US2148266A - Electron discharge modulating device - Google Patents

Description

Feb. 21, 1939. D. G HAINES ELECTRON DISCHARGE MODULATING DEVICE Original Filed March 28,- 1933 2 Sheets-Sheet l INVENTOR ATTORNEY DOA A10 6- HAM 5 h nflo i Feb. 21, 1939. D. 6. HAINES 2,148,266

ELECTRON DISCHARGE MODULATING DEVICE Original Filed March 28, 1933 2 t -sh t 2 FgH5' ATTO R N EY Patented Feb. 21, 1939 UNITED STATES PATENT OFFICE Donald G. Haines, Owensboro,

Ky., assignor to Radio Corporation of America, a. corporation of Delaware Application March 28, 1933, Serial No. 663,171 Renewed December 24, 1937 25 Claims. (Cl. 250-275) This invention relates to electron discharge devices for superheterodyne reception and more particularly to a multi- -electrode electron discharge tube in which local oscillations of a predetermined frequency and input oscillations of a different frequency, such as a radio signal, are mixed lit or combined directly in the tube without capacitive, inductive, or direct coupling of the oscillator and output circuits associated with the tube.

Usual methods of heterodyne reception use a first detector or mixer tube, in which the radio signal frequency and the local frequency, gem erated in an oscillator circuit by a separate tube or by the mixer tube, are both applied to the same grid of the mixer tube. These methods generally depend on coupling the oscillator and mixer circuits by either capacitive or inductive coupling which requires rather critical adjustment, and due to this coupling changes in the signal input circuit affect the operation of the oscillator and hence the local frequency.

One object of the present invention is to provide an electron discharge tube with high gain and in which heterodyne action may be obtained without undesired intercoupling between the input and oscillator circuits.

Another object is to provide an electron dis charge tube of the multi-electrode type having two electrodes which may be connected an an external circuit to constitute, in conjunction with the cathode, an oscillator which operates under substantially constant conditions and uninfiuenced by reactions from the signal input circuit connected to other electrodes in the tube.

A further object is to provide a thermionic elec tron discharge tube of the multi-electrode type in which in effect the emission from the thermionic cathode can be varied at a predetermined and usually high and constant frequency, thereby in effect modulating the space current by variation of cathode emission, and the cathode stream thus modulated is further modulated and controlled by a control grid and utilized by a plate electrode.

A still further object is to provide a tube of this type in which the effects of coupling between the signal input grid and the oscillator electrodes are reduced to such an extent that oscillator frequency radiation is practically eliminated.

Further objects and advantages will appear from the more detailed descriptionof the invention.

In accordance with my invention the electron discharge tube has a plurality of grid-like auxiliary electrodes interposed between the thermionic cathode and the plate so as to be passed successively by the electron stream from the cathode to the plate. An oscillator circuit may be connected to the cathode and two adjacent auxiliary electrodes to constitute an oscillator for producing in the tube an electron stream pulsating at the de- 6 sired local frequency and modified or modulated on its way to the plate by an alternating voltage input or radio signal applied to one of the other auxiliary electrodes which acts as a signal input grid. The electron stream from the cathode is 10 first affected by the oscillator input grid and then by the signal input grid, so that thefrequency of the current to the plate and output circuit is the resultant obtained by thus mixing in the tube the local frequency and the input or radio signal frel5 qucncy to obtain the desired beat or intermediate frequency in the output circuit.

In a commercial tube of this type a capacitive or similar coupling might exist between the oscillator circuit and signal input circuit because the signal input grid is necessarily close to the oscillator electrodes. In order to avoid such coupling and thereby eliminate reaction of the signal input circuit upon the oscillator circuit the signal input grid which is connected with the signal circuit is, in accordance with my invention, free from appreciable capacitive coupling to the oscillator electrodes. In one embodiment of myinvention this result is attained by electrostatically screening the signal input grid in the signal input circult from the electrodes in the oscillator circuit, preferably by interposing between the signal input grid and the adjacent oscillator input electrode a grid-like shield or screen maintained at a constant direct potential, which may be substantially zero with reference to the cathode of the oscillator, to prevent alternating electromotive forces on the signal input grid and adjacent oscillator electrodes causing reactions between the osciilator circuit and the signal input circuit. In

