claim is: 60 1. Means for producing phase modulated oscillations comprising, high frequency oscillation generating means, repeating means including a pair of electron discharge tubes each having, an anode, a cathode, and a control electrode, means 55 for connecting the anodes of said tubes in parallel, means for applying oscillations from said generating means in phase displaced relation to the control electrodes of said discharge tubes a thermionic device having an anode, a cath@de 60 and a control grid, a circuit resonant at modulating potential frequency, a source of modulating potentials, switchilig means for connecting said resonant circuit between the anode and control grid and cathode of said device to produce oscil65 lations, or the control gi:id and cathode of said device to said soutee of modulating potentials to act as an amplifier of the modulating potentials, and means connected between the anode of said last named device and said repeaters to 70 vary in opposite directions the operating characteristics of said repeaters. 2. A device for producing phase or frequency modulated waves comprising, the combination of a carrier wave generator, a low frequency oscilla75 tor and a source of voice frequency potentials, a phase modulator having its input circuits connected to said carrier wave generator, a frequency multiplier connected to the output of said phase modulator, a load circu-it coupled to said frequency multiplier and switching Tneans for 5 connecting said low frequency oscillator to said phase modulator to transmit phase or frequency wobbled waves to said load circuit or to connect said voice frequency potential source to said phase modulator to transmit telephony modu- 10 lated waves to said load circuit. 3. A device for producing phase or frequency modulated waves comprising, the combination of a carrier wave generator, a phase modulator having its input electrodes coupled to said carrier 15 wave generator, an audio frequency amplifier connected with electrodes of said phase modulator, a phase and frequency niultiplier coupled with the output electrodes of said phase modulator, a radio frequency amplitude increaser cou- 20 Pled with the output of said phase and frequency multiplier, a load circuit coupled with the output of said last named ampli'cude increaser, a source of signals, a source of low frequency osciliations, and circuits for impressing signals from said first 25 source on said audio frequency amplifier to transmit wave energy modulated in phase or frequency in accordance with said signals from said transmitter or for impressing oscillations from said second source on said audio frequency amplifier 30 to transmit waves wobbled in phase or frequency in accordance with said low frequency oscillations from said transmitter. 4. A device for phase or frequency modulating a carrier wave by telegraphy or telephony signals 35 comprising, the combination of a low frequency oscillator and a source of voice frequency potentials, a phase modulator having its input energized by said carrier wave, a frequency multiplier connected to the output of said phase modula- 40 tor, a load circuit coupled to said frequency multiplier and means for connecting said low frequency oscillator to said phase modulator to transmit telegraphy signals to said load circuit or to connect said voice frequency potential source 45 to said phase modulator to transmit telephony signals to said load circuit. 5. A device for phase or frequency modulating a carrier wave by telephony or telegraphy signals comprising, the combination of an audio fre- 50 quency amplifier, a phase modulator having its input electrodes energized by said carrier wave energy, an audio frequency amplifier connected with electrodes of said phase modulator, a phase and frequency multiplier coupled with the out- 55 Put electrodes of said phase modulator, a radio frequency amplitude increaser coupled with the output of said Phase and frequency multiplier, a radiating circuit coupled with said last named amplitude increaser, a source of voice frequency 60 signals, a source of low frequency oscillations, and means for impressing signals from said first source.on said audio frequency amplifier to transmit telephony signals modulated in phase or frecluency in accordance with said signals from said 65 first source from said transmitter or for impressing low frequency oscillations from said second source on said audio frequency amplifier to transmit wave energy modulated in phase or frequency in accordance with said low frequency oscilla- 70 tions from said transmitter for telegraphy signaling. 6. In a phase modulator, a pair of thermionic tubes each having an anode, a cathode, a control electrode and an.auxiliary electrode, a circait in- 75

cluding an inductance connected to the control electrode of one of said tubes, a circuit including a, condenser connected to the control electrode of the other of said tubes, means for applying ,5 high frequency oscillations in phase to said circuits to thereby energize the control electrodes of said tubes by phase displacedoscillations of like frequency, an output circuit connecting the anodes of said tubes in parallel, a source of mod10 ulating potentials, an impedance, a circuit connecting said source of modulating potentials to said impedance for impressiiig said modulating potentials across said impedance, a circuit connecting a point on said impedance to the cath15 ode of said tube, and circuits connecting points on said impedance at which said modulating potentials are of unlike phase to the auxiliary electrodes of said tubes, thereby varying in unlike manner the conductivity of said tubes in accord20 ance with potential vatiations in said impedance. 7. In a telegraphy or telephony signalling system, a circuit tuned to oscillate when energized at a modulation frequency rate, a, source of voice frequency potentials, a thermionic device having 2,036,165 7 input and output electrodes, a pair of thermionic tubes each having input electrodes, means for applyi ng high frequency oscillations of like frequenc y but displaced in phase to the input electrodes of said pair of tubes, an impedance, a cir- 5 cuit connecting said impedance to the output electr odes of said device, whereby potentials are impres sed frqlu said device onto said iripedance when the input electrodes of said device are energize d, circuits connecting different points on 10 said impedance to. like electrodes in said pair of tubes to apply modulating potentials in phase opposi tion to said like electrodes in said tubes, and a multiple switch and contacts cooperating with said source of voice frequency potentials to 15 couple the same to the input electrodes of said device in one position of said switch when it is desire d to signal by telephony, said switch cooperat ing with said resonant circuit to couple the same to the input electrodes of said device 20 to produce oscillations in said tuned circuit in anothe r position of said switch when it is desired to send telegraphy signals. GEORGE L. USSELMAN.

