustic wave device;
a second step of determining film thicknesses of the interdigital
transducers in such a way that peak temperatures of the first and second
surface acoustic wave resonators are different from one another;
a third step of finding a difference .DELTA. between a first resonance
frequency, which is obtained at the peak temperature of the first surface
acoustic resonator, and a second resonance frequency, which is obtained at
the peak temperature of the second surface acoustic resonator, according
to the film thicknesses; and
a fourth step of setting an electrode pitch of the first surface acoustic
wave resonator and an electrode pitch of the second surface acoustic wave
resonator in such a manner as to cancel the difference .DELTA. between the
first resonance frequency and the second resonance frequency.
21. The method according to claim 20, wherein in the fourth step, the
electrode pitch P is set according to the following equation:
P=Vs/{2(f.+-..DELTA./2)}
where Vs designates the velocity of a surface acoustic wave; f the
resonance frequency; and .DELTA. the difference between the first
resonance frequency and the second resonance frequency.
22. A method for designing a surface acoustic wave device having first and
second surface acoustic wave resonators placed on a piezoelectric plate in
such a way as to be proximate to one another, each of the first and second
surface acoustic wave resonators consisting of at least one interdigital
transducer and at least one pair of reflectors, the surface acoustic wave
resonators being adapted to have different frequency-temperature
characteristics, a transverse elastic coupling of the surface acoustic
wave resonators being performed in such a way that the surface acoustic
wave resonators compose a single coupling resonator, the method
comprising:
a first step of determining a resonance frequency (f) of the surface
acoustic wave device;
a second step of determining line widths of fingers of the interdigital
transducers in such a way that peak temperatures of the first and second
surface acoustic wave resonators are different from one another;
a third step of finding a difference (.DELTA.) between a first resonance
frequency, which is obtained at the peak temperature of the first surface
acoustic wave resonator, and a second resonance frequency, which is
obtained at the peak temperature of the second surface acoustic wave
resonator, according to the film thicknesses; and
a fourth step of setting an electrode pitch of the first surface acoustic
wave resonator and an electrode pitch of the second surface acoustic wave
resonator in such a manner as to cancel the difference (.DELTA.) between
the first resonance frequency and the second resonance frequency.
23. The method according to claim 22, wherein in the fourth step, the
electrode pitch P is set according to the following equation:
P=Vs/{2(f.+-..DELTA./2)}
where Vs designates the velocity of a surface acoustic wave; f the
resonance frequency; and .DELTA. the difference between the first
resonance frequency and the second resonance frequency. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a surface acoustic wave
(hereunder sometimes abbreviated as SAW) device for achieving frequency
stabilization and frequency control by using surface acoustic wave, and
more particularly to a SAW device whose frequency-temperature
characteristics are noticeably improved.
2. Description of the Related Art
Among conventional SAW devices, one-port SAW resonators and two-port SAW
resonators are well known as those used for frequency stabilization and
frequency control. Especially, a SAW resonator using an ST-cut quartz
crystal as a substrate has a zero temperature coefficient at ordinary
temperature and is of high accuracy. However, the secondary temperature
coefficient .beta. of the aforementioned ST-cut quartz crystal substrate
is -3.3.times.10.sup.-8 /.degree. C..sup.2, so that the
frequency-temperature characteristics or change is approximately .+-.60
ppm in the temperature range of -35 to 85.degree. C., at which mobile
communication systems or devices are used. Therefore, the SAW resonator
using an ST-cut quartz crystal substrate has inadequate characteristics
and thus cannot be used in a mobile communication device. There is a known
method for improving the frequency-temperature characteristics of a SAW
device against this by which two SAW devices are used by being
electrically connected in parallel with each other (see Japanese
Unexamined Patent Publication No. 55-121729 and Japanese Unexamined Patent
Publication No.58-39105).
However, in the case of the aforementioned conventional method, the
coupling coefficients of each of the two SAW devices is determined by the
capacitance of a capacitor (the load capacitance CL), which is an external
or outboard device, so that the frequency-temperature characteristics are
realized. Thus, in the case of an oscillation circuit comprising two SAW
devices, the frequency of a signal generated by the circuit is determined
by the same capacitance CL. Consequently, the circuit lacks flexibility in
regulating the frequency. Further, in the case of the SAW devices used as
a filter, the filter lacks flexibility in setting terminal impedance and
is, therefore, very inconvenient.
