US2054431A - Modulation - Google Patents

Description

Sept 15', 1936. N. E. LINDENBLAD MODULATION Filed Nov. 29, 1953 3 Sheets-Sheet l WW my un www Filed Nov. 29, 1935 3 Sheets-Sheet 2 N X T f u N n b F\ Qs u i lIl" |lll' ls u; l la n lNvENToR n u S, Nus E. UNDENBLAD w Q u w ATTORNEY Spt. 15, 1936.

N. E. LINDENBLAD MODULATION Filed Nov. 29, 193:5 s sheets-sheet :5

INVENTOR 'Il- 5 NlLs E. UNDENBLAD 34 ATTORNEY Patented Sept. l 15, 1936 UNITED STATES MODULATION Nils E. Lindenblad, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application November 29, 1933, Serial No. 700,252

15 Claims.

The present invention relates to ultrashort wave signalling and in particular to a novel method of and circuit for wobbling and/or modut stage.

lating ultrahigh frequency oscillations to increase the effectiveness thereof and/or to signal therewith. The invention, which is equally applicable to longer waves and to phase modulation as well as frequency modulation, is a continuation in part of United States application Serial No. 651,809, led January 14, 1933.

In order to frequency modulate a master oscillator, or crystal drive transmitter, either the frequency of the frequency determining circuit of the master oscillator itself must be variable or a modulating frequency must be added in one of the stages of the amplifier following the oscillator. The latter method has been used in the transcontinental test between Bolinas and Riverhead. As this method calls for considerable additional circuit equipment, it was deemed less practical in connection with ultrashort wave transmitters which already require a considerable number of multiplier circuits when using crystal control to increase the constant frequency crystal controlled oscillations to the desired frequency. Under certain conditions it may be more practical to vary the frequency of the crystal stage. n account of the high multiplication of the frequency required between the crystal drive and the output stage, a variation in output frequency of a certain number of cycles required a very slight variation in crystal frequency. This is because the said variation is multiplied in the frequency multipliers between the crystal stage and nal It is furthermore only practical to vary the crystal frequency a relatively small amount and therefore it may be stated that this method is perhaps limited in application. According to tests, however, it seems that the variations obtained are not only suiiicient in ultrashort wave technique but are also suflicient for commercial short wave frequency modulation. y

At first I tried to modulate a crystal frequency involved by varying the air column, and inherently the capacity, of the crystal electrode space.

- One of the electrodes was mechanically connected to the moving coll system of a dynamic loudspeaker. In this test one of the crystal electrodes and the coil' system were supported from a metal diaphragm suspended between rubber rings akin to the suspension used in phonograph sound boxes. In a crystal circuit the resonance of the air gap between the crystal surface andthe electrode surface plays a very important part. A va-A riation of the air 'gap therefore varies the ampliy tried. The result was that the UX-84l, which is tude of the crystal oscillations' in the vicinity of the resonance of this air column. Some investigators have shown that the crystal will completely stop its oscillations at these points. This therefore means that the gap should-not be varied so much that it passes through any such resonance point. The distance between two nodal points in air at 2000 kilocycles, one crystal frequency, is only 3.3 thousandths of an inch. This limits the allowable motion of the crystal electrode to such an extent that very little fre- "quency variation could be obtained. I partly overcame this difllculty by perforating the moving electrode and thus partly eliminating air column resonance. In order to get the greatest variation, however, it was found necessary to keep the swinging electrode quite close to the crystal. Compromising the various factors and working within the linear portionof the apparatus I obtained a frequency variation of plus and minus 4:00 cycles at a crystal frequency of 2000 kilocycles.

I then tried voltage variations on the crystal stage tube. Several different tubes were tried, UX-210, UX-841, UX-865, and the heater type UX-210. In the case of the UX-865 external coupling capacity was used between plate and grid to make the tube active in spite of stabilizing action of the screen grid. 'It was found that best results with this tube were obtained when this coupling capacity was fairly high, the plate of relatively low potential and the screen of relatively high potential.

