US1958027A - Emission valve modulation system - Google Patents

Description

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H. A. WHEELER EMISSION VALVE MODULATION SYSTEM Filed Jan. so, 1935. 5 Sheets$heet 1 flare/d A Wee/er Y I P W ATTORN EYS R E Y L E E H W A H EMISSION VALVE MODULATION SYSTEM Filed Jan. 50, 1933 5 Sheets-Sheet 2 INVENTOR Harm/a /4. Wheeler ATTORNEYS May 4%, 19334 H A E E 1,95&@27

EMISSION VALVE MOD LATION SYSTEM Filed Jan. 30, 1933 3 Sheets-Sheet 3 iakalm l v ATTORNEYS hit Patented May 8, 1934 UNITED sTA'ras PATENT OFFICE to Hazeltlne Delaware Corporation, a corporation oi.

Application January 30, 1933,v Serial No. 654,326

19 Claims. (o1. est-2o) This invention relates to vacuum tube circuits, and more particularly to circuits for controlling the electronic flow in vacuum tubes.

In the use of thermionic vacuum tube circuits, particularly radio receiving circuits, it is common practice to impress alternating signal volt age upon the input electrodes of a tube and to apply another oscillating voltage in such a way that the currents at the output of the tube are modulated. An example of this type of modulation is found in the well-known vacuum tube modulators.

The present invention resides in an improvement over the above-described general type of modulator. This improvement depends on forming a virtual cathode in a vacuum tube, and on utilizing the virtual cathode as a source oi electrons for the signal translating or modulating section of the tube. The properties of the virtual cathode, particularly its electron density, and hence its available emission, are varied periodically in order to modulate the signal. The arrangement is therefore designated an emissionvalve modulator system.

The invention is preferably effected by utilizing a tube having a cathode, an anode and a modulator control grid therebetween upon which is impressed a source of signal voltage; Whereas the actual cathode furnishes the electron $2018! sion ion the entire tube, the modulator'control grid operates more directly on the virtual cathode, which is a cloud of electrons transferred from the cathode to a position nearer the modulator control grid. The virtual cathode is formed just outside of a screen located between cathode and modulator control grid, this screenbeing held at a positive voltage relative to the actual cathode. The number of electrons reaching the virtual cathode is controlled, and hence its density is caused to fluctuate,

cathode and the virtual cathode.

An important embodiment of this invention lies] in its application to a combined oscillator and; screen--is-"- "greatly increased modulator. In this arrangement, the inserted in the space path between-the inner and outer control grids, the screen acting also as the anode of an oscillator. Thus, the device has, in effect, two anodes with the outer or modulator control grid situated therebetween. An appropriate oscillatory circuit is associated with the cathode, the inner control grid and the screen to produce the oscillations. The effect thus produced in the space path causes by its valve action a modulation of the signal voltage impressed output of the bythe valve action .of an inner control grid located between the actual "and which is ordinarily maintained at a upon the outer or modulator control grid; there is thus provided an oscillator-modulator requiring the use of only one tube having a single actual cathode in a, unitary electrode structure. An advantage of this form of oscillator-modulator so lies in the fact that only a single tube is required to perform the two functions of producing oscillations and modulation. Although these two functions have previously been performed in a single tube, they have not been performed by automatic, if desired, as when an automaticvolume control is employed.

A particular advantage is realized in the oscillator-modulator form of the invention when a varying biasing potential, such as that of an automatic volume control is used to control the modulator; for the control can readily be applied to the outer control grid or to some other outer electrode without affecting the performance of the oscillatory system.

A very useful form of the invention is one which utilizes a. modulator tube in which there is provided a modulator control element which provides a"gradual cut-ofi; that is, the characteristic of gain versus biasing potential. has the form of a gradual curve. This is generally known as a variable-mu characteristic, and may have an exponential variation.

The various embodiments of the invention may conveniently be carried-out by the use of an emission-valve modulator tube having live or six, ormore, electrodes. I

The output of the;.modulator system can be by the use of an outer positive screen -in the modulator section ofthe tube, between anode and outer control grid.

The term fgrid or grid-like electrode as used. in this specification and in the claims denotes an electrode by, or through, which electrons may pass'. The term"screen, as used in the specification and claims, denotes an electrode by, or. through, which electrons may pass su 11o stantial positive potential with respect to the actual cathode.

