US2545982A - Television pickup tube - Google Patents

Description

March 20, 1951 P. K. WEIMEQ TELEVISION PICKUP TUBE 2 Sheets-Sheet 1 Filed Dec. 20, 1947 INVENTOR Paid/c Wain A ORNEY March 20,1951 P. K. WEIMER 2,545,932

TELEVISION PICKUP-TUBE Filed Dec. 20, 1947' 2 Sheets-Sheet 2 )6 DARK 5 L/Gl-ITZ DAR/n INVENTOR Paul K W'ez'mer A ORNEY Patented Mar. 20, 1951 TELEVISION PICKUP TUBE Paul K. Weimer, Princeton, N. J., assignor to Radio Corporation of America, a. corporation of Delaware Application December 20, 1947, Serial No. 792,944

15 Claims.

This invention relates to pickup tubes used for converting photo images into electrical impulses and more especially to tubes in which the cathode ray beam approaches the target at substantially zero velocity but the invention is not limited thereto. The targets in such tubes may be either single or two sided such as in the orthicon and image orthicon described in the patent to Albert Rose, No. 2,407,906, September 17, 1946, and my Patent No. 2,433,941, filed September 16, 1944, Serial Number 554,949, respectively.

In these and other cathode ray pickup tubes the electrons leave the thermionic cathode of the gun at an irregular rate which produces spurious signals called shot noise. The photoelectrons are also emitted from the photocathode at an irregular or random rate, and produce additional shot noise. It is well known that the electron current emitted by such photocathode has a noise current associated with it given by the formula:

(1) In=(2e,fI)

where In is the root mean square noise current in amperes e is the electronic charge I is the total emitted current in amperes J is the frequency band width The maximum signal to noise ratio which may i be obtained with a signal current I is given by what resembling flakes of falling snow in the picture which is objectionable. Spurious signals are also superimposed on the video signals by the amplifiers to which pickup tubes are coupled and in order to increase the ratio between signal and noise it has been the practice to greatly increase the signal before it reaches the amplifier by electron multipliers in the tube as in my said application. Since electron multiplication increases the shot noise as well as the signal, maximum signal to total noise ratio is reached, as a practical matter, when the shot noise materially predominates over the amplifier noise. The multiplier gain required for this is approximately 1300. There is, however, a iundamental difiiculty in orthicon pickup tubes which prevents the attainment of maximum signal-to-noise ratio at all light levels of the picture. This is due to the inverted polarity of the modulated electron stream entering the multiplier. In both the orthicon and image orthicon the charge image on the target varies in a positive direction with the brightness of the areas of the picture and the low velocity beam scanning the target deposits sufiicient electrons to discharge the areas. The fraction of the beam not landing on the target forms a modulated beam which returns toward the cathode ray beam gun and is directed into the multiplier. The current entering the multiplier, and hence the shot noise, is maximum in the dark parts of the picture and minimum in the bright parts which gives a signal-to-noise ratio in the low lights which is poorer than that permitted in the low lights by Equation 1.

There are some types of pickup tubes in which the signal-to-noise ratio at all light levels with electron multiplication is substantially that given by the Equation 1. One example is the dissector tube in which the photoelectrons are scanned over an aperture such as in the patent granted to Farnsworth No. 1,970,036, August 14, 1934. Another is flying light spot illumination with photomultiplier cell pick up. Flying spot illumination is well known in the art, an example of which may be found in the book on Television by Zworykin and Morton, p. 218, published by John Wiley and Sons, Inc., New York 16, New York.

Such types of prior art tubes do not provide storage of the charge during the entire scanning cycle and this is obviously a great disadvantage. It is an object of this invention to provide a pickup tube that possesses full storage properties and has the maximum signal-to-noise ratio given by Equation 1 at all light levels of the picture.

Another object of the invention is to provide a pickup tube in which the electrons landing on the target and escaping therefrom as scattered electrons are the only ones utilized for producing the useful signal. 4

Another object of the invention is to provide a pickup tube in which helical motion is imparted to the beam electrons to provide efficient separation of the returning reflected electrons from the returning scattered electrons.

Another object of the invention is to provide a tube in which substantially modulation of the current entering the multiplier can readily be obtained.

Another object of the invention is to provide a pickup tube having increased signal-to-noise ratio.

