US2323250A - Signaling system - Google Patents

Description

June 29, 1943.

J. C. SMITH SIGNALING SYSTEM Filed 195s 2 Sheets-Sh t 1 J 50m 6351mm, By (J ms 7'TOEWE June 29, 1943. J. c. SMITH SIGNALING SYSTEM Filed Jan. 31, 1953 2 Sheets-Sheet 2 /)v Vf/V TOP- me G Swat T TOENE J]? BY 53g;

HIS

Patented June 29, 1943 SIGNALING SYSTEM Jerome C. Smith, Merchantville, N. 1., assignor to Radio corporation of America, a corporation of Delaware Application January 31, 1933, Serial No. 654,421 55 Claims. (01. 250-20) The present invention relates to signaling systems, and more particularly to signaling systems wherein two separate signal channels are united through a suitable mixing means and provided with a common signal output channel.

Signaling systems of the above character are employed in superheterodyne receivers and the like wherein the radio signal and the oscillator signal channel are combined through a frequency changing device and applied to a common output circuit or channel in the intermediate frequency amplifier. The invention, therefore, relates more particularly to the superheterodyne type of receiving system wherein its advantages are more fully realized.

In signal channel mixing systems of the character above referred to as heretofore known, considerable difliculty has been experienced in preventing interaction between the circuits and material changes in load upon one or both of the channels leading to the common output circuit, resulting in changes in the channel gain, or in the channel frequency response characteristic, depending upon the type of transmission system employed.

It is, therefore, a primary object of the present invention to provide an improved signal transmission system embodying apparatus and methods whereby two separate signal channels may be combined and provided with a common output circuit without appreciable interaction the one upon the other, and whereby said channels function independently of each other.

It is a further object of the present invention to provide a signaling system of the above character wherein the signal amplitude in the common output circuit may be controlled without atfecting certain operating characteristics of either of the input signal channels.

A further and more specific object of the present invention is to provide an improved detectoroscillator system for a heterodyne type of receiver employing a single electric discharge device as a combined detector and oscillator.

It is a still further and specific object of the invention to provide an improved signaling system wherein two signal channels are electronical- 1y coupled while being maintained electrically independent one with respect to the other.

A still further object of the present invention is to provide an improved signal transmission system wherein a single electric discharge device i employed as a mixing means between two separate signal channels and a common output signal channel and wherein the transconductance between certain of the control electrodes of the device and the anode may each be controlled by the signals to be mixed, and wherein means are provided to prevent interaction between said two electrodes and the circuits connected therewith.

A further specific object of the present invention is to provide a modulator, or modulated oscillator system, wherein two signal channels in connection therewith are prevented from interacting one upon the other to modify certain of the operating characteristics of either of said channels.

Another object of the invention is to provide a method of modulating electrical alternating current in a thermionic tube having a cathode, an anode, and an electron stream space path there-,

between, which comprises attracting an electron ing the alternating current to vary the further' attraction of said part of the stream, all in such manner that the modulation of the density of the virtual cathode thereby causes modulation of the altematlng current in the vacuum tube.

Still further objects of my invention are to provide such a method in accordance with which the density of the virtual cathode is modulated by producing oscillations in the portion of the space path of the tube between the actual and the virtual cathodes to thereby vary the attraction ofthe electron stream in accordance with such oscillations, and the current in the output of the tube is controlled by controlling the further attraction of electrons from the virtual cathode to the anode without substantially affecting the production of oscillations in the space between the actual and virtual cathodes.

In accordance with the invention, two signal channels to be combined and provided with a common output channel are connected with two, preferably negative, control electrodes or grids of an electric discharge device which has, as other electrodes, a cathode and anode, together with positive shielding electrode means between said first named grids or electrodes and preferably between said ilrst named electrodes and the output electrode or anode. The grids are interposed in the electronic stream 'between the cathode and anode in series relation to each other, that is, whereby the electronic stream traverses first the one grid and then the other, and in such a manner that variation in the potential of one of the control grids serves to vary the transconductance of the other control electrode with respect to the anode, or in other words, the gain between the other control electrode and said anode.

Stated in other words and more specifically, in accordance with the invention, two preferably negative and high input impedance grids are arranged in a common envelope of an electric discharge device to control, in series relation to each other, a common electronic stream, the grids being shielded the one from the other by another electrode, or other electrodes, at a positive potential, and said control grids being utilized to perform different functions and/or to operate at different signal frequencies.

The operation of the device may be considered to be such that there is formed between the positive electrode and the outer negative grid a cloud of electrons or a space charge, which can be considered as a virtualcathode capable of supplying electrons at a variable rate (as determined by the instantaneous potentials of the inner grid), which can then be acted upon by the outer negative grid in the usual manner, to produce the desired frequency conversion in a superheterodyne receiver for example, wherein the inner grid is the oscillator grid and the outer grid is the signal grid. J

A signaling system in accordance with the invention contemplates as a signal mixing means, a multi-element electric discharge device having between its anode and cathode a plurality of grid electrodes, and it has been found that in certain specific applications; such as its application in a superheterodyne receiving system, the connection of the oscillator and signal channel to certain of the grid electrodes provides an advantage, in addition to the independent frequency operating characteristic for each of the signal channels, of permitting a change of bias potential of one of the controlled grids to control the signal output through a common anode output circuit, also independently of the operating characteristics of the input signal channels.

By way of example, it has been found that a multi grid electric discharge device or tube may be arranged as above described,to function as a combined detector and oscillator in a superheterodyne receiver whilepermitting negative bias for one of the control grids to be varied, thereby to control the volume or amplitude of the signal output therefrom without appreciablyaifecting the average signal amplitude or frequency response characteristic of the oscillator portion of the system. With this arrangement, the control grid may be controlled manually or may be connected with an automatic volume control circuit in substantially the same manner as any other electric discharge device in the system to be controlled automatically in response to changes in the signal amplitude.

Likewise, it has been found that a radio frequency signal applied to a mixing device of the type above described through one of the input channels may effectively be modulated by audio frequency signals supplied through another input channel, without interaction between the two channels and with a relatively high gain.

In a superheterodyne receiver, a system of this character has .the further advantage that the oscillator may operate at a lower output voltage, the oscillator output is not coupled into the radio frequency system and only a single device or tube is required.

The invention will, however, be better understood from the following description when taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawings,

Fig. 1 is a schematic circuit diagramgof; a signaling system embodying the invention,',being specifically a combined detector-oscillator system for a superheterodyne type of receiver;

Figs. 2, 3, and 4 are curve diagrams indicating certain operating characteristics of the system shown in Fig. l; and

Figs. 5, 6, 7, and 8. are modifications of the system shown in Fig. 1. Like parts throughout the various figures are designated by like reference numerals.

Referring to Fig. 1, I0 is an input transformer for a radio frequency signal input channel II, and I2 is an output transformer in" an intermediate frequency signal output channel l3, be-

tween which channels is connected a combined in the form of a usual cathode l5, and an anode I6 between which, in spaced series relation, along the electronic stream, are interposed three other electrodes or grid elements l1, l8, and I9, being,

respectively, a first or inner negative control grid, a screen element or grid, and a second or outer negative control grid. The first control grid I1 is connected, as indicated, with the signal or radio frequency input channel through the secondary 20 of the input transformer Ill, and the grid circuit therefor, indicated at 2|, is returned to the cathode l5 through a suitable source of negative biasing potential 22 and a return ground lead 23.

