US2284545A - Piezoelectric device and neutralizer - Google Patents

Description

May 26, 194 G. 1.. USSELMAN PIEZOELECTRIC DEVICE AND NEUTRALIZER Fi Ied Feb. 27, 1941 INVENTOR AT TORNEY Patented May 26, 1942 3 PIEZOELECTRIC DEVICE AND NEUTRALIZER George L. Usselman, Port Jefferson, N. Y., assign.-

or to Radio Corporation of America, a corporation of Delaware Application February 27, 1941, Serial No. 380,899

8 Claims.

This invention relates to piezo-electric devices and more particularly to a combination of a piezo-electric crystal holder and a neutralizing capacitor.

Heretofore it has been the practice, where crystal neutralization was required, to employ a separate neutralizing capacitor in circuit with the piezo-electric device and the other elements of an oscillation generator. Perfect neutralization of the piezo-electric crystal and its holder was very diiiicult, if not impossible, to obtain because balanced conditions were not present. It is a principal object of this invention to overcome the difficulties heretofore encountered in this respect.

The objects of my invention more specifically stated are as follows:

1. To provide a crystal holder having self-com tained crystal capacitance neutralization.

2. To provide an improved crystal holder and neutralizing capacitor in combination.

3. To provide a novel piezo-electric device having inherent means for neutralizing the crystal capacitance.

I am aware of the fact that devices have been constructed which make provision for the mounting of a plurality of piezo-electric crystals in one holder. Usually such devices are intended for use with crystals having different resonant characteristics. I am also aware of the fact that crystal holders have been constructed which make provision for the use of a plurality of electrodes certain of which are specifically crystal electrodes, while one or more others are used by way of compensation for certain capacitive effects. So far as I am aware, however, no one has utilized a non-resonant replica of a piezoelectric crystal as a neutralizing dielectric where the piezo-electric device and the neutralizing dielectric are symmetrically mounted in a crystal holder so as to obtain exact uniformity of capacitive conditions between the piezo-electric electrodes and electrodes used as a neutralizing capacitor. The advantages of such a symmetric arrangement have been found to be considerable. I will now, therefore, describe my invention in detail, reference being made to the accompanying drawing the sole figure of which shows schematically a preferred embodiment.

My improved combination of crystal, crystal holder and neutralizing capacitor is shown connected to a conventional push-pull oscillator circuit arrangement. The piezo-electric crystal 1 rests upon a bottom electrode 2 and beneath another electrode 3.

A second dielectric element 4 rests upon the electrode 3 and beneath a capacitive electrode 5. The element 4 is preferably made of non-piezo-electric quartz or other satisfactory dielectric material. The dielectric properties of this element'should be sufiiciently close to those of the piezo-electric crystal I so that these two elements may be made of substantially the same dimensions. The spacing between the electrodes 3 and 5 should also be as nearly as possible equivalent to the spacing between electrodes 2 and 3. Differences in such spacings would be required only when the dielectric constant of the element 4 differs materially from that of the piezo-electric crystal I.

Suitable air gaps over the piezo-electric crystal I and over the neutralizing dielectric element 4 may be obtained by the use of spacing elements 6 which are clamped between the metal electrode plates I, 3 and 5. These spacing elements 6 are preferably made of quartz or other material having an equivalent coefficient of expansion. The entire construction of the crystal holder is made with due regard for the desired symmetry of parts above and below the electrode 3. In this way the capacitance of the piezo-electric crystal and of the electrode plates 2 and 3 is completely balanced by the capacitance of the neutralizing element 4 and its electrodes 3 and 5.

The utility of my improved piezo-electric device and neutralizing capacitor is illustrated by the circuit arrangement shown in the diagram. A resonant tank circuit comprising inductance I and series capacitors 8 and 9 is shown constituting a part of an input circuit for two discharge tubes l0 and H. The electrode plates 2 and 5 are connected respectively at points along the inductance coil 1 symmetric with respect to its center. This coil has a mid-tap which is connected through a resistor l2 to an interconnection between the capacitors 8 and 9. The center-tap of the inductance winding 1 is also connected through a resistor I3 to ground and to the negative terminal of a direct current source which is provided for operating the tubes Ill and II.