The above and other features and adyantages of the invention'will become more apparent from the following detailed description when considered with the accompanying drawings of which:

Figure 1 is a schematic circuit diagram illustrating the principles and operation of the invention;

Fig. 2 illustrates an oscillator-modulator system in accordance with the invention, employing a pentode tube coupled to an antenna, the tube circuit being arranged to produce selfoscillations;

Fig. 3 shows a system similar to Fig. 2, but varying in details of the oscillatory system;

Figs. 4 and 5 show systems somewhat similar to Fig. 3, but differing therefrom in providing difierent operating potentials for the anode and for the oscillator screen;

Fig. 6 illustrates an arrangement similar to Fig. 2 except that a hexode tube is employed instead of the pentode of Fig. 2;

Fig. 7 is in general similar to Fig. 3, but employs a hexode type of modulator tube;

Fig. 8 is similar to Fig. '7, but in this arrangement the modulator screen is provided with a voltage difierent from. that of the oscillator screen;

Fig. 9 shows a system like Fig. 8 except for details in the oscillator feedback arrangement;

Fig. 10 illustrates a complete superheterodyne radio receiver employing a manual grid bias type of gain control applied to a hexode oscillatormodulator system in accordance with this invention;

Fig. 11 shows the oscillator-modulator portion .of the receiver of Fig. 10 to the left of dividing line '7878, which may be used if desired instead of the arrangement shown in Fig. 10; and

Fig. 12 shows a complete superheterodyne receiver equipped with an automatic volume control system applied to the modulator control grid of a hexode oscillator-modulator system in accordance with this invention.

Figure 1 is a schematic circuit diagram presented to illustrate the principle and operation of the circuit of the invention in a generalized way. There is shown an emission-valve modulator tube 10 provided with an electron-emitting cathode 1, and an anode 6. In the space path between the cathode and anode there are placed four grid- like electrodes 2, 3, 4 and 5, these four electrodes being situated in the space path at increasing distances, in the order named, from the cathode toward the anode. The schematic diagram shows the cathode and anode located at opposite sides of the tube, but this conventional form of illustration is only for the purpose of schematic representation. Any tube actually used for this purpose would be likely to have a more orthodox construction, such as a centrally located cathode, helical grids surrounding the cathode, the successive grids having increasing diameters with respect to each other, and an anode in the form of a cylindrical plate surrounding the grid structures, such as that described in my copending application Serial No. 654,327 filed concurrently herewith.

' A source of signals, designated S1, is connected between the grid 4 and the cathode 1, while a source of local oscillations S2 is connected between the grid 2 and the cathode. The output circuit of the tube is designated Z and is connected to the anode 6 and the cathode. For the purpose of rendering the system operative, there are provided sources or operating voltage making anode 6 and screens 5 and 3 positive relative to cathode. Thereare also provided voltage sources making grids 4 and 2 negative relative to cathode. Batteries 80, 81 and 82 represent sources of positive voltage for anode 6, screen 5'and screen 3, respectively; and batteries 83 and 84 indicate sources of negative potential for grids 4 and 2, respectively.

The positive voltage on screen 5 should gener-.

ally be less than that on anode 6 with respect to the cathode. This screen may often be omitted, but in many embodiments of the invention its inclusion serves to improve the general operation.

In the operation of the above-described system, electrons emitted from the cathode l are attracted through the meshes of grid 2 to the screen 3, by virtue of the positive voltage applied to the latter. The electrons approaching screen 3 are traveling at high speed and most of them, therefore, pass on through the screen and approach the grid 4 which is ordinarily negative. The grid 4 therefore serves to retard the progress of the electrons and many of them are attracted back toward the positive screen 3. This cloud of retarded electrons hovering between the electrodes 3 and 4 may be said to constitute a virtual cathode with respect to the succeeding electrodes 4, 5 and 6 of the modulator, because of the fact that electrons can be easily drawn away from the cloud in the same manner that they were originally drawn away from the actual cathode. Electrodes 3 and 4 are a pair of electrodes relatively close together, the said electrode nearer the actual cathode having impressed thereon a substantially more positive voltage than the other of said electrodes. The virtual cathode and its approximate position is indicated by the dotted line 7, it being understood that the line 7 does not represent a physical element of the tube. The positive potential of the anode 6 and screen 5 serve to attract electrons from the virtual cathode to the anode, through the input grid 4 and the screen 5, in the usual manner. The tube therefore has, in efiect, two anodes, namely elements 3 and 6, with the inner grid alone controlling the entire emission from cathode 1 and the space current to both anodes. It will therefore be observed that the electrodes 4, 5 and 6 together with the virtual cathode '7 function as an ordinary signal translating vacuum tube of which the input circuit is the source S1 and the output circuit Z is connected between anode 6 and the virtual cathode (in so far I as alternating currents are concerned). I'he signal translating electrodes are 4 and 6.

Modulation results in the system in the following manner. When the grid 2 is only slightly negative, or is somewhat. positive, there is an abundant supply of electrons at the virtual cathode 7 available to supply an electron stream in the modulator section of the tube. When the grid 2 swings considerably negative, the virtual ing a frequency equal to the sum or difference of the source frequencies.

