Oct. 30, 1923. 1,472,583 W. G. CADY . METHOD OF MAINTAINING ELECTRIC CURRENTS OF CONSTANT FREQUENCY Filed May 28. 1921

Patented Oct. 30, 1923. 1,472,583 UNITED STATES PATENT OFFICE, WALTER GUYTON CADY, OF MIDDLETowN, connECTICUT. METHOD OF MAINTAINING ELECTRIC CURRENTS OF CONSTANT FREQUENCY, To all whom it may concern:Application filed May 28, 1921. Serial No. 473,434. REISSUED Be it known that I, WALTER G. CADY, a tric resonator that I take advantage of for citizen of the United States of America, my present purpose are-first: that prop residing at Middletown, in the county of erty by virtue of which such a resonator, 5 Middlesex, State of Connecticut, have in whose vibrations are maintained by im vented certain new and useful Improve pulses; received from one electric circuit, ments in Method of Maintaining Electric may be used to transmit energy in the form B) Currents of Constant Frequency, of which of an alternating current into another cir the following is a full, clear, and exact de cuit; second, that property which it posses 0 scription. ses of modifying by its reactions the alter The invention which forms the subject nating current of a particular frequency or of my present application for Letters Patent frequencies flowing to it; and third, the fact that the effective capacity of the resonator andis an maintainingimprovement alternatingin the art ofcurrents producing of depends, in a manner which will more fully 5 constant frequency. It is well known that hereinafter appear, upon the frequency of heretofore the development of such currents the current in the circuit with which it may to any very high degree of precision has be connected. been unattainable by ordinary means and In the description and explanations of my great difficulty has been experienced in invention which follow, I have assumed the producing alternating currents of high and piezo-electric resonator to comprise a sin constant frequency and free from fluctua gle suitably prepared plate cut from a piezo tions due to disturbances in or near the electric crystal, and provided with the usual generating system. coatings, and utilizing the so-called trans The useful applications of my invention verse effect; but all statements made apply 25 are numerous. It may be employed in the equally to crystal preparations utilizing transmission or the reception of intelligence the longitudinal effect, and in general, to by means of high frequency currents, or it any mechanical vibrating system whatever, may be used for the testing and measure having suitable preparations of piezo-elec ment of such currents and of those in cir tric crystals for setting the system into vi 30 cuits associated therewith in all cases where bration and for utilizing its reactions. the frequency may be controlled by the The nature and purpose of the invention electrical constants of the system, and, in may be most readily comprehended by ref general, the invention is applicable to cur erence to the diagrams which I have used in rents of any frequency. - its explanation and which are exhibited in 35 In an application filed by me on Jan. 2S, the accompanying drawing. 1920, Serial No. 354,659, which has ma In this drawing:- 90 tured into Patent No. 1450,246, April 3, Fig. 1 is a diagram of the well known 1923. I have shown and described what I and universally recognized Armstrong os 40 have termed a piezo-electric resonator, cillating circuit, selected for illustrative pur which, in general, connprises a plate of piezo thereto:poses and showing my invention applied electric crystal with coatings on its opposite 95 face. Such a device has a natural period of Fig. 2 is a diagram of a three-stage am vibration, but when set in vibration by a plification system of well-known type, with source of alternating current connected to my invention applied, thereto; its coating, the amplitude of such vibrations Figs. 3 and 4 are other diagrams similar is very slight unless the frequency of the to Fig. 1 illustrating further modifications; alternating current approximates or equals and the natural or critical frequency, in which Figs. 5 and 6 are diagrams used in ex case the reaction of the deformed crystal plaining the principles of the invention. upon the circuit may be such as to practi The above described diagrams are illus cally choke back the alternating current. trative examples of various ways in which 05 In carrying out my present invention I uti a piezo-electric resonator may be employed lize this piezo-electric resonator in the man to maintain constant frequency in an os ner hereinafter to be described. cillatory electric circuit. It will be under S5 The special properties of the piezo-elec stood that they do not purport to show the only possible arrangements for securing 10 1472,583 and 16, which in turn alter the potential dif this result, and even as to those shown it will ference across the condenser 10, and hence be further understood that the illustrated the potential of the grid itself. arrangements may be modified in various Whether amplitude of the potential of the ways which, however, are obyious to those grid be thereby increased or diminished, de 70 skilled in the art and familiar with high pends upon which of the coatings 15 or 16 frequency apparatus and which will not al is connected to the grid, and upon the phase ter the essential part played by the piezo relations in the electric circuits and in the electric resonator. piezo-electric resonator itself. I shall assume, for purposes of this case, The phase of the vibration of the reso 10 that in each case the high frequency oscil. nator is modified by the fact that the po lations are produced through the agency of tential difference between the coatings 15 and the three electrode vacuum