The second control grid i9 is connected with an input circuit coupled to the anode l6 through a coupling transformer 24 having a primary winding 25 of low impedance connected in the anode or output circuit 26 of the device, and having a secondary 21 tuned by a suitable variable capacitor 28. The tuned secondary is connected between the grid IS and the cathode l5 through a suitable source of negative biasing potential 23 and the common return lead 23.

The intermediate frequency output transformer I2 is tuned to the intermediate frequency by suitable condensers 30 and is interposed between the intermediate frequency output circuit l3 andthe anode circuit 26, the primary of the intermediate frequency output circuit being connected with the anode IS in series with the primary 25 and a source of anode potential 3| which is in connected at its negative side to the cathoderreeturn lead 23, all as indicated.

The screen electrode l3 interposed between negative potentials, while the anode and interfrom each other by at least one other positive I electrode.

asaaaso The eifect of this arrangement is such that the two control grids have no interaction one upon the other and, therefore, do not serve to load the associated circuits and thus vary the frequency response characteristic or amplitude of signals applied to the device or tube I through either the transformer l and th radio frequency signal channel II or the transformer 24. In this sense the control grids l1 and I9 act as two impedance controlling elements for independently varying the impedance presented to the electron stream between the cathode l6 and the anode l6.

In the present example, through the intercoupling of the anode and the electrode I! through the tuned transformer 2|, the grid ll may be caused to operate as an oscillator at any suitable frequency within the tuning rang of the condenser 26 and secondary 21. In a superheterodyne receiver, this frequency is ordinarily such that, combined withthe input signals, by the electron coupling in the tube, it will cause an intermediate or beat frequency, equal to the difference between the oscillation and incoming signal frequencies,to be delivered to the output circuit or output or intermediate frequency channel represented by the terminals ii.

The control grids I1 and I9 operate preferably on a linear portion of their anode-current, gridbias characteristic curve and are spaced with rapect to each other and the anode l6 common to them so that the transconductance from each s of them to the anode are of values such that the one grid, which is the grid I! in the present example, may be utilized to receive radio frequency signals, while the other. grid, which is the grid I9 in the present example, may be utilized to obtain oscillations.

The relation of the transconductances and the eifect of varying a signal potential on one control grid to vary the transconductance or mutual conductance between the other control grid and the anode may be more clearly understood by referring to the curves indicated in Figs. 2, 3 and 4. In each of these curves the transconductance or mutual conductance is represented as the ordinate and the variation in grid bias potential on one of the control grids is indicated as the abscissa.

Referring to Fig. 2 particularly, there is shown the transconductance Gml between the anode current Ip and the signal potential E on the first grid ll as the potential E8: on the third or other control grid is varied as indicated by the curve 33; a potential X, for example, applied to the third grid l9, producing a mutual conductance Y between the potential on the first grid i1 and the anode l6.

In a like manner, the curve 34 in Fig. 3 represents the mutual conductance Gm: between the anode current In and the signal potential EgIi'OH the third grid l9, as the potential Egl on the first grid I1 is varied.

From a consideration of these two curves 33 and 34 above, it will be seen that the combined efiect of two signals applied to the control grids I1 and I9 will appear in the output circuit connected with the anode l6. However, from a consideration of Fig. 4, wherein the mutual conductance Gm3 between the grid current In of the second grid or shield l8 and the signal potential E 1 applied to the first grid H, for example, is

. varied in accordance with changes in the potential E 3 applied to the third grid i9, we see that the mutual conductance curve 35 is substantially a straight line, indicating that there is substantially no interaction between the screen and the signal applied to the third grid i9.

From a further consideration of the curves shown in Figs. 2, 3 and 4, and the foregoing description, it will be seen that the current flow through the second grid or electrode is substantially constant with respect to variations in the potential applied to the control grid II. The second grid may, therefore, not only serve as a screen between the two negative control grids or electrodes, but it may also serve as an anode in circuits wherein its substantially constant anode current may be utilized to advantage. Such circuits will hereinafter be described.

From the foregoing description it will be seen that the two, high impedance, negative, signal input grids are shielded from each other by a positive electrode interposed between them in a common envelope, and that the two control grids operate to control the electronicstream between the anode and cathode in series, so that the variation in potential of one negative'grid controls the transconductance of the other negative grid with respect to the anode and thereby,

in effect, controls the gain between the other control grid and said anode.

I Stated in other words, the .signal applied to one control grid serves to control the gain through the electric discharge device from, the other grid, and since each grid, in effect, forms a terminal means for a separate signal channel, the signals transmitted through said channels to said grids are thus-eifectively united or mixed and transferred to the output circuit. In thepresent example, the oscillator grid and the signal grid receive signals at such diflering frequencies that the resulting signal delivered to the output or intermediate frequency circuit I6 is at the desired intermediate frequency, the combined signals on the control grids l'| and II being rectified in the main anode circuit of the tube.

In certain applications of the system of Fig. 1 it is desirable-to increase the anode impedance and the transconductance between the second or outer control grid l9 and the anode l6, and at the same time to remove from the output anode circuit 26 and its connection with the intermediate frequency circuit l3 the oscillator function introduced through the coupling provided through the transformer 24. To this end, the tube It may be provided with a fourth electrode or a screen element 36 between the anode l6 and the second control grid [9, as shown in Fig. 5

to which attention is now directed.

' circuit of Fig. 1. Otherwise, except for adiffer ence in the potential supply system, the circuits and the device H are identical with those of Fig. 1, as will be seen from a further inspection of Fig. 5.

It will be seen that the bias potentials for the control grids l1 and I! are derived from a self bias source 31 in the anode return lead I6, and that the cathode I5 is provided with a heater 39. Likewise, the positive potentials for the screen elements It and 36 are derived from a suitable potential divider source 46. Asshown, electrodes l6 and 36 are less positive than the anode l6.

'By providing the transformer 24 with a low impedance primary winding 25, the electrode 36 provides a more effective shield or screen between the anode l6 and the control electrode l9. This arrangement has thefurther advantage that the-output circuit 26 from the anode l6 may be led directly through the output device l2 and to the utilization circuit. In apparatus having shielding, this has the advantage that the output circuit may be more completely shielded beginning directly with the anode I6.

The signaling system shown and described in connection with Fig. l, embodying an electric discharge device having two separate control grids in series relation to each other, along an electronic stream in a common envelope, -with a positive shield or screen member between them, is readily adapted by slight modification to provide an effective modulator system, as will be seen by referring to Fig. 6. The tube It with its positively energized anode l6, cathode l5 and inner and outer control grids l1 and I9 separated by the screen electrode l8 which electrostatically isolates them from each other is shown in connection with a modulating channel or signal input circuit having terminals 4| connected to the control grid l9 through a coupling transformer 42. The modulating channel is adapted to operate at audio frequencies, being provided with an audio frequency input transformer. However, other modulating signals may be used.

