GB2325336A

GB2325336A - Selected overtone resonator with two or more coupled resonant thicknesses

Info

Publication number: GB2325336A
Application number: GB9810543A
Authority: GB
Inventors: Thane Smith
Original assignee: M Tron Industries Inc
Current assignee: M Tron Industries Inc
Priority date: 1997-05-15
Filing date: 1998-05-15
Publication date: 1998-11-18
Also published as: GB9810543D0; CA2237497A1

Abstract

A dual overtone resonator is formed that allows easier establishment of a resonant frequency, which is an overtone of the fundamental frequency of at least one of the sections of the crystal. The resonator includes a first section 504 having a first thickness and a second section 502 having a second thickness. The first thickness and the second thickness are selected to cause the nodes and the antinodes of the crystal to align when the crystal is stimulated. The first section 504 has a thickness an even number of half wavelengths thicker than the second section 502. The first and second sections have a thickness equal to a prime number of half wavelengths e.g. 5#/2 and 7#/2. The alignment of the nodes and antinodes allows the first section 504 and the second section 502 to resonate at the same frequency.

Description

SELECTED OVERTONE RESONATOR WITH TWO OR MORE COUPLED RESONANT THICKNESSES BACKGROUND OF THE INVENTION 1. Field of the Invention.

This invention relates in general to resonators, and more particularly to a piezoelectric resonator tuned to respond at a specific overtone of its parallel plate resonant frequency.

2. Description of Related Art.

Crystals are widely used in frequency control applications because of their unequaled combination of high Q, stability and small size. The Q values of crystal units are much higher than those attainable with other circuit elements. In generalpurpose crystal units, typical Q's are in the range of 104 to 106.

Some crystals, in particular quartz crystals, are highly anisotropic, that is, their properties vary greatly with crystallographic direction. For example, when a quartz sphere is etched in hydrofluoric acid, the etching rate is more than 100 times faster along the fastest etching rate direction, the Z-direction, than along the slowest direction, the slow-X-direction. The constants of quartz, such as the thermal expansion coefficient and the temperature coefficients of the elastic constants, also vary with direction. That crystal units can have zero temperature coefficients of frequency is a consequence of the temperature coefficients of the elastic constants ranging from negative to positive values.

The locus of zero-temperature-coefficient cuts in quartz 100 is shown in Fig.

la. The X 102, Y 104, and Z 106 directions have been chosen to make the description of properties as simple as possible. The Z-axis 106 in Fig. la is an axis of threefold symmetry in quartz; in other words, the physical properties repeat every 1200 as the crystal is rotated about the Z-axis 106. The cut may comprise a singly rotated cuts 120 and double rotated cuts 130 having angles 140 and e 142.

Fig. lb illustrates the relationship 150 of several different quartz cuts. The cuts usually have two-letter names, where the "" in the name indicates a temperature-compensated cut. For instance, the AT-cut 160 was the first temperature-compensated cut discovered. The FC 162, iT 164, BT 166, and RT 168 cuts are other cuts along the zero temperature coefficient locus. These cuts were studied before the discovery ofthe SC cut 170 for some special properties, but are rarely used today. Today, the highest-stability crystal oscillators employ SC cut 170 or AT cut 160 crystal units.

A crystal typically includes suitably mounted and electroded plates of crystalline quartz using bulk acoustic wave (BAW) vibrations. The plates, also called wafers or blanks, are fabricated at a precise orientation with respect to the crystallographic axes of the quartz material. Originally quartz plates were made from natural quartz, but today cultured quartz is used almost exclusively.

The cut and geometry of the quartz plate determine the resonator frequency for a chosen mode of vibration of the plate. In general the plate resonant frequency is inversely related to a plate dimension. Extension, face shear, flexure and thickness are typical types of vibration. The cut and type of vibration are intimately related and can be classified as low frequency or high frequency depending on the range of resonant frequencies over which they are commonly used.

When designing an oscillator with stringent stability requirements, the stability and accuracy of quartz frequency control is required. Quartz plates show a mechanical movement or strain when subjected to an electrical charge. Conversely, they show a potential difference between the two faces when subjected to a mechanical stress. This relationship is known as the piezoelectric effect. Because of its electro-mechanical properties, a crystal placed in an oscillator circuit can be made to oscillate both mechanically and electrically, with its resonant frequency determined primarily by its mechanical dimensions.