tube of the type 16 is influenced by the periodic voltage, al commonly used in radio-telegraphy, but I ready existing across the condenser 10. In may state that this is not essential and that order to effect the maximum reinforcement 80 5 the only requirement is that the source of of the oscillations, it may be advisable to energy shall be capable of generating cur control the phase of the vibrations of the rents of a frequency determined or con resonator. This may be done, for example, trolled by the electric constants of the oscil by giving the coil 8 a certain resistance, lating circuit, and when I use the term “coat or by inserting in series with the coil 8 an 85 ings' in referring to the resonator I mean other inductance and a resistance and con 20 either thin layers of metal on the crystal necting the coatings 13 and 14 in parallel itself, or ei plates in fixed relation to with such inductance and resistance instead the crystal, or, in general, any means where of in parallel with the coil 8. by an electric charge may be conducted to The frequency of the electric oscillations 00 2 5 the resonator in such a Way as to produce an as determined by the inductance of coil 7 electric field in the proper direction through and the capacity of the condenser 9, may be that of the fundamental vibration of the theWith crystal. the above understanding I now refer- piezo-electric resonator, or of one of its to Fig. 1. In this figure the numeral 1 harmonics. In any case, the two pairs of 95 30 represents the filament of the vacuum tube, 2 coatings on the resonator should have such is the filament battery, 3 a regulating re a size and position relatively to the ends of sistance, 4 the grid and 5 the anode of the the crystal plate, as to cause the greatest vacuum tube. The battery in the anode possible amplitude of vibration at the de circuit is designated by 6. These are the sired frequency. It is also of great impor 00 main essential parts of one of the numerous tance that the coating be so disposed, and types of circuit commonly used for the gen the plate so supported that the vibrations eration of high frequency oscillations, the shall be damped as little as possible. other elements being the coils 7 and 8 in the If, under the conditions assumed, while grid and anode circuits respectively, 9 a. the oscillations are being reinforced through 05 4. variable condenser in parallel with the coil. the vibrations of the piezo-electric resonator, 7 for the purpose of controlling the fre and at any of the frequencies mentioned quency of the oscillations, 10 the grid con above, the coupling between coils 7 and 8 be denser, and 11 the leak. All these are old. loosened to such a degree that the circuit, and well known. with the resonator absent, would just fail 10 4. The plate or crystal of the piezo-electric to oscillate, with the resonator present it resonator is indicated by 12 and this plate will be found that the oscillations still per has four coatings 13, 14, 15 and 16, the two sist; and, moreover, that a variation in the former being connected to the terminals of capacity of the condenser 9 over a certain the coil 8, the two latter to the grid circuit range has no appreciable effect upon the fre 5 O around the condenser 10. quency in the circuit. In other words, this The operation of the system is as follows: frequency is determined solely by the period When the coupling between the two coils 7 and 8 is of proper character or value, the ofas vibrationsa matter of of fact, the resonatoroften an advantageitself. It is,to circuit oscillates with a frequency deter make the capacity of condenser 10 very 20 55 mined, in the main, by the capacity of con Small, or even to omit it altogether. denser 9, and the self-inductance of the coil I have found that all of those factors that 7. If the capacity of condenser 9 be varied, usually have a disturbing effect upon the the frequency changes, and when a rate is frequency, as for example, variations in the reached corresponding to one of the natural 125. modes of vibration of the piezo-electric voltage of either of the batteries 2 or 6, or 60 resonator, the latter begins to vibrate by bymovements the use of of the the piezo-electric hand near the resonator circuit, are,and reason of the electric field existing between the coatings 13 and 14. These vibrations Within wide limits, without effect upon the through the piezo-electric action, cause elec frequency. 30 tric charges to be induced on the coatings 15 The piezo-electric resonator may be used /

1,472,583 3 in connection with other types of oscillating circuit than the one above described. In ac capacity of the resonator be sufficiently cordance with the principle of operation large, the condenser may even be dispensed set forth above, it may be used with any with. generator of alternating current, the fre In explanation of the action which takes quency of which is capable of being deter place in this case, let it be assumed that the 70 mined by the varying potential difference coupling between the coils 7 and 8 be loos between two fixed points in the circuit or ened, as by increasing the distance between system of circuits, the two, until the circuit just fails, in the 0. The greater the amplification constant of absence of the piezo-electric resonator 12 to. the vacuum tube, the more widely may the oscillate. When the resonator is present, 75 electrical constants of the circuit be altered however, and the capacity of the condenser without affecting the frequency. For this 9 is not too far from the value which would, reason a still greater degree of stability may with closer coupling, make the circuit oscil 5 be attained by the use of a plurality of tubes late at the same frequency as that of the vi connected