US2362056A - Piezoelectric crystal - Google Patents

Description

1944- w. F. DREws ET AL I 2,362,056

PIEZOELECTRIC CRYSTAL Filed March 9, 1943 INVENTORS W. FDREWS A. f. SW/c/rARD 1x4 1 M ATTOR/Vf) Patented Nov. 7, 1944 UNlTElD STATES PATENT OFFICE' PIEZOELECTRIC CRYSTAL William F. Drews, River Forest, and Andrew E. Swickard, Chicago, 111., assignors to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York 1 Application March 9, 1043, Serial No. 478,504

(01. TIL-327) 7 Claims.

This invention relates to piezoelectric crystals, and particularly to piezoelectric crystals having a face shear mode of vibration.

An object of the present invention is to provide an efficient and effective piezoelectric crystal having a face shear mode of vibration.

In accordance with one embodiment of this invention, piezoelectric crystals having a face shear mode of vibration and a frequency in the range between 370,000 cycles and 473,000 cycles, are cut to a thickness between .0160" and .0175, while similar crystals in the frequency range above 473,000 cycles are cut to a thickness between .0192" and .0205".

Other objects and advantages of the present invention will be apparent from the following detailed description taken in conjunction with the drawing, wherein:

Fig. 1 is a diagrammatic view showing the relative positions with respect to its electrical, mechanical and optical axes of a piezoelectric crystal having a face shear mode of vibration, and

Fig. 2 is a perspective view of a mounted crystal.

In the manufacture of certain types of electronic communication equipment, it is sometimes the practice to employ piezoelectric crystals as frequency control devices. In certain cases, where it is desirable to have a strongly driven crystal having a low frequency temperature coefllcient, it is sometimes the practice to employ crystals having a face shear mode of vibration. A face shear crystal having a low "frequency temperature eflicient may be obtained from a block of quartz by cutting the crystal at a suitable angle to the optical axis to obtain a crystal plate, as shown in Fig. 1, having its width axis along an electrical axis a: and its major plane and opposite electrode or major surface parallel to an electrical axis and inclined approximately 38 with respect to th optical axis 2 to obtain a substantially zero temperature coeflicient of frequency when vibrated at a frequency determined by the large dimensions Z and w. The frequency of such a crystal is a function of its length, width, density and the elastic constant of the crystal material, that is, the quartz. This relation may be expressed by the formula:

1.25 1 f lx+lz pS55 wherein 1 refers to the frequency in cycles per second, 11: refers to the length in the 2: axis direction and is substantially equal to the length in the z axis direction indicated by la and 9 refers to the density and 8'55 is the elastic constant.

Since such a crystal is designed to have a face shear mode of vibration, it has been thought that the thickness of such a crystal need not be considered and, thus, the precise thickness used has been selected in many cases primarily with a view to two factors; on the one hand, economy of cutting and on the other hand, the physical strength of the crystal plate. In general, the procedure was to make the crystal as thin as practicable in order to conserve materials and, in accordance with one practice, crystals in the frequency range from 370 kc. to 472 kc. were cut to have a thickness on the order of .0192". Occasionally some crystals in this range, dimensioned as stated, exhibited certain defective characteristics, particularly low activity, frequency shift and frequency drift. In some cases these defects could be overcome by individual adjustments in the crystal mountings. However, where the crystals were to be manufactured on a large scale, such individual adjustments were not feassible even where they would be successful in remedying the conditions.

An understanding of the adjustments that were made will be more apparent upon referring to Fig, 2 wherein a quartz crystal l 0 of substantially rectangular parallelepiped shape is illustrated. IAS is sometimes the practice with crystals having a face shear mode of vibration, lead wires H are attached to electrodes 12, comprising thin metallic coatings on either side of the crystal, by solder cones [3 which are positioned substantially at the nodal point of the crystal. Each of the outer ends of the lead wires are mounted on supporting spring wires l 5 by a solder ball l6 and the springs ii are, in turn, soldered to terminals I! supported on a base It. This type of mounting is described in the co-pending application of R. E. Brooks, Serial No. 471,884, filed January 9, 1943. Such adjustments as have been made to compensate for defects in the crystals, as noted, comprise, in the main, moving the solder balls I 6 with respect to the solder cones I3, the theory being that the mode or standing wave pattern of vibration in the lead wires interferes with the vibration of the crystal and that by moving the solder balls, the mode of vibration of the lead wires could be altered,

In some cases. such adjustments were successful in eliminating the defects. However, in other cases, adjustment of the relative position of the solder balls and the solder cones has no apparent N beneficial effect on the crystals. Other adjustments were made to cure the defects noted, such as increasing the thickness of the lead wires and varying the size of the solder cones and solder balls. However, substantially no improvement was obtained.