In the circuit shown in Fig. 6, the grid l9 receives signals from the modulating channel ll as one source of electrical oscillations, while the radio frequency signals are supplied through the channel II from another source of electrical oscillations to the inner or first control grid ii. The combined signal, i. e., the modulated radio frequency signal, is thus supplied to the anode l6 and the output circuit 26, thence to the utilization circuit l3 through the output transformer I! which. in this case, is tuned to the signal frequency.

In the event that the modulating signal and the input signal are of the same frequency, such as in the case that the signal channel II conveys a modulated intermediate frequency signal and the modulating channel ll conveys the same intermediate frequency signal without modulation, then the output circuit will receive the audio frequency component of the modulated intermediate frequency signal. A system of the above character will hereinafter be described in connection with Fig. 8.

The circuit, therefore, does not differ fundamentally from that shown in Fig. l. A difference is that grid I9 is not utilized to generate local oscillations, but instead is utilized to receive signals at the modulation frequency. The control effect of each control grid upon the mutual conductance between the other grid and the anode is the same as described in connection with the circuit of Fig. 1. It will, therefore, be seen that the system is readily adapted for use as a modulator, wherein the two signal channels may be operated without interaction one upon the other. Like the circuit shownin Fig. 1, the system has the advantage that modulation may be effected through the use of relatively low audio frequency modulating potentials. Also, no audio frequency power is necessary for the reason that the grid I9 is a negative, high impedance control electrode. This may be expressed in other words by statingthat grid I9 is so negatively polarized or energized that substantially no current is thereby drawn from the elecembodiment and in the preceding embodiments of the invention, the radio frequency signal is placed upon the first or inner grid, while the modulation signal in the present example, and the oscillator signal in the preceding embodiment, is placed upon the second or outer control grid. This is a preferred arrangement, particularly in a modulator, for the reason that the lower frequency signals, such as the audio frequency modulation signals, are less effected by the anode to grid capacity than are the radio frequency signals. In other words, with the grid arrangement shown, the radio frequency signal circuit receives no appreciable change in load, whereby its operating characteristic is changed in response to variations in the potential existing in the output or anode circuit.

Referring now to Fig. 7, a combined detectoroscillator system is shown, wherein, as distinguished from the circuit arrangement of Figs. 1, 5 and 6, the connections for the signal'or control grids are reversed, thereby placing the radio frequency signal on the outer or second control grid and the oscillator signal on the first or inner control grid.

In Fig. 7 a radio frequency signal circuit 45 is supplied with radio frequency signals from a channel, represented by input terminals 46, through a coupling transformer 41, the secondary I 66 of which forms a part of the tuned radio frequency circuit. Locally generated oscillations for beating with the incoming signals are provided in a tuned oscillator circuit 49 which is connected through a compensated filter 50 and suppressor resistor 5| to the inner or first control grid electrode 52 of a multi grid electric discharge device or tube 53 and serves as an input circuit to the grid 52. The tube 53 is provided with an anode 54 and an output anode circuit which is common to the input circuits 45 and 49 of the control grid electrodes 52 and 66 respectively and is coupled to an output or intermediate frequency signal channel, represented by terminals 56, through a tuned coupling device 51. Anode operating potential is supplied to the anode circuit 55 through a supply lead 58 connected with a suitable source of direct current 59 having supply terminals 60-40.

The cathode of the tube 53 is indicated at 6| and is connected through a self bias resistor 62 and a grounded cathode return lead 63 to an intermediate tap point 63a on the potential source 59. The polarity of the potential source 69 is indicated in the drawings, and the radio frequency input circuit is connected therewith through a coupling resistor 64 to a variable tap point 65, normally indicated as more negative than the cathode connection point 630, whereby the outer signal control grid indicated at 66 and connected with the signal input or radio fre- .through'a' suitable supply lead 13.

- bias resistor 62,while the second electrode 61 operates at a positive potential whereby it forms the plate or anode electrode of the oscillator. It also operates as a screen between the first control grid 52 and the second control grid 66 and may be termed the first or auxiliary anode or the inner screen of the tube. The electrode 61 and the control grid 52 may also be referred to as auxiliary electrodes interposed between the cathode and the grid 66 and operating as oscillation-producing means.

The screen 61 substantially prevents interaction between the oscillator and the radio frequency portions of the system. The screening function of the electrode 61 is materially increased by maintaining the primary winding 69 in circuit with it at a relatively low impedance value only suflicient to provide the necessary electrical coupling with the tuned circuit 49. As above pointed out, the screening electrode or grid 61 functions as an anode with respect to the first grid 52 and the cathode 6|, through the feed-back winding 69, for maintaining oscillations. The grid 12 functions as a screen grid adjacent to the anode 54 and may be referred to as the outer screen or shielding element. 6

It will be seen that in accordance with the description of the operation of the circuit of Fig-1 in connection with Figs. 2, 3 and 4, the anode current to the electrode 61 will not vary appreciably in response to variations in the signal input potentials on grid 66-, and for this reason it may be utilized as the oscillator anode, thereby to permit the operation of the device as a combined detector-oscillator.

Stated in other words, the change in transconductance from the first grid -or oscillator grid 52, to the second grid or oscillator anode 61 is relatively low in response to variations in the The'circuit arrangement isfundamentally the same as that of Figs. ,1, 5-and .6 in that two. preferably negative, high. *impedance, :control grids or impedance controlling electrodeszareine, imposed in .the {electronic stream .in series :rela-.-

tion-to each other and are. separated byxa'screen electrode or grid: operatingyat-a positive'poten 'tial. However, .it wills-be-n'oted that the. control 7 .grid connections are reversed, whereby the inner I grid receives thezoscillationf signal,= :and the sec-- ond control grid or thirdelectrode receives the radio frequency signal;. Ther radio frequency signal control grid is furthermore-shielded or screened from the :anode-and :is substantially shieldedor screened from: thewoscillator.-1.grid.- .The electronic. coupling, whereby :the separate signal input channels are substantially independent 'of each other and are yet:provided ,with 5520011111101! output circuit is, otherwise, the same as: described in. connection with Fig. 1. The circuit described,

or condenser 83..-;

however. my a were lat-tam new; over any of thepreceding circuits as be seen from. a further consideration-ofthecircuit. It will;be-noted that the :grid bias-potential applied to the signal control grid66 is derived in part from the self bias resistor 62 and in part from a portion.'|4 of the potential supply source through the variable tap connection 65. v The combined potentials are applied to the grid circuit through the fixed resistor64. With this arrangement the bias potential applie'dto the grid 66 varies with changes in the anode current through the self :bias resistor 62 and may be varied manually byadjustment of the; tap 65.-.

It may also be caused to vary by varying a flow of current through the resistor 64 through the medium of any suitable means such. as an'automatic volume control device or tube, as will hereinafter be described.

