US2874328A - Voltage sampling apparatus - Google Patents

Description

' Feb. 17, 1959 CRQST 2,874,328

VOLTAGE SAMPLING APPARATUS Filed ec. 31, 1954 DEFLEGTION CIRCUIT I INVENTOR. MUNSEY acnosv EK VI I E(;' v BY A TTOR/VE itc States 2 ,874,328 VOLTAGE SAMP ING APPARATUS Munsey Cms Ash n Pa k N- 5-, assis er o the United States of America as represented by the Secretary of the Army The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to voltage Sampling circuits and more particularly to an apparatus for sampling any one of a plurality of discrete voltages at a common output connection.

A primary object of the present invention is to provide an apparatus for sampling any one of a plurality of voltages at a common output terminal.

Another object is to provide an apparatus for sampling any one of a plurality of voltages ata common output connection employing a cathode-ray tube having a deflectable electron beam and an array of targets therefor, each of said targets being associated with a voltage to be sampled.

A feature of the invention resides in the combination of a cathode-ray tube of the defiectable beam type having an array of spaced target cells, each of said target cells being substantially perpendicular to the path of said electron beam, each of said target cells having a collector electrode connected to a discrete reference voltage and a floating target electrode, the potential of which is controlled by said collector electrode.

A further feature relates to a voltage sampling system of the deflectable electron beam type, wherein the voltage being sampled does not have to be removed or presampled, and wherein the level of sampled voltage may be accurately observed for an unlimited duration.

In accordance with the present invention, there is provided an apparatus for sampling any one of a plurality of discrete reference voltages at a common output termi-. nal comprising a cathode-raytube having means to gen erate a deflectable electron beam, a plurality of discrete target cells in spaced relationship, each of said target cells being disposed substantially perpendicularly to the path of the beam, each of the cells being associated with 2,874,328 Patented Feb. 17, 1959 ice " Figure 5 is a curve similar to that of 'Fig. 4 wherein" the abscissa is expanded to indicate a greater scale of electron beam voltage.

Referring now more patricularly to Fig. 1, there is shown in cross-section a cathode-ray tube 10 of any well known construction, having the usual evacuated glass envelope with a neck'portio'n 12. and an enlarged funnelshaped portion 14, which is closed by the usual end wall 16. Along a portion of the neck and inner walls of the tube there is provided a coating of aquadag or other suitable material, which serves as the final anode 18 of the tube.

Suitably mounted within neck portion 12 is means 20 for generating a beam of electrons which can be focused one of the reference voltages and including a collector electrode connected to a'reference voltage and afloating target electrode connected to the common output terminal, and deflecting means for directing the beam against any one of the cells to cause secondary emission from the target electrode, the secondary emission being controlled by the collector electrode, whereby the target electrode assumes the potential of the collector electrode.

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 drawings, and its scope will be pointed out in the appended claims.

In the drawings, v

Figure 1 is a schematic depiction of a preferred embodiment of the present invention, including a crosssection of a cathode-ray tube and its internal components, and target cells;

igure is a pe s e i of a tar e cell;

Fi u 3 i a ec ion taken alon in s 3- of Fig.

