US2138228A - Electron discharge device - Google Patents

Description

R. J. ERICHS EN ELECTRON DISCHARGE DEVICE 7 Nov. 29, 1938.

Filed 'Aug. 15, 1937 INVENTOR RUTH J. 5/?! CH5 EN B ATTORNEY Patented Nov. 29, 1938 ears-NT tries,

nresne assignments, to Radio Qorporation oi America, New York, N. Y., a corporation ct Delaware Application August 13, 1937; Serial No; 158,834-

4 Claims.

invention relates to electron discharge devices for superheterodyne reception, and more particularly to a multi-electrode electron discharge. device in which-local oscillations of prezr determined frequency and input oscillations of a; cli iierent frequency, such as a radio signal, are mixedor "combined within the device.

In-radio-receivers employing the superheterodynemethod of 'receptien' alternating voltages of two-difie-rent' frequencies must be" combined; that is; the' signalvoltage of one frequency received bythe-antennaandusually amplified in the preceding stage mustbe combined with the local oscillator voltage at" a; different frequency producedbythe-l'ocal"oscillator in the receiver proper so that aheterodyneactionwill produce an alternating voltage of an intermediate frequency modulated in the same manner as the signal voltage;

Usual methods of a heterodyne reception employ afirst detector. or mixer tube in which the radio signal (input frequency) and the local oscillationsof a different frequency generated in an oscillator circuit usually by a separate tube '1 are both applied tothe same tube but on different grids. This mixing action in modern receivers maybe accomplished by means of two different typesoftubes, one the so-called pentagrid converter, which has several grids, the oscillator grid being the inner grid near the cathode and the signal grid being an outer grid. This tube performs both the functions of the oscillator and mixer. The converter type of tube in which both functions are performed meets with several difcoupling between the oscillator and signal sections, and the change of oscillator frequency when automatic volume control voltage is applied to the tube. These diificulties were overcome to a certain extent by placing the signal voltage on an inner grid and the oscillator voltage on an outer grid. This method of operation, however, prevented a tube design which could combine the functions'of' an oscillator and'rnixer in the same section of the tube due principally to the fact that the oscillator would cease functioning when the signal or control grid became sufiiciently negative to cut off the electron stream to the oscillator portion of the tube. For high frequencies, therefore, it was found more desirable to use the so-called straight mixer tube having asignal grid and an oscillator grid to which the oscillator voltage is applied by a separate tube or a, separate group of electrodes in the sameen ficulties', the two major ones being space charge velop-e eifecti-ve as" a separate tube toprovide the oscillator voltage.

When the higher frequencies were encountered, however; it'was found that even the mixer tube was subject to certain objections because of the transit time effect-in the tube which caused grid current to how inthe first or signal grid when the second-or oscillator grid voltage was high in frequency; This effect-is explained as due to the electrons which"are periodically repelled by theoscillator grid'when it goes negative; that is; electrons whicha-re in' the space between the-signal grid and the oscillator grid when the oscillator grid swings'negative are returned to the signalg-rid. It was also discovered that-thesig-nal grid input resistance was quite low at the high frequencies and most of this so-called transit time loading was also due to the electronsrepelled by the oscillator gridto the vicinity of the'first grid.

A tube which eliminates the undesirable characteristics" due to transit time loading is described-and claimedin a co-pending application, Seri'aLNo: 139,375" filed in the name ofEdward W. Herold, April28', l937 and assigned to the sameassignee asthe present application; Briefly this tube comprises a cathode surrounded'by a control grid; a first screen grid, a second control grid, a second screen grid; a suppressor and an anode. The first screen grid is provided with a pair of oppositely disposed U-shaped channel members extending, along andsecured to the screen grid side rods to provide oppositely disposed apertures through which beams of electrons pass fromthe cathode to the anode. In operation electrons which are caught between the first'screen grid and the second control grid, when the second control grid goes negative, are caused tobe swept out of the space between the first screen grid and the secon-d'control grid and toward the channel" members which intercept theseelectrons andprevent them from returning to the first control grid;

It is the principal object of my invention to provide an improved type of mixer tube for use in superheterodynereceivers, more particularly to provide a-still further improvedtube of the type described'in the Heroldapplication in which the transit time effects-described above are substantially eliminated;

The: novel. features: which I believe to be characteristic of my-inventioniareset forthwith particularity inthe appended claims, but. the invention itself willbestbe understood-byreference to the qfollowingdescriptiomtaken imconnection with the accompanying drawing in which Figure 1 is a transverse cross section of the mount of an electron discharge device illustrating and embodying the principles of my invention; Figure 2 is a practical form of a mount of an electron discharge device described in the Herold application; Figure 3 is a perspective view partially in section to show details of construction of an electron discharge device made according to my invention and which has been found most satisfactory; Figure 4 is an end view of the electrode mount assembly of Figure 3, and Figure 5 is a conventional circuit in which an electron discharge device made according to my invention is incorporated.

