US 2251124 A
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Patented July 29, 1941 CATHODE RAY AMPLIFYING TUBE Philo T. Farnsworth, San Francisco, Calif., as-
signor, by mesne assignments, to Farnsworth Television & Radio Corporation, Dover, Del., a corporation of Delaware Application August 10, 1935, Serial No. 35,596
My invention relates to a cathode ray amplifying tube, and more particularly to a cathode ray tube suitable for the production, when energized, of a brilliant optical image which is greatly to be desired in oscillographic work or in the reception of television signals.
The device of the present application is an improvement on my former invention described and claimed in application Serial No. 655,782, filed February 8, 1933, for a Luminescent screen and method of use; and has, in addition to the structure outlined in the former case, a struc ture allowing the utilization of the broad method of electron image amplification described and claimed by me in my application Serial No. 2 9 24- filed July 1, 1935, for an Electron image amplifier.
In the former application for a Luminescent screen and method of use, I describe and claim a tube having a luminescent screen comprising thin refractory material or a refractory fabric which is positioned with respect to a source of electrons of high velocity in such a manner that when the electrons hit the luminescent screen, the points of impact are raised to incandescence. If the beam is then moved in two directions over the screen, and the beam modulated by a television signal, an incandescent image is produced which, if sufiicient power is supplied to the beam, is well adapted for use as a projection light source whereby television pictures may be projected upon the usual type of viewing screen.
In this particular type of tube, the main objection is that the electron gun must of necessity have extremely high power in order to supply a sufiicient number of high velocity electrons in the electron beam to raise the screen to incandescence, and it is not always practical to build guns having sufiicient power to attain the brilliancy desired.
It is therefore among the objects of the present invention: To provide a cathode ray tube having a pair of electron guns and a charge storage electrode upon which a charge image may be formed to control the image of the other gun; to provide a cathode ray tube utilizing a heat screen; to provide a cathode ray tube of extreme brilliancy; to provide a cathode ray tube having an amplifying characteristic; to provide a cathode ray tube of exceptionally high power; to provide a cathode ray tube wherein a moving electron beam may be of relatively low power and utilized to produce an optical image of relatively high power; to provide a cathode ray tube having structure adapted to produce both high and low velocity electrons; to provide a cathode ray tube producing an image of suflicient brilliancy for projection; to provide a cathode ray tube of simple construction capable of producing an intense optical image; to provide a cathode ray tube having a moving beam capable of producing a charge image; to provide a charge storage electrode in a cathode ray tube; and to provide a simple and eflicient cathode ray amplifying tube.
My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not limit myself, in any way, to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claim.
In the drawing which accompanies this specification and forms a part thereof:
Figure 1 is a longitudinal sectional view of a preferred embodiment of my invention.
Figure 2 is a cross sectional view looking toward the flood gun of Figure 1.
Figure 3 is a graph representing a characteristic curve of the tube shown in Figure 1.
Figure 4 is a diagrammatic hookup reduced to lowest terms, showing a preferred circuit at energization of the tube shown in Figure 1.
Figure 5 is a cross sectional view of a prefcrred form of insulating grid.
Other broad aspects of my invention may be more thoroughly understood by a direct reference to the drawing and particularly to the apparatus shown in Figures 1, 2 and 4, the discussion of the operation of the device being deferred.
An envelope l is provided at one end with an electron gun assembly comprising a perforated anode 2, a control electrode 4 and an indirectly heated cathode assembly 5. These elements are supported in the usual manner by leads sealed through a stem 6. In this respect it should be pointed out here that the particular type of electron gun utilized is immaterial in the practice of my present invention, any such apparatus capable of producing a beam of electrons having a relatively small cross section at high velocities being perfectly satisfactory.
Immediately after the electronsleave the aperture of the anode 2, a deflection system is provided positioned to influence the beam. In this particular embodiment I show horizontal deflection plates 1 energized by oscillator 9, and vertical deflection plates l energized by oscillator II, the two sets of plates being positioned so as to deflect the beam in two directions when energized, magnetic deflection systems of course being fully equivalent. Around this portion of the beam I also prefer to position a focusing coil l2 energized by focusing source l4 under the control of a rheostat l5, and prefer to adjust the current in the coil so that the beam emitted from the electron gun will be focused at the far end of the tube in the plane of an insulated grid l6 positioned perpendicular to the axis of the tube and to the beam when at rest.
The grid I is provided with a metal base 17 and is covered with an insulating layer l9, so that its exposed surface is of insulating material.