another embodiment, means are provided for establishing in the space between the input grid and the oscillator electrodes a space charge which pulsates at the frequency of the oscillator and is in effect a virtual cathode which varies in emis- 5 sion. The preferred construction in this embodiment of my invention is an ancillary grid-like electrode interposed between the oscillator input electrode and the signal input grid and maintained at a positive potential sufiiciently to attract 5 the electrons emitted by the thermionic cathode, and of sufficient open mesh to permit a large part of the electron stream to pass through it. Where the electrodes are coaxially mounted in the usual way this ancillary grid-like electrode surrounds the oscillator electrodes and in effect encloses them in a kind of open mesh cage.

The signal input grid surrounding the ancillary electrode modulates the discharge substantially as through a pulsating space charge or virtual cathode between the ancillary grid and the signal input grid were the only cathode in the tube. A grid interposed between the plate and the input grid may be maintained at a positive potential somewhat below that of the plate in order to screen the signal input grid from the plate and may, if desired, be connected, preferably within the tube, to the ancillary grid between the oscillator input electrodes and the signal input grid.

In the preferred construction only sufilcient energy for suitable operation of the oscillator circuit is abstracted from the electron stream by the electrode which acts as an oscillator anode and which is connected in the oscillator circuit through a tickler or feed-back coil in the usual way, the remainder of the electron stream passing through the tube to the plate and to the output circuit.

The oscillator anode electrode may be either a grid of very open construction, or consist of one or more rods so positioned with reference to the other electrodes that only a portion of the elec-- tron stream from the cathode is intercepted by it. Modulation of the electron stream received at the plate is effected by both the oscillating voltage on the oscillator input grid, and by the signal voltage applied to the signal input grid.

The signal input grid may to advantage be of the multi-mu type, particularly where automatic volume control is desired. It has been found that where a signal input grid of this type is used the distance of the signal input grid from the ancillary electrode enclosing the oscillator electrodes must be greater than the distance at which an input grid should be placed from an equi-potential cathode in the position of the ancillary electrode.

The novel features which I believe to be characteristic of my invention are pointed out with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof'willbest be understood by reference to the following description taken in connection with the accompanying drawings in which;-

Fig. 1 shows, partly in longitudinal section, one embodiment of my invention in an electron discharge tube; Fig. 2 is a. cross-section of the tube along the line 22 of Fig. 1; Figs. 3 and 4 show, in longitudinal section, modifications of parts of the tube shown in Fig. 1; Fig. 5 is a circuit dia-- gram for the tube shown in Fig.1; Fig. 6 is a circuit diagram for the modification shown in Fig. 3; and Fig. '7 is a circuit diagram for the tube shown in Fig. 1 when made with a filamentary cathode.

The electron discharge tube shown in Fig. 1 comprises a highly evacuated bulb ill with the usual re-entrant stem II on which the electrode assembly is carried. Two parallel support rods I2 project from the stem and carry two parallel insulating bars or spacers l3, preferably of mica, which extend transversely of the support rods and are secured to them by metal fasteners H. A cylindrical plate l5, preferably of blackened or carbonized nickel, is fastened to the support rods. The grids and the cathode, all carried on and supported by the transverse bars l3, are mounted between the bars and coaxially of the plate 15. The equi-potential cathode is of the usual type,

comprising a reverse coiled tungsten filament heater surrounded by a straight nickel sleeve 18 coated with oxides of barium and strontium, and having a cathode connector l'l joined to a bent cathode lead It which also carries the getter tab H.