In the conventional mixer tube such as shown in the Llewellyn Patent 1,876,780, issued February 7, 1933, and assigned to the Bell Telephone Laboratories the cathode is surrounded in turn by a signal grid, a screen grid, an oscillator grid to which the local oscillation is applied, and a second screen grid, and an anode. At very high frequencies the time required for the electrons to travel from the cathode to the anode is long enough so that electrons may be caught in flight between the signal grid and the oscillator grid when the oscillator grid goes negative. These electrons are sometimes repelled by the negative oscillator grid and return to the signal grid with an appreciably increased velocity. By means of my invention the electrons which are repelled by the oscillator grid are swept out of the space between the signal grid and oscillator grid and prevented from returning to the signal grid.

A better understanding of the action may be obtained by referring to-Figure 1 showing a cross section of a mount of a tube embodying the principles upon which my invention is based. A cathode IB is surrounded in turn by a signal grid ll, screen grid I 2, oscillator grid l3, screen grid l4 and anode l5. A suppressor grid l6 can be added if desired. Each of the grids is provided with oppositely disposed side rods, the side rods lying in a common plane. With the construction so far described, which is the conventional mixer tube, the electrons are projected in two more or less well defined streams or beams from opposite sides of the cathode due to the action of the signal grid side rods. In normal operation when the oscillator grid 13 is swung negative by the applied oscillator voltage, the electrons are prevented from reaching the anode l5 and are returned to the region of the signal grid through the screen grid l2 which is at a high positive potential. The effect of the returned electrons is to cause the signal grid to exhibit a very low input resistance at high frequencies. In addition, some of the electrons have appreciably increased velocities because of the change in oscil-' lator grid potential during the time the electrons are in the region between grids I2 and I3. The increased velocity is sometimes sufiicient to permit electrons to strike the negatively biased signal grid ll.

To render the returned electrons inefiective there is placed between the signal grid and oscillator grid, and in line with the side rods, a pair of auxiliary electrodes H, which are maintained at a high positive potential with respect to the signal grid and the oscillator grid and being preferably at a higher potential than the screen grid l2. The result of this applied voltage is to cause the stream of electrons on either side to travel in more or less curved paths as indicated by the dotted lines from the cathode to the anode. It will thus be seen that most of the electrons are made to pass considerably closer to the side rods than when the electrodes I1, I 1 are not present. Electrons caught between the signal grid and the oscillator grid under these conditions will not return to the region of the signal grid but will be returned as indicated by the dotted lines to the auxiliary electrode l1, II, which are at the higher positive potential. These returning electrons are indicated for only onehalf of the tube, it being understood that this action takes place on both sides. In this way the electrons which would normally be returned by the oscillator grid going negative are swept from the field between the signal and oscillator grids and are rendered ineffective to cause grid current and low signal resistance.

In Figure 2 a modified form of tube is shown which is an improvement over that shown in Figure 1. The form shown in Figure 2 eliminates the necessity for separate auxiliary electrodes and the necessary leads and results in a simpler construction in which the high potential small auxiliary anodes are replaced by lower potential but much larger auxiliary anodes. In this arrangement the cathode 20 is surrounded by a control grid 2| and screen grid 22, followed in.

turn by the oscillator grid 23, screen grid 24, suppressor 25 and anode 26. In this form the auxiliary electrodes 21 and 28 have the form of channel members attached on the outside of the screen grid to the screen grid side rods so that oppositely disposed slots are provided for the electrons issuing from the cathode. This construction also has the advantages characteristic of the form shown in Figure 1, but is in addition simpler in construction.

The preferred embodiment of my invention disclosed in Figure 3 and embodied in a tube of the metal envelope type includes a metal envelope 30 in which the mount assembly is contained. The mount is positioned between a pair of shielding members 3! and 32 supporting the mica insulating spacers 33 and 34 which insulatingly support and space the electrodes from each other. As best shown in Figure 4, the cathode comprises a tubular member 35 of rectangular cross section. The flat opposite sides only preferably are coated, which assists in the beam formation of the electrons from the cathode to the anode. The cathode in turn is surrounded by a control grid 36, the wires of which are parallel to the flat coated surfaces of the cathode, the side rods 36' being opposite the non-coated surfaces of the cathode, a screen grid 31, oscillator grid 38, screen 39, suppressor 4G and anode 4!. The collecting electrodes or auxiliary electrodes 42 are in the form of channel members electrically connected to the screen grid side rods and oppositely disposed to provide a pair of oppositely disposed slots or apertures through which the beams of electrons from the cathode are directed toward the anode.