While I do not wish in any way to be bound by any particular method of forming the insulating surface, I find that a very satisfactory combination is to smoke a nickel screen of fine mesh with fumes from burning magnesium so that magnesium oxide is deposited upon the material of the grid. I also prefer to so proportion the wires of the grid to the open spaces that when the magnesium oxide is deposited on the wires, the open portions of the grid will be substantially equal to the area of the portions of the plane occupied by the grid material. The grid I 6 is preferably provided with a lead wire 20 passing through the wall of the envelope l, and is closely adjacent a heat screen 2| facing the transparent end 22 of the tube. screen be maintained parallel and in fixed relation, I prefer to fasten them together by peripheral glass beads 24. I then provide the heat screen with a lead 25' passing through the wall of the envelope. made under the teachings of my application for a luminescent screen and method of use, supra, or with the modification described and claimed in my later application, Serial No. 67,889, filed March 9, 1935, for an incandescent light source. In either of these examples the screen is made from refractory material of extremely low mass, either in the form of an extremely thin refractory sheet or in the form of a refractory fabric, and I prefer to utilize a knitted fabric of refractory wire of approximately .00025 diameter with a mesh of at least the number of elements desired in the reproduced image. Any type of screen adapted to become luminous on electron impact is satisfactory however.
Intermediate the electron gun assembly and the grid-screen assembly I prefer to position a low velocity electron flood gun assembly. The assembly comprises two units, one on each side of the electron beam deflection path, so positioned that at extreme deflections necessary to cover the surface of the grid IS, the elements of the flood gun will not interfere with the passage of electrons through the tube.
The gun preferably comprises a pair of indirectly heated cathodes 26 formed in the usual manner around a heating element 21 and supported by heating leads 29 and cathode lead 30, the upper end of each indirectly heated cathode being also supported by a spacing bar 3| attached to the envelope by means of connection 32. Each cathode assembly is provided with a low velocity anode 34 which, in this instance, I prefer to make in the form of a spiral grid surrounding the in- In order that the grid and the heat The heat screen is preferably directly heated cathode. The low velocity anodes are provided with lead wires 35.
In operation, I prefer to energize the electrodes as shown in Figure 4, but I wish it to be distinctly understood that the numerical values given herein are simply exemplary of one particular specific tube; they are given only to show voltage relationships and are not in any way to be construed as limitations on either the means or method involved.
The anode 2 of the electron gun assembly is energized by a high voltage anode source 36; the cathode 5 is heated in the customary manner and is grounded; the grid 4 is provided with a negative bias from bias source 31; the insulated grid is provided with 1 to 5 volts positive bias, preferably variable, from a source 39; and the heat screen is provided with a positive potential from a heat screen source 49, preferably from 20 to 200 volts.
In measuring a tube hooked up as above described statically, and recording the characteristics which are produced by varying the potential on the insulated grid, a characteristic curve is obtained, as shown in Figure 3.
When the electron gun is energized, a beam of electrons having relatively small cross section issues from the anode 2 and is accelerated through the tube until it impacts the insu ated grid IS. The focusing coil I2 is so energized that the area covered by the spot on the grid I6 is of the elementary area necessary for the production of a television image. Inasmuch as the high velocity gun is adapted to produce a relatively small number of high velocity electrons they will, when they impact the insulated grid IS on the surface thereof, release secondaries therefrom. In the meantime, upon energization of the low velocity electron gun, the entire surface facing the gun,
of the grid IE, will be flooded with low velocity electrons, and these electrons will uniformly charge the insulating surface negatively and the charges will be fixed thereon, as is pointed out in my application for an electron image amplifier, supra.
An equilibrium point, however, will be reached depending in value upon the energization of the base wire and upon the leakage of electrons through the insulator to the base wire. This leakage, which will be referred to later, should be made as slow as possible or at least under one complete scanning cycle. When, however, the high velocity electrons hit the insulating surface and the electrons are knocked off, due to secondary emission therefrom, more electrons will leave that point than are received; therefore, the grid at the point covered by the spot will become positive in an amount governed by the control of the beam by a television signal applied to the grid 4. As the development of a uniform negative charge on the grid results in the formation of a space charge in the flood gun beam between the flood un and the grid, the positive potential developed at the point of impact of the spot will reduce the space charge and allow electrons from the flood gun to pass through the meshes of the grid and impact the heat screen 2| immediately behind it. There are plenty of electrons available from the flood gun, and many times the number of e ectrons pass through the screen at the point of impact than are contained in the high velocity beam. Thus a large amplification takes place and the screen is raised to incandescence over an area exactly back of that impacted by the high velocity beam and of substantially the same size as that of the spot. If, then, the high velocity beam be deflected in two directions by the defiecting plates 1 and ID, the beam can be made to traverse the entire area of a picture field upon the grid, at each successive area liberating electrons from the flood gun through the grid onto the heat screen at that particular point. If, then, the high velocity electron beam be modulated by the application of a television signal to the grid 4, a visual image will be produced upon the heat screen 2| corresponding to the picture which it is desired to reproduce.