It is, accordingly, an object of the present invention to improve the
frequency-temperature characteristics of a SAW element or device and to
provide a SAW device, which excels at frequency accuracy and is very
convenient, to the market.
SUMMARY OF THE INVENTION
To achieve the foregoing object, in accordance with an aspect of the
present invention, there is provided a surface acoustic wave device
(hereunder sometimes referred to as a first surface acoustic wave device)
that comprises a plurality of surface acoustic wave resonators placed on a
piezoelectric plate in such a way as to be proximate to one another. In
this device, each of the plurality of surface acoustic wave resonators
consists of at least one interdigital transducer and at least one pair of
reflectors. Further, the plurality of surface acoustic wave resonators are
adapted to have different frequency-temperature characteristics,
respectively. Moreover, the transverse elastic coupling of the plurality
of surface acoustic wave resonators is performed in such a way that the
plurality of surface acoustic wave resonators compose a single coupling
(or synthetic) resonator.
Thus, excellent frequency-temperature characteristics, which cannot be
realized by a single element, are realized. Thereby, a surface acoustic
wave device can be used in a portable mobile communication device which
requires high-precision frequencies. Consequently, miniaturization, the
saving of labor required for production and increased performance of the
surface acoustic wave device can be achieved. It, therefore, can be
expected that the device of the present invention will yield a great deal
of advantages in the future.
Furthermore, a plurality of surface acoustic wave devices of the present
invention have displacements that are elastically coupled to one another
and oscillate.
In the case of a second embodiment (hereunder sometimes referred to as a
second surface acoustic wave device) of the present invention, the
condition or state of the transverse elastic coupling is determined
according to the following equation:
a.omega..sub.0.sup.2 (.differential..sup.2
V(Y)/.differential.Y.sup.2)+(.omega..sup.2 -.omega..sub.0.sup.2)V(Y)=0
where .omega. designates an angular frequency; .omega..sub.0 an element
angular frequency (rad/s) of a concerned or corresponding region; V(Y) an
amplitude of a surface acoustic wave displacement in the direction of
width; Y a Y-coordinate of the surface acoustic wave device, which is
normalized in terms of the wavelength of a surface acoustic wave; and a a
constant.
Further, in the case of a third embodiment (hereunder sometimes referred to
as a third surface acoustic wave device) of the present invention, the
plurality of surface acoustic wave devices are in an oblique symmetry mode
in which a transverse elastic coupling of respective displacements is
performed and the displacements oscillate in such a way as to be of
opposite phase, respectively.
Moreover, in the case of a fourth embodiment (hereunder sometimes referred
to as a fourth surface acoustic wave device) of the present invention, the
oblique symmetry mode is a fundamental wave mode A0 as illustrated in
FIGS. 3 and 15.
Moreover, in the case of a fifth embodiment (hereunder sometimes referred
to as a fifth surface acoustic wave device) of the present invention, the
surface acoustic wave resonator is of the one-port type.
Furthermore, in the case of a sixth embodiment (hereunder sometimes
referred to as a sixth surface acoustic wave device) of the present
invention, the surface acoustic wave resonator is of the two-port type.
An example of the one-port surface acoustic wave resonator and an example
of the two-port surface acoustic wave resonator are illustrated in FIGS. 1
and 2, respectively.
In the case of a seventh embodiment (hereunder sometimes referred to as a
seventh surface acoustic wave device) of the present invention, the
frequency-temperature characteristics have different peak temperatures and
are represented by nearly-quadratic functions, each of which is upwardly
convex, respectively.
Further, in the case of an eighth embodiment (hereunder sometimes referred
to as an eighth surface acoustic wave device) of the present invention, a
difference .DELTA..theta. between the peak temperatures of the
frequency-temperature characteristics of each pair of the surface acoustic
wave resonators is within a range of 30 to 80.degree. C.
