US2486916A - Piezoelectric crystal - Google Patents

Description

NW0 11, 194% v. E. BOTTOM PIEZOELECTRIC CRYSTAL Filed Dec. 22, 1947' j I; E] ii; El

26 25 L zr ,NVENTOR Patented Nov. 1, 1949 UNITED STATES PATENT OFFICE PIEZOELECTRIC CRYSTAL Virgil E. Bottom, Fort Collins, Colo.

Application December 22, 1947, Serial No. 793,171

6 Claims. (01. 171-327) This invention relates to piezoelectric crystals, and more particularly to piezoelectric plates in which the frequency of vibration is a function of the thickness of the plate.

Thickness-mode crystals, in spite of great care in cutting and finishing, usually will be found to vibrate at a plurality of spurious or secondary frequencies in addition to the primary or fundamental frequency for which they are designed. The vibration spectra of thin plates of solid material are very complicated. Numerous attempts have been made to describe the behavior analytically. However, the problem of the vibration of an anisotropic plate with finite boundaries has yet to be completely solved. The experimental situation is different. It is quite easily possible to determine the overtone series of a quartz plate, the aggregate of such a series we call its spectrum. The various responses are not related to harmonies of contour modes, neither are they harmonies of longitudinal modes, nor due to coupling between the desired mode and some other normal mode of vibration. They are the inharmonic overtones associated with the normal vibration of every thin plate.

These overtone modes cause relatively little trouble when the plate is used as the controlling element of an oscillator. It is when the plates are used as a filter element that their most serious effects become apparent. Here the overtone modes result in passing other bands than the desired one, thereby nullifying the usefulness of the filter.

Previous attempts havebeen made to eliminate overtone modes by various means. One common method is known as dimensioning. By this method the dimensions of the quartz plate are so adjusted that the frequency of no harmonic of a contour or longitudinal mode falls near that of the required mode. This method quite effectively eliminates such modes but has no effect on those modes with which the present method is concerned.

Other experimenters have attempted to simplify the overtone spectrum by figuring or contouring the surface of the quartz plate, but no practi-- cal use has ever been made of such methods, either because of the difficulty of producing such surfaces, or their ineffectiveness in eliminating the overtone modes, or both. In no case is it possible to completely eliminate the overtone modes in a high frequency plate by any known contour, but only to displace them so that their frequencies are out of the range wherein they are troublesome.

Because of their low temperature coefficients of frequency, AT and BT out plates are most commonly used for the control of frequencies between one and ten megacycles/sec. But such plates have not been used in units designed to operate as elements in filter circuits because of the complex overtone spectrum exhibited by such plates. Furthermore, unless the lateral dimensions are very carefully adjusted, such plates often fail to operate at one or more temperatures, due to other overtone modes of vibration which couple to the fundamental mode at those particular temperatures.

The principal object of this invention is to provide thickness-mode piezoelectric crystals or plates which will be sufiiciently free of the objectionable inharmonic overtone modes above described to be useful for high frequency electrical wave filters, and to provide a practical method for producing such crystals which will be easily adaptable for large scale and economical manufacture.

Other objects and advantages reside in the detail construction of the invention, which is designed for simplicity, economy, and efficiency. These will become more apparent from the following description.

In the following detailed description of the invention, reference is had to the accompanying drawing which forms a part hereof. Like numerals refer to like parts in all views of the drawing and throughout the description.

In the drawings:

Fig. 1 illustrates a piezoelectric plate ground and lapped to the desired uniform thickness;

Fig. 2 illustrates a circular plate cut from the plate of Fig. 1;

Fig. 3 is an edge view of the plate of Fig. 2;

Figs. 4 and 5 illustrate steps in producing the improved piezoelectric plate;

Fig. 6 is an edge view of the most eflicient form of the improved plate;

Figs. '7, 8, 9, and 10 illustrate edge views of alternate forms of the improved plate; and

Fig. 11 is a cross-section through a crystal plate vibrating in a longitudinal thickness-mode.

In all views the thickness dimensions of the plates have been greatly magnified for the purposes of illustration.

Briefly, the preferred improved piezoelectric plate is a circular quartz plate vibrating in thickness shear with both of its major surfaces convex, and with its exact thickness uniformly distributed around its peripheral edge.

One form of the plate is indicated in Fig. 6, in

which the thin uniform edge is indicated at I4 lying on a medial plane between two spherical, convex, major faces I5.

A second form of plate is indicated in Fig. having an edge portion lying in a medial plane between two conical, convex, major faces 26.

The forms of Figs. 6 and 10 are best for AT, BT, or any thickness shear plates intended for filter applications. The form of Fig. 6 is the preferred form, as it presents less difficulties from a manufacturing standpoint. However, the two forms appear to be equal in efiiciency for their intended use.

