Crystal oscillator

Abstract

The present invention relates to a crystal oscillator formed of an AT-cut crystal piece having a rectangular shape in plan view with long edges and short edges; where a large number of the crystal pieces are placed within a hollow container having a curved inner periphery and the crystal pieces are ground along the inner periphery of the hollow container as the hollow container is rotated; an inclined portion is provided over the entire outer periphery of each of two principal surface sides of the crystal piece; and a central region of the crystal piece is a flatter portion; wherein the flatter-portion length ratio L2/L1 of the length L1 in the long-edge direction of the crystal piece and the length L2 of the flatter portion is between 0.24 and 0.33, and also the long-edge bevel depth ratio D1/T1 of the thickness T1 of the crystal piece and the depth D1 from each of the two principal surfaces at the center of the outer periphery of the inclined portion at each of two end portions in the long-edge direction is between 0.30 and 0.38. This provides a crystal oscillator having a greatly reduced surface area in plan view of the crystal piece, which has an inclined portion (beveling) around the outer periphery thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is illustrative of an embodiment of the crystal oscillator of the present invention, where FIG. 1A is a front view of the crystal piece in the long-edge direction, FIG. 1B is a plan view, and FIG. 1C is a side view in the short-edge direction;

FIG. 2 is illustrative of the action of this embodiment of the crystal oscillator of the present invention, where both FIGS. 2A and 2B are CI characteristics diagrams;

FIG. 3 is illustrative of the actions of the embodiment of the crystal oscillator of the present invention, where FIGS. 3A and 3B are CI characteristics diagrams thereof;

FIG. 4 is illustrative of a prior-art example of a crystal oscillator, where FIG. 4A shows the orientations of the cutting planes thereof and FIG. 4B is a perspective view of the crystal piece;

FIG. 5 is further illustrative of the prior-art example of the crystal oscillator, where FIG. 5A is a front view of the crystal piece, FIG. 5B is a plan view thereof, FIG. 5C is a side view thereof, and FIG. 5D is a section taken along the line A-A of FIG. 5B;

FIG. 6 is still further illustrative of the prior-art example of the crystal oscillator, where FIG. 6A is a front view thereof and FIG. 6B is a plan view; and

FIG. 7 is even further illustrative of the prior-art example of the crystal oscillator, where FIGS. 7A, 7B, and 7C are partial enlarged sections thereof.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An embodiment of the crystal oscillator of the present invention is illustrated in FIG. 1, where FIG. 1A is a front view thereof in the long-edge direction, FIG. 1B is a plan view, and FIG. 1C is a side view in the short-edge direction. Note that portions that are the same as those of the prior-art example are denoted by the same reference numbers, and further description thereof is either abbreviated or omitted. In the plan view, the actual dimensions are shown enlarged 15 times, in a similar manner to the prior-art example.

The crystal oscillator of the present invention is formed of the AT-cut crystal piece 1 in a rectangular shape having long edges and short edges, as previously described. In this case too, the crystal piece 1 has a thickness T1 (0.42 mm) that establishes a frequency band of 4 MHz, and is provided with curved inclined portions (beveled surfaces) on the two end sides in both the long- and short-edge directions that form the entire outer periphery of each principal surface thereof so that the external dimensions thereof in plan view are beveled to 5.251 ×1.801 mm.

These beveled surfaces are formed by placing a large number of these narrow card shaped pieces in a spherical hollow container (not shown in the figures) together with a grinding agent, then gradually grinding the pieces starting from the four corner portions of the two principal surfaces by high-speed rotation of the hollow container. To prevent coupling with the contour oscillations of the resonance energy in this case, the crystal piece 1 is inserted into the spherical hollow container after the two side surfaces thereof that are in the Z′ plane (long edges) are inclined at 5 degrees from the Y′-axis direction toward the Z′-axis direction. Note that this inclination is omitted from FIG. 1C.

The front and sideviews in the long-edge and short-edge directions of the crystal piece 1 that has been ground in this manner are such that inclined portions (beveled surfaces), which have end surfaces and which extend in all directions from a flatter portion in the central region of each principal surface, are formed in substantially the same way as in the configuration described above, where the curves from the central portion of each of the principal surfaces are curved surfaces that increase continuously. The long-edge end surfaces (the end surfaces of each long edge) are such that the thickness at corner portions of each short edge becomes gradually thicker towards the central portion thereof, to reach a maximum thickness T3 at that central portion. Since the width in the short-edge direction is so narrow, the short-edge end surfaces are of substantially uniform thickness.