Moreover, in the case of a ninth embodiment (hereunder sometimes referred
to as a ninth surface acoustic wave device) of the present invention, a
differences .DELTA..theta. between the peak temperatures of the
frequency-temperature characteristics of each pair of the surface acoustic
wave resonators is realized by making the film thicknesses of the
interdigital transducers differ from one another.
Furthermore, in the case of a tenth embodiment (hereunder sometimes
referred to as a tenth surface acoustic wave device) of the present
invention, a difference .DELTA..theta. between the peak temperatures of
the frequency-temperature characteristics of each pair of the surface
acoustic wave resonators is realized by making the line widths of the
interdigital transducers differ from one another.
Additionally, in the case of an eleventh embodiment (hereunder sometimes
referred to as an eleventh surface acoustic wave device) of the present
invention, the interdigital transducer of each of the surface acoustic
wave resonators has a first interdigital sub-transducer and a second
interdigital sub-transducer. Further, these sub-transducers are connected
in parallel with each other and are driven or excited in such a way as to
be of opposite phases, respectively.
Thus, the impedance of each of the resonators can be reduced.
Further, in the case of a twelfth embodiment (hereunder sometimes referred
to as a twelfth surface acoustic wave device) of the present invention,
the interdigital transducer of each of the surface acoustic wave
resonators has a first interdigital sub-transducer and a second
interdigital sub-transducer. Further, these sub-transducers are connected
in series with each other and are driven or excited in such a manner as to
be of opposite phases, respectively.
Thus, the electrode pattern can be simplified.
Further, in the case of a thirteenth embodiment (hereunder sometimes
referred to as a thirteenth surface acoustic wave device) of the present
invention, the piezoelectric plate is a K-cut quartz crystal.
An example of a K-cut crystal is illustrated in FIG. 4.
In the case of an embodiment of the thirteenth surface acoustic wave
device, a constant a, by which a transverse mode frequency of the surface
acoustic wave resonator is determined, is in the range of 0.01 to 0.02.
Further, in the case of an embodiment of the thirteenth surface acoustic
wave device, a distance between each pair of the surface acoustic wave
resonators is 1 to 5 times the wavelength of a surface acoustic wave.
Moreover, in the case of an embodiment of the thirteenth surface acoustic
wave device, a finger overlap (width) of each of the surface acoustic wave
resonators is 10 to 30 times the wavelength of a surface acoustic wave.
Furthermore, in the case of a fourteenth embodiment (hereunder sometimes
referred to as a fourteenth surface acoustic wave device) of the present
invention, the piezoelectric plate is an ST-cut crystal.
An example of an ST-cut quartz crystal is illustrated in FIG. 16.
In the case of an embodiment of the fourteenth surface acoustic wave
device, a constant a, by which a transverse mode frequency of each of the
surface acoustic wave resonators is determined, is in the range of 0.03 to
0.04.
Further, in the case of an embodiment of the fourteenth surface acoustic
wave device, a distance between each pair of the surface acoustic wave
resonators is 1 to 5 times the wavelength of a surface acoustic wave.
Moreover, in the case of an embodiment of the fourteenth surface acoustic
wave device, a finger overlap of each of the surface acoustic wave
resonators is 10 to 30 times the wavelength of a surface acoustic wave.
In accordance with another aspect of the present invention, there is
provided a method (hereunder sometimes referred to as a first method) for
designing a surface acoustic wave device having first and second surface
acoustic wave resonators placed on a piezoelectric plate in such a way as
to be proximate to one another. In this device, each of the first and
second surface acoustic wave resonators consists of at least one
interdigital transducer and at least one pair of reflectors. Further, each
of the surface acoustic wave resonators are adapted to have different
frequency-temperature characteristics. Moreover, the transverse elastic
coupling of the surface acoustic wave resonators is performed in such a
way that the plurality of surface acoustic wave resonators compose a
single coupling resonator. This method comprises: a first step of
determining a resonance frequency f of the surface acoustic wave device; a
second step of determining the film thicknesses of the interdigital
transducers in such a way that the peak temperatures of the first and
second surface acoustic wave resonators are different from one another; a
third step of finding a difference .DELTA. between a first resonance
frequency, which is obtained at the peak temperature of the first surface
acoustic resonator, and a second resonance frequency, which is obtained at
a peak temperature of the second surface acoustic resonator, according to
the film thicknesses; and a fourth step of setting an electrode pitch P of
the first surface acoustic wave resonator and an electrode pitch P of the
second surface acoustic wave resonator in such a manner as to cancel the
difference .DELTA. between the first resonance frequency and the second
resonance frequency.
