US2987645A - Beam control system for pickup tubes - Google Patents

Description

June 6, 1961 J. HUDGINS BEAM CONTROL SYSTEM FOR PICKUP TUBES 5 Sheets-Sheet 1 Filed Aug. 29, 1958 FIG. 1..

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A DIFFERENT [T LE VE CATHODE OR BEAM CURRENT MENTOR JOHN I HUDGINS ATTORNEY June 6, 1961 J. l. HUDGINS 2,987,645

BEAM CONTROL SYSTEM FOR PICKUP TUBES Filed Aug. 29, 1958 5 Sheet-Sheet s PHOTOCATHODE ILLUMINATION F OT CANDLES 10 TARGET TO CATHODE POTENTIAL (E F 1 G 1B F I. G 5

TO VIDEO AMP LIFIERS LONG TIME CONSTANT VOLTAGE DOUBLING 2o RECTIFIER GR D FE EDBACK 22 CONTROL sfiBR'r INTEGRATION NETWORK INVENTOR COARSE BEAM ADJUSTMENT JOHN I. HUDGINS ATTORNEY J1me 1961 J. l. HUDGINSO 2,987,6 5

BEAM CONTROL SYSTEM FOR PICKUP TUBES Filed Aug. 29, 1958 5 Sheets-Sheet 5 v mikgiffi 25 RECTIFIER TARGET POTENTIAL PRESET TO VIDEO AMPLIFIERS vo LTAGE DOUBLING 2o RECTIFIER 4| n AcK I ll CONTROL 2].

SHORT T INTEGRATION I NETWORK COARSE BEAM ADJUST.

l8 l TO VIDEO AMPHFIER AMPLIFIERS 1 xwfia T11?! GRID REC ER 1 FEDBACK CONTROL, 2 1' COARSE BEAM ADJUST. VOLTAGE DOUBLING 25 CATHODIEZ RBCTIFILR FEEDBACK 2 4: CONTROL INVENTOR JOHN 1. HUDGINS NETWORK ATTORNEY 2,987,645 BEAM CONTROL SYSTEM FOR PICKUP TUBES John I. Hudgins, Baltimore, Md., assignor to The Bendix Corporation, a corporation of Delaware Filed Aug. 29, 1958, Ser. No. 758,127 9 Claims. (Cl. 315-11) This invention has for its primary object to provide an improved system for automatically controlling the scanning beam of a television pickup tube in a manner such as to provide maximum video signal output for all light levels. The system has been successfully utilized with a pickup tube of the two-sided target, return beam type such as the image orthicon, and accordingly is herein illustrated and described in connection with such type of tube.

The operation of tubes of the orthicon type is now well known throughout the industry. In brief, a charge pattern is formed on the photocathode side of a target which produces a similar potential pattern on the opposite or scanned side of the target. The scanning beam is at substantially zero axial velocity when it reaches the target, and theoretically only sufficient electrons should land as will discharge the elementary capacitances of the target, causing the target to return to cathode potential (substantially cancel the charge or bleach the target), the remaining electrons of the beam returning to a bank of multipliers to produce an output signal proportional to the electrons landing on the target. By bleach, as used herein, is meant substantially complete or optimum target discharge. The beam should have sufificient intensity, or should be sufficiently dense, to furnish enough electrons to carry out the discharge function and no more. Increasing the intensity of the beam beyond this point does not increase the signal because the number of electrons landing on the target should ideally remain the same; it actually reduces the signal-to-noise ratio since the number of returning electrons are then out of propor tion to the number which land, and the noise in the output signal of the tube is proportional to the square root of the beam current. Thus, unless the charge on a target proportional to the highest level of illumination of the photocathode is fully cancelled for each scanning cycle, the information of the photocathode is not properly transferred to the return beam; in the case of inadequate discharge the areas which are not bleached (dark areas) will transfer little or no information, and in the case of overblcach, the signal-to-noise ratio will be reduced. Manual control of beam density is satisfactory where the light level remains substantially constant as in studio television, but when the light level is constantly varying, as where an unattended pickup tube is located at a point remote from a monitoring or control station, automatic control becomes imperative. A certain amount of control may be had under varying light level conditions by compensating for changes in light level from a given best operation value, as by the use of a light compensating iris diaphgram or a polarized light filter. Such type of regulation, however, is only effective insofar as excessive light is concerned; it affords no help when the light level drops below the best operation value, at which time the signal-to-noise ratio should be maintained at a maximum.