As aforesaid, the thickness of face shear mode crystals heretofore has been selected only with a view to obtaining the maximum economy of production without sacrificing p ysical strength. However, it has been discovered that th operating characteristics of a face shear mode crystal, and particularly the activity of the crystal, the tendency of the crystal to shift frequency and the tendency of the crystal to drift in frequency, are directly affected by the thickness dimension of the crystal. It has been further discovered that by cutting the crystal within a certain range of thickness for a given range of frequencies, substantially improved operating characteristics are obtained over similar face shear mode crystals having the same range of frequencies but cut to a larger or smaller thickness dimension.

When a face shear mode crystal vibrates, corresponding vibrations are caused in the supporting lead wires. The crystal lead wire and solder ball system i designed so that the mechanical impedance of the lead wire is small when viewed from the crystal and, thus, has no appreciable effect upon the crystal vibration. However. if the motion from one lead wire is transmitted through the crystal at certain frequencies, and imparts motion to the other lead wire, the entire lead wire vibrational system may be disturbed, and the impedance of the lead wires may be high and thus impair the crystal activity. By adjusting the crystal plate thickness, it appears, coupling through the crystal is reduced to a minimum, and the lead Wires vibrate as separate systems.

In accordance with the present invention, to avoid this adverse effect on the operating characteristics of face shear mode crystals, crystals in the range of frequencies between 370 kc. and 473 kc. are cut to a thickness between .0160" and .0175". The relationship between the thickness dimension and the length and width dimensions of the crystals may be expressed by the formula:

in which equals .l38X10 1 equals the frequency in cycles per second and w equals the width in inches. For the range of frequencies between 473 kc. and up to approximately 540 kc. a similar relationship obtains. In accordance with the present invention, such crystals are dimensioned to a thickness between .0192" and .0215". The relationship here may be expressed by the same formulasubstituting, however, the value 159x10 for the value of e in the equation. The crystals are then mounted on spring supports, as shown in Fig. 2 of the drawing.

In practice, it has been found that in the range of frequencies below 425 kc. somewhat thicker crystals may be employed than in the range above this frequency without encountering undesirable effects. Thus, crystals in the range of frequencies from 370 kc. to 425 kc. may be cut to a thickness on the order of .016" to .0199", whereas crystals in the range of frequencies from 425 kc. to 473 kc. may be cut to a thickness on the order of .0155" to .0170". It would seem likely that the relationship expressed for the two frequency ranges noted would recur, alternating at higher and lower frequencies. That is to say, in the range of frequencies obtainable from face shear mode crystals, the relative thickness dimension of such crystals would, as the frequency increased, be alternately large and then small.

What is claimed is:

l. A piezoelectric crystal having a face shear mode of vibration of a frequency in the range from 370 kc. to 473 kc. and a thickness dimension on the order of .0160" to .0175".

2. A piezoelectric crystal having a face shear mode of vibration of a frequency on the order of 473 kc. to 540 kc. and a thickness dimension on the order of .0192" to .0215".

3. A piezoelectric crystal havin a face shear mode of vibration of a frequency on the order of 370 kc. to 540 kc. whose thickness-frequency relation is T=4 fw where e equals .138x10-' for frequencies between 370 kc. and 473 kc. and .159 10 for frequencies between 473 kc. and 540 kc.

4. A piezoelectric crystal having a face shear mode of vibration of a frequency on the order of 370 kc. to 473 kc. whose thickness-frequency relatlon is T=lw where e equals .138 10.

5. A piezoelectric crystal having a face shear mode of vibration of a frequency on the order of 473 kc. to 540 kc. whose thickness-frequency relation is T=fw where e equals .159X10-.

6. A piezoelectric crystal having a face shear mode of vibration of a frequency in the range from 370 kc. to 425 kc. and a thickness dimension on the order of .016" to .0199".

7. A piezoelectric crystal having a face shear mode of vibration of a frequency in the range from 425 kc. to 473 kc. and a thickness dimension on the order of .0155" to .0170".

WILLIAM F. DREWS. ANDREW E. BWICKARD.

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