US2153949A - Cathode ray oscilloscope - Google Patents

Description

R. H. VARIAN CATHODE RAY OSCILLOSCOPE April 11, 1939-.

Filed March 15, 1935 RUSSELL H. VAR/4N.

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Patented A r. 11, 1939 umTE-o STATES PATENT OFFICE CATHODE RAY osolLLosoorn Russell H. Varian, San Francisco, Calif., assignor,

by mesne assignments, to Farnsworth Television & Radio Corporation, Dover, DeL, a corporation oi! Delaware Application March 13, 1935, Serial No. 10,888

a Glaims. (or. 250- 157) My invention relates to oscilloscope apparatus, and particularly to electrode construction for tubes used in such apparatus, in combination with means for focusing the cathode ray beam.

Among the objects of my invention are: to

provide a cathode ray tube possessing greater sensitivity than is found in tubes of like character in present use; to provide a tube which gives a greater degree of illumination at the screen thereof; to provide means, in such a tube, adapted for use in a television receiving system,for the production of a cathode ray beam having a cross section of predetermined shape; to provide, in the type of tube described, a more positive grid control for the beam, and one whichhas a. sharper cut-off; to provide a cathode ray tube in which operational diiliculties, due to residual gas in the tube and to positive ion bombardment of the cathode, have been reduced to a minimum; to

provide a cathode ray tube construction in which the careful al-inement of tube elements is not essential to the eflicient operation of the device; to provide focusing means for the cathode ray beam which, together with animproved emitter, produces a spot on the tube screen having sharper definition; to provide an improved and simplified construction for cathode ray tubes; and to provide a simple method of regulating the size of the luminescent spot in a cathode ray tube;

Other objectsof my invention will be apparent or will be specifically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of the invention herein described, as various forms may be adopted within the scope of the claims.

Referring to the drawing:

Figure 1 is a vertical sectional elevation of a cathode ray tube and associated apparatus incorporating the improvements of my invention.

Figure 2 is a plan view of one type of electron emitter adapted to .be used in the tube of my invention.

Fig. 3 is a side elevation of the electron emitter J Figure 8 is a vertical sectional view of a modified accelerating electrode, showing its cooperative relationship with others of the electrodes.

Figure 9 is a modified form of electron emitter.

In broad terms, the cathode ray oscilloscope of my invention comprises an evacuated envelope having a plurality of cooperating electrodes mounted therein. One of these electrodes is a cathode, preferably comprising a main body portion, or heater, which is a comparatively poor emitter of electrons, and a smaller treatedportion which is highly emissive, so that when the heateris energized a dense localized electron discharge will be liberated from the treated portion of the cathode.

A second electrode, or accelerating anode, preferably in the form of a screen, is provided, spaced from the cathode and provided with a positive charge, so that an intense electrostatic field is established between the two electrodes; This field causes the electron discharge from the cathode to be accelerated, at high velocity, in the formof a beam. The beam, due to its acquired velocity, passes through the pervious accelerating anode and is projected on a fluorescent screen, provided at one end of the envelope on the interior surface thereof.

Since the electrons in the beam normally tend to follow divergent paths, after escaping the electrostatic field, I prefer' to provide means, in the absence of gas, for focusing the beam so that the area illuminated by the beam, at the fluorescent screen, is of substantially the same area as that area at the cathode from which the electrons were emitted.

This focusing means preferably comprises a solenoid disposed about the envelope in axial alinement with the electron flow, so that energization of the solenoid from a direct current source will establish a steady electromagnetic field along which the electrons are projected.

, This field prevents the electrons from following divergent paths, with the result that with proper adjustment of focusing current, the spot illuminated thereby at the fluorescent'screen is a sharp definition of the area at the cathode from which the electrons were emitted.

, A third electrode, or control electrode, is proprovided closely adjacent the cathode and disposed between the latter and the accelerating anode. This control electrode, when provided with a varying charge, which may be caused, for example, by a signal train from the output of a television receiver, causes variations in the electrostatic field between the cathode and the accelerating anode, with the result that-the electron flow from the cathode to the screen is proportional to the charge on the control electrode. This variation of electron flow causes fluctuations in brilliancy of the illuminated spot projected on the fluorescent screen.

Means are preferably provided for deflecing the beam cyclically, in two directions, so that an area of the fluorescent screen will be scanned in successive elementary portions, and these means, together with the variations of spot illumination, caused by the control anode, will create an optical image on the screen, in accordance with the signal current train delivered to the control electrode.

