US1957063A - Piezo-electric crystal apparatus - Google Patents

Description

May 1, 1934- c. w. HANSELL PIEZO ELECTRIC CRYSTAL APPARATUS Filed Jan. 23, 1932 OUTPUT INVENTOR CLARENC W. HANSELL BY WVM/ ATTORNEY Patented May 1, 1934 UNITED STATES PATENT OFFICE Clarence W. Hansell, Rocky Point, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application January 23, 1932, Serial No. 588,255

8 Claims.

This invention relates to piezo-electric crystal apparatus and has especial reference to the electrode structure thereof.

It is well known in the art that the free vibra- 5 tions of a piezo-electric crystal element set up strains and stresses which are substantially proportional to the voltages applied thereto. These stresses in the crystal element are frequently unsymmetrical due to the non-uniformity of the 0 thickness of the element and the presence of different electro-motivc forces on adjacent portions of the crystal. This unsymmetry, it has been found, is an undesirable characteristic which should be avoided for best results, particularly so 5 in systems wherein a plurality of electrodes are utilized on the same side of the crystal. In this case slight differences in the thickness at various points on a crystal, and varying piezo-electric properties of the material, tend to cause consid- 0 erable variation in relative voltages set up on the electrodes. Accordingly, it is a desideratum that the electro-motive forces of each electrode be applied to the crystal over as wide an area as possible and that the areas used be variable in order that more uniform voltage ratios may be obtained with various individual crystals used with the same holder and electrodes or with holders and electrodes of the same design.

This is accomplished in the present invention by constructing the electrodes adjacent one face of the crystal element so that each electrode covers rather Widely separated areas and each is unsymmetrical with respect to any one plane passing through the crystal. This unsymmetry of the electrodes is an important characteristic of the present invention. In this manner, the respective electrodes affect simultaneously widely separated points of the crystal and aid in securing a uniform piezo-electric coupling effect between the electrodes.

A feature of this invention is the rotatable electrode mounting employed for changing the relative areas of the crystal facing the electrodes. By means of this feature one may obtain a degree of adjustment by the use of unsymmetrical electrodes which is not possible of obtainment, so far as applicant is aware, by any other type of electrode structure.

This invention is described in more detail in 0 the following description which is accompanied by drawing wherein:

Figure 1 illustrates a plan view of one particular embodiment of the present invention, showing a two-electrode structure adapted for use on 5 one side of a piezo-electric crystal element.

Figure 1A is a cross-sectional View of the electrode assemblage of Figure 1 along the line A-A.

Figures 2 and 3 are plan views of other embodiments of the present invention.

Figures 4, 5 and 6 show mountings for the crystal element which may be used with the structure of Figures 1, 2 and 3.

Figure 7 shows diagrammatically a circuit arrangement wherein there is employed a piezoelectric crystal structure embodying the principles of the present invention.

In Figure 1 is shown an electrode mounting comprising two spirally wound, metallic electrodes 1 and 2 mounted on a suitable insulating disc 3 such as bakelite. These electrodes may be attached to the insulating disc 3 either by screws or by being molded therein. The surfaces of the two electrodes 1 and 2 differ in area at different portions thereof in the manner indicated in the drawing and are arranged to be unsymmetrical with respect to any plane passing through either the mounting or the crystal. In this manner each electrode presents a surface which faces widely separated portions of the crystal so that closely spaced points of the crystal at different parts thereof face different electrodes. It will thus be apparent that any rotation or movement either of the electrode mounting or the crystal will change the relative areas of the crystal facing the two spiral electrodes.

Figure 1A is a cross-sectional view of the electrode assemblage of Figure 1 along the line A--A and shows electrodes 1 and 2 mounted on insulating disc 3. Obviously, it is within the scope of the present invention to so arrange the electrodes that their top surfaces are level with the surface of the disc.

Figures 2 and 3 are modifications of the arrangement of Figure 1 and disclose other electrode structures embodying the principles of this invention. The electrodes of these figures are also arranged to be unsymmetrical with respect to any one plane. Although only two electrodes are shown in these figures it will be understood that, if desired, other electrodes may be mounted on the insulating disc in a similar manner.

Figures 4, 5 and 6 illustrate one type of lower electrode assemblage which may be used with the arrangements of Figures 1 to 3. The crystal element 4 shown in these figures is mounted on metallic electrode 6 of the solid circular type which, in turn, is mounted on an insulating plate 8. Figure 4 discloses one way of maintaining crystal element 4 in position. In this drawing members 7, 7 firmly secure crystal element 4 to the electrode assemblage. These members may be attached to lower electrode 6 in any desired manner. Other positioning means such as brackets 5 shown in Figure 5 or pins 23 shown in Figure 6 may advantageously be employed. Brackets 5 are slidably movable in grooves shown on the plates and are held fixed to the lower electrode by means of screws. It should be understood, of course, that Figures 4, 5 and 6 shown herein are merely illustrative of any lower electrode assemblage which may be employed and form no part of the present invention.