Variations of all possible combinations were a high mu tube, was found the one giving the best results. At a plate voltage of about vtwo hundred volts the grid voltage of this tube, used as a crystal stage tube, can be varied one hundred volts in the negative direction without causing any, save a very small, plate amplitude variation. A frequency variation of plus and minus 200 cycles in the 2000 kilocycle crystal frequency was obtained. The frequency varied as the 1.4 power of the voltage.

This is not a very bad curvature, but sufficiently bad to cause distortion. My next task was therefore to devise a compensating circuit in order to make the frequency variation in linear proportion with the voltage. Besides the conventional circuits available for this purpose I have devised some new ones if possible more directly suitable to my purpose. These circuits, which will be described more in detail hereinafter, in-

' clude distorting devices which introduce a dis- 55 tortlon which is the opposite of the distortion described above and compensates the same so that undistorted frequency modulation of the 0scillations is obtained.

More specifically, I make use of a coupling circuit for applying the modulating voltages to the oscillation generator, which coupling circuit distorts vthe applied voltages in such a way as to compensate for the distortion introduced by the oscillator whose frequency is to be varied.

According to my invention further, the frequency modulated oscillations are multiplied in successive frequency multiplier stages. Each multiplier stage is so operated as to act both as a frequency multiplier and amplifier, although there is, of course, some limit to which this action can be combined.

Still other objects as well as advantages and features of my present invention, will become apparent as the fuller description thereof proceeds. This fuller description may best be understood by referring to the accompanying drawings,v

wherein:

Figures 1, 4 and 6 each illustrate a transmitter for transmitting, at a high power, exceedingly short wave length energy modulated either in amplitude, or in frequency alone; or, modulated both in amplitude and in frequency;

Figures 2 and 3 are curves explanatory of the action of my improved modulator circuit used in Figure 1, whereby linear variation in frequency of an oscillation generator may be obtained in accordance with signal voltages to be transmitted;

Figures 5, '7 and 8 illustrate alternative modulating circuits which may be used to replace the modulating frequency distorting circuits shown in Figures 1, 4 and 6; while Figure 9 illustrates an alternative source of modulating potentials which may be used to replace the modulating sources of any of the systems described hereinbefore.

As already indicated, a system for transmitting exceedingly short length frequency modulated and/or amplitude modulated waves is illustrated in Figure 1. Voice currents generated by the microphone 2 and its following audio amplifier 4, which, of course, may be a keyed tone system such as illustrated in Figure 9, are fed to a crystal controlled oscillation generator 6 through a circuit including a resistor R and coupling tube 8. Ordinarily the control potentials or voltages from the controlling source, namely, the microphone or keyed audio oscillator, would cause non-uniform or non-linear variation in the frequency of the oscillations generated by the crystal controlled generator 6. In other words, the normal characteristic of the generator is indicated as illustrated in Figure 3 by curve A where the ordinates indicate the frequency and the abscissae the grid voltage. The thus obtained non-uniform frequency variation would then, of course, introduce a corresponding undesired distortion of the signal. To remedy this defect, I couple the audio or voltage control source to the oscillator through a non-linear or non-uniform circuit including a resistor and coupling tube, the response of which varies non-linearly and in a complementary fashion as shown at B, with the voltage applied across the same, whereby the output Varies in a linear desired way, as shown at C.

The manner in which the compensating distortion of the voltage is obtained and consequently linear or nearly linear modulation of the oscillations produced in the generator will now be described in detail.