In accordance with still another aspect of the present invention, there is
provided a method (hereunder sometimes referred to as a second method) for
designing a surface acoustic wave device having first and second surface
acoustic wave resonators placed on a piezoelectric plate in such a way as
to be proximate to one another. In this device, each of the first and
second surface acoustic wave resonators consists of at least one
interdigital transducer and at least one pair of reflectors. Further, each
of the surface acoustic wave resonators are adapted to have different
frequency-temperature characteristics. Moreover, the transverse elastic
coupling of the surface acoustic wave resonators is performed in such a
way that the plurality of surface acoustic wave resonators compose a
single coupling resonator. This method comprises: a first step of
determining a resonance frequency f of the surface acoustic wave device; a
second step of determining the line widths of fingers of the interdigital
transducers in such a way that the peak temperatures of the first and
second surface acoustic wave resonators are different from one another; a
third step of finding a difference .DELTA. between a first resonance
frequency, which is obtained at the peak temperature of the first surface
acoustic resonator, and a second resonance frequency, which is obtained at
the peak temperature of the second surface acoustic resonator, according
to the film thicknesses; and a fourth step of setting an electrode pitch P
of the first surface acoustic wave resonator and an electrode pitch P of
the second surface acoustic wave resonator in such a manner as to cancel
the difference .DELTA. between the first resonance frequency and the
second resonance frequency.
In the case of an embodiment of the first or second method, in the fourth
step, the electrode pitch P is set according to the following equation:
p=Vs/{2(f.+-.L/2)}
where Vs designates the velocity of surface acoustic wave; f the resonance
frequency; and .DELTA. the difference between the first resonance
frequency and the second resonance frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features, objects and advantages of the present invention will become
apparent from the following description of preferred embodiments with
reference to the drawings in which like reference characters designate
like or corresponding parts throughout several views, and in which:
FIG. 1 is a plan view of a surface acoustic wave device according to
"Embodiment 1" and "Embodiment 2" of the present invention;
FIG. 2 is a plan view of another surface acoustic wave device according to
"Embodiment 1" and "Embodiment 2" of the present invention;
FIG. 3 is a graph for illustrating the vibrational displacement of a
surface acoustic wave device of "Embodiment 1" and "Embodiment 2" of the
present invention;
FIG. 4 is a diagram for illustrating a cutting direction employed in the
case of a K-cut quartz crystal plate used in a surface acoustic wave
device of "Embodiment 1" of the present invention;
FIG. 5 is a plan view of a surface acoustic wave device using a K-cut
crystal plate of "Embodiment 1" of the present invention;
FIG. 6 is a diagram for illustrating the electrode pattern of interdigital
transducers (hereunder sometimes abbreviated as IDT) of the parallel type
used in a surface acoustic wave device of "Embodiment 1" and "Embodiment
2" of the present invention;
FIG. 7 is a diagram for illustrating the electrode pattern of IDTs of the
series type used in a surface acoustic wave device of "Embodiment 1" and
"Embodiment 2" of the present invention;
FIG. 8 is a graph for illustrating the frequency-temperature
characteristics of a surface acoustic wave device according to "Embodiment
1" of the present invention;
FIG. 9 is a graph for illustrating the frequency temperature accuracy
characteristic of a surface acoustic wave device as a function of distance
between resonators according to "Embodiment 1" of the present invention;
FIG. 10 is a graph for illustrating the frequency temperature accuracy
characteristic of a surface acoustic wave device as a function of distance
between resonators according to "Embodiment 1" of the present invention;
FIG. 11 is a graph for illustrating a film thickness characteristic of a
surface acoustic wave device according to "Embodiment 1" of the present