Another object of the invention is to provide a pickup tube in which scanned spots on the first dynode of the multiplier are eliminated and do not appear in the transmitted picture.

Another object of the invention is to position the first-dynode of the multiplier of a cathode ray beam.tube at .an. antinode of .the. return beam.

Another object of the invention is to transmit a more uniform signal for black areas of th image.

Other objects of the invention will. occur in the following specification, reference being. had-to. the

drawing in which:

Figure 1 is a diagrammatic. illustration. by which the principle of the invention may be explained;

Figure 2 is an imaginary section of the beam illustrating the spiral paths .of electrons .endwise of Figure l at an antinode of the spiral;

. Figure 3 is a schematic axial view of a pickup tube embodying the invention;

. Figure 4.,is an endview. of the separatorplate andassociated parts shown in Figure 3;

Figure 5 is asectional elevation by which the .principlespf aisecond embodiment of thein- .vention may be explained;

Figure 6 is a diagrammatic illustration of spiral .pathsof. the reflected and scattered electrons end- .wi-sebf Figure 5;

. Figure 7 is an imaginarysection of the beam of Figure frat. the separator electrode showing the reflected. and scattered electrons .of. the return beam;

1 Figure 8 is agraph'shoWin-g the value of the total return beam and the value of the scattered .electron beam at. the separator plate;

. Figure 9 shows another and .preferredembodi- .ment .of the invention; and

. .Figurelfl. is an. end view otthe separator plate ,andassociated partsof Figure 9.

' .Whilethe invention is not. limited .to pickup tubeshavinga magnetic focusing field it will be 1 first .descr-ibedasapplied to such a tube.

...Referring to the diagrammatic illustration in Figure lthegun G projects electrons toward the .target Tin. the magnetic focusing fieldindicated by. field .lines F which is produced in away well .knownin the art.. Part of the electrons ofthe beam are emitted from the cathode so as-to enter the field with a slight. inherent component .-of .velocity. perpendicular to. the .focusing field and.this, with the longitudinal accelerating field,

;.cient energy to escape and are accelerated back ..toward. the gun. These escaping electrons are called scattered electrons. Since the electrons withtransverse' velocity leave the gun cathode in force. 3,. suchas at a,

ottheanode, or anodes such as the first anode 2 of the gun, produces spiral movement of the electrons,. eachspi-ral passing through the center line 3.-.of..the beam. The diameter of the helix traversed by each electron is proportional. to .its..transverse .velocity, an-end view .ofthe paths .traversedcan .be visualized-as in Figure 2; the line 3hr the magnetic field F being. the .one where .somepf. the [electrons enter the field with. no

.- transverse velocity. The size of the beaminthe .figuresis .otcourseexaggerated. All of the .elec itronstakethesametimefor completing a spiral and hence they-.,all.cross the.. line. 3 at thesame points I which are calledthe foci. These-.foci may also be consideredas nodal-points or nodes along .the .beam path. .The forward travel of .the .electrons,'that is, the distance between ad- ..jacent foci f,.of which five are shown, is a functionof the accelerating voltage and the strength .of the. focusing field.

ascribedfor separation of the scattered electrons from the reflected electrons as in Figure 1 be- .cause ;(a) .part' of the scattered electrons are 2lVaIiOUSTidlICtlOnS;. the electrons spiral around many centersrspaced around the center line of 5, 6, and 3. Some 'of'the scattered electrons acquire additional transverse velocities from the positive charges of the target and spiral at greater radii and follow return pathshav-ing centers at 8, 9, .13];- and.

There are, of course,- a multiplicity of spirals and only a-few are given to illustrate .the principle. Thus'the scattered electrons not acquiring sufiicient transverse velocity at the target. and the reflected electrons, spiral within the envelopelZ. The scattered-electrons acquir 'ingsufilcienttransverse velocity, spiral within envelope [3 outside the envelope l2. Both the reflected electrons, and the scattered electrons, are modulated by. the charge image on the target and thus bothcontain the signal.