The auxiliary electrodes or elements interposed between the cathode and plate so as to be passed in succession by the electron stream from the cathode to the plate are in the form of grids, preferably arranged coaxially with the cathode. Unless otherwise specified, they are the usual wound grids, each consisting of two parallel grid rods and a helix of fine cylindrical grid wire with its turns fastened to the grid rods, the ends of which fit snugly into holes in the transverse spacers or bars [3. Adjacent and surrounding the cathode is the No. l auxiliary electrode or oscillator input grid 20, preferably elliptical in crosssection. The next, or No.2 auxiliary electrode acts as the oscillator output electrode or anode, and is a grid-like anode electrde 2l constructed to draw only a fraction of the cathode current, and which preferably has no grid wire, but is merely two grid rods mounted on the stem in alignment with the grid rods of the oscillator or No. 1 grid 20, and electrically connected to each other, preferably at the top, as shown in Fig. 1. This grid-like anode electrode may be made in many diiferent ways to draw only a portion ofthe cathode current. The next, or No. 3 auxiliary electrode, is the ancillary grid 22, which, as shown in Fig. 2, is also elliptical in cross-section, with its long axis in alignment with the grid rods of the other grids. trode. is a cylindrical signal input grid 23, preferably of the multi-mu type, and shown in Fig. 1 as a helical grid of progressively varying pitch, although other well known forms of multi-mu grids may be used. The signal input grid is connected to a contact cap on the tip of the bulb. The next, or No. auxiliary electrode, is a cylindrical screen grid 24 mounted coaxially with and between the signal input grid 23 and the plate l5, and electrically connected to the ancillary grid 22 through a bent lead 25 to which both grids are connected.

In a tube constructed as shown in 'Fig. 1 and design in accordance with the usual practice for a cathode current of about 11 milliamperes and.

a plate voltage of about 250 or 100 volts, the gridlike anode electrode may be operated at about 250 or 100 volts, and the ancillary grid and screen grid at about 100 or 50 volts. In such a tube it has been found that the distance between the flattened sides of the elliptical ancillary grid 22 at the ends of the short axis of the grid and the cylindrical signal input grid 23 should be about one-eighth of an inch, a distance much greater than that at which the input grid would be placed were the ancillary grid 22 a uni-potential cathode. The exact distance depends on the cathode emission, the construction of the signal input grid, and the operating voltage of the tube. If the signal input grid is too close to the ancillary grid, the multi-mu effect is not obtained.

Fig. 3 shows a modification in which the No. 3, or ancillary grid is in effect a shielding grid 26, proportioned to act as an electrostatic screen.

It is rather close meshed, but the electrons canpass through it, and it is connected inside the tube directly to the cathode through the cathode lead l8 thereby making the grid 26 a grounded shield between the signal input grid and the two oscillator electrodes 20 and 2| adjacent the The next, or No. 4 auxiliary eleccathode. This modification shows the No. 2, or oscillator anode grid 2|, made as the usual wound grid with a pitch such that it is of very open mesh, so that it accelerates the electrons from the cathode, but intercepts only a comparatively small part of the electron stream, permitting the remainder to pass on to and through the shielding rid 26.

Fig. 4 shows a modification in which the tube is practically a trlode and a hexode side by side in the same bulb with a common cathode, as the No. 2, or grid-like anode electrode, has on the two grid rods near one end a sheet metal grid cylinder 21 much shorter than the other electrodes. The tube is practically a trlode at the end of the cathode covered by the grid cylinder 21, and a hexode at the uncovered and major portion of the cathode.

Fig. shows the tube illustrated in Fig. 1 connected in circuit for use in superheterodyne reception. A conventional input circuit 28 is con-.

nected to the signal input grid, and a conventional tuned load or output circuit 29 is connected to the plate I5, The cathode, grid 20, and anode grid 2| are connected to an oscillator circuit 30 of conventional design and comprising a feedback coil 3| connected to the oscillator output electrode or anodegrid 2|. The voltages for operating the tube may be obtained from taps on a resistor 32.

In the operation of the tube connected as shown in Fig. 5 the cathode, oscillator grid 20, and grid-like anode electrode 2| constitute the three electrodes of an oscillator. All of the electron stream from the cathode is modulated by the oscillator grid 20, but only enough of the stream is intercepted by the output electrode or anode grid 2| to supply suflicient current through the coil 3| to keep the oscillator in operation.