invention;
FIG. 12 is a graph for illustrating a line width characteristic of a
surface acoustic wave device according to "Embodiment 1" of the present
invention;
FIG. 13 is a graph for illustrating the frequency temperature accuracy
characteristic of a surface acoustic wave device as a function of distance
between resonators according to "Embodiment 1" of the present invention;
FIG. 14 is a graph for illustrating the frequency temperature accuracy
characteristic of a surface acoustic wave device as a function of finger
overlay width according to "Embodiment 1" of the present invention;
FIG. 15 is a graph for illustrating the vibrational displacement of a
surface acoustic wave device of "Embodiment 2" of the present invention;
FIG. 16 is a diagram for illustrating a cutting direction employed in the
case of an ST-cut quartz crystal plate used in a surface acoustic wave
device of "Embodiment 2" of the present invention;
FIG. 17 is a graph for illustrating the frequency-temperature
characteristics of a surface acoustic wave device according to "Embodiment
2" of the present invention;
FIG. 18 is a graph for illustrating the frequency temperature accuracy
characteristic of a surface acoustic wave device as a function of distance
between resonators according to "Embodiment 2" of the present invention;
FIG. 19 is a graph for illustrating the frequency temperature accuracy
characteristic of a surface acoustic wave device as a function of distance
between resonators according to "Embodiment 2" of the present invention;
FIGS. 20(a) and 20(b) are graphs for illustrating film thickness
characteristics of a surface acoustic wave device according to "Embodiment
2" of the present invention;
FIGS. 21(a) and 21(b) are graphs for illustrating line width
characteristics of a surface acoustic wave device according to "Embodiment
2" of the present invention;
FIG. 22 is a graph for illustrating the frequency temperature accuracy
characteristic of a surface acoustic wave device as a function of distance
between resonators according to "Embodiment 2" of the present invention;
FIG. 23 is a graph for illustrating the frequency temperature accuracy
characteristic of a surface acoustic wave device as a function of finger
overlap width according to "Embodiment 2" of the present invention;
FIG. 24 is a diagram for showing a coordinate system of a surface acoustic
wave device, which is used for illustrating an operation of each of
"Embodiment 1" and "Embodiment 2" of the present invention;
FIG. 25 is a diagram for showing the coordinate system of a surface
acoustic wave device, which is used for illustrating the operation of each
of "Embodiment 1" and "Embodiment 2" of the present invention; and
FIG. 26 is a transverse sectional view of the configuration of a surface
acoustic wave device, which is used for illustrating an operation of each
of "Embodiment 1" and "Embodiment 2" of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment 1
Hereinafter, the "Embodiment 1" of the present invention will be described
by referring to the accompanying drawings from FIG. 1 in due order. A
device according to "Embodiment 1" of the present invention is what is
called a K-cut element.
FIG. 1 is a plan view of the interconnecting pattern of
transverse-elastic-coupling one-port SAW devices which are an embodiment
of a surface acoustic wave device of the present invention. The names of
parts shown in this figure are as follows. Reference number 100 designates
a piezoelectric plate; 101 and 102 interdigital transducers (hereunder
sometimes abbreviated as IDTs); 103, 104, 105 and 106 reflectors; 107 and
108 finger overlap widths Wc; 109 the distance between SAW resonators G;
110 and 112 pads for wire-bonding; and 113 and 114 conductive pattern for
connection.
The piezoelectric plate 100 consists of a single piezoelectric crystal of,
for example, quartz or lithium tantalate (LiTaO.sub.3) and a substrate on
which a piezoelectric thin film made of ZnO or the like is formed. The
IDTs 101 and 102 and the reflectors 103, 104, 105 and 106 are made on the
piezoelectric plate 100 by forming a conductive thin film, which is made
of conductive metal such as aluminum or gold, through an evaporation or a
sputtering, and thereafter forming the pattern thereof by utilizing
photolithography techniques.
