US2304091A

US2304091A - Signal-generating system

Info

Publication number: US2304091A
Application number: US314573A
Authority: US
Inventors: Rudolf C Hergenrother
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1940-01-19
Filing date: 1940-01-19
Publication date: 1942-12-08
Anticipated expiration: 1959-12-08
Also published as: DE908864C; GB546305A; FR869218A

Description

R E H T O R N E G R E H C R S IGNAL GENERATING SYSTEM Filed Jan. 19, 1940 INVENTOR RUOLF QHERGE OTHER ATTORNEY Patented Dec. 8, 1942 SIGNAL-GENERATING SYSTEM Rudolf o. Hergenrother, medium-st, N. 2., as-

signor to azcltlne Corporation, a corporation o1! Mela Application January 19, 1940, 1 1 a I Elaims. (Cl. est-1st) get on which is focused an image of the scene to be transmitted and which is scanned by a beam of electrons to provide at the point of impact at the target a secondary-emission ratio of greater than unit. In such systems, the number of secondary electrons emitted by each incremental area of the target, which electrons are collected continuously bya suitable electron collector as the target is scanned, is determined by the charge of the area, which, in turn, is determined'by its intensity of illumination. Such systems generally comprise a suitable backing plate for the photosensitive target electrode from which the video-' frequency output of the system is derived. It has also been proposed to derive the video-frequency output of such an arrangement from the current to the electron collector of the cathode-ray tube. However, signal-generating arrangements of the general typ described, which utilize a cathoderay signal-generating tube including a photosensitive target electrode which is scanned by electrons of sufllciently high velocity to provide secondary-electron emission ration at the target, which is greater than unity, in order to derive a signal output therefrom, are subject to the disadvantage that the relation between the light input to the system and the generated signal output is not linear; that is, the system does not generate a signal with unity gamma. Such systems are also deficient in that some of the secondary electrons which are emitted from the target rain back on the target and impair the charge distribution thus decreasing its sensitivity and causing undesirable shading effects.

It has been proposed also to utilize in a signalgenerating system a cathode-ray signal-generating tube having a photosensitive target which is scanned by electrons of such low velocity that the secondary-emission ratio is less than unity, the output signal being taken from a backing plate associated with the photosensitive target. Such an arrangement is effective to eliminate certain of the disadvantages associated with systems utilizo5 ing a sumciently high velocity scanning beam to cause a secondary-electron emission ratio greater than unity at the target. A system utilizing a scanning beam of sufliciently low velocity that fewer secondary electrons are emitted, by the tar- I get than strik it is disclosed in an article entitled "The Orth'lcon in Electronics for July 1939.

However, signal generators of either of the types mentioned above provide an output signal, the amplitude of which is so minute as to necessitate a high degree of amplification to procure a useful signal. Such amplification is difiicult to produce in conventional vacuum-tube amplifier circuits for the reason that the thermal noises associated with the vacuum tube are comparable in magnitude to the signal to be amplified. Electron multipliers have heretofore been utilized in association with signal-generating cathode-ray tubes having a photosensitive target which is scanned by an electron beam of sumciently high velocity to provide a secondary-electron emission ratio at the target which is'greater than unity, in order to provide a signal output of relatively large amplitude without the disadvantages associated with the use of conventional vacuum-tub amplifler circuits. With such arrangements, it has been proposed to apply to the electron collector associated with the electron multiplier a suitable potential to attract the secondary electrons emitted by the photosensitive target during the scanning operation.

However, in all practical electron multipliers it is necessary to supply the electrons thereto for multiplication within a relatively small area. In

prior-art arrangements utilizing cathode-ray signal-generating tubes in which the target is scanned by a low-velocity scanning beam to provide a secondary-electron emission ration of less than unity, if. the collector electrode has a suitably high potential for emciently collecting electrons from the photosensitive target at a spot of restricted area, such potential would adversely affeet the scanning beam. It is particularly desirable, therefore, to provide a cathode-ray signalgenerating tube including a photosensitive target which is scanned by electrons of suficiently low velocity as to provide at the target a secondaryemission ratio of less than unity and which includes an electron collector associated therewith efiective to collect electrons at a spot of restricted area thereon and at a rate varying in accordance with the signal to be generated but having substantially no influence on the scanning beam.