In accordance with my invention the collecting electrodes 42 are provided with fins 43 which are coextensive with and lie along the edges of the channel members adjacent the beams of elecrons, being preferably bent back at an angle outwardly from the beam as shown in Figure 4. These fins give the returning electrons a much increased sideward component in their passage between the first screen grid 31 and the second control grid 38. In this manner the electrons which would normally cause grid current in the first control grid 36 are intercepted by the fins.

In actual practice this arrangement gives an increased reduction in the first grid current over the type of tube described in the Herold application without affecting the other desirable characteristics of a mixed type tube.

The application of a tube made according to my invention to a conventional circuit is shown in Figure 5 in which the tuned circuit comprising the inductance 59 and Variable capacity 5| is connected between cathode 35 and control grid 36. The screen grids 3i and 39 are connected to the usual source of voltage 46, the oscillator grid 38 being connected to a separate local oscillator 52. The output from the anode is fed through the primary of the coupling transformer 53.

While I have indicated the preferred embodiment 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 purpose for which it is employed without depart ing from the scope of my invention as set forth in the appended claims.

What I claim as new is:

1. An electron discharge device having a cathode for supplying electrons surrounded by a first control-grid, a screen grid, a second control grid and an anode for receiving said electrons, said cathode and grids cooperating to cause electrons from said cathode to move in a beam to said anode, said second control electrode being adapted to have varying negative and positive potentials of high frequency applied thereto, and a pair of oppositely disposed channel members connected to said screen grid on opposite sides of said beam, said channel members having fins extending along the edges adjacent the beam for receiving electrons returned by said second control grid when negative potentials are applied to said second control grid during operation of the electron discharge device.

2. An electron discharge device containing a cathode having a fiat surface coated with emitting material for supplying electrons, a control electrode adjacent said cathode and lying parallel to said fiat surface and provided with a pair of oppositely disposed members at opposite edges of the flat cathode for providing a beam of electrons, an anode for receiving said beam of electrons, a second control electrode in the path of said beam of electrons adapted to have varying positive and negative potentials of high frequency applied thereto, and means on opposite sides of said beam including a pair of oppositely disposed channel members on opposite sides of said beam of electrons, said channel members having fins coextensive with and lying along the edges adjacent the beam for receiving electrons returned by said second control electrode when said control electrode becomes negative during operation of said electron discharge device.

3. An electron discharge device containing a cathode having a fiat surface coated with emitting material for supplying electrons, a control electrode adjacent said cathode and having grid wires parallel to said flat surface and provided with a pair of oppositely disposed members at opposite edges of the flat cathode for providing a beam of electrons, an anode for receiving said beam of electrons, a second control electrode in the path of said beam of electrons adapted to have varying positive and negative potentials at high frequency applied thereto, and means on opposite sides of said beam including a pair of oppositely disposed channel members on opposite sides of said beam of electrons, said channel members having fins coextensive with and lying along the edges adjacent the beam for receiving electrons returned by said second control electrode when said control electrode becomes negative during operation of said electron discharge device, and a screen grid positioned between said control electrodes.

4. An electron discharge device containing a thermionic cathode having two opposite flat surfaces coated with electron emitting materiaLthe surfaces between being uncoated, a control grid surrounding said cathode and having grid wires parallel to the fiat emitting surfaces of said cath-' ode and a pair of oppositely disposed side rods positioned adjacent the non-emitting surfaces of said cathode whereby a pair of oppositely directed beams of electrons is formed, and an anode for receiving said beams of electrons, a screen grid surrounding said control grid, a second control grid surrounding said screen grid and adapted to have varying negative and positive potentials of high frequency applied thereto, and a pair of oppositely disposed channel shaped members supported by and electrically connected to said screen grid and providing a pair of apertures in said screen grid through which the beams of electrons pass, said channel shaped members having fins bent outwardly at an angle from the beams of electrons and coextensive with and along the edges adjacent the beams of electrons for receiv-' ing electrons returned by said second control grid when a negative potential is applied thereto during the operation of said electron discharge device.

- RUTH J. ERICHS-EN.

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