Abstract:
An improved AT-cut crystal oscillator for electronic watches. The AT-cut crystal oscillator is of substantially disc-shape. Furthermore, the AT-cut crystal oscillator is shaped in cross-section such that the periphery of the disc-shaped crystal oscillator is abruptly tapered. In the preferred embodiment, the cross-section is trapezoidal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to crystal oscillators and more particularly to AT-cut crystal oscillators for electronic timepieces. 
     2. Prior Art 
     In conventional crystal oscillators for watches, a tuning fork-type crystal oscillator as shown in FIG. 1 has been broadly used. This type has, however, inferior physical characteristics. Such characteristics include a frequency-temperature characteristic curve of a inverted parabola having a zero temperature coefficient at approximately room temperature as shown in FIG. 2, a total differential in frequency as much as 20 parts per million, and a frequency aging effect of 5 × 10 -6  per year. In addition, the fork-shape of the tuning fork oscillator is rather complex to manufacture thereby increasing manufacturing costs. Furthermore, its impact strength and vibration characteristics are not very good because of the complex and weak supporting structure. Accordingly, this type has not been satisfactory as a time standard for either a high quality watch or a low cost watch. 
     In addition to the tuning fork-type oscillator, shown in FIG. 3 is the configuration of an AT-cut crystal oscillator having a frequency greater than or equal to 4 MHz which is used in electronic communication instruments in general. FIG. 4 shows a distribution of a displacement in vibration mode of AT-cut crystal from the center of the crystal oscillator to its periphery as a result of touching it by means of the &#34;Probe Method&#34;. The AT-cut crystal oscillator for electronic communications instruments cannot be used for watches because of the fact that the crystal oscillator itself is too large and the supporting structure is not strong enough to protect against impact damage. In addition, if a conventional AT-cut crystal oscillator of this type is made smaller, it results in a deterioration of the characteristics of the AT-cut crystal oscillator and in particular, the temperature characteristic, the aging effect and the crystal impedance. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide an AT-cut crystal oscillator which does not have the drawbacks of a turning fork-type AT-cut crystal oscillator or those used in communication instruments. 
     It is another object of the present invention to provide an AT-cut crystal oscillator which is low in cost. 
     It is still another object of the present invention to provide an AT-cut crystal oscillator which is high in quality. 
     It is also an object of the present invention to provide an AT-cut crystal oscillator which can be securely mounted in an electronic timepiece without having any substantial effect on its characteristics. 
     In keeping with the principles of the present invention, the objects are accomplished by a unique AT-cut crystal oscillator having substantially a disc-shape. Furthermore, the AT-cut crystal oscillator is shaped in cross-sections such that the periphery of the disc-shaped crystal oscillator is abruptly tapered. In particular, in the preferred embodiments, the cross-section of the crystal oscillator is arranged and configured such that along two axes of the crystal oscillator which are 90° apart, one is rectangular in shape and the other is substantially a trapezoidal in shape or two trapezoidal shapes put together with their longer base attached. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other features and objects of the present invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings wherein like referenced numerals denote like elements, and in which: 
     FIG. 1 is a plan view of a tuning fork-shaped crystal oscillator of the prior art; 
     FIG. 2 illustrates the frequency-temperature characteristics of the tuning fork-shaped oscillator of FIG. 1; 
     FIG. 3 (a) is a plan view of an AT-cut crystal oscillator of the prior art; 
     FIG. 3(b) is a cross-section of the AT-cut crystal oscillator of FIG. 3 along the lines A--A; 
     FIG. 4 illustrates the frequency differential of the AT-cut crystal oscillator of FIG. 3; 
     FIG. 5(a) is a plan view of an AT-cut crystal oscillator in accordance with the teachings of the present invention; 
     FIG. 5(b) is a cross-section of the embodiment of FIG. 5(a) along the line A--A; 
     FIG. 5(c) is a cross-sectional view of the embodiment of FIG. 5(a ) looking along the lines B--B; 
     FIG. 6 illustrates the frequency differential curve of the embodiment of FIG. 5(a); 
     FIG. 7(a) is a plan view of a second embodiment of an AT-cut crystal oscillator in accordance with the teachings of the present invention; 
     FIG. 7(b) is a cross-section of the embodient of FIG. 7(a) along the lines A--A; 
     FIG. 7(c) is a cross-section of the embodiment of FIG. 7(a) looking along the line B--B; 
     FIG. 8 illustrates the frequency differential curve of the AT-cut crystal oscillator of FIG. 7(a); 
     FIG. 9 illustrates a supporting structure utilized with the AT-cut crystal oscillator of the present invention; 
     FIG. 10 illustrates the frequency-temperature characteristics of the AT-cut crystal oscillator in accordance with the teachings of the present invention; 
     FIG. 11 (a) is a plan view of a third embodiment of an AT-cut crystal oscillator in accordance with the teachings of the present invention; 
     FIG. 11(b) is a cross-sectional view of the embodiment of FIG. 11(a) along the line A--A; 
     FIG. 11(c) is a cross-sectional view of the embodiment of FIG. 11(a) along the line B--B; 
     FIG. 12(a) is a plan view of a fourth embodiment of an AT-cut crystal oscillator in accordance with the teachings of the present invention; 
     FIG. 12 (b) is a cross-sectional view of the embodiment of FIG. 12 (a) along the line A--A; 
     FIG. 12 (c) is a cross-sectional view of the embodiment of FIG. 12 (a) along the line B--B; and 
     FIG. 13 through 17 are additional embodiments of an AT-cut crystal oscillator in accordance with the teachings of the invention shown in plan view and cross-section along the line A--A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 5 (a ) through 5 (c), shown therein are the plan and cross-sectional views of an embodiment of an AT-cut crystal oscillator in accordance with the teachings of the present invention. The AT-cut crystal oscillator 10 is substantially disc-shaped having a flat portion 1 and downwardly sloping portions 2 and 2&#39; at its periphery. As viewed in cross-sectional along the line A--A, the shape of oscillator 10 is a trapezoid as shown in FIG. 5 (b) while the cross-section of the disc-shaped oscillator 10 taken along the line B--B is rectangular shaped as shown in FIG. 5 (c). 