It is an object of the invention, therefore. to

get electrode and an electron collector.

provide an improved signal-generating system including a cathode-ray tube having a photosensitive target adapted to be scanned by an electron beam of sumciently low velocity as to provide a secondary-emission ratio of less than unity at the target.

It is a further object of the invention to pro vide an improved signal-generating system including a cathode-ray tube having a photosensitive target adapted to be scanned by an electron beam of sufllciently low velocity as to provide a secondary-emission ratio of less than unity and having associated therewith a suitable electron collector adapted to receive from the target an electron beam varying in accordance with the signal to be generated and having substantially no influence on the scanning beam.

In accordance with the invention, a signalgenerating system comprises a cathode-ray signal-generating tube having a photosensitive tar- Means are provided for developing and accelerating toward the target electrode a beam of electrons of substantially constant intensity and having a velocity which is sumciently low at the target electrode as to provide a secondary-emission ratio of less than unity and for returning free electrons from the vicinity of the target, together with magnetic means for scanning the target in two directions normal to each other with the beam, whereby the number of electrons returning from the vicinity of each incremental area of the target, as such area is scanned by the beam,

is determined primarily by the charge of such area due to an image projected thereon. The returning free electrons travel back through the magnetic means along substantially the same path as that traversed through the magnetic means by the scanning beam. There is also provided means including a pair of beam-shifting plates adjacent the beam-developing means and having a spacing therebetween which is small relative to'the scanning deflections of the beam, and means including means for applying a steady unidirectional potential to the beam-shifting plates for directing, as an electron beam to a spot of restricted area on the electron collector, which spot is displaced from the first-named means in a predetermined direction, substantially all of the electrons returning from the vicinity of the target, whereby there is derived an output signal from the electron collector which varies in accordance 'with the number of electrons directed thereto. At the same time, the potential of the collector electrode and its arrangement in the tube are such that it does not impair the proper deflection of the electron beam.

For a better understanding of the present invention, together with other and further objects thereof, reference is bad to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

The single figure of the drawing is a circuit diagram, partly schematic, of a television transmitting system including a video-frequency signal-generating system embodying the present invention.

Referring-now more particularly to the drawing, the television transmitting system illustrated includes a conventional camera or projector Hi and a cathode-ray signal-generating unit ll having associated therewith an electron multiplier i 2 which may be of any suitable welllmown construction. Connected in cascade to the output circuit of electron multiplier i2, in the order named, are a video-frequency amplifier and mixer. it, an oscillator-modulator It, a radiofrequency amplifier Ill, and an antenna system it, ll. The system includes a suitable timing and scanning signal generator Ill from which are derived suitable scanning fields for the signal generator II and suitable blanking and synchronizing signals for the transmitted television signal.

The signal generator H comprises an evacuated tube or envelope 22 having at one end thereof a suitable cathode or source of electrons 2! which, together with accelerating

anodes

22 and 22, comprises an electron gun edective to generate a beam of relatively low-velocity electrons. Anode 22' has a very small aperture therein so that the developed electron beam is of very small cross section. In order to provide a longitudinal magnetic field within the tube 20 effective to direct the electron beam emitted by cathode 2i to a photosensitive target electrode 22 of tube ii, there is provided a winding l9 surrounding the tube 22 and energized from a suitable source, such as a battery 22. Magnetic scanning fields for deflecting the cathode-ray beam in each of two directions normal to each other are provided by scanning

windings

2t, 2t and 26, 26, respectively, disposed successively along the tube and having their axes normal to each other, as shown in the drawing, or superposed so that one set of windings encloses the other. Target 22 preferably comprises a photosensitive mosaic deposited on a thin insulating sheet 2 on the opposite side of which is a grounded metallic coating or backing plate 9. A grounded metallic coating is provided on the inner surface of the tube between the electron gun and target 22. In order to provide a fixed deflection or offset of the cathode-ray beam. there are provided beam-shifting

plates

21, 28 having a fixed potential difierence applied therebetween from taps onbattery 22. Plates 2?, 22 have a spacing therebetween which is small relative to the scanning deflections of the beam. Electron multiplier I 2 has an opening adjacent cathode 2| and facing the target 22 in which is disposed an electron collector 29 having an aperture therein and having a suitable potential applied thereto from source 22. Since the cam: era It! projects the image to be transmitted on the side of target 22 opposite to that scanned by the electron beam which is the photosensitive surface, the target 22 must be at least semitransparent.

