US1858339A - Piezo electric oscillator - Google Patents

Description

May 17, 1932.. R. s. OHL

PIEZO ELECTRIC OSCILLATOR Filed June 24, 1926 I z z I E Frequenc Patented May 17, 1932 UNITED STATES PATENT OFFICE RUSSELL S. OHL, OF NEW YORK, N. Y., ASSIGNOR TO AMERICAN TELEPHONE AND TELE GRAPH COMPANY, A CORPORATION OF NEW YORK P IEZO ELECTRIC OSCILLATOR Application filed June 24,

This invention relates to piezo-electric devices, and particularly to arrangements in oscillation generators employing piezo-electric devices for controlling the frequency thereof, so that the oscillation generators may sustain oscillations at any one of a plurality of frequencies, each characteristic of a piezoelectric device. i

A substance, such as quartz or other device of piezo-electric character, can be made to vibrate at any one'of a plurality of frequencies. Every plate of quartz, for example, located between two metallic electrodes, can be made to sustain Vibrations of, at least, two frequencies and ver frequently of three frequencies, or more. enerally, when the piezoelectric plate vibrates transversely, i. e., along a small dimension, it vibrates at a high frequency. When it vibrates longitudinally, i. e., along a longer dimension, it vibrates at a lower frequency. It has been found by experiment that when a piezo-electric device is arranged to vibrate at a very high frequency, i. e., when the metallic electrodes are separated by a small dimension of the crystal, it can be made to vibrate at any one of a plurality of frequencies, preferably natural frequencies, by properly'choosing reactances in a vacuum tube circuit with which the piezoelectric device may be associated. This will be described in greater detail hereinafter.

It is one of the objects of this invention to provide arrangements whereby a piezo-electric device may be made to vibrate at any one of a plurality of different frequencies.

It is another object of this invention to so relate a piezo-electric device to an oscillation generator that the oscillations of said oscillation generator may be sustained by said piezo-electric device at any one of a plurality of different frequencies.

While the invention will be pointed out with particularity in the appended claims, the invention itself, both as to its further ob jects and features will be better understood from the detailed description hereinafter following when read in connection with the accompanying drawings, in which Figure 1 represents a characteristic reactance curve for a piezo-electric crystal capable of vibrat- 1926. Serial No. 118,350.

ing at a plurality of different frequencies, Fig. 2 represents an oscillatlon generator sustaining oscillations of a high frequency, which is controlled by a piezo-electric device, Fig. 3

represents an oscillation generator sustainmg oscillations of a lower frequency. controlled by a piezoelectric device, and Fig. 4 represents an oscillation generator capable of sustaining oscillations at any one of a plurality of frequencies, each controlled bv the same piezoelectric device.- 7 Referring to the drawings, Fig. '1 shows a characteristic reactance curve for a piezoelectric crystal. The curve identified by the reference character a is a characteristic ca pacity reactance curve, such as might be obtainedwhen a condenser is connected to a variable frequency source, the reactance decreasing as the frequency of the current impressed across the terminals of the condenser increases. The curve identified by the reference character 6. represents a reactance curve for an inductance connected to a similar variable frequency source, the reactance increasing as the frequency of the current impressed upon the terminals of the inductance increases.

A piezo-electric device which is located between two metallic electrodes so that it may freely vibrate therebetween, may be considered to act like a condenser having a capacity reactance at all frequencies except at frequencies at which the piezo-electric device is resonant. At a resonant frequency, the reactance differs considerably from that of a condenser, its reactance being then determinable from one of the loops shown in Fig. 1. For the particular piezo-electric device having a characteristic curve such as is shown in Fig. 1, there are two frequencies at which the reactance differs substantially from normal capacity reactance of a condenser. These frequencies are designated by the reference characters F and F F being a frequency different from and higher than F Moreover, at either of these resonant frequencies, F or F the reactance may be either capacitative or inductive, as will be readily seen upon an inspection of Fig. l. The reactance may become inductive at a frequency very slightly below the resonant frequency.

Referring to Fig. 2, a space discharge tube oscillator is shown including a circuit which is arranged to sustain oscillations of a predetermined frequency. The reference character V denotes a space discharge tube working on the thermionic principle including a cathode, an anode and an input or grid control electrode. The grid electrode is connected to one conductive plate L of the piezoelectric device. The cathode is connected to another conductive plate L of the piezo-electric device. The piezo-electric crystal PE is shown herein with one of its active faces against the conductive plate L a gap existing between another of the active faces and the conductive plate L Byproviding a gap between one orboth of the conductive plates and the active faces of the piezo-electric crystal, the crystal may be set freely in vibration therebetween.

The cathode of the space discharge tube V is heated to an electron emitting temperature by the flow of current therethrough from a battery B A source of potential B supplies the space potential to the anode of the space discharge tube V through a winding W A winding W is coupled to winding W so that alternating current'waves impressed on the Winding IV are simultaneously impressed On the winding V The winding-IV is connected to an antenna A and ground. A resistance R is connected between the grid electrode and cathode of the space discharge tube V in order to maintain the grid electrode at a. suitable-negative potential with respect to-the cathode. A condenser Co, shown in dotted lines, representsthe inherentcapacity existing between the grid electrode and the anode, including the capacity of the leads connected thereto. It is this capacity between the grid electrode and the anode which produces a reaction of the plate circuit on the grid circuit in order to produce sustained oscillations.