Two sets of beam deflecting coils are therefore provided, disposed about the envelope so that their axes lie in a plane at right angles to the electron flow, and the coils of each set thereof, disposed diametrically opposite, are electrically connected.

An oscillator, preferably capable of delivering a current having a saw-tooth wave form, is connected to each set of coils and these oscillators are adjusted to frequencies so that the fields of the deflecting coils will cause the electron beam to be deflected cyclically to cause it to traverse the fluorescent screen in successive passes, both in vertical and horizontal directions.

In the present types of cathode ray tubes, for use in Oscilloscopes, a plurality of cooperating electrodes are provided, grouped at one end of the tube, comprising a cathode adapted to institute an electron discharge, a focusing electrode, in the nature of a metallic cylinder disposed in front of the cathode in axial alinement therewith, and an anode, spaced from the focusing electrode, comprising a plate having an aperture therein in alinement with the axis of the focusing electrode.

The function of the cathode is to institute an electron discharge which is compressed into a beam by an electrostatic field created by a negative charge on the focusing electrode, and this beam is accelerated toward the anode, due to an attractive force exerted thereby on the beam caused by a highly positive charge on this electrode.

Some of the electrons in the beam, due to their velocity, will pass through the aperture and proceed to the. fluorescent screen; the quantity of electrons passing through the aperture being governed by the size thereof and the compressed diameter of the beam. A large percentage of the emitted electrons, however, will not reach the screen, being intercepted by the anode, with the result that the possible brilliancyof the illuminated spot on the screen is greatly reduced.

There is still another factor which seriously aflects the brilliancy of the spot. It will be seen that if the anode aperture is not exactly alined with the axis of the focusing electrode, the center of the beam will not coincide with the aperture and, as a result, a still greater quantity of the electrons will be intercepted by the anode and be prevented from reaching the screen.

Aside from these factors, still another detrimental feature presents itself, in that as soon as the electrons pass through the aperture, they pass out of the influence of the focusing field with the result that they tend to follow divergent paths and arrive at the screen in greater spaced relationship than when they passed through the aperture. This diflusion of the electrons causes an illuminated spot on the screen which is blurred, such a spot being wholly unsuited for use in television receiving tubes where the sharpness of detail 0! portions of the reproduced image depends on the sharpness of outline of the spot, necessitating the use of additional focusing means.

I have provided an improved cathode ray tube, for uses of the character described, in which the majority of the electrons emitted by the cathode reach the screen and contribute to the brilliancy of illumination of the spot.

I have also provided means whereby the cathode may emit a beam of rectangular cross-section, which feature is very desirable when the tube is used to reproduce images in a television receiving system.

I have also provided means for focusing the electron beam so that the electrons are prevented from diverging in their path from the cathode to the screen, with the result that the spot on the screen is a sharp reproduction of the cathode area emitting the electrons.

In greater detail, my invention comprises an evacuated vitreous envelope 2, provided with a domed end 3, which has a layer of willemite or other fluorescent material deposited on the interior surface thereof, to provide a fluorescent screen 4. The envelope is provided, at the end thereof opposite the screen, with a stem 6 having a press 7 for supporting the leads of the several electrodes contained within the envelope.

One of these electrodes is a cathode adapted to emit a localized electron discharge and, as shown in Figure 2, comprises a ribbon heating element 8, supported by the divergent ends of the leads 9 and adapted to be excited by the battery II, as shown in Figure 4:. This element is constructed of a'material which has a very low rate of electron emission, such as nickel. Provision is made for localizing the electron discharge from the cathode by the disposition of a small deposit i2, of an alkaline earth oxide, or a similar sub stance which is capable of copious electron emission, at a low heat, on the ribbon ii. It is obvious that if the temperature of the heater is maintained below a point which causes little or no electron flow from the ribbon, the emission will occur substantially only from the deposit i2. Since such is the case, it is also obvious that the size and shape of the deposit governs the size of the electron beam as it leaves the cathode.

A second screen electrode, or accelerating anode I3, is provided, spaced from the cathode, and is supported by a ring i4 carried by the lead 56 and the dummy l1 secured in the press 1. When this anode is highly charged positively, with respect to the cathode, by the battery is, an intense electrostatic field is created between the electrodes which results in the electrons emitted by the cathode being accelerated in a beam l9 toward the anode. when they reach the anode they pass through and continue on to the fluorescent screen.