for cascade amplification. I have brations of the resonator, the circuit will be 80 found, for instance, that by the use of a cas found to be oscillating. The oscillations in cade amplifier comprising three tubes, the this case are due to the vibration of the resonator 12 and are of a frequency that is 20 tirelycoils 7 anddispensed 8 and thewith. condenser 9 may be en stable against disturbing influences, pro This arrangement of circuit is shown in vided those latter be not too pronounced. Fig. 2, in which 4 represents the grid of the The theory of this segmentive action may first tube and 5 the anode of the third. Two be stated as follows: Suppose that, owing. resistances 17 and 18 are employed, and two to some slight mechanical jar or electrical 25 condensers 19 and 20. This illustrates the disturbance, the resonator is set into feeble well known “resistance amplification,” but vibrations. Through the piezo-electric ac 00 any other type of amplificator might be tion, these vibrations cause corresponding employed. In this figure, 21 is a resistance changes in the potential of grid 4, Fig. 3, or other impedance in the anode or output which in turn-owing to the amplifying ac 30 circuit, while the piezo-electric resonator 12 tion of the vacuum tube and its associated has its two coatings 13 and 14 connected to circuits, cause similar pulsating currents in the plate 5, and the ground 22, respectively, the coil 7. These pulsating currents influ and its two coatings 15 and 16 connecting the ence the changes on the coatings of the reso nator and tend to maintain the vibrations gridone to4. the ground 22, and the other to the of the latter. Since the resonator is sup The operation of this arrangement is as posed to have been set into vibration by a 00 follows: Any slight increase in the potential somewhat sudden disturbance, it follows, in of the anode 5 by altering the electric field accordance with well known principles, that 40 between the plates 13 and 14 sets the piezo its vibrations, and consequently the effect electric resonator in vibration. The charges which it produces, as indicated above in the thereby excited in the coatings 15 sand 16 coil 7, do not possess simply a single fre 5 vary the potential of the grid 4 with respect quency, but may be considered as a combina to the ground 22. This varying potential tion of many frequencies of slightly different by virtue of the amplification taking place values. Of these different frequencies, that in the system, will, if the proper coating is one predominates which is associated with connected with the grid 4, still further in maximum fluctuations of grid potential. I O crease the variations in the potential of the have shown, theoretically (Proceedings, In anode 5 and maintain the piezo-electric res stitute of Radio Engineers, vol. 10, p. 83, onator in vibration. From the terminals of April, 1922), and verified by special experi 50 the resistance or impedance 21, a small ments, that for the alternating electro-motive amount of output power at a constant fre force acting in the circuit, the current flow 5 quency may be drawn. ing to the resonator is greatest at a frequency It is possible to secure constant frequency very slightly below the natural frequency of 55 through the agency of a piezo-electric reso said resonator. Also, when the resonator nator which has only a single pair of plates current is greatest the consequent fluctuations or coatings, and this may be accomplished in grid potential are naturally greatest. 20 in various ways by taking advantage of one Hence, with the circuit shown in Fig. 3, an or the other of the electrical effects produced alternating current of practically unvarying • 60 frequency will be built up and maintained byits theresonating resonator frequencies. when approximating For instance, one ofas in the system, of a frequency slightly lower shown in Fig. 3, which is exactly like the ar than the natural frequency of vibration of rangement shown in Fig. 1, except that the the resonator. . piezo-electric resonator 12, has but one pair The following example illustrates the ap of coatings 13 and 14, which are connected plicability of the piezo-electric resonator to: in parallel with the condenser 10. If the the stabilization of the frequency when con nected across the tuning condenser of a 30 1,472,588 12. Assume, now, therefore, that the piezo racuum tube oscillating circuit. Now to un electric resonator be connected in parallel derstand this action, the theory of the piezo with the tuning condenser 9, as shown in electric resonator must be considered and Fig. 4. The diagram of this figure repre borne in mind. sents a vacuum tube circuit oscillating in the 70 5 In my former application above referred same manner as that shown in Fig. 1, to. I have shown that the apparent capacity through the mutual induction of the coils of the piezo-electric resonator undergoes ('er and 8, the frequency being determined by the tain changes in the neighborhood of reso variable condenser 9. The coatings 13 and nance, and these changes are of fundamental 14 of the resonator 12 are in parallel with 75 () importance in the example now to be con the condenser 9. sidered. - Let the capacity of the condenser 9 be Let it be assumed, in illustration, that the denoted by C. If the capacity C were con resonator is connected to a high frequency stant then a decrease in capacity C would sinusoidal electro-motive force of constant result in a continuous increase in the fre 80 voltage. When the frequency is considerably (quency f, as indicated in Fig. 6, by the curve below that at which the resonator vibrates, 1. 