-' o ma e t rget elect de 38H e ent a ly floating ee to a spot of the desired size and may include the usual electron emitting cathode, a spot or beam intensity control electrode and a final accelerating electrode. The electron source may be either of the electrostatic or magnetic focus type. Since such means are believed to be well known, the component parts of means 20 have been omitted. Each of the electrodes of the electron beam generating means 20 has applied thereto potentials as is well known in the art. Preferably the accelerating anode is at ground potential and the cathode is at a high negative potential. Final anode 18 is preferably at the potential of the accelerating electrode included in means 20, viz., ground. For deflecting the electron beam generated by means 20, there is provided deflection means 24, which may be of the electrostatic or magnetic type, and which may be disposed to deflect the electron beam in either a horizontal or vertical path, the disposition of the deflection plates as shown providing a vertical deflection of the electron beam. Disposed vertically in an array near and substantially parallel to end wall 16 are a plurality of spaced target cells 26ae. To insure that the electron beam hits each target cell perpendicularly, it is preferable to make the array slightly arcuate. The target cells respectively consist of three units as shown in Figs. 2 and 3; outer wall or shield electrodes ZSq-e which may be cylinders or rectangular parallelepipeds that are open at both ends 30, each electrode 28 being disposed substantially colinearly with the path of the electron beam with ends 30 p rpendicular to the beam path. Each of the shield electrodes 28ae are connected to a common lead 32, lead 32 being connected either externally or internally to final anode 18. Containedwithin the shield electrodes ZSa-e are collector electrodes 34ae which are similar in configuration to the shield electrodes, viz., cylinders or rectangular parablelepipeds but slightly smaller than the latter to permit electrical insulation therefrom. On the end of each collector electrode 34r-e facing the electron beam is a fine mesh metal screen 35, the opposite end of electrodes 34a-e being open. Each of the collector electrodes 34a-e is connected to a reference voltage to be sampled, the discrete reference voltages being designated respectively by the numeral 36a-36c. Within each collector electrode 3411-5 is a target electrode 38a-e which is insulated and spaced from a corresponding collector electrode 34a. e and has a face perpendicularly disposed with respect to the path of the electron beam. The face of each target electrode 38a-e facing the electron beam may consist of a conductive material which is a good secondary emitter, such as silver-magnesium alloy. Each target electrode 38ae is connected to a common lead 40 Which is in turn connected to a common output terminal 42, where reference voltages 36a-36e may be sampled. Connected between common output terminal 42 and ground is a very high resistance 44 which serves trodes. Connected to deflection means 24 is a beam deflection circuit 46 for direeting the electron beam against any of the target electrodes'BBa-e. The beam deflection circuit may be any such circuit well known in the art and its detailed description has been omitted.

The operation of the present invention may best be understood by considering the graph in Fig. 4, where i denotes electron current away from the surface of target electrode 38 in the positive direction normalized with respect to the electron beam current, and v denotes the potential of the electron beam striking. the surface of target electrode 38. The zero point, B of v in the graph is the voltage of the cathode from which the electrons originate. When an electron beam of energy determined by the voltage v strikes the target surface, secondary electrons are emitted. It is seen that at first as the potential of the beam increases from E; to higher values, the net current away from the target is negative. the fact that the surface of a target electrode 38ae first collects electrons from the primary electron beam to a greater extent than it emits secondary electrons. Then at an electron beam voltage v commonly designated as the first crossover voltage, the collection of electrons from the beam by a target electrode 38a-e is equal tothe quantity of secondary emission therefrom, or, in other words, the beam current and secondary emission current are equal. As the electron beam voltage increases, the target electrode secondary emission current becomes increasingly greater than the current arriving from the electron beam, then levels 0133 and begins a very gradual decrease. At a very high voltage the secondary emission current again becomes equal to the electron beam current, and

at still higher voltages the secondary emission current is again less than the primary electron beam current. This curve is based upon the presence in the vicinity of the target electrode of a collector electrode at a potential considerably higher than the target electrode potential, so that essentially all the secondary electrons may be collected. Now, if there is a collector electrode at a fixed potential B in the immediate vicinity of the target electrode, as the voltage of the target is raised toward the volt This is due to electrode would charge negatively. If the beam electrons are relatively few, so thatthere is essentially no space charge between the target electrode surface and the collector electrode, the target surface will attain a potential slightly positive with respect to the collector potential. This occurs because the secondary electrons have an average initial velocity, which will allow them to escape from the target electrode surface against a small retarding field. However, if the electron beam current is strong enough so that a space charge exists between the target surface and the collector, this space charge will tend to suppress the emission of secondary electrons from the target electrode surface, and the target will attain a potential which is slightly negative with respect to the collector. It is seen, therefore, that there is an intermediate value of the electron beam current at which the target electrode will attain the potential of the collector electrode.

