Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-07-19
2004-08-10
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R, C333S195000
Reexamination Certificate
active
06774536
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device used for telecommunication equipment.
2. Description of the Related Art
A surface acoustic wave device, used for conversion between an electric signal and a surface acoustic wave (SAW), comprises interdigital transducers (IDTs) formed of electrode fingers interdigitated on a piezoelectric substrate. Among various kinds of surface acoustic wave devices, a surface acoustic wave resonator (SAW resonator), in particular, has advantages such as compactness, light weight and no-adjustment requirement, and is in widespread use as a device for telecommunication equipment.
Referring to
FIG. 1
, there is shown a plan view of a basic structure of a conventional SAW resonator. On a piezoelectric substrate
101
, an IDT is formed of a plurality of electrode fingers
102
arranged in an interdigital configuration, a pair of bus bars
104
which are opposed to each other and connected with the electrode fingers
102
in an alternate fashion, input/output ports
105
and
106
, and a plurality of fingers
107
which are opposed to the electrode fingers
102
on the open node side thereof and connected with each of the opposing bus bars
104
to provide a reflector function. When a high-frequency electric signal is applied across the input/output ports
105
and
106
, an electric field is produced between the electrode fingers
102
arranged in the interdigital configuration to excite surface acoustic waves on the surface of the piezoelectric substrate
101
. In surface acoustic wave excitation, an excited surface acoustic wave having a wavelength identical to an interdigital pattern period P of the electrode fingers
102
and a surface acoustic wave having a wave number vector parallel to the direction of arrow
103
are excited most intensely since they are in phase on an electrode finger crossover area. In the SAW resonator shown in
FIG. 1
, a surface acoustic wave leaks out of the IDT through both sides thereof to cause a large energy loss, resulting in a low Q value in resonance.
Referring to
FIG. 2
, there is shown an exemplary electrode configuration of a conventional SAW resonator designed for Q-factor improvement over the conventional SAW resonator in
FIG. 1
(proposed in Japanese Unexamined Patent Publication No. H6 (1994)-85602 and “Small-Size Love-Type SAW Resonators with Very Low Capacitance Ratio” by Hiroshi Shimizu and Yuji Suzuki —The Transactions of the Institute of Electronics, Information and Communication Engineers, A Vol. J. 75-A NO. 3 pp. 458-466, March 1992). In this exemplary configuration in which a surface acoustic wave crossing over the electrode fingers
102
is excited on a rhombic area
108
(excitation area) enclosed by the broken line, apodization is made in a fashion that the cross lengths W of the electrode fingers
102
are maximum at the center of the IDT and zero at both ends thereof, thereby reducing a degree of spurious response. Further, leakage of a surface acoustic wave out of the IDT through both sides thereof is reduced since the excitation area
108
is narrowed on both sides of the IDT and surface acoustic wave reflection is made by a reflector
109
comprising the electrode fingers
107
which are so arranged on the periphery of the excitation area
108
as to oppose the electrode fingers
102
in a grating form. Thus, an energy loss can be decreased to improve the Q factor. Note that in addition to the electrode fingers
107
functioning as elements of the reflector
109
, parts of the electrode fingers
102
disposed on the periphery of the excitation area
108
also serve as reflector elements. That is to say, some parts of the electrode fingers
102
are used for excitation and the other parts of the electrode fingers
102
are used for reflection, depending on the locations thereof.
In the SAW resonator having the electrode configuration shown in
FIG. 2
, the electrode fingers on both sides of the IDT are opposed mutually in parallel. Therefore, the electrode fingers on both sides of the IDT reflect a surface acoustic wave component having a wave number vector parallel to the direction of the arrow
103
(inharmonic higher-order longitudinal mode component), causing a standing wave having a waveform such as
201
. Furthermore, in the SAW resonator having the electrode configuration shown in
FIG. 2
, the boundaries between a region of the reflector
109
and the bus bars
104
are opposed mutually in parallel. Therefore, a surface acoustic wave component having a wave number vector perpendicular to the direction of the arrow
103
(inharmonic higher-order transverse mode component) is reflected on the boundaries between the reflector
109
and the bus bars
104
, causing a standing wave having a waveform such as
202
. These standing waves produces spurious response in an impedance characteristic of the SAW resonator.
FIG. 3
shows an example of an impedance characteristic of the conventional SAW resonator shown in FIG.
2
. The conventional SAW resonator in
FIG. 2
is fabricated in the following manner: On a piezoelectric substrate made of 15°-rotated Y-cut X-propagation lithium niobate (hereinafter referred to simply as 15° YX-LN), aluminum is deposited by evaporation method, and an IDT electrode pattern is formed by photolithography and dry etching method. In
FIG. 3
, reference numeral
112
indicates a peak corresponding to a series resonance frequency of the SAW resonator. As shown in this figure, a multiplicity of spurious response peaks occur in a lower-frequency region
113
with respect to the series response frequency. These multiple spurious response peaks give rise to considerable problems, particularly in a case where the SAW resonator is employed as an oscillation element in a voltage controlled oscillator (VCO). Where the SAW resonator is employed as a VCO oscillation element, an expansion coil is connected to the SAW resonator and the lower-frequency region
113
with respect to the series resonance frequency thereof is used for resonant oscillation. Since spurious response in a VOC oscillation frequency region incurs a frequency discontinuity state, the spurious response peaks in the lower-frequency region
113
are critically problematic in operation.
Referring to
FIG. 4
, there is shown an exemplary electrode configuration of a conventional SAW resonator designed for improvement over the conventional SAW resonator in
FIG. 2
(proposed in “High-Q Wide-band SAW Resonators for VCO” by Atsushi Isobe et al.—Proceedings of the 20th Symposium on Ultrasonic Electronics, pp. 63, November 1999). In this example, the inside face of each of the bus bars
104
is formed in parallel with the periphery of the excitation area
108
so that a phase of a standing wave and a frequency incurring a standing wave are unrelated to the propagation direction of a surface acoustic wave for suppression of spurious response.
SUMMARY OF THE INVENTION
Although the conventional SAW resonator shown in
FIG. 4
is successful as far as the suppression of spurious response is concerned, it is unsatisfactory for use as a VCO oscillation element.
FIG. 5
is a graph indicating an exemplary impedance characteristic of the conventional SAW resonator shown in FIG.
4
. In fabrication of the conventional SAW resonator in
FIG. 4
, aluminum is deposited on a piezoelectric substrate made of 15° YX-LN by evaporation method, and an IDT electrode pattern is formed by photolithography and dry etching method.
In the conventional SAW resonator in
FIG. 4
, although a flat impedance characteristic having virtually no spurious response is attained in a lower-frequency region with respect to a peak
112
corresponding to a series resonance frequency thereof, a ripple
111
exists around a frequency of 207 MHz. Where the conventional SAW resonator in
FIG. 4
is used as a VCO oscillation element, an oscillation frequency discontinuity occurs in the vicinity of the frequency corresponding to the ripple
111
. For this reason, the conventional SAW res
Isobe Atsushi
Kachi Tsuyoshi
Sumioka Atsushi
Aguirrechea J.
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
Mullins Burton S.
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