The external plan-view dimensions (plan view) of the crystal piece 1 are such that the shape thereof is rectangular, with four corner portions 1a that are of an arc-shape which is larger than that of the prior art, and having an elliptical flatter portion 1b in the central region thereof. In this case, the flatter portion 1b is essentially of a shape that is close to circular, but it becomes an elliptical shape by the increase in the length of the long edges of the crystal piece 1 in comparison to the short edges. Note that the flatter portion 1b is a region within 1 μm from the maximum thickness T3 of the central portion, and each of the two end portions is a region within 1 μm from the arc-shaped leading edge thereof, as previously described.

The excitation electrode 2 is formed on each of the principal surfaces of the crystal piece 1 (see FIG. 4B), a previously described, and the two end portions from which the extracting electrodes 3 extend are affixed by an electrically conductive adhesive to the inner base surface of the main container (not shown in the figures). The crystal piece 1 is then sealed with a cover to form a crystal oscillator for surface mounting, by way of example.

CI characteristics diagrams of crystal oscillators using such crystal pieces are shown in FIGS. 2 and 3. Note that FIG. 2A is a CI characteristics diagram for the flatter-portion length ratio L2/L1 of the length L1 of the crystal piece 1 and the length L2 of the flatter portion, and FIG. 2B is a CI characteristics diagram for the long-edge bevel depth ratio D1/T1 of the thickness T1 of the crystal piece 1 and the bevel depth D1 of each of the two end portions in the lengthwise direction.

Similarly, FIG. 3A is a CI characteristics diagram for the flatter-portion width ratio W2/W1 of the width W1 of the crystal piece 1 and the width W2 of the flatter portion, and FIG. 3B is a CI characteristics diagram for the short-edge bevel depth ratio D2/T1 of the thickness T1 of the crystal piece 1 and the bevel depth D2 of each of the two end portions in the widthwise direction.

Six different crystal pieces 1 with a resonant frequency of 4 MHz were formed by varying the grinding conditions of the crystal pieces 1 thereof. These graphs are the results of measuring the flatter-portion length ratio L2/L1, the long-edge bevel depth ratio D2/T1, the flatter-portion width ratio W2/W1, and the short-edge bevel depth ratio D2/T1 of each of these six crystal pieces 1, then measuring the CI characteristics of each crystal oscillator fabricated therewith. Note that the six data points (squares) in these graphs are actual measured values and the curves are quadratic approximation curves.

To summarize, six different crystal pieces having external plan-view dimensions of 5.25×1.80 mm and a resonant frequency of 4 MHz were formed by varying the grinding conditions such as the inner diameter of the grinding container (hollow container or sphere shape container), the rotational speed, the grinding time, and the grinding agent. The CI characteristics were then obtained for each ratio L2/L1, D1/T1, W2/W1, and D2/T1. In FIGS. 2A, 2B, 3A, and 3B, the six different crystal pieces are denoted by reference numbers 1 to 6, where the same reference number denotes the same crystal piece in all the graphs. Note that the dimensions in the lengthwise direction and widthwise direction are both reduced by grinding by approximately 0.001 mm.

As is clear from these figures, each CI characteristic (quadratic approximation curve) is a parabola that depends on the flatter-portion length ratio L2/L1, the long-edge bevel depth ratio D1/T1, the flatter-portion width ratio W2/W1, and the short-edge bevel depth ratio D2/T1.

Paying attention first of all to the long-edge direction of each crystal piece 1, the CI characteristic for the flatter-portion length ratio L2/L1 (see FIG. 2A) forms a parabola with a minimum at approximately 100 Ω (the actual measured value was 70 Ω) at an L2/L1 ratio of approximately 0.28. Since the resonance displacement region also increases as the length of the flatter portion (in the X-axis direction) increases, thickness-shear resonance can be achieved simply. If the length of the flatter portion is increased beyond the minimum value of 100 Ω, the CI thereof will deteriorate (tend to increase) due to end surface reflections of the resonance energy.

The CI characteristic for the long-edge bevel depth ratio D1/T1 (see FIG. 2B) exhibits a minimum value of 100 Ω(the actual measured value was 70 Ω) when the D1/T1 ratio exceeds 0.34 (the actual measured value was 0.35), then the CI tends to increase beyond that point. This shows that the end surface reflections are prevented and the CI is low when the bevel depth D1 is made large, but the CI tends to increase when that minimum value is exceeded because the length of the flatter portion is substantially shortened thereby. Note that since the grinding is done in a spherical hollow container, the maximum value of the long-edge bevel depth ratio D1/T1 (0.35 in this case) remains unchanged after that maximum is met.