Various systems or circuitry have heretofore been proposed for automatic beam regulation to correlate the sensitivity of the pickup tube to the degree of illumination of the area being televised. One example is where the pulsating signal output of the tube is amplified and then developed across a potentiometer resistance, from which a portion is fed back and added to the grid potential of the tube via a rectifying and integration net- Work in an efiort to render the beam adequate to provide 2,987,645 Patented June 6, 1961 target bleach under low light level conditions. Such systems are based on the premise that bleach occurs at maximum video output; they are maximum video seekers in that, if bleach and maximum video output occur at the same beam density, an increasing beam at a density less than that necessary for target bleach will produce an increasing signal on the orthicon grid. This continues to increase the density of the beam until bleach occurs, whereupon the increasing beam serves to decrease signal output, reducing grid potential, and as a consequence decreasing beam density. It has been found that such type of circuitry will not operate satisfactorily with a return beam type of pickup tube. This failure is due to the fact that there has to be a certain target-tocathode potential (E at each light level for maximum video signal output and optimum target discharge to occur at the same beam density. Prior known circuitry of this general type does not utilize a target potential which is a function of the level of illumination on the photocathode of a pickup tube.

A more specific object of the present invention, therefore, is to provide in conjunction with a television pickup tube, automatic control circuitry which will select and hold a value of target-to-cathode potential such as will cause target bleach and maximum video output to occur at the same beam density.

Another object is to provide improved beam control circuitry particularly adapted for a pickup tube of the orthicon or return beam type.

In carrying out the objects of the invention, I select a value of target-to-cathode potential (E such as will cause target bleach and maximum video output to occur at the same beam density, and means are provided for automatically maintaining the E at nearly optimum value. By target-to-cathode potential is meant the potential difference which exists between the target (indicated at 11 in the drawings) and the electron gun cathode (indicated at 16). In one type of circuitry, the target-tocathode potential is maintained at the optimum value as a function of the level of illumination of the photocathode, while in other forms of the invention it is so maintained as a function of the video signal, and in still another form by a suitable load resistor in the cathode circuitry. The scanning beam electron density is then caused to vary as a function of the video output signal, to thereby produce target bleach or optimum target discharge.

In the drawings:

FIG. 1 shows a family of curves, each curve plotting integrated video output against cathode or beam current at a fixed light level on the surface of the photocathode of an orthicon pickup tube, the heavy dots on these lines indicating the occurrence of target bleach;

FIG. 1A is a curve chart illustrating how actual video output varies from the desired optimum as a result of not providing the proper E (target-to-cathode potential) for each given incident light level on the photocathode. In this chart, the heavy dash line connecting the bleach dots represents the locus of video output when operating at a single value (unchanging) E and the light-line curves plot integrated video output against cathode cur rent, each curve being plotted at a different light level;

FIG. 1B is another curve chart in which photocathode illumination is plotted against E showing the results of experiments with two different types of orthicon pickup tubes;

FIG. 2 is a schematic of one type of circuitry for carrying out the objects of the instant invention; and

FIGS. 3, 4 and 5 are additional schematic diagrams illustrating other types of circuitry capable of regulating the density of the scanning beam as a function of the level of illumination of the photocathode.

It has been found that when certain elements of a pickup tube of the orthicon type are operating at given relative potentials, target bleach and maximum output signal occur simultaneously. By providing suitable circuitry for utilizing this coincidence, the density of the scanning beam may be automatically controlled to improve tube sensitivity and the signal-to-noise ratio at all light levels.

FIG. 2 is an example of a circuit which will maintain the E substantially at optimum value, and will also maintain the density of the scanning beam substantially proportional to incident photocathode illumination at all light levels. In this figure direct current flows to the photocathode through a resistance 12, any changes in flow resulting from variations in photocathode light level producing a voltage drop across resistance 12 proportional to such changes. Since these changes may include changes of exceedingly limited magnitude, and since it is desired to convert all changes into an amplified target feedback potential having an average D.C. level, the voltage or potential drops across resistor 12 are fed into an A.C. modulator 13, Where they modulate a carrier Wave, which is amplified to the desired magnitude at 13' and then demodulated at 14. The amplifier '13 is designed to produce an output which is the inverse of its input (i.e. E Kl/E The need for this is illustrated in FIG. 1B, where the linear plot of these functions will show that target-to-cathode potential varies inversely as the level of illumination. The clamping network 14' holds the amplitude of the amplified sine wave to the desired reference level of potential for target feedback. The potentiometer resistance 15 constitutes an adjustable feedback control and hence serves as a means for setting the target-to-cathode potential at a value such that target bleach and maximum video signal output occur simultaneously, as will be more clearly described in connection with the curve charts of FIGS. 1 and 1A. The potential difference (E between the target 11 and cathode 16 is thus automatically varied inversely with variations in the level of illumination of the photocathode. Feedback polarity is arranged negative-going on' the target for an increasing photocathode current.