In Figure l the audio output of 4 terminates across a potentiometer P. The above ground side of this .potentiometer is maintained negative by 5 potentials from generator G through choke L. A loading tube 8 has its grid and cathode tapped to P as shown. A resistance Ris included ,in the cathode connection. When the superimposed audio frequency from 4 swings in the negative direction the grid electrode of the loading tube 8 swings more negative. This reduces the plate current of the tube 8 which passes by way of ground through series resistance R. The decrease of current through R causes less voltage drop 15 along resistance R and therefore produces a more rapid rise in the positive direction in the negative voltage applied to the grid of the crystal stage tube 6. That is, the grid voltageof 6 becomes less negative at a rate which is non-linear with respect 20 to the modulating potentials. When the grid of 8 goes positive theloading tube 8- draws more plate current and, due to the circuit arrangement, this causes a greater negative drop across R. In this arrangement the circuit is of inverse proportion- 25 ality. The coupling circuit just described produces a distortion which is indicated by the curve B in Figure 3. This distortion when combined with the normal distortion in the generator produces linear frequency modulation response.

The same results may be obtained by using a circuit as shown in Figure 4. Here the desired non-linear response to offset the action of the generator is attained in the resistance R and applied by way of source S and radio frequency 35 choke I to .the control grid of the oscillator tube 6. As the grid of tube 8 swings negative the anode current through R decreases. The potential drop through R decreases and produces a rise in the positive direction in the negative voltage applied to the grid of tube 6. That is the grid of tube 6 becomes less negative. By operating the tube 8 in the proper point of its characteristic curve this decrease in negative potential can be made at a rate slower than linearly with respect to the negative swing on the grid of tube 8.

The circuit of Figure 4 has an added desirable feature in that the tube 8 also ampliiies the modulating frequency potentials and simultaneously servesv as a coupling tube. The linear frequency 50 modulations may be multiplied in frequency as desired in the thermionic frequency multipliers included in I0, which is coupled to 6. The multiplied amplified frequency modulated oscillations may be translated to any work circuit for utilization.

It is not essential that the particular crystal controlled generator illustrated hereinbefore be relied upon. The generator itself may be any form of oscillation generator having any form of frequency control such as simple tuned circuits, tuning forks and the like. Also, the frequency varying potentials need not be applied to the grid but may be applied to another electrode of the oscillation generator.

Thus the modulator circuit of Figure 1 and its associated generator included between condensers 'l and 9 may be replaced by the arrangement shown in Figure 5.

The tube 5 has its control grid variably tapped to the polarizing source PS through resistor Il so that the characteristic of the tube may be rep- Vex shape, such as curve A of Figure 3. As this Accordingly, the plate voltage on the crystal controlled oscillation generator I3 of Figure 5 will not have the same form or outline as the voltage fed through capacitor 1 but will be so distorted that variations in frequency produced by variable voltages upon the plate or cold electrode of tube I3 .will cause variations in oscillation frequency which are linear with respect to the amplitudes of control potentials fed to condenser 1.

In searching for still simpler arrangements than those described above, it occurred to me that a resistance in which'the potential ,drop

I or current intensity is not linear with respect to the applied Voltage ,would furnish material for a resistance of non-linear proportionality. Experiment proved this assumption correct. By the use of such a resistance the amplifier or extra coupling tube circuit need not be used. An arrangement in which use is made of the characteristics of this special type of resistance to obtain a correct distortion of the applied modulating frequencies is illustrated in Figure 6, which will now be described. The resistance which I intend to use for this purpose is known as Thyrite.

As pointed out hereinbefore, the application of controlling. potentials to the oscillator 6 causes non-uniform or non-linear variation in the frequency of the oscillations generated therein. The

vnormal characteristic of the generator is, as indicated by curve A in Figure 3, where the ordinates indicate the frequency and the abscissa indicate the generator grid voltage. -The special resistance in the coupling circuit now to be described will tend to produce a characteristic such as illustrated by B in Figure 3 and, obviously, the combined characteristics result in the linear relation indicated by the straight line C in Figure 3. The characteristic of the resistor is illustrated ln Figure 2, where the ordinates-indicate the current passed by the.Thyrite and the abscissa applied control voltage.