Patented Mar. 31, 1936 2@036@165 UNITED STATES PATENT OFFICE 2,036,165 PHASE AND FREQUENCY MODULATION George L. Usselman, Port Jefferson, N. Y., assignor to Radio Corporation of America,, a corporation of Delaware AppHeation June 8, 1932, Serial No. 616,026 7 claims. (Cl. 250-17) This invention relates to sigzialling means and in particwar to means whereby the characteristics of high frequency oscillations, other than the amplitude, are varied in accordance with signals to be transmitted. It has been found that ordinary amplitude modulated high fr,-quency oscillations in tralismission from the sending station to the receiving station are subject to what is known as fading 10 effects. This is a decided disadvantage since it introduces drop outs and errors in the signal. Evezi where diversity receivers are used to receive the amplitude modulated signals the effect of fading is a se@-ious disadvantage. 15 I have found that if the high frequency osefllations are modulated in phase or in frequency in accordance with the signal to be transmitted they are less subje-@t to, the effect of fading than amplitude modulated waves. The reason why 20 phase or frequency modulated oscillations iKi telegraph transmission are subj'ect to a less extent to fading effects is that this type ofmodulation produces a larger number of side frequencies than that produe-,d by amplitude modulation, and the 25 result in receiving a phase or frequency modulated signal on a simple receivii:ig antenna is very similar to receiving an unmodulated signal on a diversity antenna. A phase or frequency modulated signal is of particular advantage for reduc30 ing fading when simple re,-eiving antennas are used and this type of modulated signal is also beneficial for reducing the effects of fading when diversity recei-Ving systems are used. Phase and frequency modulated high frequency '3 -5 oscillations have many characteristics in common, as will a-ppear more in detail hereinafter. If high freql,ency oscillations are phase or frequency mo,dtilated by a single modulating frequency they are physically indistinguishable. At least, at the 4 present time, with the present analyzing means, this is true. Wheii a band of modulating frequencies is used it may be shown that in phase modi-ilation all modulating frequencies of equal a--,r.plitude have equal effect upon the carrier 4 phase, Nvhereas in frequency modulation the effect upon the phase of the carrier is inversely proportional to the modulating frequency. However, where a band of @,nodulating frequencies is applied to the high frequency oscillations in phase modulatio-@a, the phase of tl-ie oscil'@ations shift an extent or angle proportional to the a.-nplitude of the applied modulating frequen,-4es. Phase modulated oscillations are, however, less subject to fading effects than frequency modulated oscillations. Obviously, this method of signalling is of the utmost importance since one of the main drawbacks to signalling is the effect of fading of the transmitted oscillations. Especially is this inethod desirable now since i-t is adaptable where diversity reception methods are not in use. 5 The present invention relates to a new and novel method of and means for producing hi@gh frequenc y oscillations, phase and/or frequency modulati ng the same, and transmitting the resultant energy. 10 More in detail, the object of the present invention is to provide a new method of and means for producing high frequency oscillations aiad phase or frequency modulating the same in a novel manner with modulating oscillations or 15 voice frequency modulations obtained from a novel thermionic tube arrangemeiit which, by a single svtitching operation, may be made to operate as a modulating frequency oscillation generator, or as an amplifier for the voice frequencies, 20 with which the high frequency oscillations are to be phase or frequency modulated. The modulating frequency oscillations geiierated by this novel tube arrangement and the arr@plified voice frequenc y potentials, whichever is used for modu- 25 lating the high frequenoy oscillations, are passed through a symmetrical thermionic stage -,vhich affects the modulation in phase or frequency of the high frequency oscillations ge@ierated. An apparent advantage to b-, gained by the 30 use of the present novel method of and means for 'phase or frequency modulating high frequency oscillatio ns is that increased efficiency in operation is obtaitied and that the range and traffle speed of the transmitter of a given power has 35 been greatly increased. Anoth er advantage arising from the use of the present invention is that the thermionic tube used to produce the modulating oscillations may, by a mere reversal of position of a switch be used 40 as an amplifier for the voice frequenc@ oscillations so that the phase or frequency modulated transmitt er may be used either f or code signalling or for voice frequencies without the use of additional tubes. 45 In additign to the above, it is noted that the present phase or frequency modulated transmitter is admirably adapted for use with receiving systems without diversity reception and, since phase or frequency modulated wave telegraph 50 signalling systems are less subject to fading effects than systems known heretofore, the efficiency of reception is greatly increased. Nu merous other objects of the present invention, and advantages to be gained by the use 55

