Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
1999-08-12
2001-11-13
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S193000, C310S31300R, C310S360000
Reexamination Certificate
active
06317014
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave resonator, a surface acoustic wave filter, a duplexer, and a communications apparatus including the same. More particularly, the present invention relates to a surface acoustic wave resonator using a surface acoustic wave including an SH wave as the main component.
2. Description of the Related Art
Conventionally, surface acoustic wave resonators have been used widely in band pass filters and other electronic components included in mobile communications equipment. As an example of such a surface acoustic wave resonator, a surface acoustic wave resonator or a surface acoustic wave filter having a configuration wherein an interdigital transducer (hereinafter referred to as IDT) including a comb-shaped electrode is formed on a piezoelectric substrate is well known. Since a surface acoustic wave including an SH wave as the main component, such as a Love wave, a leaky wave, a BGS wave, or the like can utilize edge reflection in such a surface acoustic wave resonator or surface acoustic wave filter, it is used practically in a small size resonator or surface acoustic wave filter, which does not require a reflector. Further, a piezoelectric single crystal, such as lithium niobate, lithium tantalate, or the like, is used as the material for the piezoelectric substrate of a surface acoustic wave resonator or a surface acoustic wave filter. In order to generate a surface acoustic wave including an SH wave as the main component, a 41° Y cut X propagation substrate or a 64° Y cut X propagation substrate needs to be used in the case of a lithium niobate, and a 36° Y cut X propagation substrate needs to be used in the case of a lithium tantalate. The 41° Y cut X propagation corresponds with the Euler angle indication of (0°, 131°, 0°), the 64° Y cut X propagation corresponds with the Euler angle indication of (0°, 154°, 0°), and the 36° Y cut X propagation corresponds with the Euler angle indication of (0°, 126°, 0°).
However, the temperature coefficient of group delay time temperature characteristic (hereinafter referred to as TCD) of these piezoelectric single crystals is not good. More specifically, the TCD of a 41° Y cut X propagation lithium niobate substrate is 80 ppm/° C., the TCD of a 64° Y cut X propagation lithium niobate substrate is 81 ppm/° C., and the TCD of a 36° Y cut X propagation lithium tantalate substrate is 32 ppm/° C.
In general, in order to reliably obtain good characteristics in a surface acoustic wave resonator, a material with a good TCD is needed. That is, a material with a small frequency characteristic change according to the temperature change is needed. Therefore, if a surface acoustic wave resonator for generating a surface acoustic wave including an SH wave as the main component is provided with the lithium niobate or the lithium tantalate used as the substrate material as mentioned above, a problem arises in that the frequency characteristic shifts drastically. Moreover, even though lithium tantalate has a TCD which is better than that of lithium niobate, it also experiences the same problem with the frequency characteristic shift.
For example, in a surface acoustic wave resonator having a 100 MHz center frequency, a 50° C. temperature change causes a 400 KHz frequency characteristic shift in the case of a 41° Y cut X propagation lithium niobate substrate, a 405 KHz frequency characteristic shift is generated in the case of a 64° Y cut X propagation lithium niobate substrate, and a 160 KHz frequency characteristic shift is generated in the case of a 36° Y cut X propagation lithium tantalate substrate.
In order to prevent such a frequency characteristic shift, a temperature compensation circuit has been connected to a surface acoustic wave resonator. However, a problem arises in that the device as a whole including the surface acoustic wave resonator becomes bulky because of the added temperature compensation circuit so that it is difficult to achieve a small size component.
SUMMARY OF THE INVENTION
In view of the problems described above, preferred embodiments of the present invention provide a surface acoustic wave resonator which operates using an SH wave as the main component and has an excellent TCD.
According to one preferred embodiment of the present invention, a surface acoustic wave resonator includes a piezoelectric substrate, and at least one interdigital transducer having at least one pair of comb-shaped electrodes arranged to contact the piezoelectric substrate, the resonator arranged to use a surface acoustic wave including an SH wave as the main component, wherein a langasite single crystal is used as the piezoelectric substrate.
Since the langasite single crystal is used as the piezoelectric substance and the SH wave is used as mentioned above, a surface acoustic wave resonator with a large degree of coupling is achieved.
The langasite single crystal preferably has the Euler angle (ø, &thgr;, &phgr;) of approximately 0°≦ø≦30°, 0°≦&thgr;≦25°, and &phgr;=−1.07ø+90°±5°, and more preferably, has the Euler angle (ø, &thgr;, &phgr;) of approximately 11°≦ø≦24°, and 17°≦&thgr;≦24°.
Alternatively, the langasite single crystal preferably has the Euler angle (ø, &thgr;, &phgr;) of approximately 0°≦ø≦30°, 153°≦&thgr;≦180°, and &phgr;=1.05ø+28°±5°, and more preferably has the Euler angle (ø, &thgr;, &phgr;) of approximately 5°≦ø≦30°, and approximately 153°≦&thgr;≦158.5°.
According to preferred embodiments of the present invention, a surface acoustic wave resonator has a TCD which is significantly better than that of a resonator having a lithium niobate substrate or a lithium tantalate substrate. With preferred embodiments of the present invention, it is possible to obtain a TCD value of |10| ppm/° C. or less, and even |5| ppm/° C. or less.
Moreover, since the sound velocity is low compared with other materials, a small size component can be achieved, and since the edge reflection of an SH wave can be utilized, a reflector is not required so that an even smaller size component can be achieved.
For the purpose of illustrating the invention, there is shown in the drawings several preferred embodiments which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
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Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Pascal Robert
Summons Barbara
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