The modulating potentials from a source 2 may be applied by way of the audio frequency amplifler 4 to a condenser 1. Condenser 1 is chosen sufliciently large as to pass with facility either the amplified audio tones derived by means of the microphone, or the keyed alternating currents such as obtained from a tone system shown in Figure 9. The direct application of these modulating potentials or controlling potentials to the grid of the crystal controlled oscillation or alternating current generator of Figure 1 would produce a non-linear variation in frequency of conwould tend to distort, for example, the receiver output, I provide a coupling circuit between the source 2 and the crystal controlled generator which will remedy this defect. In Figure 6 this coupling circuit consists of the serially connected source of potential I2, variable resistors I4, I6 and an impedance I8, in the form of a Thyrite resistor having a non-uniform or non-linear characteristic. This circuit is grounded for unidirectional potentials by way of conductor 20 and one end of the Thyrite resistor or impedance I8 is grounded for alternating currents by way of condenser 22. To prevent the by-passing of low frequency or signalling potentials directly to ground through source I2 and lead 20, there is also interposed in this coupling circuit an audio frequency choke 24 which forces the audio potentials or low frequency potentials to be lmpressed across the Thyrite resistor I6.

Assuming the crystal controlled generator 6 to have a non-linear grid voltage-frequency charciable hindrance to the passage of the controlling currents or potentials which are impressed, as indicated, upon the grid of the tube 6. 'I'he frequency controlling piezo-electric crystal is connected between the cathode and control electrode or grid of the tube 6; and, an `output circuit formed of .inductance and capacity is connected as shown between the anode and cathode for high frequency currents through the-action of by-passing condenser 26. Resistors I4 and I6 serve the dual function of controlling the unidirectional potential or polarizing potential applied to the grid of tube 6 and also control the amount of 6. Oscillation generator 6, though crystal controlled, is thereby swung linearly with respect to the amplitude of the applied modulating potentials and this swing in frequency, though relatively small, becomes magnified considerably by the succeeding frequency multipliers, which step Aup the absolute frequency shift, that is to say, the

shift measured in cycles per second.

Frequency modulated output energy is taken from the output circuit of tube 6 through con ductor 25 and fed through coupling condenser 9 `to the input circuit of frequency multipliers, etc.,

and from thence Vto the load circuit.

In the arrangement of Figure '1 details only of the coupling circuit' are shown. Here it is assumed that modulating potentials are applied from any source to the condenser 1 and that frequency modulated oscillations are supplied-by the condenser 9 to any utilization circuit. Where ultrahigh frequency oscillations aredesired this utilization circuitmay be preceded by frequency 2', ,modulating potential swing on the grid of tube `0 multipliers. In Figure '1, a non-linear resistance 'is connected, as Shown, in series with the source S and a radio frequency choke inductance I -between the grid of the oscillation generator 6 and the condenser 1.. As in Figure 1 an audio frequency choking inductance L is connected, as

shown, to the negative terminal of a source of potential G, the positive terminal of which is grounded. The control grid is connected to ground by way of a frequency determining piezo Aelectric crystal, while the positive terminal of the source S and one terminal of the distorting resistance are connected to ground by way of a resistance R1 as shown. If, dueto modulating potentials, the negative voltage on the left hand end of the distorting resistor is decreased, the resistance of this resistor drops and permits more current to flow through the distorting resistor and through the resistance R1.' This will produce a greater potential drop through resistance R1 and this increased'negative voltage will be applied to the control grid of the oscillator 6. The increase in voltage drop across the resistance R1 takes place at a rate greater than according to linear proportions because the resistance of the particular resistor is not constant for the various applied potentials but drops with an increase of potential.

The non-linear action of the distorting resistance I8 and the resistance R1 on the control grid compensates the inherent distortion in this oscillator so that linearly frequency modulated oscillations are produced in 6.