It has been found'thata variation of the. biasing potential applied to the signal controlgrid 66 serves effectively to vary or control between desired limitations the output volume or -amplitude of the beat frequency or intermediate fre-y quency signal and that this may be done without effecting the oscillator frequency. In other words, by controlling the bias potential on the second or outer negative grid while the flrst or inner negative grid is employed in the oscillator, the amplification or gain through the detector oscillator system may be controlled without inipairing the oscillator voltage amplitude, stability, or frequency characteristic. While a single thermionic or electron discharge device is thusu'sed in a signal channelmixing circuit, yet volume control means may be applied to the device and be used with the electroncoupling'without impairing certain desired operating character istics of one of the coupled circuit, in this case" the oscillator portion. The two separate ,inde: pendent functions of the control grids'and the operating circuits in connection-- therewith are undisturbed and unafiected by the volume con -trol arrangement.

The ability to control the volume of thesignal output in connection with a devic'e of this character in asignal system is extremelydesire able, and it .will be seenthat an automatic volume control systemnmay be {coupled to .the sig--.

nal :inputgrid to control} it I automatically 5 in. response to changesxin the average amplitude.

of incoming signals. g1:

-,In the present example'zan automatic control device or .tubelflis connected with the; resistor 64 throughitsanode circuit .16. I[ n1-.or,-

der to place a positivepotentialbn' the'anode,

itscath'ode lead, indicated at "I '|*,-;is connected to a variable tap "on thesource-59.;morenegative, than the tap, point 65,;- and the- .grid' circuit 19 is,

returned through v a couplingiresistor to a still T more:' negative -point .fl iyalso on .the. source '59.;

Controlling potentialssuch; as. radio 7 frequency signal potentiais .,are supplied to .the coupling I M resistor and hence to; :the yolumeiscontrol tube;

rronrrtmy suitable sourc esu h a term n s indie. I

suitable coupling. mean-s;

cated at 6 2, through a With this; arrangem "cent-hat variations. in the average-amplitudes f wtentials' supplied to the terminalsjfl; which receive signal energy-from a -source ,commonto them-and to 4 input terminalsdi will, cause the anode: current of the device 15. to vary; through. the resistor .64, th ry theme-s s ntial a pli d. o.

the grid 66. The present arrangement operates I in such a manner that as the signal strength increases, the anode current through circuit ll also increases, thereby increasing the potential drop through the resistor 84 and the negative bias on the grid ll, which in turn reduces the amplitude of signal output from the output circuit 55. As hereinbefore described, this action is entirely independent of the oscillator signal set up in the circuit and does not serve to vary it either in frequency or amplitude.

The filter circuit 58 and the suppressor resistor II in the oscillator. grid circuit are for the purpose of smoothing the frequency response characteristic of the oscillator throughout the operating range, and are shown in the circuit as used. They do not, however, form' any part of the present invention except incidentally in connection therewith and, accordingly, are believed to require no further description.

It will also be noted that suitable by-pass capacitors 84 are provided throughout the circuit as required, and represent well known means for preventing coupling in the supply circuits.

. As fully appears from the foregoing description and the drawings, Fig. 'I shows. a system responsive to electric oscillations comprising electron tube having a cathode 6i, grid elements I! and 86, an anode 61 which is the first anode in the sense of being nearest to the cathode, and a second anode 54, in combination with high frequency oscillation generating means including the tuned oscillator circuit 49 andthe transformer primary 68 disposed respectively between the cathode 6| and the grid 52 and between the cathode and the first anode element 81 for modulating the electron stream of the tube at a frequency determined by the oscillation generating means, means including control grid II for modulating the electron stream in accord ance with the received electric oscillations, means including the output circuit of anode H for rectifying the resultantvariations in the electron stream, means including tuned output circuit I! responsive to the rectified variations and disposed in series between the cathode ii and the second anode N, and means in the form of screen grid 12 for shielding the portion of the electron stream of the tube between the cathode and the first anode 81 from the influence of the second anode 54 for high frequency variations of potential upon the second anode, such shielding means comprising the screen grid 12 for the second anode and a circuit between the screen grid 12 and the tuned output circuit. such that any portion of said last mentioned circuit which is common to the high frequency portion of the generating means is of negligible impedance at .the frequency of the generated oscillations in comparison with the internal tube impedance between the cathode and the anode element .81. By-pass'capacitors ll effectively assure that any part of the circuit between shield element .12 and the anode output circuit II, II and II, which is common to any portion of the circuits of the similar to the circuit shown in Fig. "l is further 1 shown in connection withthe electric discharge device It. All of the circuit connections to the various, electrode elements are the same and the automaticvolume control connection has been omitted for simplicity of illustration. Like parts are provided with like numerals throughout. A slightly different oscillator circuit 8! and a single suppressor resistor 88 are shown in connection with the grid -52.

.Itwill also be noted that the screen grid means I! is provided with a second electrode portion 81, whereby it further screens the second control grid I. The grid 12, therefore, forms a complete shield for the signal control grid 66 and it has been found in practice that this is desirable in certain circuits. It has furthermore been found that the grid or electrode 81 may be provided by a simple rod placed in the tube envelope between the first control grid and the screen 12-" when the function of the element 61 is no longer required as a screen.

In Fig. 8, as in Fig. 7, by-pass capacitors l4 ell'ectively assure that any part of the circuit between shield element I2 and the anode output circuit 55, I1 and 58. which is common to the circuits of the oscillation generating means is of negligible impedance at the frequency of the generated oscillations.

The embodiment of the invention shown in Fig. 8 represents the present preferred embodiment thereof and has been found in operation to give a combined detector-oscillator action without interaction between the circuits connected with the control electrodes, and a complete electronic coupling, while permitting a complete control of the output signal amplitude independently of functions of either of the signal input circuits. A circuit of this character is of importance and permits the effective use of an automatic volume control means therewith in a superheterodyne receiver, for the reason that the detector-oscillator operation is thereby renderedentirely independent of the volume control changing output with varying bias voltage in the grid 86 over a wide range of output and bias voltage.

In the language of claims'awarded to me in interference with the patent to Wheeler 1,958,027, there is produced in the exercise of my invention a-virtual cathode composed of a cloud of elec-' trons and acting as an electron source for anode I8 and grid l8 of-Figs. 1 and 6, for anode i8 and grids l8 and 38 of Fig. 5, and for anode 54 and grids l6 and I2 ofFigs. 7 and 8. This cloud of electrons may be considered in Fig. 6 by way of illustration as occupying a position on the cathode side of grid l9 and between that grid andthe positive grid electrode II. The positive potential of grid electrode l8 draws from the cathode elections for the replenishment of the cloud of electrons constituting the virtual cathode and the retarding effect of the negative potential on control grid ll relative to cathode ll tends to causethe cloud of electron to collect just beyond the electrode I 8. The density of the electron supply at the virtual cathode varies in accordance with the alternating current potential impressed on the inner grid l1. When the poten-' cathode, and when the potential on grid ll be- I assure comes more negative the electron supp y at the Vil'tllfl cathode 18 decreased. :Similarly, the V11- tual cathode in Figs"! and 8 may be considered as the cloud of electrons occupying the space on heiatiiede side or grid at and between it and the next adjacent positive electrode.