In Figure 7 is shown an oscillation generator circuit of the type described claimed and shown in Fig. 3 of the drawing in an application, Serial No. 531,684, filed by J. L. Finch, April 21, 1931, wherein this invention may be employed. In this circuit there is illustrated a vacuum tube oscillation generator 10 having the usual anode electrode 11, grid 12 and filament 13 provided with the usual filament current supply source 14 and anode voltage supply source 15. Condenser 16 is provided to shunt the radio frequency currents in the anode circuit around the voltage source 15. Associated with anode 11 of the generator 10 is a plate impedance 17 connected to the out put circuit. To this impedance is connected lead 19 which connects with spiral electrode 2 of the piezo-electric crystal apparatus. The grid 12 of the generator 10 is shown connected to the other spiral electrode 1 of the apparatus. Both electrodes l and 2 are of the unsymmetrical type and are mounted on rotatable insulating disc 3. A portion of this disc and upper electrode as semblage has been shown cut away in order to provide a semi-sectional perspective view of the entire crystal apparatus. The upper electrode assemblage, including insulating disc 3, is supported on a ring 9 which provides proper spacing between crystal 4, which is attached to lower electrode 6 by means of pins 20, and upper electrodes 1 and 2. It is to be understood that the spacing between the crystal element 4 and the upper electrodes shown in the drawing is greatly exaggerated and is merely for the purpose of illustration since, in actual practice, the spacing would usually be from approximately one to three thousandths of an inch.

The operation of this circuit is as follows:

Voltage waves impressed on grid 12 of the tube 10 are amplified in the tube and are, in turn, impressed upon the resistor circuit 17 by anode 11. Energy is fed from the output circuit 19 and impressed on the electrode 2, setting up a voltage between electrodes 2 and 6. If this energy is periodic and has components corresponding in frequency to some mode of mechanical vibration of the crystal member 4, it will set up oscillations in the crystal. Due to these oscillations and also due to the piezo-electric properties of the crystal, voltages of this same frequency will be impressed upon electrodes 1 and 6. These voltages are introduced in the input circuit 18 which is arranged to impress voltage waves upon the grid 12 identical with those originally assumed. Thus, this circuit will sustain oscillations and, if other conditions are correct, will build up oscillations, from an initial non-oscillating state, at the frequency at or close to the resonant frequency of the crystal. The output of this generator may be taken off by lead 21.

In this circuit one may obtain optimum operation of the circuit by moving the upper electrode assemblage of the crystal control apparatus through rotatable member 3 to obtain the desired relative voltages on the primary and secondary electrodes 1 and 2 for building up and sustaining oscillations in the circuit. Consequently, fewer crystals must be rejected and the uniformity of operating adjustments obtainable helps to make the frequency of the oscillations less subject to influences tending to change the frequency.

It should be noted that, in the circuit of Figure 7, the piezo-electric crystal and its electrodes really constitute a filter between the anode and grid circuits which passes an extremely narrow band of frequencies and it is this filter action which prevents production of oscillations at a frequency much different from the natural period of the crystal. This arrangement of variable unsymmetrical electrodes really serves to make variable the effective characteristic impedance inside the crystal filter and its effective relative terminal impedances at its input and output ends.

It is to be understood that this invention is not limited to the precise arrangement of parts illustrated and described herein, but that many modifications of the crystal mounting and holder are possible without departing from the spirit and scope of the invention. For example, if desired, an arrangement similar to Figures 1, 2 and 3 may be utilized for a lower electrode assemblage. Also, other configurations of the electrodes may be resorted to in order to give the desired results, provided, of course, that the electrodes observe the important characteristic of being unsymmetrical with respect to any one plane.

What is claimed is:

1. An electrode mounting for a piezo-electric crystal, comprising an insulating member, a plurality of electrodes positioned on the same side of said member and arranged unsymmetrically with respect to any one transverse plane passing through said crystal.

2. An electrode mounting for piezo-electric crystal apparatus, comprising a plurality of spirally wound electrodes afiixed to an insulating plate and positioned on one side of said crystal, said electrodes being arranged unsymmetrically with respect to any one transverse plane passing through said crystal structure.

3. In piezo-electric crystal apparatus, a piezoelectric crystal, a plurality of electrodes in electrical relation with said crystal, at least two of said electrodes being on one side of said crystal and arranged unsymmetrically with respect to any transverse plane passing through said crystal.

4. A mounting for a piezo-electric element, comprising, in combination, a piezo-electric crystal, two spirally wound metallic electrodes spacially positioned on the same side of said element and attached to an insulating member, said member being rotatably movable with respect to said crystal.

5. In combination, in a piezo-electric crystal holder, an insulating electrode support, two E shaped metallic. electrodes mounted on a single insulating element and separated from each other, the projecting portions of each electrode being arranged to intermesh with corresponding portions of the other electrode.

6. A piezo-electric crystal holder comprising in combination a mounting for a piezo-electric crystal element comprising, a crystal, a plurality of unsymmetrically arranged electrodes positioned on the one side of the crystal, said electrodes being rotatable with respect to said crystal.

7. A piezo-electric crystal controlled oscillator electrode arranged to retain a piezo-electric crystal, said upper insulating member having a plurality of unsymmetrically arranged electrodes retained within the inside of said upper insulating member and adapted for rotation with respect to said lower member.

CLARENCE W. HANSELL.

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