2 2,036,165 thereof, will become apparent from the following detailed description thereof and therefrom when read in connection with the attached drawings, throughout which like reference characters indicate like parts, and in which: Figure I illustrates diagrammatically the several 'Lu-iits included in a transmitter arranged in accordance with the present invention; while, Figure 2 shows the circuit arrangements of 10 certain units of Figure 1 which are constructed in accordance with the present invention. In Figure 1, C indicates a phase or frequency rnodulating unit connected on the one hand to a sgurce of constant high frequency oscillations 15 D, which may include an output buffer stage, on the other hand to a frequency doubler E or several frequency doubles in cascade which may feed an antenna system by way of an amplifier and frequency multiplier F. The high frequency oscil20 lations produced in D are supplied to C where they are caused to be modulated in phase or frequency by an amplitude moduiating stage B, which is supplied with either low frequency modulating oscillations or with voice frequency po25 tentials from a source A. The frequency multiplier or multipliers in E may be of anY tYPe known today, and, since this unit forms no part of the present invention, description thereof is thought unnecessary at this point. The same re30 marks apply to the frequency raultiplier and/or amplifier F and the radiating system connected therewith. The oscillation generator D may be of any type so long as the oscillations produced thereby are 3,5 constant in frequency and of constant amplitude. This oscillator preferably is of the long line frequency control type or of the crystal control type. The oscillator D may include a buffer output or, if it does not include a buff er output, a buff er 40 output may be interposed between the oscillator D and the phase or frequency modulator C if necessary. The phase modulator C comprises means including thermionic repeater tubes in which the 45 oscillations generated at D are repeated and varied in phase or frequency in a manner characteristic of the voice frequency potentials or the modulating frequency oscillations supplied from A by way of B. 50 The unit B includes thermionic repeaters or amplitude modulators by means of which the low frequency oscillations or audio frequencypotentiais originating in A are utilized in a novel manner to impress on the high frequency oscil55 lations repeated in C characteristic phase or frequency modulations. The unit B includes also the grid biasing filter network and the grid biasing potentiometers necessait to supply biasing potentials to the units A, B and C. This filter 6o network and potentiometer system is connected to a source I of direct current potentials. The unit B also includes the alternating current transformers necessary to supply filament current to the tubes in the units B and C. 65 The unit A includes a thermio.-iic tube and novel circuits cooperating therewith in such a manner that the tube may fun--tion as a low frequency oscillation generator or as a voice frequency ainplifier merely by a single movement of 70 a switch cooperating with the circuits of the tube. Tlae unit A also includes the audio frequency transformer necessary to impress voice frequencies from the source 2 on to the thermionic tube th6rein when the latter is to be used as a voice 75 frequency amplifier. Alternating current for the thermionic tubes in the units A, B, and C is supplied from an alternating current source 3 connected by way of lea&@ to the unit A, which is in turn c-.niiected by leads to the unit B, in which the filament transformers 5 are included. The filament transform--rs in B are connected by filtered leads to the thermionic tubes in the unit C. The necessary plate voltages for the therrnionic tubes of the units A, B, and C are sup-,lied fro@n 10 a direct current potential source I by way of a unit G, wliieh includes filtering means for the direct current potentials, and a Dotent:c--,-,ieter arrangement whereby the proper voltages may be supplied to the different thei@mionic tul3es. 15 The manner in which phase or frequency modulations are impressed on the carrier w4Al "low be described. Reference v,7ill first be made to the unit C of Figure 2. A pair of ther--ri-@"@onic repeaters K and L, of the screen grid type, hpve 20 their control electrodes C) and co-nnect-d through leads to a blocking condenser IO, whiia is in turn connected with any source of constant high frequency oscillations, as, for exar-ipie, the source D. Oscillations from the source D are 25 impressed by way of condenser IC on to the el@,-ctrodes 6 and 7 substantially cophasally. T-.