----I have disclosed herein a method of modulating an alternating current signal in a vacuum tube having a cathode. an anode, and an electron space path therebetween, which comprises, again with reference to Fig; 6 by way of illustration, attracting an electron stream to an intermediate position in said path as by the positively energiaed grid ll, retarding the electron stream to form a virtual cathode at a position beyond such intermediate position as by the negative repelling action of the negatively biased control grid is,

modulating the density of the electrons at the virtual catho'deby varying the attraction of the electron stream as by impressing alternating current potential on the inner grid l1; further attracting a part of the electron stream from the virtual cathode to the positively energized anode I! (the other part being absorbed by the positively energized grid l8), and causing the alterthe positively energized electrode next adjacent to and on the side of grid 66 toward the actual cathode 6|. Such positively energized electrode and the negatively biased control grid 66 set up a virtual cathode between them. Figs. '7 and 8 also involve the further step of producing oscillations in the space path between the actual and virtual cathodes by varying the attraction of the electron stream, thereby effecting modulation of the density of the virtual cathode in accordance with the generated oscillations, As previously described, the oscillations are produced in the circuits associated with cathode GI and grid electrodes 52 and 61. I also eifect the further step of controlling the current in the output path associated with the main anode 54, by controlling the further attraction of electrons from the virtual cathode to the anode, without substantially affecting the oscillations in the spacebetween the actual and virtual cathodes.

- As already pointed out above, in describing the tubes and circuits of Figs. '7 and 8,, the production of the local oscillations between the actual cathode and the virtual cathode permits the lastmentioned step to be performed in that the production of the oscillations in the tube is not effected by the controlling potentials by which the last-mentioned step is effected. More particularly this permits use of automatic volume control means in a superheterodyne receiver for the reason that the oscillator portion of the detectoroscillator operation is substantially independent of the automatic volume control operation. In other words, variations of the negative bias on the control grid 56 do not affect the supply of electrons from cathode 6! to the oscillator electrodes 52 and 61. When, for example. signals of large amplitude are being received and grid it is driven strongly negative bythe circuits-of the automatic volume control tube ll; such negative potential on grid 66 does not prevent a plentiful supply of electrons from cathode I to oscillator electrodes 52 and 61 to assure maintenance of the oscillations.

The desirable features of the circuit of Fig. 8 may be utilized in a modulated oscillator by ap plying to the outer grid 66 a modulation signal in the same manner as shown in connection with Fig. 6. This involves merely the substitution of the input transformer 42 for the tuned circuit 45. With such an arrangement the control grids will operate at different signal frequencies and through independent input circuits, while serving to electronically couple the circuits and to provide a modulated signal output.

Furthermore, as pointed out in connection with Fig. 6, a device of the character described therein, and the modification thereof shown and described in-Fig. 8 may be utilized as a detector of modulated signals, the two cc-nt'rolgrids receiving signals at the same frequency, the one being modulated and the other unmodulated.

The circuit of Fig. 8 has the additional ad- I vantage that the output volume may be adjusted by operation of the tap connection 65 as described in connection with the. preceding figure. It will be appreciated, however, that any other suitable arrangement for controlling the grid bias on the signal control grid may be provided and, likewise for the oscillator or first control grid, ias means may be provided other than the self bias resistor 62. The potential sources 59 and other potential sources shown throughout the drawings in connection with the various figures, have been shown only by way of example, as representing any suitable sources of operating potential having the polarities as described.

An additional advantage of the screen grid 12 employed in the circuits of Figs. 7 and 8 lies in in the fact that it materially increases the plate impedance and the transconductance between the signal control grid 66 and the anode 54. It also tends to prevent degeneration or other interaction between said last named elements.

In a combined detector-oscillator system of the character shown and described herein, in addition to the independent operation of the electronically coupled circuits, the oscillator portion functions at a lower radio frequency voltage thanwas customarily used prior to my invention and in the then current superheterodyne receiver circuits employing a local oscillator tube separate from the first detector, thereby decreasing the tendency toward undesirable radiation, since only about onetenth of the voltage output is required for the oscillator. Furthermore, a relatively high radio frequency to intermediate frequency gain is provided. I

I claim as my invention:

1. In a signaling system, a multi-electrode electric discharge device having a pair of negative control grids arranged to operate in series relation to each other in the electronic stream of said device, a positive screen electrode between said grids, a signal input circuit connected with one of said control grids, a third grid, and means for coupling said third grid and the other of said control grids.

2. In a signal receiving system, a combined dcanode, a cathodepand a. plurality of grid electrodes therebetween, an oscillator circuit connected with and coupling two oi said electrodes more adjacent to the cathode, a signal input circult connected with another of said electrodes, more adjacent to the anode, and a screen electrode for said signal input electrode.

3. In a signaling system, an electric discharge device having an anode, a cathode, and a plurality of grid electrodes interposed in spaced relation to each other along the electronic path be tween said first and second named electrodes, a

signal input circuit connected with one of said electrodes more adjacent to the cathode, means for applying a negative bias potential thereto whereby it opertaes as a control grid, a second signal input circuit connected with another of said electrodes more adjacent to the anode, and means for applying a variable negative bias potential thereto whereby it operates as a second control grid, said control grids being separated by another of said electrodes, means for applying a positive potential to said last named electrode, and means connected with said last named electrode for feeding back energy therefrom to one of said signal input circuits.

4. In a signal receiving system, an electric discharge device having a pair of control grids, an input ciruit connected with each of said control grids, an output anode circuit for said device, a screen grid interposed between said control grids, means connected with said screen grid whereby it may function as a screen electrode and as an anode, and means providing an electrical coupling between said screen grid and one of said control grids.

5. In a signal receiving system, an electric discharge device having a pair of control grids, an input circuit connected with each of said control grids, an output anode circuit for said device, a screen grid interposed between said control grids, means connected with said screen grid whereby it may function as a screen electrode and as an anode, means providing an electrical coupling between said screen grid and one of said control grids, and means for applying a variable biasing potential to the other of said control grids.

6. In a signaling system, an electric discharge device having a pair of control grids arranged to operate in series relation to each other in the electronic stream of said device, a third electrode interposed between said grids, means connected with said third electrode whereby it may function as a screen electrode between said grids and as an anode, means for applying a negative biasing potential to each of said control grids, at least one of said potentials being variable, and means for coupling said third electrode to one of said control grids for the transmission of signal potentials.

7. In a signal receiving system, a combined detector oscillator means, including in combination, a single electric discharge device having an anode, a cathode, and a plurality of grid electrodes therebetween, an oscillator circuit connected with and coupling two of said electrodes more adjacent to the cathode, a signal input circuit connected with another of said electrodes, 'means for applying a negative bias potential to said last named electrode whereby it operates as a control grid, and means for applying a potential to at least one other electrode adjacent to said control grid whereby said last named electrode functions as a screen for said control grid.

8. In a superheterodyne receiver, a combined detector-oscillator comprising an electric discharge device having a cathode, an anode, a signal input grid more adjacent to the anode, an oscillator grid more adjacent to the cathode, means providing a screen-electrode between said grids and between said signal grid and the anode, means providing an oscillator anode between said oscillator grid and the screen electrode, a tuned oscillator coupling means between said oscillator anode and grid, a tuned signal input circuit connected with said signal input grid, means for applying a variable negative biasing potential to said signal input grid. means for applying a negative biasing potential to said oscillator grid, means for applying positive potentials to said screen electrode and oscillator anode, and an intermediate frequency output circuit connected with the anode.

