US2198340A - Electron discharge modulating device - Google Patents

Description

' 23, 1940. w. A. HARRIS ELECTRON DISCHARGE MODULATING DEVICE Filed Sept. 25, 1937 2 Sheets-Sheet 1 L% ::IIIIIIIIIIIIII% Lb EIIIIIIIIIIIIIIW a Y mm H mm W I -M M m April 23, 1940. w, A HARRIS 2.198.340

smc'rnon DISCHARGE MODULATING DEVICE Filed Sept. 25, 1937 2 Sheets-Sheet 2 INVEN TOR. WILLIAM A. HARRIS ATTORNEY.

Patented Apr. 23, 1940 j UNITED STATES PATENT OFFICE Delaware Application September 25, 1937, Serial No. 165,633

7 Claims. (01. 250-27) taken along line 2-4 of Figure 1, Figure 3 is a .My invention relates to electron discharge devices for superheterodyne reception, and more particularly to a multi-electrode electron discharge device in which local oscillations of predetermined frequency and input oscillations of a different frequency, such as a radio signal, are mixed or combined within the device.

In radio receivers employing the superheterodyne method of reception alternating voltages of two different frequencies must be combined; that is, the signal voltage of one frequency received by the antenna and usually amplified in a preceding stagemust be combined with thelocal oscillator voltage at a different frequency produced by the local oscillatorin the receiver proper so that a heterodyne action will produce an alternating voltage of an intermediate frequency modulated in the same manner as the signal voltage.

Usual methods of heterodyne reception employ a mixer tube 'having a numberof grids between the, cathode and anode, in which the radio signal (input frequency) and the local oscillations of a, different frequency. generated in an oscillator circuit, usually by a separate .tube, are both applied to the same tube but on different grids. This mixing action in modern receivers may be accomplished by means of a so-called converter tube, which has several grids, the oscillator grid being the inner grid near the cathode and the signal grid being an outer grid nearer the anode; This tube performs both the functions of the oscillator and mixer. .The. converter type of tube in which both functions are performed may be.

subject to the difliculty more fully discussed below, of so-called space charge coupling between.

the oscillator and signal sections, resulting in a' degenerative action in the tube, reducing its out put and making it less sensitive to applied signal voltages. a

It is the principal object of my invention to. provide an improved electron discharge device of. the converter or mixer type for use in superheterodyne receivers, more particularly to provide such a device in which the space charge coupling effects are minimized.

The novel features which I believe to be char: acteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawings in which Figure l is a longitudinal section of anielectron discharge device of the conventional converter type, Figure 2 is a cross. section of the device.

transverse section of an electron discharge device embodying my invention, Figure 4 is a circuit diagram for the tube shown in Figure 1, and Figures 5 and 6 are circuit diagrams using a tube made in accordance with my invention.

The conventional converter tube comprises a cathode for supplying electrons and an anode for receiving electrons, and a plurality of grid-like auxiliary electrodes interposed between the oathode and the anode, which are passedsuccessively by the electron stream from the cathode to the anode. Anoscillator circuit may be connected to the cathode and two adjacent auxiliary electrodes to constitute an oscillator for producing in thetube an electron stream pulsating at the desired local oscillator frequency, the electron stream being modified and modulated on its wayto the plateby 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 thus firstv affected by the oscillator input grid and then by the signal input grid so that the frequency of the current to the plate and output circuit is the resultant obtained by thus mixing in the tube the local oscillator frequency and the input or radio signal frequency to obtain the desired beat or intermediate frequency in the output circuit.

In the usual tube the grids positioned between the cathode and the anode are in succession, the oscillator grid, which may be the conventional heli'callywound grid, the oscillator anodes which may be only a pair of rods, the screen grid for electrostatically shielding the oscillator section from the rest of the tube, the signal grid and second screen grid, each of the last three grids usually being of the helically wound type. Another grid may or may not be used for suppressing secondary electrons from the anode.

'It is believed that the space charge coupling is produced as follows. In the operation of a tube of the type described a cloud of electrons or so-called space charge is formed between the first screen grid and the signal grid. This space charge varies in accordance with the oscillator frequency and in effect induces a voltage and a corresponding charging current in the signal grid circuit. This charging current is also of the local oscillator frequency and produces a voltage across any impedance offered to it by the grid circuit. The phase relationship of this .voltage may be opposite to that of the applied oscillator voltage so that there is interference with the flow of electrons from the cathode through the signal grid to the anode; in other words, there is a degenerating action in the tube. In the usual broadcast receiver where the local oscillator frequency is comparatively far removed from the signal frequency, for example 15 or 20%, the impedance offered by the signal grid circuit to the current of local oscillator frequency is very small and the degenerating action not particularly noticeable. At higher frequencies however, at 20 megacycles or higher (15 meters or lower) the local oscillator frequency is removed from the signal frequency by only 1 or 2% and the degenerating action is serious. ,By means of my invention I substantially reduce this degenerating action.