The two parallel rods projecting from the stem and constituting the anode electrode 2| are in alignment with and shielded from the cathode by the side rods of the oscillator grid 20 to such an extent that only a fraction of the electron current emitted by the cathode reaches the anode electrode 2|. The remainder of the electron stream or current from the cathode, modulated by the oscillator grid 2|, passes directly to the ancillary grid 22, which is sufficiently positive, usually about 100 volts, to draw the electrons to and through it, and to form between the ancillary grid 22 and the controlgrid 23 a space charge which responds to the signal input grid like a virtual cathode with an emission pulsating at the frequency of the oscillator.

The ancillary grid 22 is'connected to the posi tive voltage source through a low impedance connection, so that it is held at constant direct current potential which does not change with the modulation. It is not an anode in the usual sense, as it draws only grid current, which is a very small portion of the cathode current, but it is essentially an accelerating electrode for the modulated electron stream from the cathode. It .is not an element of the oscillator and takes no part in its operation. So far as capacity coupling of the oscillator current to the signal input grid is concerned, the oscillator electrodes inside the ancillary grid are practically non-existent, and in effect the cathode for the signal input grid and the plate is the virtual cathode around the ancillary grid. It has been found that the use of the positive ancillary grid also reduces to a marked extent the localfrequency radiation.

It has been found advantageous by adjustment of the circuit to operate the oscillator grid at a bias of about 60 volts negative, and when so operated the cathode current flows only while the oscillator grid is swinging between 20 volts negative and enough volts positive to utilize the oathode current to the best advantage. The result is a pulse of cathode current through the tube during the middle portion of that half cycle of the oscillator when the anode grid 22 is positive, and no cathode current through the tube during the remainder of the cycle, as no current flows during the other half cycle of the oscillator when the anode grid is negative. These pulses of cathode current flowing practically unimpeded to and through the ancillary grid 22, produce near the outside of the ancillary grid a space charge which pulsates at oscillator frequency and acts as avirtual cathode.

The load condenser in the plate circuit should be of suflicient size to limit the magnitude of the radio frequency voltage built up across the load and thus prevent radio frequency voltage feed-back between the plate and the signal input grid and consequent degenerative effects.

Converter circuits such as shown in Fig. 5 and employing a tube constructed in accordance with my invention may easily be designed to have a translation gain as high as approximately 60.

The circuit in Fig. 6 is substantially the same as in Fig. 5, except that the shielding grid 26 is connected, preferably inside the tube, directly to the cathode and hence is at zero potential .with reference to it.

vention has the advantages of requiring lessspace, as one tube takes the place of the two tubes heretofore required; high conversion conductance; no radiation of the oscillator frequency with the signal system; simplified associated circuits without any coupling; and a constancy of oscillator frequency unaffected by wide variation in the voltage on the signal input grid.

While I have illustrated and described the preferred embodiments of my invention it is by no means limited to the precise construction or circuit arrangement shown, as many variations in the structural details as well as the manner in which it is used may be made without departing from the scope thereof as set forth in the appended claims.

Having thus described my invention, what I claim is:

1. In an electron discharge device, the combination with a thermionic cathode, a plate, and two input electrodes located between said cathode and said plate at different distances from .said cathode, of a grid-like anode electrode of onic cathode, a plate, and a plurality of auxiliary electrodes interposed between said cathode and said plate to be passed successively by the electron stream from said cathode to said plate and comprising an input grid adJacent said cathode, a gride-iike anode electrode adjacent and of more open mesh than said input grid, an

ancillary gridlike electrode adjacent and of closer than said output electrode interposed between said cathode and said output electrode, a signal input grid between said output electrode and said plate, and a shield grid electrode of finer mesh than said output electrode interposed between said output electrode and said signal grid and connected inside said envelope to said cathode.

4. An electron discharge device comprising an evacuated envelope enclosing a thermionic cathode, a plate, an output electrode adjacent said cathode, an input electrode between said cathode and said output electrode, an elliptical ancillary grid surrounding said output electrode and said cathode, and a circular input grid of non-uniform pitch surroundingv said ancillary grid and spaced from said ancillary grid 9. distance suiiicient to cause said device to operate as a multi-mu device.