A first one-port SAW resonator is constituted by the reflectors 103 and 105
and the IDT 101. Further, a second SAW resonator is constituted by the
reflectors 104 and 106 and the IDT 102. A large number of fingers (namely,
electrodes) and conductive strips of the IDTs and the reflectors are
placed in such a way as to be orthogonal to a phase advance direction of a
utilized surface acoustic wave (for instance, a Rayleigh wave and a leaky
wave) and to be in parallel to one another and periodic. The first and
second SAW resonators are adapted to have displacements which are
elastically coupled to each other in an oblique symmetry mode.
Next, transverse-elastic-coupling two-port SAW resonators, which are
another example of "Embodiment 1" of the present invention, will be
described hereinbelow by referring to FIG. 2. In this figure, reference
number 200 designates a piezoelectric plate; 201, 202, 205 and 206
reflectors; 203, 204, 207 and 208 IDTs; 209, 210, 211, 212, 213 and 214
electrode pads for wire-bonding; 215 and 216 input terminals 1 and 1'; and
217 and 218 output terminals 2 and 2'.
As shown in FIG. 2, a first two-port SAW resonator is constituted by the
reflectors 205 and 206 and the IDTs 207 and 208. A second two-port SAW
resonator is constituted by the reflectors 201 and 202 and the IDTs 203
and 204. The first and second two-port SAW resonators are adapted to have
displacements which are elastically coupled to each other in the direction
of the width thereof in an oblique symmetry mode.
In the case of the surface acoustic wave device according to "Embodiment 1"
of the present invention, the one-port SAW resonators consisting of one
IDT and a pair of reflectors, the two-port SAW resonators consisting of a
plurality of IDTs or a transversal-type SAW filter using only two IDTs may
be employed as first and second composing elements thereof.
FIG. 3 shows the amplitudes of the vibrational displacements in the
transverse direction or in the direction of width (the Y-direction), which
are exhibited by the surface acoustic wave devices of FIGS. 1 and 2. In
FIG. 3, reference number 300 denotes a piezoelectric plate; 301 and 302
IDTs (transverse sections of SAW resonators); 303 a fundamental wave
oblique symmetry mode A0; and 304 a first order oblique symmetry mode A1.
It is characteristic of both of the amplitudes of the vibrational
displacements in the oblique symmetry modes 303 and 304 to be nearly
symmetric with respect to the central point O between the two SAW
resonators. In the case of the embodiment of the present invention, it is
preferable that the fundamental oblique symmetry mode A0 is used.
Next, the case of a SAW device employing a quartz crystal K-cut element
will be described hereunder as a more practical example of application of
the embodiment of the present invention. FIG. 4 is a diagram for
illustrating a cutting direction employed in the case of a quartz crystal
K-cut element used in the SAW device according to this embodiment of the
present invention. In this figure, reference number 401 denotes an
electrical axis of the crystal; 402 a mechanical axis thereof; and 403 an
optical axis thereof. These axes compose a right-hand orthogonal
coordinate system. Further, reference number 404 denotes a quartz crystal
plate that is obtained by rotating a Y-cut plane, which is perpendicular
to the Y-axis, 402, around the electrical axis 401 counterclockwise by an
angle .theta. of 6.51.+-.1 degrees. In the case of a surface acoustic wave
element 405, a cutting direction is set in such a manner that the phase
propagation (or advance) direction of a surface acoustic wave is in
agreement with the direction of X'-axis 406 obtained by rotating the
electrical axis 401 of the quartz crystal plate around Y'-axis 407 by an
angle .psi. of 32.43.+-.2 degrees.
FIG. 5 is a conceptual diagram showing an electrode pattern formed on the
surface of a K-cut element obtained by setting the cutting direction as
illustrated in FIG. 4 on the surface of a piezoelectric plate 500. In FIG.