. Insteadof locating the first dynode of the multiplier approximately at anode 2 as in my said application which collects the entirebeamof scattered and reflected electrons, I place an apertured first dynode diagrammatically shown at M at or near an antinode which is substantially midway between two nodes or nodal points as indicated in Figure 1, the imaginary .cross section of the beam being depicted in Figure 2. The scattered electrons outside the envelope I3 then strike the first dynode, as in my said application, while the reflected electrons pass through the aperture in first dynode IQ and are collected by anode 2 and discarded, which anode 2 would not be a dynode "roughly proportional to the number of electrons landing on the target and are proportional to "the brightness of the target areas.

The signal current,-carried by thescattered electrons is therefore of direct, and not reversed,- polarity and 'the modulation. can be made substantially I prefer. not to rely on the inherent transverse velocities -of the "emitted electrons as just dewithin the reflected beam envelope [2 as is evident from Figure 2, and hence they will pass through the aperture in the first dynode as at 14* and are not utilized and (b) the electrons .which land in the gray areas are not scattered beyond the reflected beam envelope i2 and hence cannot-be separated from the reflected electrons.

The disadvantage of the first operation will be apparent. The disadvantage of the second is that it amounts to clipping the signal current for gray areas. Greatly reducing the section of the beam eliminates the disadvantageous effects but this is not easily carried out. I therefore prefer to obtain the results by deliberately introducing additional helical motion into the primary beam so that all of the scattered electrons of the return beam are projected from the target so as I to be outside the envelope [2 containing the reflected electrons. The increased spiraling may be induced in a number of ways.

One form of cathode ray beam tube for forming a return beam of scattered electrons that do not appreciably intermingle with the returning reflected electrons is shown in Figure 3. In this figure the evacuated envelope l5 of glass or other suitable material contains the gun G, having the usual grid and cathode not shown but shown in my said application to which reference is made for these details. Surrounding the anode l9 of gun G is a block diagram indicating a multiplier M having a plurality of multiplying dynodes,

and a collecting electrode shown in detail in my said application. All these elements do not per se constitute my present invention and reference is made to my said application for details of their construction.

The image section of the tube comprises a twosided target T consisting of a semi-conducting glass 21, or equivalent, as in my said application, closely adjacent to which isthe so-called signal screen 28 conductively attached to the electrode cylinder 29 across one end of which the glass target 21 is placed in taut condition. The inside surface of the end of the transparent envelope I5 is coated with a thin semi-transparent conductor on which a light sensitive mosaic is formed, constituting a well known form of photo cathode 30. Between the photocathode and the electrode cylinder 29 is placed accelerating cylinder anode 2911. This image section of the tube is constructed as described in my said application.

A field modifying anode cylinder 3| called the persuader is positioned in front of the gun G to help direct the secondary electrons from the first multiplying dynode, which in this case is the end of anode l9, into the multiplier unit 29. The anode edge for separating the reflected electrons from the scattered electrons may be edge 32 of the opening of a disc 32a conductively secured to the persuader 3!. This aperture may have either a straight edge or a circular edge, the latter being shown (see Figure 4).

A wall coating electrode may be used, as in my said application but for convenience of manufacture I prefer a cylinder anode 33 such as disclosed in my application filedFebruary '7, 1946, Serial Number 646,076, now U. S. Patent 2,452,- 619, issued November 2, 19.48, which performs the same function as a wall coating.

A decelerating anode is positioned between the wall coating anode 33 and the glass target 2?. This may be an anode ring 34 as in my first mentioned application, but I prefer to use a fine mesh screen 35 across the ring so as to produce a uniform and steep decelerating field in front of the target, which also reduces scanning of the first dynode by the return beam which is desirable in my present invention. The screen 35 and ring 34 may be connected to the anode cylinder 33 or they may be separately supplied with voltage less or more than that of the ring.

"An axial magnetic focusing field is produced 1 cated) in the tube by solenoid 36 energized by direct current. A scanning yoke 31 is placed outside the envelope. This comprises a vertical scanning coil and a horizontal scanning coil (not indi- These coils are connected to sawtooth generators 38, 39 having the required frequencies and variable voltages to scan the beam across the target. The focusing and scanning coils and the sawtooth generators may be of standard form well known in the art.

An axially adjustable compensating coil 4|] may be used toinitially align the beam as disclosed in the patent granted to Albert Rose, No. 2,407,905, September 17, 1946, as well as in my said application.