In the operation of the present invention, each collector electrode 34 is connected to a corresponding reference voltage 36a36e, which is to be reproduced at the interconnected common output terminal 42. The potentials of electrodes 34 are all preferably somewhat higher in I potential than the potentials of shield electrodes 28, al-

age of the collector electrode, the secondary emission current from the target surface will begin to decrease just before the potential of the collector electrode is reached. At the potential of the collector electrode, the secondary emission current from the target and the current received by the target from the electron beam will tend to become equal. At electron beam voltages higher than the collector electrode potential, more current will be received from the beam by the target electrode than will be emitted therefrom by secondary emission, and thus the value of the current received by the target electrode from the electron beam Will approach as a limit the actual value of the beam current at still higher voltages. The reason for this phenomenon is that the secondary electrons, being principally of low energy, will not be able to escape from the vicinity of the target electrode in the absence of a collecting field. This collecting field will decrease as the target voltage approaches collector voltage from the negative side and will reverse as the target voltage exceeds the collector voltage.

Now, if the surface of the target electrode is either unconnected to any external circuit or is connected to ground through a very high resistance, so that it is essentially at a floating potential, its surface would find itself in equilibrium at the collector potential. If the potential of the surface deviated slightly from the collector potential in the negative direction, the secondary emission current would exceed the current collected fromelectron beam and the target electrode would charge positively; while if the surface potential of the target electrode deviated from the collector potential in the positive direction the secondary emission current would be less than the current collected from the electron beam, and the target electrodes 34 are somewhat negative with respect to shield electrodes 28. The electron beam is directed toward any one of the target electrodes 38, and, in accordance with the described mechanism of secondary emission, the speeific target electrode 38ae will attain the potential of its associated collector electrode 34a-e. Then by shifting the deflection voltage on deflection plates 24, the electron beam can be directed toward any other target 38a-e and, consequently, terminal 42 will in turn attain the potential of the associated collector electrode 34ae. It is, of course, obvious that beam deflection circuit 46 is to be calibrated so that the electron beam can be readily switched from one target electrode 38 to another. It is thus seen, with the present invention, there is provided a device wherein voltages may be quickly and accurately sampled at a common output terminal.

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. An apparatus for sampling any one of a plurality of discrete reference voltages at a common output terminal, comprising a cathode-ray tube having means to generate an electron beam, said cathode ray tube including a plurality of elements at prescribed potentials, a conductive coating on the inner surface of said tube at the most positive of said potentials, a plurality of discrete target cells in spaced relationship, each of said target cells being disposed substantially perpendicularly the path of said beam, the spacing between said cells being greater than the width of said beam each of said cells being associated with one of said reference voltages and including a collector electrode directly connected to one of said reference voltages and a floating target electrode connected to said common output terminal, and deflecting means for directing said beam against any one of said cells to cause secondary emission from said target electrode, the secondary emission being controlled by the collector electrode, whereby the target electrode assumes the potential of said collector electrode.

2. An apparatus for sampling any one of a plurality of discrete reference voltages at a common output terminal,

rality of elements at prescribed potentials, a conductive coating on the inner surface of said tube at the most positive of said potentials, a plurality of discrete target cells in spaced relationship, each of said target cells being disposed substantially perpendicular to the path of said beam, each of said cells being associated with one of said reference voltages and including a collector electrode connected to a reference voltage and a floating target electrode connected to said common output terminal, each of said target cells comprising a first open-ended hollow shield member coaxially aligned with the path of said beam and at the same potential as said conductive coating, 2. second hollow collector electrode within and insulated from said shield member and connected to one of said reference voltages, said collector electrode having a screen at the end opposed to the path of the beam and being open at its other end, said floating target electrode being within and insulated from said collector electrode,

and deflecting means for directing said beam against any one of said cells to cause secondary emission from said target electrode, the secondary emission being controlled by the collector electrode, whereby the target electrode 5 assumes the potential of said collector electrode.

References Cited in the file of this patent UNITED STATES PATENTS

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