Since the electrons in the beam will tend to follow divergent paths, as soon as they escape the influence of the electrostatic accelerating field, I provide means for restraining the electrons from following such paths, so that they will arrive at the screen within an area corresponding to the area at the cathode from which they were emitted. Disposed about the envelope is a solenoid 2| which is supplied with current from a. direct current source 22 through a variable resistor 23. Energization oi the solenoid will create a powerful. magnetic field substantially parallel.

to the electron flow, with the result that the tendency of the electrons to deviate from parill allel paths is controlled. By proper adjustment of the current through the focusing solenoid 2i, the electrons are focused in the plane of the screen, thus giving rise to a light spot of the size and shape of the emitting area.

While the foregoing description deals with focusing means for prevention of diversion of the electrons in the beam, 1 may prefer to use a non-divergent accelerating field which would produce a practically non-divergent beam without the aid of the focusing means. This field could be produced in a number of ways; for example, by providing a cylindrical electrode about the path of the beam and charging the electrode to create a uniform electrostatic field through which the electrons of the beam travel on their' way to the screen. The field effectively prevents divergence of the electrons from parallel paths, and assures their arrival at the screen within an area corresponding to the defined area from which they were emitted at the cathode.

Means are provided for deflecting the beam I9 cyclically so that it will traverse an area of the fluorescent screen in successive passes, both in horizontal and vertical directions. Beam deflecting coils 24 and 26, shown diagrammatically in Figure 1', are provided and are disposed in pairs about the envelope so that their axes lie at right angles to the electron flow. Each pair of coils is connected to oscillators 21 and 28, respectively, which are capable of producing an alternating output current having a saw-tooth wave form. The alternating fields of these coils will cause the beam to be deflected so that it will traverse successive linear portions of the fluorescent screen, having a width depending on the diameter of the beam. 1 A- screen electrode, or control electrode 29, is provided between the accelerating anode and the cathode, and is supported by a ring 3! which-is carried by a lead 32 and a dummy 33 secured in the press I. The lead 32 is connected with a tap 34 of the battery l8, and this places a charge'on the anode 29 which is slightly positive with respect to the cathode, and negative with respect to the accelerating anode.

By connecting the electrode 29 through a blocking condenser 36 and a lead .31, with the output of apparatus, which may be, for example, a receiver of television signals, and connecting the return circuit from the apparatus to the negative-terminal oi the battery li-by alead 38, the varying amplitudes of the apparatus output current applied to the control electrode will cause modifications oi the electrostatic field, existing between the accelerating anode and the cathode, with the r'esult that the quantity of electrons allowed to pass the electrode 29, at a given instant; is proportional to the charge on the control electrode caused by the amplitude of the signal at-that instant. This variation of electron flow causes fluctuations in the brilliancy of illumination of the spot at the screen I, and, in

combination with the scanning action, will produce an image on the screen which is an optical translation of the train of signals.

In Figure 3 I have illustrated a cathode construction by means of which an electron beam, having a rectangular cross section, may be produced. This cathode comprises a pair of ribbons 39 and ll which together make up the heater.

The ribbon 39 is provided with a rectangular aperture l2, and a thin thoriated or oxide coated plate 43 is disposed between the ribbons so that a portion thereof is exposed by the aperture.

and the least This exposed portion is the surface from which the electrons are emitted, and it will be seen that the electron beam, as it leaves the cathode, will have a cross section governed by the shape of the aperture 42. This rectangular beam produces a spot of corresponding shape at the screen, and produces a completed television image of greater brilliancy and sharpness than one produced by a .beam of circular cross section, as greater coverage ofthe screen area is achieved. It will be readily seen that the electron distribution of a beam of circular cross section, taken in successive planes from the center to the periphery thereof, with respect to the direction of travel of the beam, decreases as the chordal distance decreases, hence the greatest illumination of a path traversed by such a beam will be located at the center thereof amount of illumination will be obtained at the edge of thepath. This sparcity of electrons produces the familiar linear shadow areas observed between the lines of television images. By the useof a beam of rectangular cross section, the electron distribution in any plane parallel with the path of beam travel isuniform, and by adjusting the deflecting oscillators to move the beam, in steps not greater than the thickness thereof, it will be seen that there [will be no visible line of demarcation between successive passes of the beam and consequently the reproduced image will be sharper and possessed of a greater degree of brilliancy.