9. In a modulation system. a vacuum tube com-,- prising 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.

10. 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. i

11. In a signal receiving and translating arrangement, an oscillator-modulator system com prising 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.

12. 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 created between said second and third electrodes, a tuned circuit system connected to said actual cathodeand having oscillation-producing coupling to said first and second electrodes. and

asource of,signals connected between said third electrode and said, actual cathode, and an output circuit connected between said cathode and an 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 altemating 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 i maintained more nearly constant than the input over a wide range of received intensities.

14. In a superheterodyne receiver, an oscil later-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 electrodesand 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 efiects of the bias voltage on the oscillation are minimized by the location of said grid and anode.

15. Ina superheterodvne 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 be tween 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, 8. source of signal voltage coupled between said control grid and said cathode, means fordeveloping a bias control 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 aflecting the intensity of said oscillations,

16. In a superheterodync receiver, containing anoscillator-modulator system comprisinga ,vacuum tube having an anode an electron emit-,-- tingtcathode, a control electrode, a.signal inpu circuit connected to the control electrode, means forimpressing On said control electrode a bias control voltage whichis,negative relative to-the. cathode, {go-maintain a-high. tube inDut*resist-= ance between said signal-control electrode and; the cathode-and whichvaries automatically in accordancewith the intensity of the receivedsignals, means including a plurality of additional: electrodes interposed between-the-cathode andthe control electrodeforproducing'local oscillations and for. shielding the signal; electrode "from the oscillation-producing electrodes, whereby'zthe flow .of electrons to --the oscillation-producing electrodes is substantially unafiected by variations in the bias control voltageand;radiation, of locally produced-oscillations from circuits associated, with the signal. control electrode ;is .pre- I vented, andan intermediate frequency. output circuit connectedtotheanode. I

17. In a superheterodyne ,receivenc ontaining an oscillator-modulator system comprising a vac uum tube having an anode, an electron emitting cathode, a control electrode, a, signal input circult-connected to the. control electrode, means for impressing on said control electrode a bias control voltage which is negative relativeto the cathode to cause the control electrode to act toward the signal input circuit as a high imped ance electrode, means including a plurality of additional electrodes interposed between the cathode and the control electrode for producing local oscillations and for shielding the signal electrode from the oscillation-producing elec-I trodes, whereby the flow of electrons to the oscil-l iation-producing electrodes is substantially unaflected by variations in the bias control voltage and radiation of locally produced oscillations from circuits associated with the signal control electrode is prevented, and an intermediate frequency output circuit connected to the anode.

18. In a superheterodyne receiver, an oscillator-modulator system comprising a vacuum tube having a cathode, an anode and at least three grids lccatedin the space path between said cathode and anode, oscillation-producing feed-back 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 twogrids, a signal input circuit coupled between a thirdof said grids and saidcathode, and'an output circuit coupled to said anode, causing said oscillatory currents to modulate the signal from said input circuit, means for developl ing a bias voltagewhich becomes more negative when the signal intensity increases, and means for impressing said bias voltage on said third grid, thereby maintaining the output: intensity in said output circuit more nearly constant than the input intensity over a considerable range 01' input signal intensities, and at the same time pre-, venting any substantial variation ot saidwoscilla-f torycurrents. ,t r 19. A modulation system comprisinga vacuum, tube having an input circuit and an output cir cuit, said vacuum tube. having a space-current, path in which are included; a cathode and at, least 'two grids, one of said rids having a source pr alternating voltage connected thereto, the other. or said grids having said input circuit-co nected thereto and having impressed hereon a variable,

bias voltage more negative than said cathode;-

.the space,path, in the ordernamed, .frO i fS d" cathode o said mo em s na in ut air-gl b, clone nected between,- said third electrode ,and s'ai dl f cathode, an output oircuit connectedf betwe n saidca-thode and said anode, a;source ot" lations connected; between said firstelectrode n said cathode, :a, source 01-,positiveyoltag n nected to said second; electrode zand a sou ce 1, variable :negative .bias-: woltage applied, 5t third-electrode fonzcontrolling t output rs a ,1 system, said-.thirdelectrode being oi the type which provides a very gradually changing output 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 the output of said tube may be varied over a wide range.

22. 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.

23. A system having the means according to claim 22 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.

24. The method of modulating a signal in a vacuum tube having a cathode, an anode and a 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, 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.

25. The method of modulating a signal in, and controlling the modulated output of, a vacuum tube having an actual cathode, an anode and a 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 dlflerence beween 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.

- 26. -An electron discharge device comprising a thermionic cathode, an anode, an output circuit connected to said anode, two impedanc controlling elements for independently varying the impedance presented to an electron stream between said cathode and said anode, an auxiliary anode interposed between said cathode and said firstmentioned anode, oscillation-producing means comprising circuits coupled together and connected respectively to said auxiliary anode and one of said controlling elements, and means comprising a grid-like element interposed between said two controlling elements whereby electrostatic coupling between said two controlling elements is substantially eliminated.

27. A heterodyne detector comprising an electron discharge device having a thermionic oath-- ode, an anode, two input grids interposed between said cathode and anode and passed successively by the electron stream from said cathode to said plate, positively energized electrode between said input grids and proportioned to intercept only a fraction of the electron current from said cathode, an oscillation generating circuit connected to said cathode, said output electrode, and said input grid adjacent said cathode, a signal circuit connected to said cathode and to said other input grid, an output circuit connected to said cathode and to said plate, and a positively polarized screen between said other input grid and said output electrode connected to said oscillation generator circuit.

28. In an electrical system the combination of a continuous electron discharge path comprising an electron emitting cathode and a positively energized anode, a plurality of negatively energized electrodes spaced between said cathode and anode and electrostatically isolated from each other by an electrode interposed therebetween, a plurality of sources of oscillations, and means for impressing diflerent ones of said oscillations upon difierent ones of said negatively energized electrodes, the negative energizing of a one of said electrodes between said interposed electrode and said anode being so proportioned that substantially no current is thereby drawn fom said electron discharge path during operation, whereby fidelity of operation is enhanced.

29. In anelectrical system the combination of an electronically active path comprising an electron emitting source and an appropriately spaced therefrom positively energized anode, differently spaced control electrodes interposed between said source and said anode, said control electrodes being electrostatically isolated one from the'other by an electrode interposed therebetween, separate sources of electrical oscillations, separate means for impressing one of said oscillations between said electron emitting source and one of said control electrodes, separate means for impressing another one of said oscillations between said electron emitting source and a control electrode farther removed therefrom, and means for so negatively polarizing said farther removed electrode that in operation substantially no current is thereby drawn from said electronically active path, whereby fidelity of operationis enhanced.