Normally quartz crystals have a uniform thickness between the electrodes.

However, a quartz plate will resonate vigorously when the driving frequency results in an odd number [1,3,5,7, etc.] of acoustic half-wavelengths between the plates.

The number of half wavelengths in a uniformly thick area is known as the "overtone number". At resonance "standing waves" occur that include, at fixed positions, nodes (planes of zero amplitude), and antinodes (plains of maximum amplitude).

The lowest resonant frequency of a crystal, known as the "fundamental frequency", is inversely proportional to the thickness of the crystal. There are practical limits to how thin a crystal can be made. Thus, for high frequencies, a high "overtone," which is very nearly an integer multiple of the fundamental frequency, is desired. This integer multiple of the fundamental frequency is referred to as the overtone number. The term " l "14 overtone" will be used herein to mean the fundamental frequency since one times this frequency is the same frequency.

Quartz crystal accuracy and stability far surpass the performance obtained by circuits utilizing conventional capacitors, inductors, and resistors. A typical twopoint mount package 200 for a crystal is shown in Fig. 2. The crystal consists of a quartz blank 202 with a metal electrode 204 on each of the two major surfaces. The electrodes 204 are connected to mounting clips 206 at a bonding area 208. The mounting clips 206 couple the crystal to a leaded header or base 210. At a later stage, the crystal is encapsulated by welding a cover 220 over the assembly.

This crystal behaves electrically as the circuit in Fig. 3. This circuit is called the equivalent circuit for the crystal. The mechanical losses of the crystal appear as an equivalent series resistance, R1 302, while the mechanical elasticity of the crystal is equivalent to a series capacitor, C, 304. C0 306 is the parallel capacitance associated with the holder and the electrode capacitance. The frequency of a crystal operating at series resonance is given by:

At series resonance the reactance of Cl 304 and L1 310 are equal and opposite.

Thus, the net reactance of the series circuit is zero. At this point the crystal appears as a resistance Rl 302.

It is known to tune resonators so that they respond to a specific overtone of its parallel plate resonant frequencies. One approach is reduce the blank diameter to degrade its response at the fundamental and higher modes below the desired overtone frequency and to increase its response at the frequency of the desired overtone. Another approach is to apply mass loading in the form of a metal ring.

Fig. 4 illustrates this mass loading structure 400. In Fig. 4, a metal ring 402 substantially circumscribes a top electrode 404 over a quartz blank 406. A similar pattern is also applied to the bottom of the quartz blank 406.

Nevertheless each of the above methods has its disadvantages. The first method leads to the blank diameter becoming inconveniently small thereby causing problems in manufacturing. In the second method, the metal ring 402 introduces unwanted parasitic capacity in the resonator circuit which can reduce the output power and degrade the oscillator quality factor (Q).

It can be seen that there is a need for a resonator that allows easier establishment of a resonant frequency.

It can also be seen that there is a need for a resonator tuned to respond at a specific overtone frequency.

SUMMARY OF THE INVENTION To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a dual overtone crystal.

The present invention solves the above-described problems by providing a resonator that allows easier establishment of a resonant frequency. While the present invention is primarily described with reference to quartz crystals, those skilled in the art will readily recognize that the invention is equally applicable to any piezoelectric material. Moreover, those skilled in the art will recognize that the present invention is equally applicable to a dielectric resonator.

A system in accordance with the principles of the present invention includes a first section having a first thickness and a second section having a second thickness.

The first thickness and the second thickness are selected to cause the nodes and the antinodes of the crystal to align when the crystal is stimulated.

Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the first section has a thickness an even number of half wave lengths thicker than the second section.

Another aspect of the present invention is that the first and second sections have a thickness equal to a prime number of half wave lengths.

Another aspect of the present invention is that the alignment of the nodes and antinodes allows the first section and the second section to resonate at the same frequency.