@e relative phase of these oscillations is sbi'@'Lrd, however, since the oscillations reaci--ng '@he grid 7 pass through a phase advane-i@,ig means in the 30 form of a variable condenser II, while the high frequency oscillations reaching the grid 6 pass through a, phase retarding means in 'uhe form of an inductance 12. The anode electrodes 13 and 14 are coiinected 'D 5 together and to the termir-al of a tank circuit @ 5 which includes an inductance I and a variable capacity 16. The tarik cir--uit 15 is tuned to resonance at the frequency of the oscillations suppliedfromd. Thehighfrequencyoscillatioi,)s, 40 slightly shifted in phase, and impressc(-l on to the control electrodes 6 and 7, are repep-ted and amplified in the tubes 7, and L, and the energy from K and L appears in the tal:ik eircui-t @ 5 shifted in phase an amount determii-ed by the 45 phase shift impressed on the energ-y by the elements 12 and I i. This energy may be suT3plied from the tank circuit to the radipting system by way of the units E and F through P, direct current blocking condenser 17. Anode potential for 50 the tank circuit 15 is supplied by v@Tay of a lead 18 from a potentiometer P included in the uiiit G and supplied with energy from the source @'@ by way of filter circuit FC. Energy for the fflaments of the tu'lles K aDd L 55 is supplied by leads 19 and 20 respectively froll'i the secondary windings of transformers T and Ti respectively included in the unit B and supplied by way of leads 21 from the source 3 connected with unit A. In order that the amount Of low 60 frequency hum impressed on the thermi.onic repeaters K and L from the fllc,,ment energizir).g source may be held at a minimu.,-n, the filament circuits thereof include hum filtering means in 65 the form of resistances r and i" connected in allel with the filaments of tubes K and L res@ectively, said resistances being tapped at the eiectrical center and connected by way of space current indicating meters MA and by-pass cand,-ns- 70 ers C' to ground GL. The resist,9,iices r and r' also form a low impedance path to ground for the low frequencies appearing in the input or output circuits. The low frequencies ae in this manner shunted around the high impedance of 75

the filament transfomer windings, and by-pass condensers C'-C' A low inipedance path for the high frequenry oscillations repeated in K and L is provided by by-passing condensers C' connected, as shown, in series across the terininals of the filaments af each tube and having their electri-cal center connected, as shown, to gr6und GL. Voltmeter V, shunted by by-pass condensers C', may be cGn10 nected across the terminals of the fflament to indicate the voltage applied thereto. Norirngl diree't current biasing potentials for the control electrodes 6 and 7 of tubes K and L is supplied by way of resistances Ri, R2 from lead 22' connected to 15 a movable point on a potentiometer resistance Pi associated with a filtering circuit P"C' comprising choking inductances and parallel condensers and connected with the direct current biasing souxce 0. The eurrent drawn by the control eleotilodes 20 6 and 7 of tubes K and L may be indicoted by meters M'A' inserted in the biasing current paths between 22 and the terniinals of P and P' alid shunted by by-pass condensers C'. The direct current biasing potential applied to the control 25 electrodes is indicated by a voltineter Vi. Applicant has now described the manner in which the high frequency oscillations originoting in D are applied shifted in phase, but in a sense cophasally, to the control electrodes of the tubes 30 K and L, amplified and repeated therein, and transferred therefrom substantially in phase to the tank circuit 15, from which they may be utilized. The manner in which these oscillations are 3 @. modulated in phase or frequency at signal frequency will now be pointed out. In the present embodiment of the invention, the modwation is accomplished by iinpressing the modwating frequencies iii phase opposition 4,-' on the screen grid electrodes 24 and 25 of tubes K and L respectively by way of sliielded leads 2 and 27 connected, as shown, tG the anodes. 28 and 29 ofthermionic tubes M and N respectively. The manner in whir-h the modulating potentials 45. are caused to, appear at the terrninals of leads 26 and 27 will be described hereinafter. The effect of said potentials being applied in phase opposition to the screen grid electrodes 24 and 25, and the manner in which said signal 50 oscillations are applied thereto in opposition, rqodulated in phase or frequency, and the high frequency oscillations repeated therein, will be set forth now. Assume that no modulating frequencies are ap55 plied to the screen grid electrodes of tubes K and L. The same direct current. biasing potential is supplied to the control electrodes, and the high frequency oscillations impressed on the control electrodes are substantially in phase, being shift60 ed only in difflerent senses by the impedanceg of the inductance 12 and capacity II. The anode circuit of tube K will supply, therefore, to, the tank eirewt, energy equal in amount to the energy supplied to the tank circuit 15 from the 65 anode of tube L. The energies from