30. 'In an electrical system, the combination of an electron discharge path comprising an electron emitting cathode and a cooperating posi-- tively energized anode, a pair of differently spaced grid electrodes interposed in said path, said grid electrodes being electrostatically isolated one from the other by .an electrode interposed therebetween, a pair of separate sources of electrical oscillations having separate connections for separately impressing them one between said cathode and the nearer thereto one of said grid electrodes, and the other between said cathode and the farther removed therefrom grid electrode, and means for so negatively energizing at least said farther removed grid electrode that substantially no grid current is drawn thereby from said path during operation, whereby fidelity of operation is enhanced.

31. In an electrical system, the combination of an electronically activepath comprising an electron emitting source and an appropriately spaced therefrom positively energized anode, differently spaced electrodes interposed between said source and said anode, an electrode between two of said interposed electrodes positively energized to a lesser degree than said anode, separate sources of electrical oscillations, separate means for impressing one of said oscillations between said electron emitting source and' one of said interposed electrodes located between said source and said positively energized interposed electrode, separate means for impressing another one of said oscillations' between said electron emitting source and a control electrode on the far side of said positively energized interposed electrode, and means for so negatively polarizing said far side electrode that in operation substantially no current is thereby drawn from said electronically active path, whereby fidelity of operation is enhanced.

32. The method of modulating electrical alternating current in a thermionic tube having an electron emitting source, an anode and an electron stream space path therebetween, which comprises constantly attracting electrons oi said stream to an intermediate position in said path, negatively repelling electron movement at a position beyond said intermediate position, modulating the density of the attracted electrons by variably polarizing that part of said electron stream subjected to said constant attraction, further attracting electrons to said anode, and causing said alternating current to vary said negative repelling of electron movement less than enough on the positive half cycles of the alternating current to draw substantially no electrons fromsaid electron movement, whereby the fidelity of modulation is enhanced.

33. The method of modulating electrical alternating current in a thermionic tube having an electron emitting source, an anode and an electron stream space path therebetween, which comprises attracting electrons of said stream to an intermediate position in said path, negatively repelling electron movement at a position beyond saidintermediate position, modulating the density of the attracted electrons by variably polarizing said stream at a position ahead of said intermediate position, further attracting electrons to said anode, and causing said alternating current to vary said negative repelling of electron movement less than enough on the positive half cycles of the alternating current to draw substantially no electrons from said electron movement, whereby the fidelity of modulation is enhanced.

34. The method of modulating electrical alternating current in an electronically active path having an electron emitting source, an anode and an electron stream space path therebetween, which comprises constantly attracting a substantialpart of the electrons of said stream to an intermediate position in said path, modulating the density of said constantly attracted nlec trons, further attracting electrons to said anode, causing said alternating current to vary said further attraction, and sufllclently negatively polarizing the varying of said further attraction that substantially no electrons are drawn from said electron stream on the positive half cycles of the altematingcurrent, whereby the fidelity of modulation is enhanced.

35. The method of modulating electrical alter-p nating current in an electronically active path having an electron emitting source, an anode and an electron stream space path therebetween, which comprises attracting electrons of said stream to an intermediate position in said path, modulating the density of the attracted electrons by variably polarizing said stream at a position in advance of said intermediate position, further attracting electrons to said anode, causing said alternating current to vary said further attraction, and sufliciently negatively polarizing the varying of said further attraction that substantially no electrons are drawn from said electron stream on the positive half cycles of the alternating current, whereby fidelity of modulation is enhanced.

36. In a system for modulating electrical alternating current, a thermionic tube comprising an electron emitting source, a pair of electrodes relatively close together, the said electrode nearer said electron emitting source being positively energized and the other one of said electrodes being negatively energized, a positively ener gized anode beyond said negatively energized electrode, a source of alternating current connected to said negatively energized electrode, an electrode interposed between said electron emitting source and said pair of electrodes, and a second source of alternating current connected to said last mentioned electrode, said negatively energized electrode being sufficiently so energized that substantially no current is drawn thereby during the positive halt cycle of the impressed thereon alternating current, whereby the fidelity oi. modulation is enhanced.

37. In a system for modulating electrical alternating current, a thermionic tube comprising an electron emitting source, a pair of negatively energized electrodes and an interposed therebetween positively energized electrode, a positively energized anode beyond said electrodes, and separate sources of alternating current connected to said negatively energized electrodes, the one of said electrodes nearer to said anode being sutficiently negatively energized that substantially no grid current is drawn thereby during operation, whereby fidelity of modulation is enhanced.

38. A system responsive to electric oscillations comprising an electron tube having cathode, grid, anode and second anode elements, high frequency oscillation generating means disposed between said cathode, grid and first mentioned anode elements for modulating the electron stream of said tube at a frequency determined by said oscillation generating means, means for modulating the electron stream in accordance withthe received electric oscillations, means for rectifying the resulting variations in the electron stream, and means responsive to said rectified variations disposed in series between said second anode and cathode elements, and means for substantially shielding that portion 0! the electron stream of said tube between said cathode and first mentioned anode elements from the influence of said second anode element for high trecomprising an electron tube having cathode, grid,

anode and second anode elements, high frequency oscillation generating means disposed between said cathode, grid and first mentioned anode elements for modulating th electron stream of said tube at a frequency determined by said oscillation generating means, means for modulating the electron stream in accordance with the received electric oscillations, means responsive to the resulting variations in said electron stream disposed in series between said second anode and cathode elements, and means for electrostatically shielding said grid element from the infiuence of said second anode element for high frequency variations of potential upon said second anode, said shielding means comprising a shield element for said second anode and a circuit between said shield element and said responsive means such that any portion of said last mentioned circuit which is common to the high frequency portion of said generating means is of negligible impedance at the frequency of the generated oscillations in comparison with the internal tube impedance existent between said cathode and first mentioned anode elements.

40. A system responsive to wave signaling energy comprising an electron tube having oscillation-generator-elements and at least one auxiliary element, means including said generatorelements for the self generation of oscillations by said tube and for varying the electron stream of said tube at a frequency determined by said oscillations, means for simultaneously varying said electron stream at a frequency determined by said signaling energy, means for utilizing the resulting variations in said electron stream to set up a; flow of current at a third frequency in an auxiliary circuit disposed between said auxiliary element and one of said generator elements, and means comprising a shield element having a connection with said auxiliary circuit of low reactance at the frequency of said oscillations whereby the electron stream between said generatorelements is shielded electrostatically from the tube elements other than said generator elements.

41. A system responsive to wave signaling energy comprising an electron tube having oscillation-generator-elements and at least one auxiliary element, means including said generator-elements for the self generation of oscillations by said tube and for varying the electron stream of said tube at a frequency determined by said oscillations, means for simultaneously varying said electron stream at a frequency determined by said signaling energy, means for utilizing the resulting variations in said electron stream to set up a flow of current at a third frequency in an auxiliary circuit disposed between said auxiliary element and one of said generator-elements, and shielding means within said tube having a connection to said auxiliary circuit comprising a path of relatively low reactance at the frequency of said oscillations and signaling energy.