The parts thus far described are the same as, or are known equivalents of, those described in my said application 554,494, now U. S. Patent 2,433,941 issued January 6, 1948, and to produce separation of substantially all of the returning scattered electrons from the returnin reflected electrons, I introduce a spiralling velocity component into the beam. As shown in Figures 3 and 4, this is produced by two electrodes 4|, 42 very close to the aperture of the beam to which is connected a direct current source of potential that is adjustable so as to control the transverse force applied to the electrons of the beam, such as a bleeder resistance 43. These electrodes are spaced apart equal distances from the beam from the gun which is projected between them. The field produced by these electrodes gives the electrons a transverse component of velocity dependent upon the potential applied thereto. This transverse component produces spiralling of the electrons above the normal spiralling of Figures 1 and 2, as indicated in Figures 5 and 6. Between the separating disc 32a and the anode 33 is placed lifter anodes 44, 45 (Figure 4), to deflect the beams so that the return beam B1 is out of the path of the original beam B. In the axial magnetic field this deflection will be in the plane of the paper in Figure 5. These lifter anodes 44, 45 are known in the art and may be either planar or curved, the latter being shown. The end 45 of the anode [9 may constitute the first dynode 46 of the multiplier.

The operation of the embodiment in Figure 3 may be described as follows:

The various electrodes are given various properly proportioned voltages as will be known to those skilled in the art which particular voltages need not be given. The electrodes 4|, 42 cause the electrons of the beam to spiral to a greater extent than indicated in Figure 1. This is illustrated in Figure 5 in which the outgoing beam B is shown in imaginary longitudinal section. The tube is shown broken away to reduce the size of the illustrations. The production of helical movement of the electrons by the electrostatic field of electrodes 4|, 42 causes part of the energy of the electrons to be associated with the helical movement and the energy associated with the longitudinal movement thereof is decreased by that amount. The target T may therefore be biased slightly positive with only a few electrons landing in the dark. The polarity of the lifting plates 44, 45, appearing in Figure 3 but not in Figure 5, is such as to bend the beam B down so that electrons spiral through the line 3 instead of 3 as shown in Figure 5. The polarity of the field produced by electrodes 4|, 42 is such that electrons of the beam B spiral below a plane perpendicular to the plane of the drawing at the aces-cam 7E:- axis 1.3 through Jwh-ich,:of course,- :.all Tspirals "passer at .thefiocal points -:;f.

When-thetbeamis overa white ar.ea','which has a .moreipositive potential, moreelectrons land, and part: of theseescape and are. scattered. .The. scattered electrons escape in all directions and spiral: as:.at-4l-in:Figure- 6 all around the focal line-ii; .The.paths.:of the reflected and scattered electronsas-they areaccelerated back toward the gun, are different in Figure 3, fromithose in Figure las-the plates. 4], 42. produce a transverseenergy component: inetherbeam. inone. direction, down as shown in Figure.5..-. .When the :return beam. passes 1 throughthe field oilifterplates 44, 45 it is moved. downwards to the new line 3". When the: re:-. turn -beam. .reaches the. separator .disc 52a. -the scattered.electronslBi'. pass throughthe opening, therein and .the reflected electrons .-.B'. .land on. thewdisc... 32a below the edge. 32. as indicated .in Figure 7. .The scatteredv electrons then .land. on. the firstdynode 45which is the end of the anode- I9 but a separate plate .could' be usedas the first dynode if'desired.

The paths followed by'the scattered electrons at all points, other than the foci, are not sub-1 stantially intermingled with the reflected electronsasisevident' from Figure in which B is thebeam of reflected electrons and B" is the beam ofscattered electrons. Thus the return beams can be separated at points other than the foci; In Figure 5 theyare separated by the disc 32w shown as circular, but not necessarily so, which collectsthe reflected electrons and passes the scattered electr'onsabove the'edg'e' 32. The collector pla'te 32'a isplaced at an antinode; All of the reflected. electrodes strike the disc 32a. below the separationedge 32 and are discarded. The scattered electrons pass through the opening and land on the first dynode 45 located substantially at a node, and. bombard secondary electrons therefrom. These. bombard additional secondary electrons from'the' succeeding dynodes. inrsuccession in the multiplier M as in my said application. 554,494 .now U. "s. Patent 2,433,941,. issued -January6,.1948,' and current intthe output line L isxgreatly multiplied.