these tubes will be of different phase and the energy supplied from the tank circuit to the frequency doubler or ne3ct stage will have a resultant phase determined by the phase of the separate energies supplied 7( from the tubes K and L so that in this case the carrier oscillations will have no change or shift in phase. The energy supplied to. the tonk circuit 15, from tl-ie tube which has the lughest modwating 75 potential applied to the screen grid electrode, 3 will be greater in amplitude than the energy supplied to the tank circuit 15 from the tube having 9, lesser modwating potential applied to its screen g-rid. This is due to the fact that the effective poten-tiai applied to the screen grid of a screen 5 grid tube governs the internal impedance of the tube, the arnpli-fication factor thereof, and other characteristics. Each of these variables in turn effect the amount of energy supplied to, the anode eirr-uit of the tube. Therefore, varying in p"@-.ase 10 opposition the effective potentials of the screen grid electrodes of the tubes K and L, by modulating potl-ntials from the lines 26 and 27, varies the amount of energy said tubes will supply to tho tank circuit 15. 15 The maximum possible phase deviation of the carrier is determined by the adjustment of impedances II and 12 of the grid phase shifting circuits. The phase shift, therefore, of the resul-tant energy in the tank circuit 15, is limited 20 by the adjustments applied to the phase shifting circuits. The frequency of the phase de-viatio.-i of the catrier is determined by the -@,nodulating frequency. The a-rnount of phase deviation of the carri4@r is determined by the airpli.tude c)f the 25 modulating frequengy and is proportionol to the amplitude of the modltlating frequengy. The amount of phase shift or deviation of the carrier or the tank eire-uit oscillations may, accordingly, be lirnited by the adjustments of the phase 30 sbifting circuits connected with the input of the tubes. The amount of phase shift or deviation of the carrier or the tank circuit oscillations is deterfnined by the amount or difference of powe,@ delivered by one tube over the power delivered 35 by the other tube, and by the phase difference of the energy delivered by both tubes. In other words, the phase shift of the oscilla;Lions in the tank Gire-Wt 15 is the result of the increase of energy delivered by one tube wbile th@-- energy 40 delivered by the otlier tube decreases a like aniount because of the phase diff er,-cnce in the alternating ettrrent energy delivered by each tube, and is proportional to said increase and decrease. 45 The phase shifts which take place, as indicated above, will be multiplied or increased by a, freciuency niwtiplier in case the oscillations from the tank circuit 15 are fed by way of a frequ-en--y multiplier or multipliers to th-@ utilization cizeuit. -50 As pointed out briefly hereinbefor.-, one of the novel features of applicant's inventioll resides in the means for supplying, by the use of a single tube, -either low frequency oqc:'Ilations or aliplifted voice frequency piotentials to the unit B, 5,5 wherein they are utilized to apply in phase. o.,oposition the modulating potentials to the screen grid electrodes of the phase modulator C by way of leads 26 and 27. The low frequency oscillation genei,ator or am- 60 plifier comprises a thermion,'@c tub@a X having a cathode 30, control electrode 3,1, a-.,@d anode 32. When the tube is to be used as a low fr.3quency oscillations generotor the movable blades of the switch 34 are moved to the left and the colltrol 65 electrode 31 is connected by way of parasi4@-ic prevention resistor 33, switch 34, and lead 35 including blocking condenser 36, to one temlinal of an ind-uctailee 37, tlle other terminal o'L ivhich is connected by way of lead 38 and switch 34 to lead 39, 7 Which i8 connected to the anode 32. The direct cutrent grid bias eireliit is coinpieted by @vay of a grid leak resistance 40 connected between one terminal of the condenser 36 and ground, which is the same as egnnecting the grid 31 to the 75

4 2,036,165 ground by way of resistance 40. The voltage drop across grid leak resistor 40, caused by the rec'bified grid current passing through it, supplies grid bias voltage to the grid of tube X when it r acts as an oscieation generator. The direct current anode circuit which furnishes the charging potential to the anode 32 is completed by a lead Iii connected to a movable point on the inductance 37 and passing by way of switch 34, resistlo ance 42, and current indicating device M2A2 to a point on the potentiometer resistance P. This point on P is moved to a position at which chargiiig potentials, of a value such that sufficiently strong oscillat;@ons are produced in the tube X, are 15 applied to the anode of the tube. The rneter M2A2 in the lead 41 permits checking of the direct current floiving in the anode circuit of tube X ,,@7hen it acts as an oscillator. The filament 30 of tube X is supplied with alternating current from 20 the secondary winding of a transformer T2, the primary winding of wmeh is connected with the