9, 2, 4, 6, 8 and 7. Suppose, however, and the latter behaves like a simple condenser. observing that the corresponding points in having a certain capacity, which may be Figs. 5 and 6 are similarly numbered, that termed its “normal' capacity. If and as the the piezo-electric resonator begins to vibrate O frequency is raised to a value at which the perceptibly at a frequency corresponding to piezo-electro resonator begins to vibrate per the point 2, thereupon the capacity C begins ceptibly, the elongation of the resonator to be abnormally large and therefore tends plate is at first nearly in phase with the to diminish the frequency, with the result mechanical stress, which, in turn, is always that the frequency increases less than it 90 5 exactly in phase with the Voltage across the otherwise would. Continued decrease of the resonator. lence, in accordance with the capacity C' causes capacity C to increase well known laws of piezo-electricity, the still further, so that the curve bends down piezo-electric polarization resulting from the along the portions 2, 3. At point 3 the ap elongation is nearly in phase with the im parent capacity C has reached its greatest 95 3) pressed voltage, thus causing the total dielec possible value, and if C be further de tric displacement in the piezo-electric reso creased the resonator plate suddenly ceases nator to be greater than normal. to vibrate, or at best it vibrates feebly at Since the apparent, or as it may be termed, a much higher frequency, namely, the fre the equivalent capacity of the resonator is, quency corresponding to the point 8. In 00 35 other things being equal, proportional to the other words, the curve springs abruptly total dielectric displacement, it follows that from the point 3 to the point 8. From this so long as the frequency is below the resonant on, any further decrease in C gives rise alue, the apparent capacity is abnormally merely to the undisturbed portion 8, 7 of the large. It may, in fact, be many times in curve. If, on the other hand, the capacity kit) excess of the normal value. But after pass C be increased, beginning at the point 7, ing through a luaximum, the apparent capac. the path 7, 8, (3, 5, 9, 1 will be traced, for ity decreases, and close to the resonant fre reasons strictly analogous to those given for (uency it returns to its normal value. pon the curve described on decreasing the ca a further increase in frequency the apparent pacity C as stated above. capacity becomes abnormally small, in some The less damped the vibrations of the cases even attaining a considerable negative piezo-electric resonator, the more nearly con value. If the frequency continues to in stant will be the frequency over the por crease, a minimum in the apparent capa ?ity tions of the curve 2, 3 and 5, 6 in Fig. 6. 5 is reached, after which it gradually returns Hence it is manifest that a resonator made to its normal value. from materials of good mechanical proper I have represented these changes in the ties, such as quartz, or a combination of diagram of Fig. 5 in which the apparent Quartz, and steel suitably prepared and capacity of the piezo-electric resonator, which mounted, as has been set forth in my prior may be denoted by C in plotted as a func application, will exert a marked stabilizing 120 5 5 tion of the frequency f. Starting at a low value of frequencies the apparent ca effect upon the frequency of the generating pacity has its normal value represented circuitturbing with agent, which as for it isexample, connected. small Any changes dis by the point indicated by 1. As the in the value of C or in other capacities in frequency f increases, the apparent ca the circuit, or variations in the filament cur (3) pacity C remains approximately constant rent, will have almost no effect at all upon until at a frequency corresponding to the the frequency, so long as such disturbing point 2. the resonator begins to vibrate per agent is not so pronounced as to cause the ceptibly, and the curve begins to rise. The operating point on the curve to fall out 30 points 3, 4, 5, 6 and 7 are reached in suc side of the ranges 2, 3 and 5, 6. 65 cession, in accordance with the theory stated. 1472,583 I claimHaving is: now described my invention what pair being so connected to the alternating 1. The combination with an alternating current system that the said resonator is current system of such nature that the fre maintaned in a state of vibration, while the 25 quency of the alternations is governed by other pair is connected to the said two fixed potential variations in a portion of the points so as to cause the generator to pro same, of a means for keeping the frequency duce alternating current of a frequency constant within very narrow limits, consist identical with that of the mechanical vibra ing of a piezo-electric resonator having its tions of the said resonator. 30 10 two conducting coatings so connected to the 3. The combination with an alternating system that the said resonator is maintained current amplifier of a piezo-electric reson in vibration by the electric oscillations in ator having two pairs of conducting coat the system, and which by its reactions causes ings, one pair connected to the output circuit the frequency of said potential variations to of the amplifier, the other pair connected to 5 be identical with that of its own mechanical the input circuit so as to cause in the out vibrations. put circuit an alternating current to flow the frequency of which is determined by 2. The combination with a generating sys the mechanical vibrations of the said res tem of alternating current the frequency of onator. which is capable of being determined by the 40 20 varying potential difference between two In testimony whereof I hereto affix my fixed points therein, of a piezo-electric res signature. onator having two pairs of coatings, one WALTER GUYTON CADY.