5, reference numbers 501, 503, 504 and 506 designate reflectors; and 502
and 505 IDTs. The elements of this figure constitute
transverse-elastic-coupling one-port SAW resonators. In this figure,
reference character Wc denotes a finger overlap width of each IDT; and G
the distance between the resonators. Both the first resonator consisting
of the elements 501, 502 and 503, and the second resonator consisting of
the elements 504, 505 and 506 is configured in such a manner that a
direction, in which the resonator is formed, is inclined at an angle
.delta. of 3.+-.2 degrees to the phase propagation direction of the
surface acoustic wave (the direction of the X'-axis of FIG. 4). Thereby,
the direction in which the power or energy of the surface acoustic wave
propagates can be the same as the direction in which a region of the
resonator is formed. Consequently, the Q-factor of the resonator can be
enhanced. In FIG. 5, the Y-axis is set in such a way as to be orthogonal
to the X'-axis, the direction of which is the aforementioned phase
propagation direction of the surface acoustic wave.
Next, the configurations of the IDTs for exciting the oblique symmetry
modes A0 and A1 will be described by referring to FIGS. 6 and 7.
FIG. 6 illustrates the placement of the electrodes of the IDTs of the
parallel-connected type for performing the excitation. In this figure,
reference number 601 designates the IDT of the first resonator; 602 the
IDT of the second resonator; 603 to 608 electrode fingers (electrodes);
609 a positive-electrode signal input terminal; 610 a negative-electrode
signal input terminal; 611 a conductor or conductive wire for connecting
the positive electrodes of the first and second IDTs with each other; and
612 a signal source. In this figure, arrows denote vectors indicating the
excited electric fields. It is very important that in this case, the
direction of the electric field produced by the fingers 604 and 605 is
opposite to the direction of the electric field produced by the fingers
606 and 607, wherein the fingers 604 and 605 and the fingers 606 and 607
are positioned nearly in the same place in the longitudinal direction of
the resonators.
Thereby, the electric field of the fundamental oblique symmetry mode A0 is
excited.
Next, the configuration of the electrode finger pattern for exciting the
oblique symmetry mode in the case of connecting the first and second IDTs
in series with each other will be described hereinbelow by referring to
FIG. 7. In this figure, reference number 701 designates a first IDT; 702 a
second IDT; 703, 704, 705, 706, 707 and 708 electrode fingers; 709 a
positive-electrode signal input terminal; 710 a negative-electrode signal
input terminal; and 712 a signal source. In this case, the adjoining
fingers of the two resonators are connected with one another through a
common conductor 711. Further, in this case, the corresponding fingers
possessed by the first and second IDTs are positioned nearly in the same
place in the longitudinal direction of the resonators.
In the case of the IDTs of the parallel-connected type, the impedance of
the aforementioned transverse-elastic-coupling resonator is small. In
contrast, in the case of the IDTs of the series-connected type, the
impedance thereof is large, while the electrode pattern is simple.
Next, the frequency-temperature characteristics of various surface acoustic
wave devices realized by the aforementioned "Embodiment 1" of the present
invention will be described.
FIG. 8 illustrates the frequency-temperature characteristics of the
embodiments of FIGS. 1 and 2. In FIG. 8, the horizontal axis represents an
ambient temperature T (.degree. C.); the vertical axis the rate of change
in frequency .DELTA.f/f (in ppm). Further, reference numbers 801 and 802
designate upward-convex nearly-quadratic-function curves which
respectively represent the frequency-temperature characteristics of the
first and second resonators in the case that there is no elastic coupling.
The curve 801 has a peak temperature .theta..sub.max1. Further, the curve
802 has a peak temperature .theta..sub.max2. The frequency-temperature
characteristics 801 and 802 are represented by the following function:
.DELTA.f/f=.beta.(.theta.-.theta..sub.max).sup.2
+.gamma.(.theta.-.theta..sub.max).sup.3 (1)
where .theta. designates a temperature; .beta. a second order temperature
coefficient; .gamma. a third order temperature coefficient. In the case of
"Embodiment 1" of the present invention, the term associated with .gamma.
is often negligibly small.