With the plates of Figs. 6 and 10 it is possible to remove the nearest overtone mode from the primary or desired mode by ten per cent or more of the frequency of the main mode, which is more than suflicient for filter circuit use.

Figs. 8 and 9 illustrate what will be herein designated as frustra plates. Fig. 9 illustrates a plate having a medial thin edge portion I9 and two similar major surfaces, each consisting of an annular, spherical surface 2| arising to a fiat surface 20, the two surfaces 20 being parallel.

Fig. 9 illustrates a somewhat similar plate having a relatively thin peripheral edge 22 and two opposite, parallel plane surfaces .23. In this plate, however, the edge 22 and the surfaces 23 are separated by annular, conical surfaces 24.

The frustra plates of Figs. 8 and 9 donot remove the overtone modes as far from the primary mode as the plates of Figs. 6 and 10, but they are a great improvement over the present filter and oscillator plates, and will produce a clear primary frequency over a range of approximately five per cent of the frequency of the fundamental mode.

In other words, as the areas of the center fiat plane surfaces 20 and 23 are reduced, the value of the equivalent series capacitance is reduced, and the equivalent resistance is increased. The precise nature of the contour depends upon the purpose for which the unit is intended, the frequency of the unit and the diameter of the plate.

The frequency of the plate is determined by its thickness at the center, but the overtone spectrum is determined by the contour of the two principal surfaces. For example, to produce filter plates in the frequency range between four thousand and five thousand kilocycles per second, using AT plates having a diameter of 0.500 inch, the edge of the plate should be about 0.010 inch thinner than the center of the plate.

One method of producing the improved contoured plates is as follows:

A fiat section of quartz crystal vibrating in thickness shear, such as indicated at I0 in Fig. 1, is ground and lapped to the required frequency thickness on conventional lapping machines. A section of the flat plate I0 is then cut or turned out to form a circular disc I I, as shown in Fig. 2. A magnified side view of the disc II is shown in Fig. 3, and it will be noted that it has two plane, parallel major surfaces. The disc I I is then placed in a receiving socket in a lapping tool I2 and held against a concave lap I3.

The tool I2 and the lap I3 may be of the usual types employed for grinding optical lenses, and the abrasives employed may be the usual lens grinding abrasives and rouges. Optical laps having curvatures of from one to five diopters have been used with excellent results. The selection of the lap depends upon the frequency of the plate and the diameter of the plate. In other words, a lap should be selected to produce a difference of 0.005 inch rise at the center of the plate over the edge thereof. After perfect lapping, the disc II is removed from the tool I2 and reversed. to place its lapped face upwardly in the tool l2. The opposite surface is then lapped against the lap I3, as shown in Fig. 5, to produce a plate having the contour illustrated in Fig. 6.

To produce a contour such as illustrated in Fig. 10 a conical lap is employed, and to produce 'contours as shown in Figs. 8 and 9, either a larger diameter disc I I is used so that the edge thickness will be reached before the central planes have been lapped, or a lap with a greater curvature or a greater angle of cone is employed.

If the plate is intended simply for the purpose of improving the quality of an oscillator without the necessity of controlling the overtone spectrum for filter purposes, a less drastic contour is adequate.

A plate for this purpose is illustrated in Fig. '7 consisting of a flat plate having two fiat, parallel major surfaces ll beveled around their peripheries, as shown at I 8, to form a relatively thin peripheral edge I6. Such a procedure eliminates the necessity for accurately dimensioning the major surfaces, reduces the erratic fluctuations which result from changes of temperature, and eliminates the adverse clamping effects usually encountered in oscillator plates.

To accomplish the desired results, the plate must be contoured as described uniformly on both faces. Experiments with plates having one face optically flat and the other convexly contoured did not produce the desired results, evidently due to the inherent harmonics of the remaining flat surface.

The above-described method is applicable to any quartz plate vibrating in thickness shear, to which this invention is more specifically directed. For plates vibrating in a longitudinal thickness-mode, the lapping must be reversed, that is, a convex lap is employed, producing two similar concave major surfaces 28 on the crystal, as shown at 21 in Fig. 11. In such a case the primary frequency is determined by the edge thickness, and the overtone spectrum is determined by the contour of the surface.

The same procedures may be applied to square or rectangular discs. Circular discs, however, are best because their spectra are inherently simpler, and a circular shape is more easily handled and mounted. Plates constructed according to the above process have been found to eliminate the troublesome overtone modes usually encountered in thickness-mode crystals.

Piezoelectric plates formed as shown in Figs. 6 and 10 have no overtone modes within a frequency range of plus and minus 10% of the frequency of the fundamental mode, whereas crystal units fabricated according to the techniques in common use at the present time have from five to twenty modes in a frequency interval of plus and minus 1% of the fundamental mode.