The description now turns to the short-edge direction of the crystal piece 1, where the CI characteristic for the flatter-portion width ratio W2/W1 (see FIG. 3A) exhibits a minimum value of 70 Ω (the actual measured value was 70 Ω) when the W2/W1 ratio exceeds approximately 0.42 (the actual measured value was 0.29), then the CI shows an upward trend beyond that. In this case, the resonance region (electrode surface area) in the widthwise direction increases and the CI decreases, but the CI deteriorates after the minimum value is achieved, due to causes such as reflection of the resonance energy at the end surfaces.

Similarly, the CI characteristic for the short-edge bevel depth ratio D2/T1 (see FIG. 3B) exhibits a minimum value of 70 Ω (the actual measured value was 70 Ω) when the D2/T1 ratio is 0.12 (the actual measured value was 0.12), then the CI tends to increase beyond that point. This shows that the end surface reflections can be prevented and the CI is low when the bevel depth D2 is made large, but the CI tends to increase when that minimum value is exceeded because the length of the flatter portion is substantially shortened thereby, in a similar manner to that seen in the long-edge direction of the crystal piece 1. Note that since D2/T1 in the widthwise direction of the crystal piece 1 stays at 0.12 after reaching that maximum, the effect of inclining the side surface (Z′ surfaces) by 5 degrees is maintained. Note also that each beveled portion is a curved surface, as described previously.

The description now turns to extracting a range of up to 200 Ω, which is the target for actual use, from the quadratic approximation curves in the CI characteristics for each ratio L2/L1, D1/T1, W2/W1, and D2/T1. In other words, the minimum value of 0.28 for the flatter-portion length ratio L2/L1 is within a range of 0.24 to 0.34 and the minimum value of 0.34 for the long-edge bevel depth ratio D1/T1 is within a range of 0.30 to 0.38. Similarly, the minimum value of 0.42 for the flatter-portion width ratio W2/W1 for short-edge bevel depth ratio D2/T1 is within a range of 0.25 to 0.55 and the minimum value of 0.12 is within a range of 0.8 to 0.14.

The various ratios that produce the minimum CI value of 70 Ω in this embodiment are compared with similar values of the prior-art example below.

	This Embodiment	Prior-art
Thickness T1 of crystal piece	0.420 mm	0.420 mm
Length L1	5.250 mm	8.240 mm
Width W1	1.800 mm	1.720 mm
Dimensional ratio W1/L1	0.349	0.209
Flat portion length ratio L2/L1	0.280	0.185
Long edge bevel depth ratio D1/T1	0.350	0.260
Flat portion width ratio W2/W1	0.290	0.300
Short edge bevel depth ratio D2/T1	0.120	0.060

The description now turns to studies of the external plan-view dimensions and flatter portion of the crystal piece 1 which have shown that the width W1 of the crystal piece 1 of this embodiment is substantially the same as that of the prior-art (actually slightly larger), the length L1 thereof is slightly shorter, but the dimensional ratio W1/L1 is increased from 0.209 to 0.349. Similar studies have shown that both the long and short edges are generally shortened to the same degree (called “uniform shortening”), but in this case the dimensional ratio W1/L1 is increased so that the surface area of the plate is relatively larger than a case in which uniform shortening occurs.

The flatter-portion length ratio L2/L1 is set to 0.280, which is a 1.5-times increase (50% increase) over the 0.185 of the prior-art example. This makes the length L2 of the flatter portion relatively larger, even though the length L1 of the crystal piece 1 is shortened. Since the lengthwise direction (X-axis direction) of the crystal piece 1 is the displacement direction for thickness-shear resonance, therefore, resonance is simplified and the CI can be reduced.

In such a case, since the flatter-portion width W2 (0.522 mm, giving a W2/W1 ratio of 0.290) is substantially the same as that of the prior-art (0.516 mm, giving a W2/W1 ratio of 0.300), the surface area of the flatter portion that is the resonance region is relatively larger in comparison with a case in which the dimensions are shortened uniformly (see above). In short, since the flatter-portion width W2 of this embodiment is substantially the same, or even greater, than that of the prior-art, the surface area of the flatter portion is relatively larger. Thus the resonance energy trapped within the flatter portion that is the resonance region also increases and the CI decreases.

Next, comparisons of the inclined portions (beveled surfaces) of the crystal piece 1 show that the horizontal length L3 (1.890 mm) in the lengthwise direction of this embodiment is shorter than that of the prior-art (3.358 mm) and the depth ratio D1/T1 (0.350) thereof is greater than that of the prior-art (0.260), in other words, the beveled slope has become steeper which increases the energy trapping effect. Therefore, if the two end portions of the crystal piece 1 are restrained, by way of example, leakage of resonance energy from those end portions is prevented and the CI can be kept small.