In Figure 8 the same or similar results are obtained by coupling any source of modulating potentials, as shown, by way of a modulating frequency transformer T and blocking condensers C to the terminals of the distorting resistor, which is connected in series with a resistor R1, as shown. Changes in the potentials applied to the terminal of the Thyrite resistor from the modulating frequency transformer T alter the resistance thereof in a non-linear fashion. This permits a variation in the flow of current through R1, which is 'non-linear with respect to the modulating potentials. The drop in potential through Ri, as in the prior arrangement, is applied to the control grid of the oscillation generator 6 to insure compensating distortions of the frequency modulated oscillations produced therein. 4

The frequency modulated oscillations may be utilized in any manner by applying the same from condenser 9 to a utilization circuit directly or by way of any necessary added units, for example, frequency multipliers or amplifiers.

An advantage of the modulation system described herein is that the frequency modulation circuits do not decrease in any manner the stability of the crystal control of the oscillation generator since the crystal itself is left untouched.

In place of the microphone and audio amplifier 4 of Figures 1, 4, 5, 6, 7.and 8 an arrangement such as shown, for example, in Figure 9 may be used.

Referring to Figure 9. screen grid tube 28 connected regeneratively, as shown, to function as an audio frequency oscillator, is turned on and off by the action of switch 30. The latter switch is in turn operated by means of a key 32 causing the electromagnet 34 to move contact 36 to its upper position. Opening of key 32, of course, allows 36 to fall upon its lower contact. In its lower position, a large negative bias is impressed upon the grid of tube 28 as a result of which little current is drawn through resistor 40 allowing sufficient plate potential to be applied to tube 28 to cause it to oscillate at an audio frequency rate depending upon, in. general, the tuning of grid circuit 42. In its upper position, contact 36 causes the grid of tube '38 to assume such a potential as to allow passage of current therethrough as a result of which there is such a fall of potential across resistor 40 asto reduce the plate potential of tube 28 to an inoperative value. Consequently, in its upper position, armature 36 will prevent the generation of audio frequency currents and their subsequent passage through condenser 44. If desired, this primary audio frequency so generated and so keyed on and oif, may be fed through conductor 46 to condenser 1 of Figures 1, 4, 5, 6 and 7, or through the primary of the modulation potential transformer of Figure 8 to the generator 6.

However, if desired, the audio frequency generated by tube 28 may be frequency multiplied before use in the transmitter.

One way of doing this is to feed the audio frequency generated through condenser 44 to a distorting resistor 48, the biasing current through which is supplied by source of potential 3| and controlledby variable resistor 52'. The resistor 48 has a characteristic such as illustrated in Figure 2 wherein a relatively smallA increase in potential causes a disproportionate increase of current through it. Consequently, application of audio frequency current to the distorting resistor 1 48, will-cause the wave form o1' current passing therethrough to become distorted as a result of which, in the serially tuned circuit 33, preferablyI tuned to a desired harmovic, harmonic frequency energy may be derived which may be fed by means of conductor 31 to the condenser 'l of Figure 1.

However, i1' desired, the harmonicv frequency energy may be resonated in a parallel tuned circuit 39 and amplified by means of electron discharge device 4l. By a suitable choice of bias from source 3| for device 4|, as disclosed in the copending application of C. W. Hansell, Serial No. 177,505, filed March 23, 1927, a further harmonic may be generated in the parallelly tuned output circuit 43 of tube 4I. may be desired to use tube 4I merely as an amplifier, in which case the output energy appearing in circuit 43 may be fed into a further Thyrite resistor polarized by source 49 and maintained at a suitable operating point by ad- Justment of resistor' 5I. A further harmonic will then be generated in parallelly tuned circuit 50 keyed or modulated in accordance with the actuation of keying switch 32. If desired this harmonic may be fed through conductor 52 to condenser 'l of the prior figures, or, it may be -ampliiied by means of tube 54 and then fed through condenser 1 of Figures 1, 4, 5, 6 and 7 or through the primary winding of the modulation frequency transformer of Figure 8 to the generato-r 6.