The piezoelectric crystal PE is so arranged that it may freely vibrate between the conductive plates L and L a small gap existing between one of the surfaces of the crystal and the correspondingly conductive plate.

The crystal is preferably disk-shaped, cut from a substance such as quartz, preferably along an aXis of symmetry. It is the property of a crystal so cut that when subjected to aphysical stress from. the sides of the crystal, plus and minus charges appear on the faces of the crystal, and conversely, when the crystal is subjected to an electrostatic field, a physical deformation occurs along the axis of the crystal. The signs of the charges produced in the first case depend upon the character of the stress, that is, whether the stress is tension or compression, whereas in the second case, the physical deformation is due to the direction of the electrostatic field. If the physical stress changes from tension to compression, or vice versa, the signs of the electric charges on the faces of the crystal are reversed. On the other hand, if the direction of electrostatic field is alternately reversed,'the crystal undergoes contraction and elongation, alternately in the direction of the field. In other words, the crystal vibrates in an electrostatic field in which the charge is alternately reversed.

The arrangement of Fig. 2 is set up so as to sustain oscillations of a high frequency, such as the frequency F designated in Fig. 1. The inductive reactance of the winding W is high, preferably :equalto or greater than the plate-filainent impedance of the space discharge tube V. At any frequency lower than F the impedance of the winding 7 would be too low to permit the arrangement to sustain oscillations at that frequency. Consequently, the arrangement of Fig, l sustains oscillations of the high frequency, for eXample,'F l

Moreover, itis a well known property of space discharge tube-oscillators that when the coupling between the plate and gridcircuit is capacitative, the reactance in the plate circuit and that in the grid circuit must be inductive, in order that oscillations may be sustained. Conversely, when the coupling between the plate and grid circuits is inductive, the reactance in the plate circuit and that in the grid circuit must be capacitative. In Fig. 3, a similar arrangement is shown which is capable of oscillating at a lower frequency, such asfrequency F designated in Fig. 1. In this arrangement, a variable condenser C is bridged across the terminals of the windings winding lV is very high, preferably equal to or greater than the plate-filament impedanceof the space discharge tube V at the higher frequency F yet,'this impedance is lower than the plate filament impedance at the lower frequency F By shunting the winding WV by the condenser C the impedance in the plate circuit can be considerably increased. Inasmuch as the reactance in the plate circuit at the lower frequency F 5 is inductive, the arrangement sustains oscillations at the lower frequency F while it can not sustain oscillations at the higher frequency F because at that frequency, the reactance is capacitative.

In Fig. l, a similar arrangement is shown, in which windings W and W are in series relationship with each other, each of a substantially high impedance.- The variable con denser C is connected acrossthe terminals of i the winding W A winding V5 is coupled to the windings l/V and W and is connected to the antenna A and ground. When the capacitative reactance of the condenser C1 is very low, the inductive reactance in the plate The impedance'of the circuit is of such magnitude that oscillations are sustained at the lower frequency F Moreover, the reactance is too high to sustain oscillations at the higher frequency F lVhen the capacity of the condenser C is increased to a sufficiently large value, then the winding l/V becomes eflectively shunted by the condenser G at the higher frequency F Consequently, the impedance of the winding VV matches the plate-filament impedance of the space discharge tube V at the higher frequency F and therefore the arrangement sustains oscillations at that higher frequency F At the lower frequency, the reactance is capacitative, and the inductive reactance of the winding is too low to enable the arrangement to sustain oscillations at that lower frequency F Thus, in the arrangement of Fig. i, it has been shown that oscillations may be sustained at either one of two or more substantially different frequencies.

It is to be distinctly understood that while it has been particularly shown that oscillations of a piezo-electric device may be sustained at any one of two frequencies, it is within the scope of this invention to sustain the oscillations of a piezo-electric device at one or more of a plurality of frequencies, which frequencies may be substantially different from each other.

The term piezo-electric device and the term piezo-electric crystal as employed herein, refer to any body or substance capable of setting up an electromotive force in response to pressure, and vice versa.

While this invention has been shown in certain particular embodiments herein merely for the purpose of illustration, it is to be understood that the general principles of this invention may be embodied in other and Wide- 1y varied organizations without departing from the spirit of the invention or the scope of the appended claims.

What is claimed is:

1. The method of producing oscillations of predetermined frequency with a piezoelectric crystal capable of being set into vibration in a plurality of modes, which consists in selecting the desired mode of vibration and changing the effective reactive value of the crystal to a capacitance of definite magnitude to correspond to the predetermined frequency.

2. The method of producing oscillations of predetermined frequency with a piezo-electrio crystal exhibiting positive and negative reactances over each of a plurality of different bands of frequencies characteristic of the different modes of vibration of the crystal, which consists in sustaining the piezo-electric crystal in vibration in one of said modes to the exclusion of all other modes and changing the effective reactive value of the piezoelectric crystal from a positive value to a June,1926.

RUSSELL S. OHL.

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