line 2 1. When the electrodes of the tube X are energ@Ized, sustained oscillations ai:e produced in the 25 circuit including the inductanee 37 and capacitors TC which are coupled between the anode 32 and control electrode in a well known manner. The frequency of these oscillations generated is determined by the amount of capacity connected 30 in parallel with the adjustable inductance 37. The amount of capacity connected in parallel with the inductance 37 may be changed in pred,,termined increments by connecting one or more of the circuit tuning condensers TC in parallel 35 with the inductance 37 by closing one or more of th@. keys S, SI, S2 and S3. The oscillations produced in the anode grid circuit of tube X are short-circuited around the anode potential source and associated resistances by a pair of by-pass 40 condensets C' connected, as shown, between the lead 41 and the two leads 43 betwe.-n the f!Iament 30 aiid the secondary winding of the transformer T2. In order to prevent any hum at the filament energizing current frequency from occurring in 45 the tube X and associated anode circuit, and also to act as a by-pass to ground for low frequency ewaent, the electrical center of the balancing resistar@c-- 7-2 is connected to ground GL. The low frequeiicy alternating current potentials devel50 oped in @Lhe taik circuit including the inductance 37 and the selected tuning condenser TC are suppl-"ed by way of coupling condenser 44 to the ter1-.Liinal of a potentiometer resistance 45, the other terminal of Nvhich is connected directly to ground 55 C-L, thereby supplying a potentiometer load circuit. Tl,iese potential variations at the frequency determined by the oscillation circuit 37, TC, are fed by way of leads 46 from potentiometer 45 alid 60 switch 34 to a line 47 which terminates in the primary winding of a modulation frequency transformer MT, from which they may be utilized to act through the unit B to. modulate in frequelicy or in phase the oscillations from unit D 65 repeated in the unit C in a manner NvMch will be pointed out hereinafter. In order that the ampl,itude of these modulations may be adjustable or varied, one of the leads 46 is connected to a movable point on the resistance 45, as shown. 70 Most of the oscillations which appear in the di,,ect current charging circuit 41, are filtered out by way of a. by-pass condenser C' connecting one terminal of the resistance 42 to ground. In the description of the unit A, given above, 75 it was assumed that low frequency oscillations were to be developed therein and impressed on the unit B, from which they were to cause frequency or phase modulation in the oscillations repeated in C. It will now be assumed that instead of rnodulating 'with low frequency oscilia- 6 tions other audio, frequency potentials, as, for example, voice frequencies, a-re to be used to modulate the carrier lepeated in C and, further, that the tube X of the unit A is to amplify these potentials or voice frequencies before they are fed 10 to the unit B to be used to modulate the high frequency energy repeated in C. To use the tube X as an ampl-"fier the blades of the multiple switch 34 are moved into a right hand position to close the circuits associated with the right hand 15 row of contacts. In this position the grid electrode 31 of tube X is connected through switch 34 in series with the secondary winding of a transformer T3 and by way of lead 48 to a point on a resistance P2 connected in parallel with the 20 filter F'C' which is energized from the direct current source 1. The connection 48 supplies direct current biasing potential to the grid electrode 31 by way of parasitic prevention resistance 33 to charge the grid to a point at which the tube X 25 operates as a good linear amplifier. The transforiner T3 has its primary winding connected in parallel with a potentiometer resistance 49, which is connected by way of leads 50 and jack 51 to the source of audio frequency osc,"Ilations 2. 30 In order that the amplitude or magnitude of the Pudio frcaueiicy oscillations applied to the T)rimary winding of transformer T3 may be adjustable, one of the leads 50 is connected to a r-r-ovable point on resistance 49. The input cir- 35 cuit for the modulating frequencies impressed on to the secondary winding of the transformer T3 is co,,rpleted by ivay of an audio frequency bypassii-ig condenser C' connecting the low potential terminal of the secondary winding of T3 to 40 gtound GL and to the electrical center of the cathode 1,0. In this manner modulating frequeilcies from the source I are impressed 1.lpon the input circuit of the tube A for amphfication. The modulating potentials or currents amplified 45 in the tube X appear on the anode 32 and are fed by way of lea,d '@>9 and switch 34, and one of the pair of leads 47, to the primary winding of traisformer MT, and from the primary winding of transformer MT by way of 'uh-, other I.