Neglecting for the moment the operation of the cathode-ray signal-generating unit ii, the

apparatus just described comprises the features of a conventional television transmitting system. The construction and operation of such apparatus being well known in the art, a detailed description thereof is deemed unnecessary herein. In brief, however, in the operation of the apparatus, an image of the scene to be transmitted is projected by camera It on the photosensitive surface of target electrode 22 in order to develop a video-frequency signal output from electron multiplier l2 which varies in accordance with variations in illumination from point to point of the target as it is scanned. The video-frequency signal output of the electron multiplier I2 is amplified in video-frequency amplifier l2 and mixed therein with the synchronizing signals and the blanking signals to be transmitted.

iromsuch area that portion will efiectively re- The composite signal output of the video-freuuency amplifier and mixer It is utilized to modulate a suitable carrier wave in oscillatormodulator H, the modulated-carrier signal being thereafter amplified in radio-frequency amplifier l5 and supplied to transmitting system It, H for radiation.

:w now to the operation of the portion oi the system involving the present invention, a beam of electrons of substantially constant intensity and having a velocity which is sumclently low as to provide a secondary-emission ratio of less t unity at the target electrode 22 is (11- rected thereto by means of the magnetic field developed by winding iii. In the absence of all deflect potentials, such electron beam would follow in a tight spiral the lines ofjforce ofthe his it from the aperture in accelerating anode 22' to the target electrode 22. The acceleratt anodes 2t and 22', having apertures therein, are, therefore, included in a means for accelerating as well as in a means for developing and accelerating toward .the target electrode 22 a beam of electrons which is perpendicular to the plane of the target. Under the influence of the magnetic scanning fields provided by coils 2t and 22, however, the beam is first deflected in one direction by means of the field developed by windings 2t, 2t and is then deflected in another direction at right angles thereto by means of the field developed by windings 2t, 2t and is thus edective to scan the target electrode 22 in a held or series of parallel lines.

Neglecting the action of beam-shifting, plates 22, 2, the electron beam from the aperture in the accelerating anode 22' is deflected in two directions normal to each other and is at all times incident upon target electrode 22 substantially perpendicular thereto so as to prevent deiocusing of the beam. .The operating potentials oi the tube are so adjusted that the velocity of the scanning beam is very small near the surface of target electrode 22; that is, so that the seconda'ry-electron emission ratio from target 22 is less than unity. Under these conditions, and under a condition of equilibrium when none oi the electrons of the scanning beam is utilized to neutralize a charge on the target, the target is substantially at the potential of the cathode. When the target is thus at cathode potential, 0 electrons from the beam approaching the target normal to its surface will be brought to rest just at the target surface by the electric field and then will return by the same path to the aperture in collector electrode 22' under the influence of 5-) the electric field. That is, the returning free electrons travel back through the magnetic means along substantially the same path as that traversed through the magnetic means by the scanning beam. The electron beam follows the magnetic lines of force of the combined focus and scanning field both in its travel toward the target and in its return from the target and since the time of this travel is so small that noappreciable change occurs in the scanning field in this inter- 15 val, the beam returns to substantially its starting point. If there is a positive charge on the portion of photosensitive target 22 upon which the beam is incident due to photoelectric emission 0 tain a sumcient number of electrons from the beam to neutralize the charge, thereby correspondingly diminishing or modulating the intensity of the return beam.

In accordance with the invention, howeverj' there is provided a suitable deflecting potential upon

plates

22 and 28 which is efiective to displace the electron beam coming from cathode 2| in a given direction parallel to plates 21 and 23, as illustrated by the arrows a, and to effect a displacement of the returning cathode-ray beam, as illustrated by the arrows b, so that the resultant over-all lateral displacement can be a made equal to the distance between cathode 2| and the aperture in electron collector 29, which are adjacently positioned. That is, the

beamshifting plates

21, 28 provide a constant electrostatic field perpendicular to the longitudinal magnetic field provided by winding is and perpendicular to a plane through the centers of the source 2i and the electron collector 29. The electron collector is of a restricted area. Therefore, the returning electrons are directed to the aperture in the electron collector 29 of electron multiplier l2 after which electron multiplication may be eflected in conventional manner. That is, a means is provided for directing, as an electron beam to a spot of restricted area on the electron collector 29, substantially all of the electrons which leave the target 22 and, inasmuch as the number of electrons which return from the vicinity of each incremental area of the target 22 as such area is scanned by the beam is determined only by the charge of such area, the