These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance withthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS Referring now to the drawings in which like reference numbers represent corresponding parts throughout: Fig. 1 a illustrates the locus of zero-temperature-coefficient cuts in quartz; Fig. lb illustrates the relationship of several different quartz cuts; Fig. 2 illustrates a typical two-point mount package; Fig. 3 illustrates an equivalent circuit for a crystal; Fig. 4 illustrates a mass loading method for increasing the response of the blank at the frequency of the desired overtone; Fig. 5 illustrates a face view of a dual overtone resonator according to the present invention; and Figs. 6a-i illustrate different embodiments of a dual overtone resonator according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION In the following description of the exemplary embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration the specific embodiment in which the invention may be practiced.

It is to be understood that other embodiments may be utilized as structural changes may be made without departing from the scope of the present invention.

The present invention provides a plate having a thinner and a thicker portion that causes the nodes and the antinodes in the two portions to align.

Fig. 5 illustrates a face view 500 of a dual overtone resonator according to the present invention. A first charge electrode area 502 has a thickness associated with a first overtone. A second cliarge electrode area 504 has a thickness associated with a higher overtone. In Fig. 5, the thickness of the first charge electrode area 502 is seven half wave lengths. The thickness of the second electrode charge area 504 has a thickness of five half wave lengths.

Thus according to the invention, a crystal oscillator structure may be formed having a thin area 504 and a thicker area 502 under the electrode plates. The thickness of the thicker portion 502 must be an even number of half wave lengths greater than that of the thinner section 504. The thinner section 504 must be aligned with the thicker section 502 such that the nodes and the antinodes in the two sections 502, 504 will be perfectly aligned. If the half wave length numbers in each section are prime, then the frequency spectrum of the resonator will be restricted to the lowest frequency that resonates simultaneously in both section and the odd numbered overtones of that frequency. In this case, both areas 502, 504 of the crystal will resonate. If the entire area between electrodes can resonate at a given frequency, establishment of resonance at this frequency is much easier than if only part of this area can resonate. Thus, for the case of three or more half wavelengths in the thinner area 504, the lowest resonance that is easily established is an overtone of the fUndamental frequency of the thinner section 504.

Figs. 6a-i illustrate different embodiments 600 of a dual overtone resonator according to the present invention. In Figs. 6a-i, the thicker portion 602 always has a thickness of seven half wave lengths. However, those skilled in the art will recognize that the invention is not limited to a dual overtone resonator having a thick portion of seven half wave lengths. Other embodiments may be developed which are consistent with the teaching of the invention, i.e., forming a crystal having two different thicknesses which result in the nodes and the antinodes in the two sections becoming perfectly aligned. Furthermore, those skilled in the art will recognize that the placement of thicker and thinner portions described herein below could be reversed.

Figs. 6a-b illustrate 7,5 dual overtone resonator 610. In Fig. 6a, the thinner portion 612 of the dual overtone resonator is formed by machining a flat circular hole 614 of two half wave lengths in a first face 604 of the plate resulting in a thinner portion 612 of five half wave lengths. The depth chosen in machining the circular hole 614 should be uniform over the area ofthe bottom. Fig. 6b illustrates a 7,5 dual overtone resonator wherein flat circular holes 616, 618 of one half wave length each are machined on both sides 604, 606 of the resonator. Those skilled in the art will recognize that the machining method is not meant to be limited to any method in particular. For example, machining is most often accomplished by masked chemical etching, but a laser or ion-beam machining method could also be used.

Figs. 6c-e illustrate 7,3 dual overtone resonator 630. In Fig. 6c, the thinner portion 632 of the dual overtone resonator is formed by machining a flat circular hole 634 of four half wave lengths in a first face 604 of the plate resulting in a thinner portion 632 of three half wave lengths. Fig. 6d illustrates a dual overtone resonator whercin a first hole 636 is machined on a first side 604 of the resonator to a depth of three half wave lengths. Then a second hole 638 is machined on the second side 606 of the resonator to a depth of one half wave length. Thus, the thinner portion 632 has a thickness of three half wave lengths.

Fig. 6e illustrates a dual overtone resonator wherein a first hole 640 is machined on a first side 604 of the resonator to a depth of two half wave lengths.