5. An electron discharge device comprising an evacuated envelope enclosing a thermionic cathode, a plate, an output electrode adjacent said cathode, an input electrode between said cathode and said output electrode, an elliptical ancillary grid surrounding said output electrode and said cathode, and a circular input grid of non-uniform pitch surrounding said ancillary grid and spaced from said ancillary grid a distance considerably greater than the distance necessary to space said input grid from an equipotential cathode in the position of said ancillary grid to obtain the same multi-mu effect.

6. An electron discharge device comprising a thermionic cathode, a plate, a plurality of cylindrical grids of different diameters coaxial with said cathode and each comprising a pair of parallel grid rods and a helix of grid wire mounted on said grid rods, the grid rods of said grids being in alignment, and an anode electrode conslsting of a pair of electrically connected rods mounted between two of said grids in alignment with the grid rods thereof.

7. An electron discharge device comprising an envelope enclosing a thermionic cathode, a cylindrical plate, four cylindrical grids of diiferent diameters coaxial with said cathode and each comprising a pair of parallel grid rods and spaced grid wires transverse to said grid rods, the two grids nearest said cathode being elliptical in cross-section, and the other two grids circular in cross-section, a pair of electrically connected parallel rods between and in alignment with the grid rods of said elliptical grids, and a direct connection inside said envelope between the said elliptical grid furthest from said cathode and the cylindrical grid nearest said plate.

8. An electron discharge device comprising a thermionic cathode, a helical input grid sur-.

rounding said cathode, an ancillary helical grid surrounding said cathode and said helical input grid, a second helical input grid surrounding said ancillary grid, a grid anode interposed between amazes said first input grid and said ancillary grid and of more open mesh than any 01' said grids, and a plate surrounding said second helical input a id.

9. An electron discharge device comprising a thermionic cathode, a plate, a plurality of cylindrical grids oi! different diameters coaxial with said cathode and each comprising a pair oi. parallel grid rods and spaced grid wires extending between said grid rods, said grids being mounted with their grid rods in alignment, and an anode electrode comprising a pair of parallel rods mounted outside of and in alignment with the grid rods of said grid adjacent said cathode and inside and in alignment with the grid rods of the next grid, and a metal strip mounted on and extending transversely of said pair of parallel rods and located in registry with only a minor portion of the active surface of said cathode.

10. In an electron discharge device the combination with a thermionic cathode, of two elliptical grids of diilferent diameters coaxial with said cathode and each comprising a pair of parallel grid rods and a flattened helix of fine wire secured to said grid rods, said pairs of grid rods being mounted in alignment, and an anode electrode outside the main electron stream from said cathode to said outer grid and comprising a rod mounted between and parallel to the adjacent grid rods of said elliptical grids.

11. In an electron discharge device the combination with a thermionic cathode, of an input grid surrounding said cathode, an open anode adjacent said input grid and consisting of conductors spaced to provide a more open mesh than said input grid and to intercept only a portion of the electron stream flowing from said cathode past said anode, and a grid electrode of finer mesh-throughout its length than said anode coaxial with said cathode and surrounding said input grid and said anode;

12. In an electron discharge device the combination with a stem and an elongated thermionic cathode mounted in alignment with the axis of said stem, of an inner grid surrounding said cathode, an anode electrode outside of said inner grid and consisting of a pair of electrically connected rods extending from said stern parallel to and on opposite sides of said cathode,

and an outer grid exposed throughout its length thermionic cathode, a plate, and a plurality of auxiliary, electrodes interposed between said cathode and said plate to be passed successively by the electron stream from said cathode to said plate and comprising a multi-mu input grid of non-uniform mesh, another input grid electrode juxtaposed to said cathode, and an anode grid electrode of coarser mesh than said other input electrode interposed between said input grids.