In addition, the horizontal length W3 (0.639 mm) in the widthwise (short-edge) direction of this embodiment is substantially the same (but slightly larger) as that of the prior-art (0.602 mm) and the depth ratio D2/T1 (0.120) is larger than that of the prior-art (0.060). The effect of trapping resonance energy in the widthwise direction is therefore increased and the CI is kept low. Note that a lack of restraint on the two ends in the widthwise direction has no effect on the leakage of resonance energy due to the restraining of the inclined portions (beveled surfaces), but it is useful to also form the inclined portions (beveled surfaces) in the widthwise direction of the crystal piece 1, from the viewpoint of preventing phenomena such as standing waves due to end surface reflections, thus preventing the occurrence of spurious signals.

From this it is clear that this embodiment provides a minimum CI value of 70 Ω with a crystal piece 1 having external plan-view dimensions of 5.25×1.80 mm (4 MHz). In contrast thereto, if inclined portions are formed in the long-edge and short-edge directions at similar ratios to those of the prior-art example when the external plan-view dimensions of the crystal piece 1 are the same 5.25×1.80 mm as that of this embodiment, the CI value would be roughly 600 Ω which gives a completely different result in practice.

The external plan-view dimensions of the crystal piece 1 of the above-described embodiment were given above as 5.25×1.80 mm (for 4 MHz), but the present invention can also be applied substantially when the dimensions of the long and short edges are within ±5%, allowing for errors. For example, since the crystal piece 1 can be housed in a sealed container for a 8045-type oscillator (the inner dimensions of the container of 5.5×2.4 mm, and the outer dimensions thereof of 8.0×4.5 mm by way of example), the length of the crystal piece 1 can be increased to about 5.5 mm (an approximately 5% increase). In addition, the two end portions of the crystal piece 1 from which the extracting electrodes extend were described as being restrained, but it is also possible to employ a configuration in which the extracting electrodes extend from two sides of one end portion of the crystal piece and those two sides of the one end portion are restrained.

Claims

1. A crystal oscillator formed of an AT-cut crystal piece having a rectangular shape in plan view with long edges and short edges; where a large number of said crystal pieces are placed within a hollow container having a curved inner periphery and said crystal piece is ground along the inner periphery of said hollow container as said hollow container is rotated; an inclined portion is provided over the entire outer periphery of each of two principal surface sides of said crystal piece; and a central region of said crystal piece is a flatter portion; wherein the flatter-portion length ratio L2/L1 of the length L1 in the long-edge direction of said crystal piece to the length L2 of said flatter portion is between 0.24 and 0.33, and also the long-edge bevel depth ratio D1/T1 of the thickness T1 of said crystal piece and the depth D1 from each of the two principal surfaces at the center of the outer periphery of said inclined portion at each of two end portions in said long-edge direction is between 0.30 and 0.38.

2. The crystal oscillator according to claim 1, wherein the flatter-portion width ratio W2/W1 of the width W1 of said crystal piece in said short-edge direction and the width W2 of said flatter portion is between 0.25 and 0.55, and also the short-edge bevel depth ratio D2/T1 of the thickness T1 of said crystal piece and the depth D2 at the center of the outer periphery of said inclined portion at each of the two ends portions thereof in the short-edge direction is between 0.08 and 0.14.

3. The crystal oscillator according to claim 1, wherein an excitation electrode formed on each of the two principal surfaces of the crystal piece partially extends over said inclined portion from said flatter portion.

4. The crystal oscillator according to claim 1, wherein said crystal piece is an AT-cut piece, the X-axis direction of the crystal axes (XY′Z′) thereof is along said long edges and said Z′-axis direction is along said short edges.

5. The crystal oscillator according to claim 4, wherein the Z′-plane of said crystal piece that forms side surfaces in the lengthwise direction that intersect said Z′-axis are inclined at an angle θ degrees from said Y′-axis in said Z′-axis direction.

6. The crystal oscillator according to claim 5, wherein said θ degrees is 5 degrees.

7. The crystal oscillator according to claim 1, wherein said hollow container is spherical shape; and the thickness of each of the long-edge end surfaces that form two end sides in the short-edge direction of said crystal piece is such that the two corner portions of each of said short edges define the thickness of two corner portion of each of said long-edge end surfaces, which increases gradually towards the center of the outer periphery of said long edge.

8. The crystal oscillator according to claim 1, wherein the frequency band of said crystal piece is the 4-MHz band and the external plan-view dimensions of said crystal piece are 5.25×1.80 mm.

9. The crystal oscillator according to claim 1, wherein said flatter portion is a region within 1 μm from the maximum thickness of said crystal piece; and each of said depths D1 and D2 is the difference between the surface 1 μm inward from the leading edges of the two end portions in the long-edge direction and the maximum thickness portion thereof.