The elements of the tube, other than the actual cathode, may be considered to be situated in two groups: (1) elements 4, 5 and 6 associated with source S1; with source S2. The elements within each group are mutually adjacent, but all the elements of group (1) lie beyond those of group (2) Itis possible to control the output or conversion gain of the modulator over a wide range of intensities by varying the negative bias applied to the modulator grid 4. It will be observed that th's type of control does not materially affect the behavior of the oscillatory portion of the system because the grid 4, regardless of its potential, is incapable of cutting off the electron current to the oscillator screen 3, which is the current requiredto maintain osciflations. It is therefore an easy matter to effect automatic volume control by automatically biasing modulator grid 4 more negative when the received signal strength increases. This type of automatic volume control is shown in detail in subsequent figures.

The operation of the system can often be improved, particularly when a bias operating volume controlling system is applied to modulator grid 4, by making th's modulator grid of the variable-mu or gradual cut-ofi type. This can be easily done, for example, by constructing the grid 4 with a mesh which varies from one end to the other. The result is that the modulator is provided with a characteristic of gainversus grid bias which is in the form of a gradual curve, such as an exponential curve. This kind of grid gives a gradual variation of space current when the bias voltage is varied; and there is a very gradually changing output with varying bias voltage, over a wide range of output and bias voltages. This type of characteristic permits a wide range of negative biasing potentias to be employed, even very high negative potentials, without renderingthe modulator inoperative or subject to excessive distortion.

The reason for the above satisfactory results can be readily shown by considering what would happen in the case of a constant-mu or sharp cut-off tube. In such a case, a high negative biasing potential produces operation below the relatively sharp bend of the tube characteristic curve, whereupon variations of the anode current are greatly reduced and the tube is rendered inoperative.

Figure 2 illustrates an oscillator-modulator system constructed in accordance with this invention, employing a pentode or five-electrode type of signal translating tube 9 which is similar to the hexode or six-electrode tube 10 of Fig. 1, except that the screen 5 is omitted. The system is associated directly with the antenna system of a radio receiver. The antenna 11 is connected to ground through the usual form of antenna coil 12 which is inductively coupled with the tunable input circuit of the'receiver, comprising the tuning condenser 14, a fixed condenser 15 and an inductance 13, the latter beingcoupled to the antenna coil. The tunable input circuit is connected with the modulator control grid 4 of the oscillator-modulator tube 9 and is likewise efiectively connected through ground to the cathode 1 of the tube, by virtue of the grounding of the cathode system through resistance 21, condenser 22, and the lower end of a coil 19.

A portion of the tube '9 likewise forms an inand (2) elements 2 and 3 associated tegral part of an oscillatory system which comprises a tuned oscillatory circuit including inductance 19 and a variable condenser 18, this oscillatory circuit being connected between the oscillator grid 2 and the cathode 1. For the purpose of producing an electron flow from the cathode, and also to complete the oscillatory system, the oscillator screen 3 is connected through the voltage source 23, shunted by a bypass condenser 24, to ground; so that there is a complete circuit from the screen 3 through ground to the cathode 1, through part of coil 19 and the condenser- resistance connection 22, 21. The mutual inductance between the two portions of coil 19, and also condenser 18, constitute an oscillation-producing feed-back coupling because they are common to the cathode-screen circuit and the cathode-grid circuit of the oscillator, so there is produced feedback of energy from the screen 3 to the oscillator grid 2 which produces constant sustained oscillations. The auxiliary electrodes 2 and 3 may therefore be designated the oscillation-producing electrodes, or means for modulating the signal. It will be observed that in so far as the oscillatory system is concerned, the screen 3 acts as an anode. It is noted that the control grid and anode are located in the space path beyond said oscillationproducing electrodes.

By virtue of the negative bias applied to control grid 4, the virtual cathode exists at '7, from whence space path current flows to the modulator anode 6. The modulator anode circuit is completed through the primary winding of a modulator output transformer 20, and the voltage source 23 bypassed by condenser 24.

In operation, a signal voltage across the signal tuned circuit is impressed on the control grid 4, and modulation between the signal frequency and the frequency of the oscillatory system is produced by virtue of the valve action of the oscillator upon the electron emission stream. The oscillatory system is a means for causing the electron density of the virtual cathode to fluctuate periodically and thereby modulate the signal voltage. This may be called space path modulation. The resulting products of modulation appear across transformer 20 which lies in the modulated signal output circuit. In this case, the transformer 20 is tuned by the fixed condensers 85 and 86, across the transformer windings, to the well-known intermediate fre quency produced in superheterodyne receivers, this intermediate frequency being the difference between the signal frequency and oscillator frequency.

Tube 9 may be subjected to gain control or volume control by applying a bias control voltage to grid 4 which is negative relative'to cathode l. One way of connecting this bias voltage is indicated in Fig. 2 as follows. The bias voltage is connected to point 17, and impressed on the amplification, or gain-control grid 4 through resistor 16 and coil 13. This constitutes a biasvoltage control of amplification; and the arrangement may be termed a bias controlled modulator stage. Condenser 15 prevents the flow of alternating currents in the bias circuit.

