Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-01-27
2002-02-12
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S365000
Reexamination Certificate
active
06346761
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a surface acoustic wave (SAW) device, and more particularly to a SAW device for use in communication devices and so on, which defines a cross region of electrode fingers forming interdigital transducers (IDT), and a direction of surface acoustic wave propagation (group velocity).
SAW devices are widely used in such applications as resonators, reflectors, filters and so on as lightweight, small-size circuit elements for communication devices. The SAW devices may be classified into SAW devices which utilize Rayleigh waves, SAW devices which utilize Love waves, and so on. The SAW devices utilizing Love waves are known to have a larger electro-mechanical coupling coefficient (k
2
) than the SAW devices utilizing Rayleigh waves.
A known Love-wave type SAW resonator (Love-wave type SAW device) has the structure illustrated in
FIGS. 1 and 3
.
FIG. 1
is a top plan view illustrating the basic structure of electrodes which form part of the Love-wave type SAW resonator. An interdigital transducer composed of bus bars
2
-
1
,
2
-
2
, electrode fingers
3
, and input/output terminals
4
,
5
is formed on a surface of a piezoelectric substrate
1
. In the SAW resonator of
FIG. 1
, an electrode cross region
8
is arranged in a grating region
9
such that the width W thereof is constant in a surface wave ropagation direction. The electrode fingers connected to the bus bar
2
-
1
and the electrode fingers connected to the bus bar
2
-
2
are arranged alternatively in the electrode cross region
8
. Non-harmonic higher-order longitudinal and transversal mode oscillations exist in the surface wave propagation direction, i.e., the group velocity direction (the direction indicated by the arrow in
FIG. 1
) and in the direction perpendicular to the surface wave propagation direction, respectively. The impedance characteristic of the resonator as illustrated in
FIG. 1
disadvantageously exhibits a spurious response (indicated by a wavy line
30
in
FIG. 2
) resulting from the existence of such oscillations (standing waves), as can be seen in a Smith chart of FIG.
2
.
A SAW resonator employing an apodization technique, as illustrated in
FIG. 3
, has been proposed in order to suppress the spurious response due to the nonharmonic higher-order modes (for example, see JP-A-6-85602). Referring specifically to
FIG. 3
, the apodization technique distributes a cross region
8
a
of electrode fingers
3
in conformity to a function which defines a maximum cross width at the center of the cross region
8
a,
and reduces the cross width along the surface wave propagation direction (indicated by the arrow in
FIG. 3
) to substantially zero at paired electrode fingers located at both side ends of the cross region
8
a.
Thus, the spurious response can be suppressed by such a apodization-based arrangement (see, for example, JP-A-6-85602, or “Small-Size Love-Type SAW Resonator with Very Low Capacitance Ratio” by Hiroshi Shimizu and Yuji Suzuki, Transactions of the Institute of Electronics, Information and Communication Engineers A, Vol. j.75-A, No. 3, pp 458-466, March 1992).
A grating region
9
a
located outside the cross region
8
a
of the electrode fingers between bus bars
2
-
1
,
2
-
2
also functions as a SAW waveguide. Conventionally, the influence of the SAW waveguide has hardly been taken into account. For this reason, the piezoelectric substrate and the electrode finger (grating) region
9
of electrodes have been in the shape of a rectangle, i.e., the same shape as the optical waveguide and electromagnetic wave waveguide.
SUMMARY OF THE INVENTION
The conventional SAW resonator which has been improved by the weighted cross region in accordance with the apodization technique, though it has achieved a large effect in suppressing the spurious response due to the non-harmonic higher-order modes, still fails to completely eliminate the spurious response due to the non-harmonic higher-order modes. If such a SAW resonator is used as an oscillating element for a highly accurate voltage controlled oscillator (VCO), the resulting voltage controlled oscillator suffers from skipping of the oscillating frequency caused by the spurious response due to the aforementioned non-harmonic higher-order longitudinal and transversal modes.
It is therefore an object of the present invention to realize a SAW device which is capable of further suppressing the spurious response due to the non-harmonic higher-order modes.
It is another object of the present invention to realize a SAW device which is capable of allowing for the ease of manufacturing and a reduction in size as well as achieving the above object.
To achieve the above objects, in one aspect of the present invention, a surface acoustic wave device having an interdigital transducer (IDT) formed on a piezoelectric substrate is structured such that in significant portions of boundaries between bus bars and a grating region, which is composed of electrode fingers of the IDT, the extending directions of the boundaries are oriented non-parallel with the propagation direction of acoustic surface waves.
In one aspect of the present invention, a surface acoustic wave device comprises a piezoelectric substrate, and an interdigital transducer formed on a planar surface of the piezoelectric substrate, and having first and second bus bars, a first plurality of electrode fingers connected to the first bus bar, and a second plurality of electrode fingers connected to the second bus bar, wherein the first and second plurality of electrode fingers of the interdigital transducer have an electrode cross region in which the first and second plurality of electrode fingers are arranged alternatively, and each of boundaries between the first and second bus bars and a grating region of the first and second plurality of electrode fingers is arranged such that the boundary is not substantially parallel, with a group velocity direction (transmission direction) of surface acoustic waves excited by the interdigital transducer.
In another aspect of the present invention, a surface acoustic wave device comprises a piezoelectric substrate, and an interdigital transducer formed on a planar surface of the piezoelectric substrate, and having first and second bus bars, a first plurality of electrode fingers connected to the first bus bar, and a second plurality of electrode fingers connected to the second bus bar, wherein the first and second plurality of electrode fingers of the interdigital transducer have an electrode cross region in which the first and second plurality of electrode fingers are arranged alternatively, and the distance between the first and second bus bars along the first and second plurality of electrode fingers varies along a group velocity direction of the surface acoustic waves excited by the interdigital transducer.
In one example of the present invention, each of the boundaries is arranged such that the extending direction of the boundary is at an angle in a range of 45±27 degrees, in a significant portion thereof, with respect to the group velocity direction of the surface modes because of the influence of distorted standing waves generated between opposing bus bars, which had not been taken into account in the prior art. Then, the inventors fabricated a variety of prototype SAW resonators, each of which had bus bars determined in shape and arrangement such that standing waves differed in frequency and phase with respect to the propagation direction of surface acoustic waves to prevent the standing waves from being generated between the bus bars, and verified the effects of these SAW resonators.
In the aforementioned conventional SAW resonator illustrated in
FIG. 3
, the grating region
9
has two regions. One of the two regions is the electrode cross region
8
a
in which the electrode fingers connected to the bus bar
2
-
1
and the electrode fingers connected to the bus bar
2
-
2
are arranged alternatively. The other of the two regions is the region
9
a
in which two kinds of th
Asai Kengo
Hikita Mitsutaka
Isobe Atsushi
Sumioka Atsushi
Hitachi Denshi Kabushiki Kaisha
Medley Peter
Ramirez Nestor
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