US2109289A - High power projection oscillograph - Google Patents

Description

Feb. 22, 1938. P.AT. FARNSWORTH ET AL 2,109,289

HIGH POWER PROJECTION OSCILLOGRAPH Filed NOV. 2, 1936 I INVENTORS 5 'Pkxlc T T q' 'mmh.$\ SW01- Patented Feb. 22, 1938 UNITED STATES 2,199,289 HIGH POWER, PROJECTION osoILLoGRAPH Philo T. Farnsworth, San Francisco, and Frank J. Somers, San Jose, Calii'., assignors to Fansworth Television Incorporated, San Francisco, Calii'., a corporation of California Application November 2, 1936, Serial No. 108,723

3 Claims.

Our invention relates to projection oscillographs, and'more particularly to such oscillographs utilizing a luminous screen wherein light is produced by incandescence rather than by 5 fluorescence.

In cathode ray tubes where the image is produced on a screen by raising elementary areas thereof to incandescence by ,the impact of a cathode ray beam, the principal difllculties have 10 been first, to obtain sufllcient power to excite the heat screen at low cathode ray gun voltages, and second, to provide a screen having a sufiiciently low mass so that heat losses may be confined almost wholly to radiation.

15 Another difliculty, an outgrowth of the first two above-mentioned, is that it is desirable to make the picture area of the heat screen relatively small, preferably of standard motion picture frame size, so that economical lenses may be used 20 for projection of the image. When such a relatively small screen is used, it is obvious that a very fine scanning spot is necessary to obtain high fidelity images, and a fine spot is inconsistent with high gun currents.

25 It is therefore the main object of our present invention to provide a cathode ray tube of the heat screen type, wherein the gun current can be kept low in. order to obtain a fine scanning spot.

30 Another object of our invention is to provide a high illumination screen, even though a small scanning spot is used, and to be able to use a relatively low gun anode voltage.

A further object of our invention is to provid 35 an exceedingly high voltage on the screen itself, and to generate this voltage close to the screen and by the use of high frequency oscillators, in order to minimize distortion due to electrostatic stresses.

40 A still further object of our invention is to utilize secondary emission to increase the number of electrons reaching the heat screen; and again, it is also an object of our invention to feed power to the electrons bombarding the heat 5 screen after deflection.

Our invention possesses numerous other ob: jects and features of advantage, some of which, together with the foregoing, will be set forth in the'following description of specific apparatus 5 embodying and utilizing our novel method. It is therefore to be understood that our method is applicable to other apparatus, and that we do not limit ourselves, in any way, to the apparatus of the present application, as we may adopt various 55 other apparatus embodiments, utilizing the method, within the scope of the appended claims,

Referring to the drawing:

The figure is diagrammatic and reduced to lowest terms, and comprises a sectional view of a preferred tube embodying our invention, together with a sectional view of a preferred oscillator and circuits connecting the two for operation.

Our novel method of operating a cathode ray tube, together with one preferred means for practicing the method, may be more fully understood by direct reierence to the figure.

The cathode ray tube comprises an envelope I provided at one end with a window 2 and at the other end with a reentrant stem 4. On this reentrant stem we mount a gun heater 5, a gun cathode 6 and a thimble grid 1 having a central aperture 8 through which electrons pass to enter the beam canal 9 of a gun anode l0.

The cathode, grid and anode of the gun are alined to project a beam of electrons of low power into the envelope space H and toward a luminescent screen l2 which is preferably formed of refractory material of low mass, such as, for example, an extremely thin sheet of tantalum or tungsten, or the screen may be knitted from fine refractory wires to have a mesh smaller than the elementary area it is desired to reproduce, or again, it may comprise parallel rows of refractory Wire spirals; in fact, there are many forms in which this screen may be made, the main factor being that it shall have low mass so that heat losses may be confined almost wholly to radiation, and that it shall have an elementary structure such that all desired detail may be reproduced thereon.

A secondary emission screen I4 isprovided between anode 9 and heat screen l2, preferably closer to the heat screen than to the anode. The envelope chamber II between anode 9 and secondary emission screen It is preferably wallcoated with a silver or nickel film I5, and this wall film is connected directly to secondary emission screen M at one end and provided with an external lead It; at any convenient point. Heat screen I2 is provided with an external lead ll, anode 9 with its lead l9, and the various gun elements are provided also with their respective leads, as is customary in the art.