It is usually desirable to incorporate in radio receivers an automatic volume control system, now well-known inthe art. Such a system is claimed and described in my United States Patent 1,879,863, issued September 2'7, 1932. In ac- I cordance with the operation of the automatic volume control system there is produced a unidirectional bias voltage which varies with the tuned signal strength. This variable bias voltage is impressed upon a gain-control element of a vacuum tube. It is desirable to impress such an automatic volume control on oscillatormodulator tubes used in superheterodyne receivers, particularly in those receivers employing very few tubes, as where the oscillator-modulator tube is the first tube of the receiver, as shown in Figure 2. Figure 2 is accordingly adapted for this type of automatic volume control, the negative automatic volume control bias voltage, variable with the signal strength, being connected at terminal 17 and impressed upon the modulator control grid 4 through resistance 16..

Automatic volume control as applied to the system of Figure 2 is made practicable for the reason that the volume control bias is not applied to the oscillator grid 2, but only to the modulator eontrolgrid 4. In the previously known types of oscillator-modulator utilizing only a single tube, the oscillator grid and modulator grid are identicaL or at least are interrelated in such manner that any biasing voltage applied to the control grid affects the oscillations, and even stops the oscillations when this grid becomes greatly negative.

Figure 3 illustrates a system quite similar to Figure 2, like elements being designated by the same numerals as in Figure 2. The system of Figure 3, however, has certain refinements or modifications not found in Figure 2. There is employed in the oscillator system a grid-leak 26 and grid condenser 25 which develop a negative average bias on the oscillator grid 2, relative to the cathode, which serves to limit the amplitude of oscillation and the oscillator grid current.

In place of the single tapped oscillatory coil 19 of Fig. 2 there is provided in Fig. 3 a pair of inductively coupled coils 27 and 29, the coil 27 being included in the oscillatory circuit. Feedback from the screen 3- (oscillator anode) is conveyed through a condenser 30 to coil 29 and fixed condenser 28 to ground, the condenser 28 and the coupling between coils 27 and 29 producing combined capacitive and inductive feedback in aiding phase. In this arrangement, the amplitude of oscillation is better sustained at the lower frequencies by virtue of condenser 28, and is preferably held uniform over the tuning range. The condenser 30 serves to convey the feedback currents and yet insulate the oscillatory circuit from the voltage source 38. A resistance 31 is provided to prevent the voltage source from shunting the feedback elements 28, 29 and 30.

Condenser 28 also makes possible the use of like condensers 14 and 18, ganged together by a uni-control device as indicated by the dotted line 8, while maintaining a uniform frequency difference between signal and oscillator circuits.

In this arrangement, the feedback voltage is incidentally applied to the anode 6, but this circumstance produces no great effect on the performance.

\ Figure 4 illustrates a system similar to that of Figure 3 but differing in two details. In Figure 4, the arrangement is such that the source of operating voltage 39 applies separate voltages to the anode 6 and to the screen 3, these different voltages being provided by means of resistances 34 and 33 connected respectively to the two electrodes 6 and 3, the battery 39 being connected to the other ends of these resistances which are joined together. Condensed 35 bypasses the resistor 34 and voltage source 39 in the anode circuit. In this figure, also, a grid leak 32 is connected across condenser 28 of the oscillatory circuit, and no separate grid condenser is required as in the case of Figure 8.

Figure 5 is similar to Figure 4 except that the feedback coil 29 of Fig. 4 is replaced by a coil 36 in series with the cathode and coupled to the inductance 27 in the oscillatory circuit. In this figure, also, a choke coil 37 is used to replace the resistance 33 of Fig. 4, the use of the choke coil in this position preventing a loss of operating voltage. This permits the use of a voltage source 40 having a lower voltage than source 39 in Fig. 4.

Figure 6 illustrates the embodiment of Figure 2 except that the hexode type of tube 10 is employed in place of the pentode tube 9, the extra elec-'- trode of tube 10 being the modulator screen 5 positioned between the modulator control grid 4" cuit by a fixed condenser 48, this arrangement.

providing somewhat improved results. Oscillator screen current is supplied through resistor 46 from voltage source 45 bypassed by condenser 47. The screens 3 and 5 are provided with the same voltage. It will be noted that the feedback operates simultaneously on both screens, although this circumstance produces no great difference in the resulting performance.

It is sometimes desirable to impress different voltages on the two screens, in which case, the arrangement of Figure 8 may be employed, Figure 8 being similar to Figure 7 except for the use of the separate screen voltage for screen 5. Voltage source 45 in Fig. 7 is replaced by voltage sources 49 and 50 in series in Fig. 8, bypassed by condensers 51 and 52.