Then a second hole 642 is machined on the second side 606 of the crystal to a depth of two half wave lengths. Again, the thinner portion 636 has a thickness of three half wave lengths.

Figs. 6f-i illustrate 7,1 dual overtone resonators 650. In Fig. 6f, the thinner portion 652 of the dual overtone resonator is formed by machining a flat circular hole 654 of six half wave lengths in a first face 604 of the quartz plate resulting in a thinner portion 652 of one half wave length. Fig. 6g illustrates a dual overtone resonator wherein a first hole 656 is machined on a first side 604 of the resonator to a depth of five half wave lengths. Then a second hole 658 is machined on the second side 606 of the crystal to a depth of one half wave length. Thus, the thinner portion 652 has a thickness of one half wave lengths.

Fig. 6h illustrates a dual overtone resonator wherein a first hole 660 is machined on a first side 604 of the resonator to a depth of four half wave lengths.

Then a second hole 662 is machined on the second side 606 of the resonator to a depth of two half wave lengths. Again, the thinner portion 652 has a thickness of three half wave lengths.

Finally, Fig. 6i illustrates a dual overtone resonator wherein a first hole 664 is machined on a first side 604 of the resonator to a depth of three half wave lengths.

Then a second hole 666 is machined on the second side 606 of the resonator to a depth of three half wave lengths. Again, the thinner portion 652 has a thickness of three half wave lengths.

In summary, a resonator may be formed which has two or more sections with each having a different thickness and alignment with one another that results in the nodes and the antinodes in the two sections becoming perfectly aligned. The thickness of the thicker portion is an even number of half wave lengths greater than the thinner section. Further, if the half wave length numbers are both prime, the lowest possible frequency where perfect alignment occurs is a specific overtone of the thinner section and a higher specific overtone of the thicker section. There will also be a series of higher frequencies where perfect alignment occurs which are odd overtones of the lowest frequency. However, those skilled in the art will recognize that other embodiments having dimensions which have not been illustrated may be developed which are consistent with the teaching of the invention. Moreover, those skilled in the art will recognize that then invention is not meant to be limited to circular-shaped crystals. Other crystal shapes in accordance with the invention could be utilized, e.g., rectangular (AT strip).

The foregoing description of the exemplary embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not with this detailed description, but rather by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A resonator plate, comprising: a first section having a first thickness; and a second section having a second thickness, the first thickness and the second thickness being resonant at different overtones of a first frequency at a position of the second section with respect to the first section being selected to cause the nodes and the antinodes of the resonator to align when the resonator is stimulated.

2. The resonator plate of claim 1 wherein the first section has a thickness an even number of half wave lengths thicker than the second section.

3. The resonator plate of claim 2 wherein the first and second sections have a thickness equal to a prime number of half wave lengths.

4. The resonator plate of claim 2 wherein the alignment of the nodes and antinodes allows the first section and the second section to resonate at the same frequency.

5. The resonator plate of claim 1 wherein the first and second sections have a thickness equal to a prime number of half wave lengths.

6. The resonator plate of claim 1 wherein the alignment of the nodes and antinodes allows the first section and the second section to resonate at the same frequency.

7. The resonator plate of claim 1 wherein the first section has a thickness of seven half wave lengths.

8. The resonator plate of claim 7 wherein the second section has a thickness of five half wave lengths.

9. The resonator plate of claim 8 wherein the second section is formed by machining a flat circular hole of two half wave lengths in one face of a plate resulting in a thinner portion of five half wave lengths.

10. The resonator plate of claim 8 wherein the second section is formed by machining a flat circular hole of one half wave length on both sides of the blank.

11. The resonator plate of claim 7 wherein the second section has a thickness of three half wave lengths.

12. The resonator plate of claim 11 wherein the second section is formed by machining a flat circular hole of four half wave lengths in one face of the plate resulting in a thinner portion of three half wave lengths.

13. The resonator plate of claim 11 wherein the second section is formed by machining a first hole in a first side of the blank to a depth of three half wave lengths and machining a second hole in the second side of the blank to a depth of one half wave length.

14. The resonator plate of claim 11 wherein the second section is formed by machining a first hole in a first side of the blank to a depth of two half wave lengths and machining a second hole in the second side of the blank to a depth of two half wave lengths.