For operation, gun heater 5 may be energized from alternating current mains through transformer 20; gun cathode 6 is connected to ground; gun anode 9 and film l5 are connected together in a combined supply lead 2|; and thimble grid is provided with the usual bias through biasing resistor and battery 22 so that the gun current may be accurately regulated.

Scanning oscillators 24 and 25 energize scanning coils 26 and 21 in order that their fields may move the beam issuing from beam canal 9 in two directions over secondary emission electrode llfand a projection lens 29 is provided so that the image formed on heat screen l2 may beprojected on any convenient viewing surface. In order that the scanning spot or cross section of the scanning beam may be kept as small as possible, we prefer to surround the beam path with a focusing coil 30, energized by focusing battery 3i under the control of resistor 32.

So far we have described all connections to the tube except the voltage supply to anode l0 and secondary emitter l4, and to the heat screen l2. We prefer to supply voltage to the heat screen from a high frequency oscillator 35. This oscillator may be any oscillator capable of a high frequency output, such as three megacycles, for example, although we prefer to utilize a multipactor oscillator for this purpose. Such an oscillator has been described by Philo T. Farnsworth in his application for United States Letters Patent, Serial No. 611042, filed Jan. 27, 1936, and comprises an envelope containing a secondary emissive cathode 36, a perforated anode'31 and a central ion collector 38.

The multipactor cathode and anode are connected together through a tuned circuit 39 which forms the primary of a direct-coupled transformer, the secondary 40 of which is connected to lead l1 and thence to heat screen l2. Multipactor anode voltage is supplied by anode battery 4|, and lead 2| is connected to this battery through radio frequency choke 42, thus providing steady voltage for gun anode ID. The cathode-anode circuit of the multipactor contains a blocking condenser 43 to prevent anode voltage reaching the cathode. Ion collector 38 is connected to ground through an ion control source 44 so that ion control electrode 38 is slightly negative to pick up any free ions in the multiplier space.

Such a tube is a self-oscillator at high frequencies, oscillating power being developed by repeated impacts of primary electrons against cathode 36, which is preferably treated or otherwise formed in such a manner that for each primary impacting it, two to ten secondaries will be emitted. The action of this type of oscillator is well known in the art, due to Farnsworths publications, and no further explanation is deemed necessary.

In operation, we shall describe a particular tube and give the voltages which have been found, by experimentation, to be satisfactory in the operation of that particular tube, and it will be well within the knowledge of those skilled in the art to understand the energization and operation of other examples.

We prefer to utilize the gun to produce a gun current of about one milliampere, with the relatively low gun anode voltage of fifteen hundred volts. This means that the beam issuing from beam canal 9 may be of very small diameter and may be kept at a small diameter until it reaches secondary emission electrode M to form an exceeding'ly fine spot thereon. In addition, due to the factthat the gun anode has such a low voltage, and because there is a small diameter beam, the scanning sensitivity of the cathode ray beam .to deflection is maintained high.

We then prefer to form the secondary emission screen I in such a manner that it will generate copious secondary emission when the electrons in the beam bombard it, and we have found that we can so treat this screen that secondaries in the ratio of one to one may be obtained with impact voltages as low as twenty volts, with a total secondary production for each primary of as high as ten to one. We form the screen I4 of relatively fine wire so that even though the mesh count of the screen I4 is greater than the number of elements desired in the image, there is still plenty of aperture area.

When oscillators 24 and 25 are set in operation, the beam is moved to produce a picture area on the screen ll. Obviously, some of the primaries in the beam will pass through the apertures in screen i4 without creating secondaries. Other primaries in the beam will, however, impact the wires of screen I4 and generate secondaries. Consequently, there may be available, in the plane of the screen ll, both primaries and secondaries. If, then, oscillator 35 is set into operation so that the transformer 39-40 will supply the heat screen l2 with approximately fifty to seventy five kilovolts R. M. S. at a frequency of about three megacycles, both primaries and secondaries will be pulled through screen l4 and tremendously accelerated before they impact heat screen l2. The radio frequency current applied to the heat screen is, of course, self-rectified within the tube. The frequency of the multiplier oscillator is chosen high enough so that there will be several cycles at least of the radio frequency per picture element, and there will, therefore, be no matte effect produced in the picture from this source. At the same time, frequencies in the neighborhood of three megacycles are low enough to allow the use of high Q coils, eflicient oscillator operation and eflicient voltage step-up in the transformer 39-40.