Figure 9 shows an arrangement which is in general similar to that of Figure 8, the principal feature of Figure 9 residing in the rearrangement of the oscillator feedback system. In Figure 9, two condensers 54 and 55, in series, are used to replace the feedback coupling condenser 28 of Figure 8, the condensers 54 and 55 therefore each being made larger than condenser 28, but having the same resultant capacity. The voltage across only one of the condensers, namely condenser 55, is used for capacitive feedback in the oscillator screen circuit. The inductive feedback, in this embodiment, is provided by the coil 53 in the cathode lead; coupled to the oscillatory tuned coil 27, as in the case of Figure 5.

Figure 10 illi1strates a complete superheterodyne receiver employing a hexode oscillatormodulator system essentially like that of Figure 8. In this receiver, a manual amplification or gain control is provided in the form of a rheostat 61 connected between ground and a point 77 in the cathode circuit. When the rheostat 61 is adjusted for zero resistance, that is, for maximum gain, point 77 is grounded and the resulting circuit is exactly like that of Figure 8, in so far as the oscillator-modu ator performance is concerned. A slight difference from the arrangement of Figure 8 lies in the use of two resistors 59 and 60 the point between these resistors. This arrangement avoids the use of the two batteries 49 and 58 shown in Figure 8.

In the operation of the receiver, when the resistance of rheostat 61 is increased, the principal efiect is to make the cathode voltage more positive with respect to ground, at the same .time leaving the bias on modulator grid 4. connected to ground. The effect is that of placing an increased negative bias on the modulator grid relative to cathode,

thereby reducing the gain of the tube. The general operation of the system is not otherwise affected by this manual gain control.

The remainder of the receiver is of a conventional type and comprises a second detector 56 having signal rectifying electrodes for developing the audio signals by rectifying the intermediate frequencies present in the output transformer 20 of the oscillator-modulator. An audio amplifier 57 having signal amplifying electrodes is coupled to the output of the detector 56 from whence the signals are utilized in anydesiredmanner such as by a loudspeaker coupled to the output thereof.

Figure 11 illustrates a form of oscillator-modulator system well adapted for use in the receiver of- Figure 10. In Figure 11, only that portion of the receiver to the left of the dividing line 7878 in Figure 10 is shown. The oscillator-modulator of Figure 11 is very similar to that of Figure 10 with the exception that the biasing potential on the oscillator screen 3 is applied to the modulator screen 5 through a resistor 62 connected between the modulator screen and resistor 46. This arrangement obviates the use of the tworesistors and 59 of Fig. 10, but still prevents appreciable shunting of the feedback elements 28 and 48.

. Figure 12 illustrates a superheterodyne receiver embodying the invention, complete with an automatic volume control system applied to the oscillator-modulator tube. The oscillator-modulator system is similar to that of Figures 8 and 10.

The intermediate-frequency output transformer 20 serves to couple the oscillator-modulator 10 with. an intermediate frequency amplifier 63. The output-of amplifier 63 in turn is coupled through transformer 65 to a diode type of rectifier in tube 64. This tube is a well-known type of tube comprising both a diode and a triode having a common cathode 76, the diode elements being anode 66 and cathode 76, and the triode elements being cathode .76, control grid 72 and anode '79. A form of a satisfactory diode rectifier is more fully described in my above-mentioned Patent 1,879,863 and in my copending application Serial No. 526,857, filedApril 1, 1931.

.The output of the diode detector is coupled to an audio frequency amplifier 57 in any' well-known manner. 1

By virtue of the'rectifying action of the diode detector, there is developed at point 17 a unidirectional rectified signal bias voltage which varies in accordance with the signal strengthimpressed upon the diode. When' the signal strength increases, the rectified current flowing through resistances 67 and 68 makes the bias voltage of point 17 more negative with respect to ground. This bias is applied through resistor 16 and inductance 18 to the gain-control grid 4 of modulator 10, thereby biasing the modulator grid in proportion to the signal strength. As previously-explained, the automatic volume control arranged in this manner is especially satisfactory across the secondary coil of transformer 65 is impressed between the diode anode 66 and the point 17, the latter point being connected to the diode cathode 76 through resistors 67, 68 and 71 in series. These resistors are bypassed for radio-frequency currents by condensers 69 and 70, both having veryhigh impedance at audio frequencies. The rectified audio frequency'voltage, freed of radio-frequency compounds, which exists across resistor 68, is connected to the grid 72 of the triode section of tube 64, The cathode 76 of tube 64 is biased positively relative to ground by resistor 71. This bias prevents grid' current in tube 64 and also renders the diode inoperative at very small signal intensities where static or noise interference is objectionable. The audio voltage is amplified in the triode portion of tube 64, and applied from the ,anode 79 to voltage divider 73 which controls the volume level in the succeedin audio-frequency circuits, the volume level in tube 64 being controlled automatically.