42. A system responsive to wave signaling energy comprising an electron tube includin cathode, grid, anode and second anode elements positioned in the order named, circuits for the self-generation of oscillations disposed between said cathode, grid and first mentioned anode element for varying the electron stream of said tube at the frequency of said oscillations, means for varying the electron stream of said tube in accordance with the frequencies of said signaling energy, an auxiliary circuit disposed between said second anode and said cathode elements and including an output device, means for maintaYning said second anode electro-positive with respect to said cathode, means for setting up a flow of current in said output device which varies at frequencies equal to the difference between the frequency of said oscillations and the frequencies of said signaling energy whereby a potential across said output device is caused to vary in accordance with said flow of current, and means for substantially electrostatically shielding that portion of the electron stream of said tube between said cathode and first mentioned anode elements from the influence of. said varying potential across said output device.

43. A system responsive to wave signaling energy comprising an electron tube including cathode, grid, anode and second anode elements, circuits for the self-generation of oscillations disposed between said cathode, grid and first mentioned anode element for varying the electron stream of said tube at the frequency of said oscillations, means for varying the electron stream of said tube in accordance with the frequencies of said wave signaling energy, a circuit disposed between said second anode and cathode elements and including an output device, means for maintaining said second anode electropositive with respect to said cathode, means for setting up a flow of current in said output device which varies at frequencies determined by the combined eifect upon the electron stream of the frequency of said oscillations and the frequencies of said wave signaling energy, whereby the potential across said output device is caused to vary in accordance with said flow of current, and means for substantially electrostatically shielding that portion of the electron stream between said cathode and anode elements from the influence of said varying potential across said output device.

44. A system responsive to wave signalin energy comprising an electron tube including cathode, grid anode and second anode elements positioned in the order named, circuits for the selfgeneration of oscillations disposed between said cathode, grid and first mentioned anode element for varying the electron stream of said tube at and first mentioned anode elements from the iniluence or said variable electrical constants of 'the output device.

45. 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 oscillations 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 oscillations, said means comprising an electrode interposed between said actual cathode and said virtual cathode.

46. The method of modulating an oscillation in a vacuum tube having a cathode, an anode and a 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, 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 oscillation 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 oscillation in said vacuum tube.

47. The method of modulating electrical alternating current in a thermionic tube having an electron emitting source, an anode and an electron stream space path therebetween, which comprises attracting electrons of said stream to an intermediate position in said path, retarding the electron movement in said stream to form a virtual cathode just beyond said position, modulating the density of the virtual cathode by variably modifying the attraction of the electrons of said stream, further attracting electrons from said virtual cathode to said anode, and causing said alternating current to vary the further attraction of electrons, the modulation of the density of the virtual cathode thereby causing modulation of the alternating current in said tube.

48. The method of modulating electrical alternating current in a thermionic tube having an electron emitting source, an anode and an electron stream space path therebetween, which comprises attracting electrons of said stream to an intermediate position in said path, retarding the electron movement in said stream to form a virtual cathode therein, modulating the density of the virtual cathode by variably modifying the attraction of the electrons of said stream, further attracting electrons from said virtual cathode to said anode, and causing said alternating current to vary the further attraction of electrons, the modulation of the density of the virtual cathode thereby causing modulation of the alternating current in said tube.

49. In a system for modulating electrical alternating current, a thermionic tube comprising an electron emitting source, means for producing a virtual cathode composed of a cloud of electrons, said means comprising a pair of electrodes relatively close together, one of said electrodes having impressed thereon a potential positive with respect to the other one of said electrodes, a positively energized anode near said virtual cathode creating an electron stream therebetween, a source of altematingcurrent, means for impressing the alternating potentials .of said current on the electron stream between said "anode and virtual cathode, and a second source of alternating current including means for impressing the alternating potentials thereof on the electron stream between said electron emitting source and said virtual cathode.

50. In a high frequency signalling system, an electron discharge tube provided with an actual cathode and an anode, means for producing a virtual cathode within the tube composed of a cloud of electrons, said means comprising a pair of grids relatively close together, means for maintaining the grid of said pair nearest the actual cathode at a substantially more positive potential than the other grid of the said pair, means for maintaining said last named grid at a substantial negative potential with respect to said actual cathode, means independent of said grids for causing the electron density of said virtual cathode to fluctuate, the last means including an auxiliary grid electrode interposed between the actual cathode and the virtual cathode, and a source of varying high frequency voltage coupled to said auxiliary grid electrode.

51. The method of modulating the output of an electron discharge tube provided with a cathode, anode and a space current path therebetween, comprising attracting an electron stream to an intermediate position in said path, retarding said stream to form a virtual cathode just beyond said position, attracting a part of said stream, from said virtual cathode to said anode, and modulating the density of the virtual cathode by varying the attraction of said stream to said intermediate position and thereby causing modulation of the output of said tube.

52. In a signal reception network including a tube provided with an electron emission element, an anode and a signal input electrode, a positive screen grid between the signal electrode and the anode, an auxiliary electrode adjacent the screen grid and the anode at a negative potential, said screen and negative electrode cooperating to provide a virtual cathode between the signal electrode and the anode, by virtue of a high value of said negative potential, a signal input circuit connected to the input electrode, and an outpu circuit connected to the anode.

53. A method of regulating the amplification of signal waves which consists in impressing the waves upon the signal grid of an amplifier tube having an effective plate current signal grid voltage characteristic whose slope is dependent upon the potential of a gain control grid, separating the signal and gain control grids by a positive screening field, and varying the bias of said gain control grid.

54. In a superheterodyne receiver, containing an oscillator-modulator system comprising a vacuum tube having an anode, an electron emitting cathode, a control electrode, a signal input circuit connected to the control electrode, means for impressing on said control electrode a bias control voltage which is negative relative to the cathode, to maintain a high tube input resistance between said signal control electrode and the cathode, and which varies automatically in accordance with the intensity of the receive; signals, means including a plurality of additionaelectrodes interposed between the cathode and the control electrode for producing local oscillations and for shielding the signal control electrcde from the potentials oi the local oscillations produced in the tube, said last-named means utilizing one of said additional electrodes as an oscillator grid and another of said additional electrodes as a shield between the signal control electrode and the oscillator grid,.whereby the flow 'of electrons to the oscillation-producing electrodes is substantiallyunaflected by variations in the bias control voltage and radiation of locally produced oscillations from circuits associated with the signal control electrode is prevented, and an intermediate frequency output circuit connected to the anode.

55. In a superheterodyne receiver, containing an oscillator-modulator system comprising a vacuum tube having an anode, an electron emitting cathode, a control electrode, a signal input circuit connected to the control electrode, means for impressing a negative bias control voltage on said signal control electrode to maintain a high tube input resistance between the signal control electrode and the cathode, shielding means interposed between the. signal control electrode and the anode, means including a plurality of additional electrodes interposed between the cathode and the signal control electrode for producing local oscillations and for shielding the signal control electrode from the potentials of the local oscilations produced in the tube, said last-named means utilizing one of said additional electrodes as an oscillator grid and another of said additional electrodes as a shield between the signal control electrode and the oscillator grid, whereby the flow of electrons to the oscillation-producing electrodes is substantially unaffected by variations in the bias control voltage and radiation of locally produced oscillations from circuits associated with the signal control electrode is prevented, and an intermediate frequency output circuit connected to the anode.

JEROME C. SMITH.

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