-ad 60 of the pair of leads 47, switch 34, resistance 42, current indicating means M2A2, and lead 41' to the movable point on the potentiometer resistance P. In this manner a charging potential is applied to the anode of tube X in the 55 same manner in which it was applied when the tube X was used as an oscillations generator. The alternating current anode circuit is completed by way of audio frequency by-pass condenser C3 connected between one of the leads 47 60 and ground and cathode 30. Condenser C' prevents oscillations appearing in this anode circuit -from being transferred by way of lead 41 to the resistance P to modulate therein any otber oscillations which may appear from the other units. 65 Further and more complete filter-Ing of this circuit is insured by a by-pass condenser C' connected betw.-en one terminal of resistance 42 and ground GL. The modulating frequencies appearing in the 70 source 2 are impressed by way of transformer T3 on to the input electrodes of tube X, are repeated and amplified therein, and are impressed en the primary winding of the transforriier MT for a purpose which will appear hereinafter. 7A

Any ra.dio frequency osrillations appearing in the lines 50 from source 2 are prevented f-kom reaching the transformer T3 by means of radio frequency by-pass condensers 52 and 54 connected as shown. Condenser 53, connected in series with the primary winding of transformer T3, prevents any direct current from source 2 from reaching said winding and also acts as an impeda,nce to i:educe the volume of the low frelo quency voice currents which are normally overemphasized in an audio frequency trangformor. The manner in which the oscillationg at m6dulating frequency appearing in the piiihary winding of the ti7ansformer MT in the unit B, 15 whether they originate in the fube X, oeting as an oscillation generator, or originate in the source 2 and are amplified in X, are utill2ed by the unit B to produce frequency br phase Inodulation of the carrier repeated in the unit C 20 ivill now be desgribed. Two thernlionic tubes M and N have thdir contro-I electrodes, 60 and 61 respectively coni'leeted by way of parasitic preventionrosistonces 62 and 03 respectively and normo.Jly closed con25 tacts of the jack 64 and leaclg 66 to the opposite terniinals of the secondary winding of trtnsformer MT. In this nianner modulating frequencies or voltages are apt)lied in opposition to the control electrodes of tubes M and N4 Equal "O direct current biasing potentialg for the control electrodes 60 and 61 are supplied by way of a lead 66 connected orl the one hand to a movable point on the resistance P4 connected in parallel wit ' h the filter circuit F'C' supplied with energy 3;5 from the source 1, and on the other hand through a clirrent indicating device M3A3 to the electrical center of the secondary winding of the transformer MT, from which winding it is. applied by way of leads 65 and jacks 64 to the control elec40 trodes 60 and 61. The filaments 67 Arid 68 of tubes M and N are supplied with alternating he,iting current over lines 69 connected With the secondary winding of a transformer T4, the pri@ mary wind-.ng of which is connected With the 45 su-Dply leads 2 1. The thermionic tubes M and N are balancect with respect to the oscillationq in the filam0iit heating circuit by connecting the electrical midpoint of balancing resistancos r3 aiid r4 corinected 50 across the cathodes 67 and 68 rOspeotive!Y tb ground GL. These resistances r3 and r4 furiiish lo,w impedance by-pass paths around tra@sz former T4 to ground for low frequency oscihations. These resistances also telid tb iotevent 55 any hum at the frequency of the enetgi2itig source from being developed. The modulating potentials impressod ift oppo@ sition from the transformer MT to the i@ontr6l electrodes of the tubes M and N are roijeated 60 in inverse sense and ampl@ified I thereiii aiid @ippear in phase oppositi6n on the anbd6 eler,trodes 28 and 29 thereof respectively. 'Phe@ diroct current aiiode circuit of tube U is completed by way of a current indicating deviee DA4A4, lead 65 2 6, resistance 7 0, and lead 4 I' to, a point on the resistance P connected in paxallel With the filtering circuit FC supplied ftom the high voltage source 4. The anode direct eurrent dircuit of the tube N is completed by WAY oi current indi@ 70 cating device M4A4, lead 27, resiqtance 7 1, and lead &I' to a point on the resistance P. most of the audio and substantially all of the higher potential oscillations are by-passed atound the anode direct current energizing sottrde by way of 75 a by-pass condenser C' connected between the terminals of the res!8tances 70 and 71 and the gr6und GL@ In some cases the resistances 70 and 71 may be replaced by choke coi1g. However, in most cases the resistance give,,@ the more desirable wave form and is, accordingly, used. 5 The conductivity of the tubes M and N depends itpon the effective potential applied to the coiitrol eledtrodes there6f. The effective potential applied to thb control electrodes 6f the tubes M and N depends upoia the constant 10 direct current potential and the modulating poentia applied adding or in oliposition thereto. The current which flows in the anode circuits (yf the tubes M and N is deterniined by the condiletivity of the tubes' It