After elementary areas of the grid have been traversed by the high velocity beam and the area changed in charge in accordance with the modulation of the high velocity beam and the beam passes on, these areas -gradually return to the equilibrium potential in accordance with the leakage factor of the insulator.
Therefore, in addition to the amplification obtained by the release of large quantities of low velocity electrons from the space charge in front of the grid, I also obtain an amplification effect as far as the optical result is concerned because of the fact that the charge image on the grid 18 remains the-re after the scanning beam has passed, thus allowing electrons from the low velocity gun to pass through to impact the screen after the scanning beam has gone by.
In order for the device to be most effective the leakage time should be adjusted so that any elementary area will return to the equilibrium potential just before the scanning beam passes over it again, and when this happens the elementary area is recharged to a new potential corresponding to the next picture cycle.
In practice it is not of course possible to adjust this time exactly, but it is possible to obtain a very definite storage of the charge during the scanning cycle. In practice I find I can easily attain amplifications of more than 1000; that is, for every electron in the high velocity scanning beamI can release 1000 or more electrons through the grid to the heat screen, thus greatly reducing the difficulties attendant upon producing an electron beam of the intrinsic intensity necessary to produce the results obtained with the present device.
Reference to the characteristic curve shown in Figure 3, however, will immediately suggest another mode of operation. While I do not wish to be bound by the theory herein, I believe that the explanation of the characteristic curve is relatively simple. It will be seen that amplification is obtained which is both positive and negative, line 4| representing zero amplification, maximum positive amplification in the tube described taking place at 2.8 volts positive on the grid base wire and maximum negative amplification taking place at approximately five volts positive thereon. Over the region where amplification is positive I believe that electrons are leaving the insulating surface due to secondary emission, the electrons emitted probably joining the space charge created by the equilibrium charge.
As the positive potential on the insulated grid is increased, the electrons leaving the portion of the insulator facing the high velocity gun are not allowed to join the space charge but return through the grid to the back side thereof and charge .the insulator there negatively. As the control on the 10W velocity electrons will be greatest for the back side of the insulator the curve becomes reversed, the negative charge acquired overcoming the positive charge on the front of the insulator, thus producing the curve shown in Figure 3. It is therefore possible to utilize an u'nmodulated high velocity beam, an unmodulated low veloc-' ity beam, and'control the tube entirely by television signals applied directly to the base wire of the insulated grid..- There are a number of portions of the curve of Figure 3on which the tube will operate-as can readily be seen by those skilled in the art.
The advantages of such a tube are readily apparent. High intensity images are produced without the use of guns requiring a large amount of electrons with restricted beam sections; large quantities of electrons are immediately available from the low velocity fiood gun which does not require a production of a beam having a restricted cross section; large currents are available irom the flood gun under full control of the charge image produced on grid l6 by the interaction of the high velocity gun and the bias on the insulated grid. The leakage lag occurring in the insulator provides not only an increase in the apparent brilliancy of the image but also prolongs the fade-out of each individual picture to such a point that flicker is, almost completely eliminated, there being only a gradual dying down of one image before the next one appears. The amount of amplification available is extremely high and there are no critical adjustments necessary in the tube except for the adjustment of the bias on the insulated grid, and this adjustment needs only to be suificiently critical as to place the tube on a proper operating portion of the curve shown in Figure 3.
In the particular embodiment shown I have been able to release ten to twenty milliamperes of current in the elemental areas of the heat screen with a scanning beam supplying only a few microamperes; and While the velocity of the electrons in the scannnig beam must be sufiiciently high to create secondaries upon impact, the velocities need not be high as compared .to the velocities which would be necessary were the heat screen to be impacted directly.
A cathode ray tube comprising an envelope containing aluminescent screen defining a picture area, an apertured grid adjacent and parallel to said screen, an electron gun including a cathode of relatively small area and a co-operating anode having a relatively small aperture therein, co-operating to define an electron beam of elemental cross-section directed at said picture area, and additional electron sources including a cathode having an extended area positioned on each side of said beam between said first electron gun and said screen, and an open mesh accelerating electrode surrounding each of said electrodes.
PI-IILO T. FARNSWOR'IH.