Resistors 74 and 75 in series furnish a reduced positive voltage for the screens of tubes 10 and 63, relative to the anodes.

. In the arrangements described herein, the precise structure and number of electrodes may be varied over a wide range. The diagrams indicate tubes of five and six electrodes (pentodes and hexodes). Additional electrodes may be provided without departing from the spirit of the inventionand without destroying its advantages.

. Pentode tubes of type 57 or type 58 are operative in the circuits of Figures 2, 3, 4and 5. These tubes are characterized by having a cathode and an anode separated by three successive grid-like electrodes, the middle electrode being a screen of fine mesh, which serves well as the oscillator screen 3 in the pentode circuits described herein. The grid adjacent to the anode in these tubes has a uniform wide mesh and serves well as modulator control grid only in those cases where a wide range of gain control is not required in the modulator-tube. v 1 Best-results are obtainable with special sixelectrode tubes or hexodes. Referring to Fig. 1. grid 2 may be of uniformor varying mesh,.but uniform.meshis generally preferred. Screens 3'and 5 arepreferab'ly of fine mesh. The preferred mesh of grid 4 depends on whether a high gain or a wide For high gain. grid 4 is preferably of .uniform fine mesh. For a wide range of gain control, as

range of gain control is required.-

5 (if present) the average cathode current, the cathode current being substantially the sum of the screen and anode currents. This relation provides that changes in anode current have a negligible effect on the performance of the oscillator circuit.

The average negative bias on grid 2 relative to cathode l,'which is provided by resistor 26 or 32 in Figures 3, 4, 5, 7, 8, 9, 10, 11 and 12, is prefer ably related to the voltage on the oscillator screen 3, to provide that the space current in the tube is substantially zero during the negative half-cycles of the oscillator voltage on grid 2, that is, the space current is periodically reduced to zero. For this purpose, the oscillator control grid 2 having the constant alternating voltage thereon should be of the common sharp out-off type. This insures a complete valve action of the oscillator voltage as affecting the density of the virtual cathode and the performance of the modulator. This condition is referred to as complete modulation in the oscillator-modulator tube.

In Figures 3, 4, 5, 7, 8, 9, 10, 11 and 12, combined capacitive feedback 28 (55 in Fig. 9) and inductive feedback 29 (36 in Fig. 5 and 53 in Fig. 9) are utilized to maintain oscillations. These two feedback elements are preferably related to maintain the oscillation voltage on grid 2 nearly uniform over the tuning range. In the same figures, a condenser 30 or 48 (55 in Fig. 9) is shown in the feedback circuit. This condenser relative to all other elements in the oscillator screen circuit, is preferably proportioned to maintain the impedance between screen 3 and ground as small as possible without too far reducing the oscillator feedback or weakening the oscillations.

Figs. 2 and 6 have a negligible impedance in the oscillator screen circuit, the cathode and oscil lator grid being associated with the oscillator tuned circuit in a feedback relation. This arrangement minimizes the coupling of oscillator currents to the tuned antenna circuit which would otherwise occur through inter-electrode capacitive coupling between grid 4 and screen 3. This arrangement, however, is somewhat less effective as a modulator circuit, compared with the other arrangements shown.

In Figs. 10, 11 and 12, it is noted that the bias between grid 2 and cathode l is distinctly independent of the volume-control bias between grid 4 and cathode. This feature is desirable in most cases and is often essential to the successful operation of the oscillator-modulator system in the presence of gain control means.

I claim:

1. In a modulation system, a vacuum tube comprising an actual cathode, means for producing a virtual cathode composed of a cloud of electrons, said means comprising a pair of electrodes relatively close together, the said electrode nearer said actual cathode having impressed thereon a substantially more positive voltage than the other of said electrodes, an anode near said virtual cathode, a grid between said anode and said virtual cathode, a source of signal voltage coupled to said grid, and means independent of said grid and anode for causing the electron density of said virtual cathode to fluctuate periodically and thereby modulate said signal voltage, said means comprising an electrode interposed between said actual cathode and said virtual cathode.

2. In a signal translating arrangement, an oscillator-modulator system comprising a cathode, an anode and a grid between said cathode and anode, a source of signal voltage coupled to said grid, an output circuit coupled to said anode, and oscillation-producing means for modulating said signal in said tube, said means including two auxiliary electrodes interposed between said cathode and said grid, the said auxiliary electrode furthest from said cathode having a positive voltage applied thereto, and a tuned circuit coupled between one of said auxiliary electrodes and said cathode.

3. In a modulator vacuum tube having a cathode and a gradual-cutofl control grid, means for efiecting complete modulation which comprise a sharp cut-oi! control grid interposed between said cathode and gradual cut-off control grid, and a source of constant alternating voltage impressed on the interposed grid which periodically reduces to zero the space current of said vacuum tube.