intensity of the returning electron beam at electron collector 29 varies in accordance with variations of illumination of the successive incremental areas of the target 22 and is representative of the signals to be generated. At the same time the shading efiects and lack of sensitivity accompanying the use of a scanning beam of sumciently high voltage to produce a secondaryelectron emission ratio of greater than unity at the target are avoided while the potential of the collector electrode 29 approximates that of anode 22' so that its eflect on the scanning beam is negligible.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A signal-generating system comprising, a cathode-ray signal-generating tube having a photosensitive target electrode and an electron collector, means for developing and accelerating toward said target electrode a beam of electrons of substantially constant intensity and having a velocity which is sufficiently low at said target electrode as to provide a secondary-emission ratio of less than unity and for returning free electrons from the vicinity of said target, magnetic means for scanning said target in two directions normal to each other with said beam, whereby the number of electrons returning from the vicinity of each incremental area of said target as such area is scanned by said beam is determined primarily by the charge of such area due to an image. projected thereon and such returning free electrons travel back through said magnetic means along substantially the same path as that traversed through said magnetic means by said scanning beam, means including a pair of beamshifting plates adjacent said beam-developing means and having a spacing therebetween which is small relative to the scanning deflections of said beam, and means including means for applying a steady unidirectional potential to said beam-shifting plates for directing as an electron beam to a spot of restricted area on said electron collector, which spot is displaced from said first-named means in a predetermined direction, substantially all of the electrons returning from the vicinity of said target, whereby an output signal may be derived from said electron collector which varies in accordance with the number of electrons directed thereto.

2. A signal-generating system comprising, a cathode-ray signal-generating tube having a photosensitive target electrode and an electron collector, means for developing and accelerating toward said target a of electrons of substantially constant intensity, which is perpendicular to the plane oi. said target, which has a velocity which is sumciently low at said target electrode as to provide a secondary-emission ratio of less than unity, and for returning free electrons from the vicinity of said target, magnetic means for scanning said target electrode in two directions normal to each other with said beam, whereby the number of electrons returning from the vicinity of each incremental area of said target as such area is scanned by said beam is determined primarily by the charge of such area due to an image projected thereon and suchreturning free electrons travel back through said magnetic means along substantially the same path as that traversed through said magnetic means by said scanning beam, means including a pair of beam-shifting plates adjacent said beam-developing means and having a spacing therebetween which is small relative to the scan ning deflections of said'beam, and means including means for applying a steady unidirectional potential to said beam-shifting plates for directing as an electron beam toa spot on said electron collector, which spot is displaced from said first-named means in a predetermined direction, substantially all of the electrons returning from the vicinity of said target, whereby an output signal may be derived from said electron collector which varies in accordance with the number of electrons directed thereto.

3. A signal-generating system comprising, a cathode-ray signal-generating tube having a photosensitive target electrode, an electron collector of a restricted area and a source oi electrons adjacent said collector, means for accelerating toward said target. electrode a beam of electrons from said source of substantially constant intensity andhaving a velocity which is sufiiciently low at said target electrode as to provide a secondary-emission ratio of less than unity and for returning free electrons from the vicinity of said target, magnetic means for scanning said target in .two directions normal to each other with said beam, whereby the number of electrons returning from the vicinity of each incremental area of said target as such area is scanned by said beam is determined primarily by the charge of such area due to an image projected thereon and such returning free electrons travel back through said magnetic means along substantially the same path as that traversed through said magnetic means by said scanning beam, means including a pair of beam-shifting plates adjacent said accelerating means and having a spacing therebetween which is small relasoaoci.

and means including means for applying a a unidirectional potential to said beam-shifting plates for directing as an electron beam to a spot of restricted area on said electron collector, which spot is displaced from said first-named means in a predetermined direction, substantially all of the electrons returning from the vicinity of said target, whereby an output signal may be derived from said electron collector which varies in accordance with the number of electrons directed thereto.