15. The resonator plate of claim 7 wherein the second section has a thickness of one half wave lengths.

16. The resonator plate of claim 15 wherein the second section is formed by machining a flat circular hole of six half wave lengths in one face of the plate resulting in a thinner portion of one half wave length.

17. The resonator plate of claim 15 wherein the second section is formed by machining a first hole in a first side of the blank to a depth of five half wave lengths and machining a second hole in the second side of the blank to a depth of one half wave length.

18. The resonator plate of claim 15 wherein the second section is formed by machining a first hole in a first side of the blank to a depth of four half wave lengths and machining a second hole in the second side of the blank to a depth of two half wave lengths.

19. The resonator plate of claim 15 wherein the second section is formed by machining a first hole in a first side of the blank to a depth of three half wave lengths and machining a second hole in the second side of the blank to a depth of three half wave lengths.

20. A resonator, comprising: a blank having piezoelectric properties, wherein the blank oscillates when stimulated; a plurality of electrodes, coupled to the blank, for providing a signal to the blank to cause the blank to become stimulated and oscillate at a fundamental frequency; a base for providing leads to the resonator; and mounting clips, coupled to the electrodes, for providing a signal path between the leads of the base and the electrodes and for holding the blank in position; wherein the blank further comprises a first section having a first thickness and a second section having a second thickness, the first thickness and the second thickness being resonant at different overtones of a first frequency at a position of the second section with respect to the first section being selected to cause the nodes and the antinodes of the blank to align when the blank is stimulated.

21. The resonator of claim 20 wherein the first section has a thickness an even number of half wave lengths thicker than the second section.

22. The resonator of claim 21 wherein the first and second sections have a thickness equal to a prime number of half wave lengths.

23. The resonator of claim 21 wherein the alignment of the nodes and antinodes allows the first section and the second section to resonate at the same frequency.

24. The resonator of claim 20 wherein the first and second sections have a thickness equal to a prime number of half wave lengths.

25. The resonator of claim 20 wherein the alignment of the nodes and antinodes allows the first section and the second section to resonate at the same frequency.

26. The resonator of claim 20 wherein the first section has a thickness of seven half wave lengths.

27. The resonator of claim 26 wherein the second section has a thickness of five half wave lengths.

28. The resonator of claim 27 wherein the second section is formed by machining a flat circular hole of two half wave lengths in one face of a plate resulting in a thinner portion of five half wave lengths.

29. The resonator of claim 27 wherein the second section is formed by machining a flat circular hole of one half wave length on both sides of the blank.

30. The resonator of claim 26 wherein the second section has a thickness of three half wave lengths.

31. The resonator of claim 30 wherein the second section is formed by machining a flat circular hole of four half wave lengths in one face of the plate resulting in a thinner portion of three half wave lengths.

32. The resonator of claim 30 wherein the second section is formed by machining a first hole in a first side of the blank to a depth of three half wave lengths and machining a second hole in the second side ofthe blank to a depth of one half wave length.

33. The resonator of claim 30 wherein the second section is formed by machining a first hole in a first side of the blank to a depth of two half wave lengths and machining a second hole in the second side of the blank to a depth of two half wave lengths.

34. The resonator of claim 26 wherein the second section has a thickness of one half wave lengths.

35. The resonator of claim 34 wherein the second section is formed by machining a flat circular hole of six half wave lengths in one face of the plate resulting in a thinner portion of one half wave length.

36. The resonator of claim 34 wherein the second ction is formed by machining a first hole in a first side of the blank to a depth o. . ire half wave lengths and machining a second hole in the second side of the blank to a depth of one half wave length.

37. The resonator of claim 34 wherein the second section is formed by machining a first hole in a first side of the blank to a depth of four half wave lengths and machining a second hole in the second side of the blank to a depth of two half wave lengths.

38. The resonator of claim 34 wherein the second section is formed by machining a first hole in a first side of the blank to a depth of three half wave lengths and machining a second hole in the second side of the blank to a depth of three half wave lengths.

39. A resonator substantially as herein described, with referee to Figures 5 and 6a-6i of the accompanyincj drawings.