The .liitersplates l i, 45-, .see Figures;3 and 4, de.-;. fiect both the outgoing gandtzreturning. beams. down infthe planeofthepaper 'in.;Figure 3 as well known since they are immersed in'amag netic.-field.-: Thedefiection of the outgoing-beam deflects the rasterv on the target, -but'this. is only half the deflection of the return beam. This is; usually not objectionable. However, the displacement of Ltheraster on-th'e target may. be. cor-i rected, if: desired,. by;-adjustmentv ofv the .sawtooth generator 3Bproducing the frame scansion;..-.

The advantage my invention has-over the standard image orthicon such .asdisclosed in my firstrnentioned application willbe apparent from Figure 8. In this figure the total: return beam (reflected; electrons plus scattered electrons) which enter the multiplier in the prior typesof orthicons is given by 54 for black areasand 55 Y for-whiteareas. In .my' improvement; thebeam enteringthe multiplier .is substantially. nil for. black area-sand. is indicated at .55, and that for white. areas is indicated at 5?. Thus, in my improvement the noise is a minimum, practically zero, in black areas, forthe shot noise varies as; a currentof the beam. The scattered electrons utilized are. proportional to the illumination on thepicture areas and hence the signal-to-noise ratio may approach the value given by Equation 1. In.obtaining. this .desirable. result, storage; of .the v 8f". signal-flora full :frame ctime: iSiZiQbllElJi-Ilfid 'asathis featureis the: same as in .prior2type.orthicons;

Itv is important: thatrthereturn .beam of re=' flected electrons do not'xscan theseparation 'disc such as 32a. Thisis substantially obtainedin my improvementsibecause the separation edge is lo'-' cated at an antinode and. scanning is also re in front of the target. Scanning of the separation plate can also be reduced in-various other Ways.

In adjusting the improved tube for-operation,

the various 'adjustments'are made as in 'the standard image 'orthicon so as to obtain cor-" rect focus of the beam on the target, and withthe current directed into'the multiplier. In doing 1 this the voltage on the plates 4!, 42 is omitted,

and the voltage on lifting plates 44,45 may be adjusted to direct the'current into the multiplier. The picture is then noted in a monitor receiving tube. This picture will be negative with maximum output current in the dark as in my ap-' plication 554,494 now U. S. Patent 2,433,941, issued January 6, 1948. plates 4|, 42 of the rightpolarity, and. varied. At the correct value the polarityof the picture reverses and changes to positive.

Another embodiment of the improvement is shown in Figure 9.

at one side of anadditional anode'cylinder 53 in front of the gun, .(c) the gun is mounted above the axis of. the tube. The remaining parts in this figure, beingthesameasthose in Figure 3, are giventhe same .reference characters as'in that figure. Hence, onlythenewly arranged parts will be described.

The anode cylinder 53 carries at the front end an integral or conductively connected disc 54 having an opening 55 for. the outgoing beam (see Figure 10) and an opening 56 therebelow for the I scattered electrons. The reflected electrons land on the disc 54. Back of the opening 55 is the first dynode 5'! of the multiplier. This may be conveniently made by cutting and bending back the.-v metal in forming the opening 56. The top side of the metalbeing uncut.

In this modification, adjustment of the ali'gni ment coil 45 is such thata sufficient cross field is produced to give the electrons of the beam the desired transverse velocity and in such direction that the returning. reflected electrons are below Patent 2,433,941 issued January 6, 1948, and in my application filed-June. 28, 1946, Serial Number 680,002, in which the multiplier surfaces are located at the bottom instead of around the en tire circumference. The results obtained by this embodiment are in general the same as in Figure 3, except thatv I obtain here; the. additional ad- Now voltage is .put on The tube is the-same. as shown in Figure 3 except (althe. helical mo.-.. tion. is produced by ,adjustment. of alignment. coil .45, (b) themultiplier .units are positioned...

vantage that the first dynode is at or near an antinode and thereby out of focus.

The beam may be caused to have the desired helical motion in various other ways than those referred to in the above embodiments, and the invention is not limited to any particular construction. For example, helical motion may be imparted to the outgoing beam by slight misalignment of the gun relative to the axial focusing field.