i. A signal-generating system comprising, a cathode-ray signal-generating tube having a photosensitive target electrode, means for developing and accelerating toward said target electrode a beam of electrons of substantially constant intensity and having a velocity which is sumciently low at said target electrode as to provide a secondary-emission ratio of less than unity and for returning free electrons from the vicinity of said target, magnetic means for scanning said target electrode in two directions normal to each other with said beam, whereby the number of electrons returning from the vicinity of each incremental area of said target as such area is scanned by said beam is determined primarily by the charge of said area due to an image projected thereon and such returning free electrons travel back through said magnetic means along substantially the same path as that traversed through said magnetic means by said scanning beam, an electron multiplier associated with said tube and including an electron collector, means including a pair of beam-shifting plates adjacent said beam-developing means and having a spacing therebetween which is small relative to the scanning deflections of said beam, and means including means for applying a steady unidirectional potential to said beam-shifting plates for directing as an electron beam to a spot of restricted area on said electron multiplier for multiplication therein, which spot is displaced from said first-named means in a predetermined direction, substantially all of the electrons returning from the vicinity of said target electrode, whereby an output signal may be derived from said electron multiplier which varies in accordance with the number of electrons directed thereto.

5. A signal-generating system comprising, a cathode-ray signal-generating tube having a photosensitive target electrode and a source of low-velocity electrons therein, means for developing and accelerating toward said target electrode a beam of electrons from said source of substantially constant intensity which has a velocity suiilciently low at said target electrode as to provide a secondary-emission ratio of less than unity and for returning free electrons from the vicinity of said target, magnetic means for deflecting electrons from said source in two directions normal to each other to scansaid target with said beam, whereby the number of elecative to the scanning deflections of said beam,

trons returning from the vicinity of each incremental area of said target as such area is scanned by said beam is determined primarily by the charge of such area due to an image projected thereon and such returning free electrons travel back through said magnetic means along substantially the same path as that traversed through said magnetic means by said scanning beam, an electron collector, means including a pair of beam-shifting plates adjacent said beamdeveloping means and having a spacing therebetween which is small relative to the scanning deflections of said beam, and means including means for applying a steady unidirectional potential to said beam-shifting plates for directing as an electron beam to a spot of restricted area on said electron collector, which spot is displaced from said first-named means in a predetermined direction, substantially all of the electrons returning from the vicinity of said target, whereby an output signal may be derived from said electron collector which varies in accordance with the number of electrons directed thereto.

6. A signal-generating system comprising, a cathode-ray signal-generating tube having a photosensitive target electrode and an electron collector, a source of low-velocity electrons in said tube, means for accelerating toward said target a beam of electrons from said source of substantially constant intensity and for returning free electrons from the vicinity of said target, the electrons of said beam having a velocity which is suihciently low at said target electrode as to provide a secondary-emission ratio of less than unity, means for developing a longitudinal magnetic field between said source and said target electrode, magnetic means for deflecting electrons from said source in two directions normal to each other to scan said target with said beam, whereby the number of electrons leaving each incremental area of said target as such area is scanned by said beam is determined primarily by the charge of such area due to an image projected thereon and such returning free electrons travel back through said magnetic means along substantially the same path as that traversed through said magnetic means by said scanning beam, means including a pair of beam-shifting plates adjacent said accelerating means and having a spacing therebetween which is small relative to the scanning deflections of said beam, and means including means for applying a steady unidirectional potential to said beam-shifting plates for directing as an electron beam to a spot of restricted area on said electron collector, which spot is displaced from said source of electrons in a predetermined direction, substantially all of the electrons returning from the vicinity of said target, whereby an output signal may be derived from said electron collector which varies in accordance with the number of electrons directed thereto.

7. A signal-generating system comprising, a cathode-ray signal-generating tube having a photosensitive target electrode, an electron collector, and a source of electrons adjacent said collector, means for accelerating toward said.

target electrode a beam of electrons of substantially constant intensity which has a velocity sufiiciently low at said target electrode to provide a secondary-emission ratio of less than unity and for returning said beam to the vicinity of said source of electrons, means for developing a longitudinal magnetic field between said source and said target electrode, means including a. pair of beam-shifting plates adjacent said accelerating means and having a spacing therebetween which is small relative to the scanning deflections of said beam, and means for applying a steady unidirectional potential to said beam-shifting plates for providing a constant electrostatic field perpendicular to said magneticfield and perpendicular to a plane through the centers of said source and said electron collector, and magnetic means for deflecting said beam in two directions normal to each other to scan said target, whereby the number of electrons returning from the vicinity of each incremental area of said target as such area is scanned by said beam is determined primarily by the charge of such area due to an image projected thereon and such returning free electrons travel back through said magnetic means along substantially the same path as that traversed through said magnetic means by said scanning beam, said electron source and said electron collector being spaced by such a RUDOLF 0. mm