14. An electron discharge device comprising a thermionic cathode, a plate, and a plurality of auxiliary electrodes interposed between said cathode and said plate to be passed successively by the electron stream from said cathode to said plate and comprising a helical input grid juxtaposed to and surrounding said cathode, an anode grid surrounding and of more open mesh than said input grid, a second helical input grid surrounding said anode grid and of non-uniform pitch, and a screen grid of finer mesh than said aniolde grid and surrounding said second input gr I 15. An electron discharge device comprising a thermionic cathode, two tubular grid electrodes surrounding and coaxial with said cathode, an output electrode consisting of a pair of rods parallel to and on opposite sides of said cathode and positioned between said two grid electrodes, another tubular grid electrode of non-uniform pitch surrounding and coaxial with said two grid electrodes, and a tubular plate surrounding and coaxial with all said grid electrodes.

16. An electron discharge device comprising a thermionic cathode, two cylindrical grids of successively increasing diameters coaxial with said cathode and elliptical in cross-section, a gridlike anode electrode interposed between and of more open mesh throughout its length than said two elliptical grids, a, third cylindrical grid circular in cross-section and surrounding said ellip-. tical grids and having non-uniform grid spacings, and a cylindrical plate surrounding said circular grid. i

17. An electron discharge device comprising an envelope enclosing a thermionic cathode, a cylindrical plate, four cylindrical grids of successively increasing diameters coaxial with and surrounding said cathode, a grid-like anode electrode interposed between the first and second grids from the cathode and of more open mesh throughout its length than said second grid, and a direct connection inside said envelope between said second grid and the fourth grid from the cathode.

18. An electron discharge device comprising a thermionic cathode, a plurality of tubular grid electrodes coaxial with and surrounding said cathode and of successively increasing diameters, a grid-like anode electrode between the first and second grid electrodes from the cathode and of more open mesh throughout its length than either of said grid electrodes, 9. third grid electrode having non-uniform grid spacings and surrounding said second grid electrode, and a tubu lar plate surroundingsaid third grid electrode.

19. An electron discharge device comprising a thermionic cathode, a cylindrical plate, a plurality of grid-like electrodes interposed between said cathode and said plate to be passed in succession by the electron stream from said cathode to said plate and including three tubular grids of successively increasing diameters, the smallest diameter grid being close to the cathode, thev intermediate diameter grid elliptical in cross.- section, and the largest diameter grid circular in cross-section and surrounding said intermediate elliptical grid. I

20. An electron discharge device comprising a thermionic cathode, a cylindrioalplate, five gridlike electrodes interposed between said cathode and said plate to be passed in succession by the electron stream from said cathode to said plate, the first grid-like electrode being nearest said cathode, the second grid-like electrode from said cathode constituting a grid-like anode of more open mesh throughout its length than the first grid-like electrode, the third grid-like electrode from said cathode being curved to the surface of an ellipse and-of finer mesh than said first grid-dike electrode, the fourth grid-like electrode from said cathode being curved to the surface of a circular cylinder and having non-uniform grid spacings, and the fifth grid electrode from said cathode being nearest said plate.

21. An electron discharge device comprising a thermionic cathode, a plate, a pair of input grids in series relation in the path of the discharge from said cathode to said plate, a grid-like anode of more open mesh than said input grids mounted between said grids in the path of said discharge, and a screen grid. of finer mesh than said anode mounted between said anode and the second input grid to electrostatically isolate said anode from said second input grid and said plate.

22. An electron discharge device comprising a thermionic cathode, an input grid surrounding said cathode, two other tubular grids of different diameters coaxial with and surrounding said input grid, the inner one of said two tubular grids being elliptical and the other circular in crosssection, and a tubular plate surrounding said grids.

23. An'electron discharge device comprising a thermionic cathode, an input grid surrounding said cathode, two other tubular grids mounted coaxial with and surrounding said input grid, the inner grid being elliptical and the other circular in cross-section, one of said tubular grids being of non-uniform mesh, and a tubular plate surrounding said grids.

24. An electron discharge device comprising a thermionic cathode, an input grid surrounding said cathode, a grid electrode surrounding said grid and imperforate over part of its length and comprising over another part of its length a pair anode. v v

' DONALD G. mm.

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