The invention is not limited to tubes having magnetic focusing fields. Other means for focusing the beam electrons on the target may be used such as electrostatic focusing fields which are, of course, well known in the art.

Various equivalents of the embodiments disclosed may be used without departing from the spirit of the invention.

What I claim as new is:

1. A signal generating apparatus comprising an envelope, means for forming an axial magnetic focusing field within said envelope, a cathode ray beam gun within said envelope having a cathode and positioned to project an electron beam into said field, some of the electrons of said beam having an inherent velocity component transverse to the axis of said field whereby they spiral along said field through common nodal points, a target within said envelope adapted to have substantially the potential of said cathode and positioned in the paths of the electron spirals at a nodal point thereof, and a multiplying dynode positioned in said envelope in the path of electrons landing on the target and scattered therefrom and out of the path of electrons reflected by said target.

2. A signal generating apparatus comprising an envelope, means for forming an axial magnetic focusing field within said envelope, a gun including a cathode and anode electrodes for projecting electrons into said field angularly cf the lines of force thereof, whereby the electrons spiral along said focusing field through common nodal points, a target within said envelope adapted to have substantially the potential of said cathode and positioned in the path of the electron spirals at a nodal point thereof, and a multiplying dynode within said envelope positioned in the path of electrons landing on the target and scattered therefrom and out of the path of electrons reflected by said target.

3. A signal generating apparatus comprising an envelope, means for forming an axial magnetic focusing field therein, a target within said envelope adapted to'have a charge image thereon and positioned in said field substantially perpendicular thereto, a gun within said envelope including a cathode and anode electrodes for forming and projecting a beam of electrons into said field at an angle to the lines of force thereof whereby the electrons of the beam spiral along said focusing field through common nodal points, electrode means adjacent said target for decelerating the electrons of said beam to substantially zero velocity at the target, said target being at a nodal point of the electron spirals, and a multiplying dynode positioned in the path of electrons landing on the target and scattered therefrom and positioned out of the path of the electrons reflected by said target.

4. A signal generating apparatus comprising an envelope, means for forming an axial magnetic field therein, a target positioned within said envelope in said field substantially perpendicular thereto, a gun in said envelope including a cathode and anode electrodes for forming and projecting a beam of electrons into said field at an angle to the lines of force thereof whereby the electrons spiral along said focusing field through common nodal points, electrode means adjacent said target for decelerating the electrons of said beam to substantially zero velocity at the target, said target being at nodal points of the electron spirals, and a multiplying dynode positioned in the path of electrons landing on the target and scattered therefrom and positioned out of the path of the electrons reflected by said target.

5. A pickup tube comprising an envelope, means for forming a uniform axial magnetic field within said envelope, a target positioned within said envelope in said field substantially perpendicular to the lines thereof, a cathode ray beam gun within said envelope for projecting a beam of electrons along said field towards said target, said gun having a cathode adapted to have substantially the same potential as said target, electric field producing means for causing said beam to enter the axial field at an angle thereto whereby said beam electrons follow spiralling paths passing through common nodal points, said target being positioned at a nodal point of the electron spirals, an electrode having a surface positioned between said gun and said target substantially in the plane of an antinode of the spirals, said electrode having an edge lying between the electron reflected by said target and those landing thereon and scattered therefrom, said surface being in the path of the electrons landing on the target and scattered therefrom and positioned out of the path of the scattered electrons, and a multiplier between said electrode and said gun in the path of scattered electrons passing said edge.

6. A signal generating device comprising an envelope, a target electrode mounted within said envelope, a source of electron emission within said envelope, means for directing said emission as a beam along a normal path to and from said target electrode, an electrode within said envelope for collecting electrons of said beam reflected from said target, and a second electrode within said envelope for collecting beam electrons scattered from the surface of said target.

7. A signal generating device comprising an envelope, a target electrode mounted within said envelope, at source of electron emission within said envelope, means for directing said emission as a beam along a normal path to and from said target electrode, means including an electrode positioned between said electron source and said target electrode for separating the electrons of said beam reflected from said target electrode from said beam electrons scattered from the surface of said target electrode.