Aside from the benefits derived directly by the elimination of the overtone modes by this improved method of contouring, there are still other benefits which result therefrom. The contoured plate vibrates in a manner much simpler than does a fiat plate. This results in much smaller stresses and strains being produced in the plate as it vibrates. Consequently, for a given area, the contoured plate will handle larger currents than will the flat plate, thus enabling it to control more powerful oscillators.

Another benefit derived from this improved technique of contouring is that the plates are more easily mounted. There are two methods of mounting the vibrating quartz plates. In one the quartz plate is clamped between fixed electrodes which clamp the plate at three or more points around the circumference. The other method is one in which the electrodes are electroplated or otherwise deposited directly on the surface of the quartz plate, which is then supported on wires which are attached to the quartz plate. The first method is called pressure mounting, the second wire mounting. In either method, the mounting is quite critical with fiat plates, due to the nature of the vibration of the plate. However, with these improved contoured plates the periphery is a nodal line, i. e., it does not partake of the vibration of the plate. Therefore, the plate may be supported by its edges in any manner whatsoever without affecting its vibrating characteristics.

With the use of the improved plates, the representation of the crystal unit by the customary equivalent circuit is quite exact for either high or low frequency units, whereas high frequency plates fabricated by the customary techniques cannot usually be accurately represented by the simple equivalent circuit because of the overtone modes which are nearby and coupled to the fundamental mode. This fact greatly simplifies the design of filter and oscillator circuits utilizing quartz crystal units.

While a specific form of the improvement has been described and illustrated herein, it is desired to be understood that the same may be varied, within the scope of the appended claims, without departing from the spirit of the invention.

Having thus described the invention, what is claimed and desired secured by Letters Patent A piezoelectric filter element for exhibiting a substantially single mode of vibration within a frequency range of from 1 to megacycles per second, comprising: a circular quartz blank; two opposed major electrode surfaces on said blank forming a convexo-convex contour, with the greatest thickness at the center, and with the least thickness around the periphery.

2. A piezoelectric filter element for exhibiting a substantially single mode of vibration within a frequency range of from 1 to 10 megacycles per second, comprising: a circular quartz blank; two opposed major electrode surfaces on said blank forming a double-convex contour with the greatest thickness at the center, and with the least thickness around its periphery, both surfaces being exact duplicates, and" the edge being of uniform thickness throughout its periphery.

3. A piezoelectric filter plate in which the frequency of vibration is a function of the thickness of the plate, comprising: a circular disc of piezoelectric crystal; two opposite, uniform spherical convex faces on said disc; and a pcripheral edge portion of uniform thickness surrounding said disc the thickness at the center of the plate being such as to have a fundamental vibration of from 1 to 10 megacycles per second.

4. A piezoelectric filter plate in which the frequency of vibration is a function of the thickness of the plate, comprising: a circular disc of piezoelectric crystal; two opposite, uniform convex faces on said disc; and a relatively thinner peripheral edge portion of uniform thickness surrounding said disc, the center portion of said disc having a thickness to exhibit a fundamental thickness-mode vibration of from 1 to 10 megacycles per second, the peripheral edge of said disc having a thickness of from .002 to .010 inch less than said center portion.

5. A piezoelectric filter plate in which the frequency of vibration is a function of the thickness of the plate, comprising: a circular quartz disc having a diameter of substantially 0.500 inch; two convex major surfaces on said plate, said surfaces being convex and corresponding in contour, the thickness at the center exhibitin a fundamental frequency of vibration of from 1 to 10 megacycles per second, the thickness at the peripheral edge being approximately 0.010 inch thinner than the center of the plate, the edge portion lying in a plane positioned exactly in the middle between the high points of the two faces.

6. A piezoelectric filter plate in which the frequency of vibration is a function of the thickness of the plate, comprising: a circular disc of piezoelectric crystal; two opposite, uniform, spherical, convex faces on said disc; and a peripheral edge portion of uniform thickness surrounding said disc, the thickness at the center of the plate being such as to have a fundamental vibration of from 1 to 10 megacycles per second, the thickness at the peripheral edge portion being such as will exhibit a nodal vibration to the fundamental vibration of the extreme thickness.

VIRGIL E. BOTTOM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,076,012 Allsopp Oct. 21, 1913 2,149,796 Hawk May 23, 1939 2,371,303 Liebowitz Mar. 13, 1945 2,375,003 Kent May 1, 1945 FOREIGN PATENTS Number Country Date 501,121 Great Britain Feb. 21, 1939 OTHER REFERENCES Ser. No. 288,502, Bechmann (A. P. 0.), pub. June 22, 1943.

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