Curves 804 and 805 represent the frequency-temperature characteristics of
the first and second resonators coupled to each other in the fundamental
oblique symmetry mode A). The difference between the shapes of the curves
804 and 805 varies with the combination of the frequency-temperature
characteristics of the first and second resonators
.DELTA..theta.=.theta..sub.max2 -.theta..sub.max1 and depends upon the
distance G between the resonators and the width Wc of each of the
resonators. The curve 804 corresponds to the case where the elastic
coupling is weak. In contrast, the curve 805 corresponds to the case where
the elastic coupling is strong. The optimum combination of conditions for
making the frequency-temperature characteristics of the resonators, which
are in the coupled state, flat depends on the material of the
piezoelectric plate and the cutting direction. This is due to the
difference in magnitude of effective shear stiffness constant a between
the resonators, which determines the degree of the elastic coupling
therebetween, and to the difference in frequency-temperature
characteristics therebetween, which is present when a single resonator is
composed. In the case of the quartz crystal K-cut element, the constant a
ranges from about 0.01 to 0.02 under normal design conditions. Further,
the constant a is a parameter contained in the following differential
equation which prescribes the displacement of the resonator in the
transverse-mode such as an oblique symmetry mode and a symmetry mode.
a.omega..sub.0.sup.2 (.differential..sup.2
V(Y)/.differential.Y.sup.2)+(.omega..sup.2 -.omega..sub.0.sup.2)V(Y)=0(2)
where .omega. designates an angular frequency (rad/s); .omega..sub.0 an
element angular frequency (rad/s) of a concerned region; V(Y) an amplitude
of a surface acoustic wave displacement in the direction of width; Y a
Y-coordinate of the surface acoustic wave device, which is normalized in
terms of the wavelength of a surface acoustic wave, as illustrated in FIG.
3. The derivation of this equation and the aforementioned conditions will
be described later in detail.
Moreover, it is preferable for optimizing the frequency-temperature
characteristics that the finger overlap width Wc of the resonator is 10 to
30 times the wavelength of the surface acoustic wave, that the distance G
between the resonators is 1 to 5 times the wavelength of the surface
acoustic wave and that the difference .DELTA..theta. between the peak
temperature of the frequency-temperature characteristics is in a range of
30 to 80.degree. C. Thereby, in the temperature range between -30 to
90.degree. C., the frequency accuracy can be 20 ppm at best and 70 ppm at
worst. When such best frequency accuracy is realized, the difference
.DELTA..theta. is 70.degree. C. and the peak temperatures .theta..sub.max1
=-10.degree. C. and .theta..sub.max2 =60.degree. C. Meanwhile, in the case
of the temperature characteristics of the single K-cut resonator, the
second order temperature coefficient .beta.=-2.5.times.10.sup.-8 /.degree.
C.sup.2. Thus, in the temperature range between -30 to 90.degree. C., the
frequency accuracy is -90 ppm. Therefore, the frequency accuracy can be
improved by a factor of 4.5. Furthermore, at the central temperature of
the operating temperature range, 30.+-.30.degree. C., the rate of change
in frequency .DELTA.f/f can be within .+-.3 ppm (at that time,
.DELTA..theta.=50.degree. C.) by suitably combining the ranges of the
aforementioned parameters with each other.
The hereinabove-mentioned characteristics of the frequency temperature
accuracy at the time of coupling the resonators (namely, the change in
frequency in the temperature range of -30 to 90.degree. C.) is illustrated
in FIGS. 9, 10 and 13, in each of which the abscissa represents the
distance G between the resonators and among which the parameters Wc,
.DELTA..theta. and a are changed. In FIG. 9, lines 901 and 902 represent
the cases where the overlap width Wc of the resonator is 10 times and 30
times the wavelength of the surface acoustic wave, respectively. In FIG.
10, lines 1001 and 1002 represent the cases where the difference
.DELTA..theta. between the peak temperature of the frequency-temperature
characteristics is 30.degree. C. and 80.degree. C., respectively . In FIG.
13, lines 1301 and 1302 represent the cases where the constant a is 0.01
and 0.02, respectively. Further, the frequency temperature accuracy at the
time of coupling the resonators is illustrated in FIG. 14 in which the
abscissa represents the finger overlap width Wc of the resonator and in
which the parameters G, .DELTA..theta. and a are changed.
Next, a practical method of designing the surface acoustic wave device
according to "Embodiment 1" of the present invention will be described
hereinbelow by referring to FIGS. 11 and 12. When designing the surface
acoustic wave device, it is necessary to make the frequencies, which
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