     In FIG. 6 is shown the curve of the change in frequency of the oscillator of FIG. 5(a) from the center to the periphery thereof as a result of touching the crystal oscillator 10 by means of the &#34;Probe Method&#34;. FIG. 6 clearly shows that the displacement of vibration mode of AT-cut crystal has been reduced by forming the slopes 2 and 2&#39; as compared to the conventional type of crystal oscillators shown in FIG. 4. Accordingly, it is possible to avoid the adverse effect due to the supporting which was suffered by the conventional type crystal oscillator if the oscillator 10 is provided with slopes 2 and 2&#39; and supported at its slopes 2 and 2&#39;. 
     Referring to FIGS. 7(a), (b) and (c), shown therein is a second embodiment of an AT-cut crystal oscillator in accordance with the teachings of the present invention. The second embodiment is shaped such that the cross-section taken along the line A--A is substantially equal to two trapezoids with the longer of the two bases placed together as shown in FIG. 7(b). Similar to the first embodiment, the second embodiment is of substantially rectangular cross-section along the line B--B as shown in FIG. 7(c). 
     In FIG. 8 is shown the frequency differential of the second embodiment of the AT-cut crystal oscillator. FIG. 8 clearly shows that the differential decreases drastically as one gets closer to the periphery of the AT-cut crystal oscillator. 
     In FIG. 9 is shown a plan view of a mounting structure for an AT-cut crystal oscillator in accordance with the teachings of the present invention. The AT-cut crystal oscillator 10 has a diameter of 6mm or less and slopes 2 and 2&#39;. An electrode 3 is provided on both sides of the AT-cut crystal oscillator 10 and AT-cut crystal oscillator 10 is coupled to a seal end 5 by an elastic ring 4. 
     Referring to FIG. 10, shown therein is a curve of the frequency temperature characteristics of an AT-cut crystal oscillator 10 in accordance with the teachings of the present invention which illustrate the improved quality of this invention over the tuning fork-type crystal oscillator. In particular, there are two points where the temperature coefficient is zero. Furthermore, the frequency differential is less than three parts per million. In addition, experiments have shown that AT-cut crystal oscillators in accordance with the teachings of the present invention have other advantages in connection with the crystal impedance and aging effects. 
     Referring to FIG. 11, shown therein is a third embodiment of a crystal oscillator in accordance with the teachings of the present invention. In FIG. 11, the slopes 2 and 2&#39; are formed in the shaped of a cresent as shown in FIG. 11(a). Otherwise, the embodiment of FIG. 11(a), (b) and (c) is substantially the same as that shown in FIGS. 5(a), (b) and (c). 
     Referring to FIG. 12, shown therein is a fourth embodiment of an AT-cut crystal oscillator in accordance with the teachings of the present invention. The embodiment of FIG. 12 is substantially the same as that shown in FIG. 7 except that the sloping regions on both sides of the AT-cut crystal oscillator are crescent shaped. Otherwise, the embodiment shown in FIGS. 12(a), (b) and (c) is substantially the same as the embodiment shown in FIGS. 7(a), (b) and (c). 
     Referring to FIGS. 13 through 17, shown therein are other embodiments of an AT-cut crystal oscillator in accordance with the teachings of the present invention shown in a plan view and a cross-sectional view taken along the lines A--A. As shown in FIG. 13, this fifth embodiment comprises a circumferential step cut out in the periphery of the AT-cut crystal oscillator. As shown in FIG. 14, this sixth embodiment comprises a circumferential step cut out of the periphery of both sides of the AT-cut crystal oscillator. 
     As shown in FIG. 15, the seventh embodiment comprises a disc-shaped AT-cut crystal oscillator having two straight step portions cut out of two opposite edges of the periphery of the disc-shaped AT-cut crystal oscillator. As shown in FIG. 16, the eighth embodiment comprises pairs of straight step-like portions cut out of opposing edges of the periphery of the AT-cut crystal oscillator. As shown in FIG. 17, the ninth embodiment comprises in general, a trapezoidal shape wherein the long base is curved. 
     As stated above, when using an AT-cut crystal oscillator in accordance with the teachings of the present invention, it is possible to avoid adverse effects upon the AT-cut crystal oscillator due to the supporting structure, to reduce the manufacturing costs because of a simple configuration as compared to the conventional tuning fork-type crystal oscillators and to improve the preciseness of the frequency by having better frequency characteristics over the prior art crystal oscillators. 
     In all cases it is understood that the above described embodiments are merely illustrative of but a few of the possible specific embodiments which represent the applications of the principles of the present invention. Numerous and varied other arrangements can be readily devised by those skilled in the art without departing from the spirit and scope of the invention.