4. In a signal receiving and translating arrangement, an oscillator-modulator system comprising a vacuum tube having a cathode, an anode and a first, second and third grid positioned in the space path in the order named, from said cathode to said anode, a source of positive voltage relative to said cathode impressed upon said second grid for creating an electron stream from said cathode, a signal input circuit connected between said third grid and said cathode, an output circuit connected between said cathode and said anode, and a tuned circuit connected to said cathode and coupled to said first and second grids for producing sustained oscillations.

5. In a superheterodyne receiver, an oscillatormodulator system comprising a vacuum tube having an actual cathode, an anode electrode and first, second and third grid-like electrodes located at respectively increasing distances from said actual cathode in the space path between said actual cathode and said anode, a voltage source impressing a positive voltage on said second electrode, and a voltage source impressing a negative voltage on said third electrode, whereby a virtual cathode with respect to said anode is 110 created between said second and third electrodes,

a tuned circuit system connected to said actual cathode and having oscillation-producing coupling to said first and second electrodes, and a source of signals connected between said third 115 electrode and said actual cathode, and an output circuit connected between said cathode and an electrode located beyond said third electrode.

6. In a signal receiving and translating system, a bias controlled modulator stage comprising a vacuum tube having a cathode, an anode and at least two grids interposed between the anode and cathode, one of said grids having a variable-mu structure and having impressed thereon the signal voltage and the bias control voltage, said other grid having impressed thereon an alternating voltage, whereby space-path modulation is effected in said tube, means responsive to the output of said modulator for causing said bias voltage to automatically vary in proportion to the received signal intensity, whereby the output of said modulator stage is maintained more nearly constant than the input over a wide range of received signal intensities.

7, In a superheterodyne receiver, an oscillatormodulator system comprising a vacuum tube having a cathode and two oscillation-producing electrodes near said cathode, impedance means including a tuned circuit coupled between said two electrodes and connected to said cathode for producing sustained oscillations in said tube, said tube including a control grid and an anode located beyond the path between said cathode and said two oscillation-producing electrodes in said tube, a source of signal voltage coupled between said control grid and said cathode, and means for applying a bias control voltage on said control grid, whereby the anode current is controlled and any effects of the bias voltage on 50 noaaoav the oscillations are minimized by the location of said grid and anode.

8. In a superheterodyne receiver, an oscillator-modulator system comprising a vacuum tube having a cathode and two oscillation-producing electrodes near said cathode, impedance means including a tuned circuit coupled between said two electrodes for producing sustained oscillations, said tube including a control grid and -an anode located in the space path beyond said oscillation-producing electrodes, a source of signal voltage coupled between said control grid and said cathode, means for developing a bias con trol voltage which automatically varies with the intensity of said signal source and means for applying said bias voltage to said control grid, whereby the intensity of the modulated signal in the anode circuit is maintained more nearly constant than said signal voltage over a considerable range of signal voltage at said source, without substantially affecting the intensity of said oscillations. v

9. A modulation system comprising a vacuum tube having a cathode, an anode and first, second andthird grid-like electrodes interposed in the space path, in the order 'named, from said cathode to said anode, a signal input circuit connected between said third electrode and said cathode, a modulated signal output circuit connected between said cathode and said anode, a source of positive voltage applied to said second electrode, asource of oscillations connected between said first electrode and said cathode, and.

' means for applying a. variable bias voltage upon said third electrode, whereby the output of said ,tube may be varied over a wide range.

10. In a superheterodyne receiver, an oscillator-modulator system comprising a vacuum tube having a cathode, an anode and at least three grids located in the space path between said cathode and anode, oscillation-producing feedback coupling between two of said grids causing sustained oscillatory currents to flow in said space path between said cathode and at least one of said two grids, asignal input circuit cou-' pled between a third of said grids and said cathode, and an output circuit coupled to said anode, causing said oscillatory currents to modulate the signal from said input circui means for developing a bias voltage which becomes more negative when the signal intensity increases, and means for impressing said bias voltin the space path, in the age on said third grid, thereby maintaining. the

output intensity in said output circuit more nearly constant than the input intensity over a considerable range of input signal intensities, and at the same time preventing any substantial variation of said oscillatory currents.

11. A modulation system comprising a vacuum tube having an input circuit and an output circuit, said vacuum-tube having a space-current path in which are included a cathode and at least two grids, one of said grids having a source of alternating voltage connected thereto, the other of said grids having said input circuit connected thereto and having impressed thereon a variable bias voltage more negative than said cathode, said other grid being of the type giving a gradual said bias voltage is varied.