The tubes l0 and H may be of the multi-grid type, or they may be simple triodes. As illustrated, however, the are constituted by tetrodes having control grids connected respectively to the terminals of the inductance coil 7. These tubes also possess directly heated cathodes, or, if desired, indirectly heated cathodes. The screen grids 15 are shown connected through resistor [6 to a suitable tap on the direct current source M. The screen grids are also capacitively coupled to ground or to the cathodes of the tubes It! and H, as is usual.

The anodes of the tubes I0 and l l are suitably fed with positive potential from the source I4. The connections for this purpose include a centertap on the primary Winding of a transformer 17. The secondary of this transformer is connected to any desired utilization device for receiving the output energy from the oscillator. A tap on the primary winding of the transformer I! is shown in circuit with a blocking capacitor l8 and the electrode 3 of the piezo-electric device. By means of this circuit suitable feed-back is obtained for driving the piezo-electric crystal.

This circuit arrangement is completely neutralized for every oscillation frequency except that at which the crystal I will vibrate. The inter-electrode capacitance of the discharge tubes is neutralized by the screen grids. If the crystal did not oscillate, the feed-back to the electrode 3 of the .piezo-electric device would supply equal energy to each side of the grid tank circuit 1, 8, 9. Therefore, no oscillations can build up in the tank circuit except those which are induced therein by the oscillatory state of the piezo-electric crystal.

It is a matter of precaution to arrange the crystal holder so that any unavoidable ground capacitance shall be suitably balanced. That is to say, the ground capacitance should have an equal eiTect upon the crystal I and upon the neutralizing dielectric element 4.

It is well within the scope of my invention to provide a piezo-electric device of the type generally indicated but with modifications such as 3 may be necessitated if other materials were to be used than quartz. For example, the substitution of tourmaline or Rochelle salts might be made. Furthermore, if it is found that a substance like glass should have dielectric properties sufficiently close to those of the quartz crystal, then' it ma be preferable to use glass for the neutralizing dielectric, since this material is generally not piezoelectric. Quartz plates can be made, however, which do not exhibit piezo-electric properties in any appreciable degree. In the cutting of'quartz plates for piezo-electric elements there is a large amount of material which must 'be rejected because it is unsuitable as an electro-mechanical vibrator. such otherwise-rejectable quartz.

The invention may, of course, be used with any desired oscillator circuit which requires crystal control. The scope of the invention therefore, commensurate with that of the claims.

I claim: 1. In combination, a discharge tube oscillator The element 4 might well be made from having input and output circuits, a frequency control device including a piezo-electric element and electrodes one of which is connected to said input circuit, means for causing energy to be fed 'back from said output circuit to another electrode 'of said piezo-electric device electrode.

3. The combination set forth in claim 1 in which the piezo-electric element and the nonresonant dielectric element are both composed of quartz.

4. A piezo-electric device having in combination therewith a neutralizing condenser, the combination comprising two outer electrode plates, a central electrode plate and two plates of quartz disposed on opposite sides of the central electrode plate and between the outer electrode plates, one of said quartz plates being piezo-electric and the other of said quartz plates being substantially non-resonant.

5. The combination according to claim 4 and including separators between the central electrode plate and each of the outer electrode plates, said separators having substantially the some coefficient of expansion as that of quartz.

6. In a device of the class described, a crystal holder comprising three parallel-disposed electrode plates and means for maintaining suitable separation between said plates, a piezo-electric element disposed between two of said plates, and a capacitive neutralizing element of non-resonant properties disposed between the central electrode plate and the third plate, the last said element having substantially the same dielectric constant as that of the piezo-electric element.

7. The combination according to claim '6 and including an electronic oscillator the circuit arrangement of which comprises a'resonant input circuit and connections therefrom to the outer electrode plates of said crystal holder.

8. The combination according to claimfi and including an electronic oscillator the circuit arrangement of which comprises a resonant output circuit and feed-back means coupling a suitable point on said output circuit to the central one of said electrode plates in said crystal holder.

GEORGE L. USSELMAN.

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