Inasmuch as primaries and secondaries are only emitted from screen I at the point of scansion, it is obvious that only corresponding points on heat screen 12 will be bombarded, and due to the acceleration between screens i4 and I2, as a consequence of the extreme high voltage on screen i2, the wires of screen I2 at the point of electron impact will become incandescent and illuminated in accordance with the modulation of the original scanning beam. It is therefore obvious that by supplying the majority of power in the tube as an accelerating potential on the final screen, the beam itself may be made of sufiiciently low power so that it may be sensitive to deflection and have an extremely small cross section, thus eliminating the objections to primary'guns of highpower.

There is, however, another very important feature in the use of a high frequency alternating potential on screen l2. Screen I2 is adapted to be raised to incandescence by electron impact. It is obvious, therefore, that the mass of the wire from which screen I2 is woven must be small in order to reduce heat conduction along the wires. and in order that the mass may be raised to incandescence without substantial lag. The material 'of the screen may be tungsten or tantalum wire, for example, between .005" and .0005. The type of mesh of the screen may be, and preferably is, that of a knitted fabric, in order that the individual wires thereof may be longer than the distance between their points of attachment to the support frame. By utilizing such a knitted screen or a similar inherently elastic amaaeo mesh, it is possible, as far as the diiferential heat- .ing of the screen is concerned, to maintain the screen I2. It is obvious that if this high voltage were steady, that there would be large electrostatic stresses set up between screen l2 and screen I4, or between screen l2 and charges on the ad- Jacent tube walls. Such stresses could only be eliminated by wide spacing between adjacent screens, and by the enlargement of the envelope to a point where electrostatic charges on the walls thereof would not be suillcient to place a stress upon screen I2. Such spacings, however, are highly impractical, and with spacings demanded by modern design, the electrostatic stresses set up by a steady potential on screen l2, under the voltages mentioned, are sufllcient to completely destroy the utility of screen l2, inasmuch as it is inherently elastic and will distort freely under stresses much less than those encountered in the example given.

If, however, the screen vI2 is energized by an alternating potential, particularly an alternating potential which is of sufflciently high frequency so that the screen l2 cannot move under the influence of the rapidly changing and reversing electrostatic stresses, it is obvious that the screen I 2 will stay in its predetermined plane. It is only necessary, therefore, to apply an alternating potential having a frequency that is substantially higher than the natural period of vibration of screen It, and having, in case that the s reen I2 is to be used as an image producer w ere elemental areas are to be formed thereon, the frequency sufiiciently high so that there will be several t:sycles of the high frequency per picture elemen It is, however, to be distinctly understood that we also wish to use the method of energizing inherently elastic screens with high frequency potentials in all cases where such screens would normally, under the influence of a steady potential, be subject to electrostatic stresses causing distortion of the screen, irrespective of whether or not the screen is to be used as a light source, an accelerating electrode, or in any other fashion within a thermionic tube.

We claim:

1. A cathode ray. tube comprising an envelope containing a source of electrons, at collecting electrode normally distortable by the electrostatic stresses set up when energized to a predetermined steady collection potential, an oscillator connected to said electrode, and circuits maintaining the frequency of said oscillator substantially higher than the natural period of said collecting electrode.

2. A cathode ray tube comprising an envelope containing a source of electrons, a collecting electrode comprising a fine wire screen normally distortable by the electrostatic stresses set up when energized to a predetermined steady collection potential, an oscillator connected to said electrode, and circuits maintaining the frequency of said oscillator substantially higher than the natural period of said collecting electrode.

3. The method of developing an incandescent image on a heat screen in a cathode ray tube which comprises generating a stream of electrons having an elemental cross section, scanning a picture area with said beam, generating secondary electrons with part of said beam at each elemental area scanned, combining said secondary electrons with the remainder of said beam, and energizing said screen with an alternating potential at a frequency substantially higher than the number of elements involved.

PHILO T. FARNSWORTH. FRANK J. SOMERS.

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