12.'A modulation system comprising a vacuum tube having a cathode, an anode and first, second and third grid-like electrodes interposed order named, from said cathode to said anode, a signal input circuit 4 cathode, an

of complete modulation,

variation of space current when connected between said third electrode and said output circuit connected between said cathode and said anode, a source of oscillations connected between said first electrode and said cathode, a source of positive Voltageconnected to said second electrode, and a source of variable negative bias voltage applied to said third electrode for controlling-the output of said system, said third electrode being of the type which provides a very gradually changing output with varying bias voltage over a wide rangeof output and bias voltage.

13. A modulation system comprising a vacuum tube having a cathode, an anode and first, second, third and fourth grid-like electrodes interposed in the space path, in the order named, from said cathode to said anode, asignal input circuit connected between said third electrode and said cathode, a modulated-signal output circuit connected between said cathode and said anode, a source of positive voltage applied to each of said second and fourth electrodes, a source of oscillations connected between said first electrode and said cathode, and means for applying a variable bias voltage upon said third electrode, whereby theoutput of said tube may be varied over a wide range.

14. Means for performing on an electric signal wave in one vacuum-tube space path the combined operations of modulation, amplification, and bias-voltage control of amplification, said means comprising a cathode, an inner grid, an inner screen, an outer grid, an outer screen and an anode, all situated in said space path in the order named, said grids having a negative average bias relative to said cathode, said screens and anode being considerably positive relative to said cathode, said inner grid and inner screen having oscillation-producing coupling therebetween, said outer grid having a signal-voltage source coupled thereto, said anode having an output circuit coupled thereto responsive to the difference between the signal and oscillation frequencies, said amplification being controlled by the outer grid bias.

15; Means for performing on a signal in one vacuum-tube space path the combined functions amplification, and

gradual control of amplification, said means com.-

prising a cathode, a sharp-cutoff inner grid, an inner screen, a gradual-cutoff outer grid, an outer screen and an anode, all situated in said space path in the order named, said grids having a negative average bias relative to said cathode, said screens and anode being considerably positive relative to said cathode, said inner grid and inner screen having oscillation-producing coupling therebetween which periodically reduces the space current to zero, said outer grid having a signalvoltage source coupled thereto, said anode having an output circuit coupled thereto responsive to the difierence between the signal and oscillation frequencies, said amplification being controlled gradually by a variation of the negative bias on said outer grid.

16. A system having the means according to claim 14 in combination with means for causing the outer grid bias to increase automatically with increasing signal intensity, thereby maintaining the amplified signal voltage more nearly uniform than the signal voltage of said source.

1'7. A system having the means according to claim 15 in combination with means for causing the outer grid bias to increase automatically with increasing signal intensity, thereby maintaining the amplified signal voltage more nearly uniform than the signal voltage of said source. 1

18. The method of modulating a signal in a vacuum tube having a cathode, an anode and a spacepath therebetween, which comprises attracting an electron stream to an intermediate position in said path, retarding said stream to form a virtual cathode Just beyond said position, modulating the density of the virtual cathode by varying the attraction of said stream, further attracting a part of said stream from said virtual cathode to said anode, and causing said signal to vary the further attraction of said ,part of said stream, the modulation of the density of the virtual cathode thereby causing modulation of the signal in said vacuum tube.

19. The method of modulating a signal in, and controlling the modulated output of, a vacuum tube having an actual cathode, an anode and a new space path therebetween, which comprises attracting an electron stream to an intermediate position in said path, retarding said stream to form a virtual cathode just beyond said position, producing oscillations in the space path between said actual and virtual cathodes by varying the attraction of said stream, further attracting a part of said stream from said virtual cathode to said anode as output current, causing the signal to vary the further attraction of said part of said stream and thereby to produce in the output current path a current fluctuation of the frequency difference between signal and oscillation, and controlling the current in said output path by controlling said further attraction without substantially affecting the oscillations in the space between said actual and virtual cathodes.

HAROLD A. WHEELER.

. CERTlFlCATE 0F CORRECTION.

Patent No. 1,958,027.

( HAROLD A,

May 8, 1934.

WHEELER.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 4, line 74, for "Condensed" read Condenser; page 5, line 89, for "compounds" read components; and page 6, line 10, for "out-off" read cut-off; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record oi the case in the'Pa tent Office.

Signed and-sealed this 19th day of June, A. D. 1934.

,(Seall Bryan M. Battey a A in Ween es?! taffeta? D i S C L A l M E R 1,958,027.-Har0ld A. Wheeler, Great Neck, N. Y. Emssron VALVE MODULATION SYSTEM. Patent dated May 8,

"1940, by the inventor; the assignee,

1934. Disclaimer filed September 25,

Hazeltine Corporation, assenting.

Hereby enters this disclaimer to claims 1, 2, 4 to 14, inclusive, 16, 18, and 19 of said patent.

[Qficial Gazette October 22,?1940]

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