8. A signal generating device comprising an envelope, a target electrode mounted within said envelope, a source of electron emission within said envelope, means for directing said electron emission as a beam along a normal path to and from said target electrode, means including an electrode positioned between said electron source and said target electrode for separating the electrons of said beam reflected from said target electrode from said beam electrons scattered from the surface of said target electrode, and an electrode within said envelope for collecting said scattered beam electrons.

9. A signal generating device comprising an envelope, a target electrode mounted within said envelope, a source of electron emission within said envelope, means for directing said electron -emission as a beam along a normal pathto and from said target electrode, means including an electrode positioned between said electron source,

and said target electrode for separating the electrons of said beam reflected from said target electrode from said beam electrons scattered from the surface of said target electrode, said last envelope, a source of electron emission within said envelope, means for directing said electron emission as a beam along a normal path to and from said target electrode, means for establishing on said target electrode a distribution of positive and negative charges with respect to said electron source, means including an electrode positioned between said electron source and said target electrode for separating the electrons of said beam reflected from negatively charged areas of said target surface from said beam electrons scattered from positive areas of said target surface.

11. A signal generating device comprising an envelope, a target electrode mounted within said .envelope, a source of electron emission within said envelope, means for directing said electron emission as a beam along a normal path to and from said target electrode, means for establish- ,ing on said target electrode a distribution of positive and negative charges With respect to said electron source, means including an electrode positioned between said electron source and said target electrode for separating the electron of said beam reflected from negatively charged areas of said target surface from said beam electrons scattered from positive areas of said target surface, and an electrode within said envelop for collecting said scattered beam electrons.

12. A signal generating device comprising an envelope, a target electrode mounted within said envelope, a source of electron emission within said envelope, means for directing said electron emission as a beam along a normal path to and from said target electrode, means establishing a magnetic focussing field enclosing said normal beam path between said gun and target, said field having a direction substantially parallel of the normal path of said electron beam, means including an electrode positioned between said electron source and said target electrode for separating the electrons of said beam reflected from said target electrode from said beam electrons given a velocity component transverse to said field direction upon striking said target electrode and scattered therefrom.

13. A signal generating device comprising an envelope, a target electrode mounted within said envelope, a source of electron emission Within said envelope, means for directing said electron emission as a beam along a normal path to and from said target electrode, means establishin a magnetic focussing field enclosing said normal beam path between said gun and target and having a direction substantially parallel to the normal path of said electron beam, means for separating the electrons of said beam reflected from said target electrode from said beam electrons given a velocity component transverse to said field direction upon striking said target electrode and scattering therefrom, said beam separating means including electrostatic means deflecting said beam transversely to said normal beam path to produce helical motion of the electrons of said beam about said normal electron path and on one side thereof.

14. A signal generating device comprising an envelope, a target electrode mounted-within said envelope, a source of electron emission within said envelope, means for directing said electron emission as a beam along a normal path to and from said target electrode, means establishing a magnetic focussing field enclosing saidnormal beam path between said gun and target and having a direction substantially parallel to the normay path of said electron beam, means for separating the electrons of said beam reflected from said target electrode from said beam electrons upon striking said target electrode and scattering therefrom, said beam separating means including electrostatic means deflecting said beam transversely to said normal beam path to produce helical motion of the electrons of said beam about said normal electron path and on one side thereof, and means positioned on said one side of said beam path to collect the beam electrons reflected from said target electrode.

15. A signal generating device comprising an envelope, a target electrode mounted within said envelope, a source of electron emission within said envelope, means for directing said electron emission as a beam along a normal path to and from said target electrode, means establishing a magnetic focussing field enclosing said normal beam path between said gun and target and having a direction substantially parallel to the normal path of said electron beam, means for separating the electrons of said beam reflected from said target electrode from said beam electrons upon striking said target electrode and scattering therefrom, said beam separating means including electrostatic means deflecting said beam transversely to said normal beampath to produce helical motion of the electrons of said beam about said normal electron path andon one side thereof, and means positioned on said one side of said beam path to collect the beam electrons reflected from said target electrode, and a second collector electrode for collecting the scattered beam electrons returning from said target electrode on the other side of said beam path.

PAUL K. WEIMER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,407,705 Kilgore Sept. 17, 1946 2,407,906 Rose Sept. 17, 1946 2,413,276 Wolff Dec. 24, 1946 2,433,941 Weimer Jan. 6. 1948

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