This novel means for producing oscillatory energy or energy of varyingl potentials for modulation purposes in the systems disclosed in Fig-n ures l, 4, 5, 6, 7, and 8.of the present invention, has been disclosed and claimed in my copending U. S. application Serial #81,359 filed4 May 23, 1936, which is a division of the present application and of my U. S. application Serial #651,809 filed January 14, 1933.

Having thus described my invention and the operation thereof, what I claim is:

1. In an alternating current system wherein the frequency of an oscillatory current is inherently modulated non-linearly in accordance with the amplitude of impressed controlling potentials characteristic of signals, the method of obtaining linear frequency modulation of said oscillatory current in accordance with said controlling potentials which includes distorting the amplitude of the controlling potentials in a direction to oppose said inherent non-linearity and applying the distorted potentials to the oscillatory current to vary the frequency thereof.

2. In a system for producing an alternating current of a variable frequency varying in accordance with a controlling undulating potential, a source of undulating control potentials, a

i source of oscillatory energy, means coupling said sources together whereby the frequency of said oscillatory source varies in accordance with the amplitude of the undulating control potentials, and means, connected with said undulating control source, for altering the wave form of said undulating controlling potentials, whereby the frequency of said oscillatory source varies in substantially linear relation with respect to the amplitude of said controlling potential.

3. In a system for obtaining a linear frequency response, an oscillation generator comprising an electron discharge device having within a container an electron emitting cathode and However, it

a cold electrode, a source of undulating control potentials, and a circuit the potentials at the' output ,of which are non-linear with respect to the input potentials coupling said 'sources t0- gether whereby the oscillations generated by said device vary linearly in frequency inl accordance with the amplitude of said controlling potentials.

4. In apparatus of the characterdescribed, an oscillator comprising an electron discharge device having within an hermetically sealed container an electron emitting cathode and a plurality of cold electrodes, a source of alternating controlling potentials, and a circuit the potentials at Vthe output of whlch'are non-linear with respect to the input potentials coupling said controlling source to an electrode of said oscillator, whereby the oscillations produced by said oscillator vary linearly in frequency in' response to the amplitude of the controlling potentials.

5. In combination, an electron discharge device having an anode a cathode and a grid, a circuit interconnecting said grid and said cathode, another circuit interconnecting said anode and cathode, said circuits being reactively associated whereby continuous oscillations are set up, a source of relatively low frequency controlling potentials, and non-linear means coupling said last mentioned source to one of the electrodes of said electron discharge device whereby the oscillations generated by said 'device vary linearly in frequency in accordance with the amplitude of the controlling potentials.

6. In combination. an oscillation generator comprising a vacuum tube having an anode a cathode and a grid, said oscillation generator normally varying non-linearly in frequency with varying potentials applied to one of the electrodes of said tube, a source of control potentials, and means, having a non-linear characteristic coupling said source to one of said electrodes whereby the oscillations generated by said device varying linearly in accordance with the amplitude of potentials supplied from said source.

'7. In combination, an oscillation generator comprising an electron discharge device having an anode a cathode and a grid, said device producing oscillations varying non-linearly in frequency in accordance with the unidirectional` potential applied to said grid, a source of audio frequency potentials, and non-linear means coupling said source to said grid whereby said generator produces oscillations varying linearly in frequency in accordance with the amplitude of potentials.

8. In apparatus of the character described, an oscillation generator comprising an electron discharge device having within an evacuated container an anode a cathode and a grid, a frequency controlling crystal connected between said grid and said cathode, said oscillation generator varying in frequency non-uniformly with variations in grid polarizing potential, a source of audio frequency potentials, and means for polarizing said grid from said source, said last mentioned means comprising a resistor in which the ow of current caused by applied potential is not truly characteristic of said appliedr potential and a soruce of potential in series, whereby the audio frequency controlling potentials are so altered in shape that the voltage on the grid of said oscillator is varied in such a way as to cause said oscillator to vary in frequency in a uniform fashion with respect to the 5 amplitude of the audio frequency -controlllng potentials.