therefore follows that 15 tho amouiit of current which flows in the anode circuits of tubes M %nd N varies at modulating frequency and depends upon the aniplitude of the modulating frequency. Since the current flowing in tho anode circuits of tubes M and N 20 respectively varies at modulatilig frequonc3t, the potential drop through resistances 70 and 71 also 'Varies at riloduliitiiig freqtiency. This potential drop through 10 and 71 being in negative sense to thO potential supplied by line 41 from P is 25 applied directly bY way of leads 27 and 26 respectiVeISr to fhe screen grid electrodes 25 and 24 of tubes L and K respectively. As pointect out hereinbefofo, the effect' - voltago on he screen grid 6lectrod6s of the teljeater tubes K and L de- 30 terinineg thi@ conductivitsr of said tubes and thorefdre deterniiiies thE@ ainount of current the tespective ttibi@s suplily to the tank circuit i5. obviotisly, the pbtcntial Variations at signal freqtt6iioy, aj)plibd from the terniiriqls of resistances 35 70 and 71 through- Iiiie8 26 and 27 modulate in unit C the carrier ftequency energy repeated in tubes K and L suppli6d fr(im the source D. As painted (iut before thi@ corrier frequencsr supplied to the grids 24 and 95 of tubes K and L is of 4( dqual arhplitude by shiftiiig oppo@itely an equal amount in lihase so that the carrior freqilency rbp6aied by tubbs K and L is also shifted oppositely btit eqilally in lihas6. Hotever, as the sig@ nal varies, tubes K aiid L have thoir screeii gtid 45 potoiitial s Vatied in phas6 oppbsition and consequently the amplitude of the carrier output energy bf these tubes is varied or iftodulated according to the signal. But since the anod(@s 13 and i 4 of tubes K and L ate connected in parallel 66 t6 the same point on tank circuit 15, the bmplitude modulation cancels and disappear@ blit phase iiiodwati on will appear. The phase of the o@cillations in tank circuit 15 Will approach that of the tube suliplying the most carrier energy. This 66 phase ghift of the oscillatioiis in the tank circuit 15 Will be prbpbrtional to the amount bf energy supplied by one tube over thot supphed by the other. The degree of phase deviation will be prolibrtio nal to the intensity or amplitude of th6 60 signal and the frequoney of phase shift will be the signal frequency. Alte rnating current for the filaments of all 6f the tubes in each of the units A, B, and C is supv- es 65 plied from a source 3 by way of protection de ic Po and lin6s 2 1. The amplitude of curre mAy be deti@rmined , by the variable resistbnee V . R connected in 8eries with one side of the line 21. Any oscillationg which have not been bypassed to grourid by the by-pass condensers de- 10 scribed hereinbefore are shunted atound th6 sourco 3 by by-pass condensers C' corinecting each side of the line 74 to grbund GL. To ftirther insure tha-t the o8dillat' ns, etc., 10 developed and worked with in the several units, 15 ive t

6 2,036,165 are cor.L-ined to said units, the leads, such as 41 and the leads bitween the transformers T and Ti and the filaments of the tubes energized thereby, are filtered by way of by-pass condensers C' connected as shown between said leads and ground GL. In a like manner the corresponding leads, where they enter the unit C after passing from the units B and G, are again filtered by means of by-pass condensers C', there being a by-pass io condenser connected between each line and the ground GL. In practice it is important that the tubes M and N of the unit B impart to the modulating frequency the same amplification or are other15 wise balanced in such a manner that the potentials appearing at the terminals of resistances 70 and 71 are similar in amplitude in order that the proper phase or frequency modulation is impressed on the carrier. 20 In order that the effect of tubes M and N on the modulating frequency may be determined and regulated, a jack 64 is connected, as briefly described hereinbefore, in the input circuit thereof in order that a monitoring circuit may be uti25 lized to check on the modulating characteristics of this unit. The monitoring circuit forins no part of the present invention and will not be described herein. The above description, and the drawings re30 ferred to therein, have been given merely for purposes of illustration since, obviously, many changes may be made therein without departing from the scope of the present invention. For example, different types tubes could be used in 35 the unit A wherein the modulating frequency oscillations are developed or the modulating potentials energized and, likewise, many changes could be made in the circuit thereof. In a like inanner, different type tubes, arranged in differ4 ent circuits, could be used in the unit B. Furthermore, it is clearly understood that other tubes than those shown n-light be used in place of the thermionic tubes in the unit C and that likewise the circuits in which said tubes are con45 nected may be departed from materially, without departing from the scope of the present invention. Having thus described my invention and the operation thereof, what I