In accordance with my invention I place between the first screen grid at a positive potential with respect to the cathode and the signal grid preferably biased slightly negative, for example, about threevolts with respect to the cathode, another or auxiliary grid, which is at a potential less than that of the space between the signal grid and the screen grid at the position of this auxiliary grid. It may for example be biased by means of a separate battery or it may be connected directly to the cathode. In this Way the control or signal grid is effectively shielded from the space charge so that the charging current cannot be induced in the signal grid circuit. A better understanding of my invention may beobtained'by referring to the drawings in which Figure 1 represents a longitudinal section of an electron discharge device of the conventional converter type. The electron discharge device shown inFigures 1 and 2 comprises a highly evacuated bulb 10 with the usual re-entrant stem l| on which the electrode assembly is carried.

Two parallel support rods 2 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 M. A cylindrical plate |5, 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 I 5. The equipotential cathode is of the usual type, comprising a reverse ,coiled tungsten filament heater surrounded by a straight'nick'el sleeve l6 coated with oxides of barium and strontium, and having a cathode connector I! joined to a bent cathode lead H! which also carries the getter tab l9. 1

The auxiliary electrodes or elements inter- ,posed 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 anda 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 spaceror bars l3. Adjacent and surrounding the cathode is the No. 1 auxiliary electrode or oscillator input grid 20, preferably elliptical in cross section. The next, or No. 2 auxiliary electrode, actsas the oscillator output electrode or anode, and is preferably rnerelytwo rods mounted on the stem in alignment with the grid rods of the oscillator or No. 1 grid 2|], and electrically con- This oscillator anode electrode may be made in many different ways to draw only a portion of the cathode current. The next, or No. 3 auxiliary electrode, is a screen grid '22, which, as shown in Figure 2, also may be elliptical in cross-section, with its long axis in alignment with the grid rods of the other grids. The next, or No. 4 auxiliary electrode, is a cylindrical signal input grid 23, preferably of the multi-mu type, and shown in Figure 1 as a helical grid of progressively varying pitch, although other well known forms of multimu grids may be used. The signal input grid is connected to a contact-cap on the tip of the bulb. The next, or No, 5 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 screen grid 22 through a bent lead 25 to which both grids are connected;

Figure 5 shows thetubeillustrated in Figure 1 connected in circuit for use in superheterodyne reception. A conventional input circuit 28 is connected to the signal input grid, and a conventional tuned load or output circuit 29 is connected to the plate IS. The cathode I6, grid'2ll, and anode 2| are connected to an oscillator circuit 3|) of conventional'design and includes. a feedback coil 3| connected to the oscillator output electrode or anode 2|. The voltages for operating the tube may be obtained from taps on a resistor or voltage divider 32.

In the operation of the tube connected as shown in Figure 5 the cathode l6, oscillator grid 20 and anode electrode 2| constitute the three electrodes of an oscillator. Allof 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 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 grid 20, during operation-of the tube, will become negatively biased up to from 15 to 20 volts due to the action of the grid leak arrangement connected between this grid and ground and cathode. The oscillator anode 2| is connected to the voltage divider through coil 3| so that its voltage is approximately 200 volts positive with respect to ground, the anode I5 being at about 250 volts positive. The remainder of the electron stream or current from the cathode, modulated by the oscillator grid 20, passes directly to the grid 22, which is sufiiciently positive, usually about volts which is less positive than that of the oscillator anode 2| .andoutput anode I5, to draw the electrons to and through it, and to form between the grid 22 and the control grid 23 biased slightly negative with respect to the cathode a space charge which responds tothe signal input grid like a virtual cathode with an emission pulsating at the frequency of the oscillator. It is of course possible to vary the above voltages within limits and still obtain the operation desired.

The grid 22 is connected to the positive voltagesource 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 thecathode 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. Capacity coupling between the oscillator and the signal input grid is substantially eliminated by screen grid 22, and in effect the cathode for the signal input grid and the plate is a virtual cathode formed between the screen grid 22 and the signal grid 23. In operation the oscillator grid 20 is so biased by the grid leak connection that cathode current flows only during a portion of its cycle, the result being a pulsating cathodecurrent through the tube during the portion of the half cycle when the oscillator grid is positive, no current flowing through the tube during the remainder of the cycle. These pulses of cathode current flowing to and through the screen grid 22 produce on the outside of the grid 22 the space charge which pulsates at the oscillator frequency and acts as a virtual cathode. As pointed out above this space charge formed between the screen grid 22 and signal grid 23 results in adegenerative action which at the higher frequencies is detrimental to the best operation of a circuit in which the tube is used.

In accordance with my invention I place between the screen grid 22 and the signal grid 23 an auxiliary grid 22a which is connected to the cathode or to some point on the voltage supply to make it slightly positive with respect to the cathode. then forms between the screen grid 22 and the auxiliary grid 22a and is modulated as described above at the frequency applied to the oscillator grid 29. The mesh of the auxiliary grid 22a while sufficiently small to give the grid a good electrostatic shielding characteristic is sufficiently wide so as not to interfere to any substantial degree with the functioning of the tube, the electrons from the virtual cathode 'being drawn through the auxiliary grid 22a by the resultant field of the plate I5 and screen grid 24, preferably connected to screen grid 22, whenever the signal grid 23 is at such a voltage that electrons can pass through it. My invention is also applicable to mixer tubes using a separate tube for the oscillator.