Figure '7 shows still another modified form of cathode in which the oxide deposit 44 is disposed on the surface of the heater ribbon 46 away from the fluorescent screen. An aperture 41, which may be either rectangular or circular in form, is provided in' the ribbon. I have found that electrons emitted by the oxide deposit 44 will be drawn through the aperture 41 to form the beam. It will be understood that the cross sectional shape of the beam will depend on the shape of the aperture 41. r t

Figure 5 illustrates a modified construction for the control electrode and accelerating anode. The electrodes illustrated in Figure 1 are constructed of fine wire mesh, and consequently, due to the interception of a certain percentage of the electrons by the wires, due to their being positioned in the path of electron flow, the possible brilliancy of the spot at the screen will be slightly reduced. Although the electrodes formed of fine mesh screen provide a trifle better operation of the tube, the construction shown in Figure 5 will give substantially as good results in addition to greatly reducing the electron interception by these electrodes. Each of the supporting rings l4 and 3| are provided with a plurality of fine spaced wires 43, those of the control anode being disposed at right angles to those of the accelerating anode. These wires may be spaced apart a greater distance than those of the wire mesh without noticeably affecting the efficient functioning of the electrodes. It will be seen that since there the pyramid formed by the cyclically deflecting electron beam. This type of anode, when charged, will produce an electrostatic fleld which will provide satisfactory operation of the tube and will provide the desirable feature of not intercepting any of the electrons in the beam.

In conclusion, the cathode ray oscilloscope just described ofl'ers distinct improvements over like devices in present use by providing superior cathode constructions, which permit substantially all of the electrons emitted thereby to reach the fluorescent screen and consequently produce an illuminated spot of maximum briiliancy; by eliminating the apertured anode which permits only a small portion of the emitted electrons to reach the screen; by providing electrode construction which permits positioning of the control anode closely adjacent the cathode, thereby greatly increasing the mutual conductance of the tube and permitting complete control'and sharper cut-off of the electron flow with very low electrode potentials; by providing a magnetic focusing field through which the electrons fiow,-thereby permitting sharper definition of the spot on the fluorescent screen; by providing cathode constructions which permit the construction of cathode ray tube in which the electron beam is possessed of a crosssection of predetermined shape; by providing a cathode ray tube which does not require the accurate alinement of cooperating electrodes to insure efllcient operation thereof; by providing a cathode ray tube wherein ionization, due to electron collision, is reducedto a minimum due to the small quantity of oxide used as the emitting element and the fact that the majority of emitted electrons reach the screen; by providing a simplifled and easily constructed cathode ray tube; and by providing cathode constructions, for use in cathode ray tubes, utilizing gas as the focusing medium, which, due to the small quantity of oxide used therewith, are not seriously affected by positive ion bombardment of the cathode caused by the gas.

While I have chosen to illustrate and describe the improvements of my invention in conjunction with a cathode ray tube and a television receiving system, I do not desire to be limited by such illustration and description, as uses for my improvements, in other electronic or space discharge devices, will readily be apparent to those skilled in the art.

I claim:

1. A'cathode ray tube comprising an, envelope containing a cathode support member capable of being heated and having a substantially flat surface of substantially non-electron emitting material, an area of thermionic electron emitting material deposited on said flat surface and heatable thereby to electron emitting temperature, said area being'of elemental dimensions, a planar accelerating screen having multiple apertures therein positioned parallel with and close to said flat surface, and having an area substantially larger than the area of .said emitting material, a fluorescent screen of picture size in the path of electrons passing through said accelerating screen, a magnetic solenoid surrounding the entire path between said emitting material and said fluorescent screen, and. scanning means positioned to act on electrons after they leave said accelerating screen and before they arrive at said fluorescent screen.

2. A cathode ray tube in accordance with claim 1, with a planar grid between said accelerating screen and said emitting material, said grid being of substantially greater area than said emitting material and positioned parallel to both said accelerating screen and said surface.

3. A cathode ray tube in accordance with claim 1, with a planar grid between said accelerating screen and said emitting material, said grid being of substantially greater area than said emitting material and positioned parallel to both said accelerating screen and said surface, and wherein the accelerating screen is made up of equally spaced straight wires extending in one direction only, with the grid made up of equally spaced straight wires extending only in a direction substantially at right angles to the wires of said accelerating screen.

4. A cathode ray tube having an envelope containing a fluorescent screen at one end thereof, a thermionic cathode at the other end thereof and a fiat screen accelerating electrode between said cathode and said fluorescent screen, said cathode comprising a thin flat sheet of non-emitting material adjacent and parallel to said accelerating screen and having an aperture therein of elemental dimensions, and a layer of thermionic electron emitting material confined solely to the back surface of said sheet away from said accelerating screen and adjacent the edges of said aperture, the side walls of said aperture and the front surface of said sheet being free from electron emittin'g material.

RUSSELL H. VARIAN.

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