9. In a system for producing alternating current of a frequency which varies linearly in ac-'l oordance with controlling modulating potentials, a thermionic oscillation generator of the constant frequency type including a therrnionic tube having electrodes coupled in frequency deter-l mining circuits, an impedance energized by said modulating potentials, a connection between an electrode of said oscillation generatorand said impedance, and a resistance, the drop of potential through which varies non-linearly as the applied potential connected with said i'lrst named impedance.

10. In a system for producing alternating current of a frequency which varies in accordance with a controlling modulating frequency, a thermionic oscillation generator of the constant frequency type including a thermionic tube having anode, cathode and control grid electrodes connected in oscillation generating circuits and a piezo-electric crystal frequency determining ele- .ment in one of said circuits, a source of modurent of a frequency which varies in accordance with a controlling modulating frequency, a thermionic oscillation generator of the constant frequency type including a thermionic tube having a control grid, an anode and a cathode connected in oscillation producing and frequency determining circuits, one of which includes a piezo-electric crystal frequency determining ele- 4 ment, a source of modulating potentials, an impedance connected with said source of modulating potentials to be energized thereby, a connection between the control grid of said oscillation generator and said impedance, said connection including an inductance and a resistance, the4 drop of potential through. which Varies nonlinearly with respect to the applied potential connected with said first named limpedance and with a source of potential.

12. In combination an oscillation generator comprising an electron discharge device having an anode, a cathode and a grid, said device normally producing oscillations varying non-linearly in frequency in accordance with unidirectional modulating potentials applied to said grid from a source of modulating potentials, and nonlinear means coupling said source to said grid, whereby said generator produces oscillations varying non-linearly infrequency in accordance with the amplitude of the modulating potentials comprising, a resistance, the drop of potential through which is non-linear with respect to voltages applied tothe terminals thereof, inductive means coupling said resistance to the control grid of said electron discharge device, an impedance connected in parallel with said resistance. and a circuit connecting said source of modulating potentials to said impedance.

13. In combination,- an oscillation generator comprising an electron discharge device having an anode, a cathode and grid electrodes, said electrodes being coupled in oscillation producing circuits, said circuits including a device for norlmally insuring the production of oscillations oi' constant frequency, and `means `for varying the frequency of the oscillations produced in said device and circuits linearly with respect to modulating potentials comprising, an inductance and a distorting resistance and an impedance connected in series between the control grid and cathode of said tube by way of a source of potentials, and a circuit forlapplying said modu# lating potentials to said impedance.

14. `In combination, an oscillation generator comprising. an electron discharge device havin an anode, a cathode and a grid electrode, said' electrodes being connected in oscillation producing circuits, said circuits including means to normally insure the 'production of oscillations 0f constant frequency, and means for varying the frequency of the oscillations produced in said device and circuits linearly with respect to modulating potentials comprising, a distorting resistance and an inductance connected in series between the control grid and cathode of said tube, and means for coupling said source of Vmodulating potentials in parallel with said distorting resistance.

15. In combination, an oscillation generator comprising an electron discharge device having an anode, a cathode and a grid, said device normally producing oscillations varying non-linearly in frequency in accordance with unidirectional modulating potentials applied to said grid from a source of modulating potentials, and nonlinear means coupling said source to said grid, whereby said generator produces oscillations .varying linearly in frequency in accordance with the amplitude of the modulating potentials comprising, a resistance, the drop of potential through which is non-linear with respect to vol' tages applied to the terminals thereof, a second resistance and an inductance coupling said first resistance'to the control grid of said electron discharge device, an impedance connected in parallel with said resistances and a circuit connecting said source of modulating potentials to said impedance.

NlLS E. LINDENBLAD.

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