-In Figures 5 and 6 are shown circuits which are substantially the same as that shown in Figure 1 but in which a tube made in accordance with my invention is used. In Figure 5 however the auxiliary grid is shown connected directly to the cathode, whereas in Figure 6 it is connected to the voltage divider 32 at some point between the cathode and the screen grid so that its voltage is less than that of the screen grid but higher than that of the signal grid.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the The space charge or virtual cathode.

trol electrode, a screening electrode, means for maintaining said screening electrode at a fixed positive potential, a second control electrode. and an auxiliary electrode between said screening electrode and said second control electrode, means for maintaining said control electrodes at a voltage not greater than that of said cathode, and means for maintaining said auxiliary electrode at a potential less than the potential of the space at the position of said auxiliary electrode between the screening electrode and said second control electrode.

2. An electron discharge device having a cathode for emitting electrons and an anode surrounding said cathode for receiving said electrons, and a plurality of grid electrodes positioned between the cathode and the anode and coaxial with said cathode and comprising in succession a first control electrode adapted to have an alternating voltage applied to it, and a screening electrode, means for maintaining said screen grid at a fixed positive potential, a second control electrode between said screening electrode and said anode and adapted to have a modulating voltage applied thereto, and an auxiliary electrode between the screening electrode and said second control electrode, means for maintaining said control electrodes at ,a voltage not greater than that of said cathode, and means for maintaining said auxiliary electrode at a potential less than the potential of the space at the position of said auxiliary electrode between the screening electrode and said second control electrode.

3. An electron discharge device having a cathode for emitting electrons and an anode for receiving said electrons, a plurality of foraminous electrodes between said cathode and said anode and comprising in succession a first control electrode, a screening electrode and a second control electrode, an output electrode between said first control electrode and said screening electrode, and an auxiliary electrode between said screening electrode and said second control electrode, means for maintaining said control electrodes at a negative potential with respect to said catliide, and means for maintaining said auxiliary electrode at a potential less than the potential of the space at the position of said auxiliary electrode between the screening electrode and said second control electrode.

l. An electron discharge device having a cathode for emitting electrons, an anode for receiving electrons, a plurality of foraminous electrodes between said cathode and said anode, and comprising a first control electrode and an output electrode for providing an oscillator section of the electron discharge device, a screening electrode surrounding said first control electrode and output electrode and adapted to be maintained at a positive potential with respect to said cathode, and a second control electrode 'between said screening electrode and said anode, and an auxiliary electrode between said screening electrode and said second control electrode, means for maintaining said control electrodes at a potential negative with respect to said cathdde and means for maintaining said auxiliary electrode at a potential less than the potential of the space at the position of saidauxiliary electrode between the screening electrode and said second control electrode.

5. An electron discharge device having a cathode for emitting electrons and an anode for receiving said electrons surrounding said cathode, and a plurality of grid electrodes positioned between the cathode and the anode and coaxial with said cathode and comprising in succession a first control electrode and a screening electrode, and an output electrode between said first control electrode and said screening electrode, said first control electrode and said output electrode forming the oscillator portion of said electron discharge device, a second control electrode between said screening electrode and said anode and adapted to have a modulating voltage applied thereto and an auxiliary electrode between the screening electrode and said second control electrode, means for maintaining said control electrodes at a negative potential with respect to said cathode and means for maintaining said auxiliary electrode at a potential less than the potential of the space at the position of said auxiliary electrode between the screening electrode and said second control electrode.

6. An electron discharge device having a cathode for emitting electrons and an anode for receiving said electrons surrounding said cathode, and a plurality of grid electrodes positioned between the cathode and the anode and coaxial with said cathode and comprising in succession a first control electrode and a screening electrode, and an output electrode between said first con- 7 trol electrode and said screening electrode, said first control electrode and said output electrode forming the oscillator portion of said electron discharge device, a second control electrode between said screening electrode and said anode and adapted to have a modulating voltage applied thereto and an auxiliary electrode between the screening electrode and said second control electrode connected tothe cathode.

7. An electron discharge device having a cathode for emitting electrons and an anode for receiving said electrons surrounding said cathode, and a plurality of grid electrodes positioned between the cathode and the anode and coaxial with said cathode and comprising in succession a first control electrode and a screening electrode, and an output electrode between said first control electrode and said screening electrode, said first control electrode and said output electrode forming the oscillator portion of said electron discharge device, a second control electrode between said screening electrode and said anode and adapted to have a modulating voltage applied thereto and an auxiliary electrode between the screening electrode and said second control electrode connected to the cathode, and a second screening electrode positioned between said second control electrode and said anode.

WILLIAM A. HARRIS,

Images