The video output signal is taken from the final dynode 17 of the electron multipliers, amplified at 18 and a portion of the amplified video fed back to the control grid 19 of the pickup tube across a potentiometer resistance 20, voltage doubling rectifier 21, and an integrating network 22. Tube 21' clamps the feedback voltage to the desired level. By using a voltage doubling rectifier, feedback becomes proportional to peak video output, a feature common to all circuitry illustrated herein.

When the output signal increases in response to an increase in photocathode illumination, the bias on the control grid 19 will be reduced and the density of the beam increased, and this increase will continue until bleach occurs. For a beam current slightly greater than that required for optimum target discharge, signal output is decreased and the bias on the control grid is increased, thereby reducing beam density. Thus by combining the target feedback and grid feedback circuitry in the manner illustrated, thetarget-tocathode potential and beam density are both caused to vary as a function of the photocathode illumination, to in turn provide maximum video signal output for all light levels.

The curve charts of FIGS. 1, 1A and 1B illustrate the theory of operation on which the invention is based. FIG. 1 shows a family of curves plotting average video signal output against cathode current at a fixed incident light level on the surface of the photocathode 10, each curve representing a different value of E It will be noted that there is an optimum E (so labeled) where target bleach occurs at maximum video output, which is maximum under any condition for a given incident light level. The proper value of E for the particular light level in FIG. 1 is at the top of curve F. In curves A to E, inclusive, overbleach will occur, since E is less than optimum and video output remains at substantially the same level over a given range following bleach. Here the signal to-noise ratio will deteriorate since the number of electrons in the return beam will be out of proportion to those that land on the target. In curves G, H and'I, E is greater than optimum and bleach occurs following maximum video signal output and here, again, the electrons of the return beam will be out of proportion to those landing on the target.

In FIG. 1A the thin full lines plot average video signal output against cathode current with E held constant,

each line representing a different light level.

FIGURE 3 If reference is had to FIG. 1, it will be noted that within the region prior to target bleach at any given value of beam current beyond threshold, the video output becomes peaked concurrently with proper E Such would be the case if from a stable condition of photocathode illumination, target and grid potential settings, the level of illumination then increased. 7

FIG. 3 illustrates a circuit in which video output is fed back to' the target as a DC. potential, causing the target to assume the correct potential for each level of photocathode illumination; and this feedback is regenerative prior to maximum output and degenerative beyond maximum output, and as such is a maximum video seeker.

In FIG. 3, the basic beam-control feedback circuit is similar to that of FIG. 2 and other maximum video seeking types of control circuits found in the prior art. However, instead of feeding the photocathode current back to the target as in FIG. 2, a predetermined portion of the video output is amplified and then fed back to the target across a regulating potentiometer 23, voltage doubling rectifier 24 and integration network 25. Assuming operation at some light level under conditions of proper beam density and E for optimum bleach, upon reducing light level, the E 'is initially too low in value and the target is then beyond bleach as illustrated at the point X on curve A, for example. Thus in FIG. 1 it will'be seen that if E responded at the same rate or faster than the beam current responded, one could conceivably end up in a stable condition at the second hump X of the curve I, which occurs when E is greater than optimum. Therefore it becomes necessary for the'beam to respond to a change in the video signal at a rate faster thanthe target responds to such change (follow curve A to the vicinity of its bleach point prior to changing E so that the target can change 'in the near bleach region where optimum E is a function of maximum video output,

FIGURE 4 Due to the low transconductance exhibited by the orthicon type of pickup tube, it becomes feasible to utilize the cathode as the point of reception of the governing potential instead of the target as in FIGS. 2 and 3. A

FIGURE 5 A simplification of the circuit of FIG. 4 can be achieved by applying the basic beam-controlling feedback to the grid of the electron gun section of the pickup tube and causing variations in the beam current to develop a voltage in the cathode circuit which will approximate the change in E potential met when small changes in light level are encountered. FIG. 5 illustrates a circuit of this gen eral type. In this instance, the basic feedback circuit applies a portion of the video output to the grid 19 across the voltage doubling rectifier and integration network heretofore described in connection with FIGS. 2, 3 and 4 to provide the necessary beam control. The cathode circuit has therein a resistance- capacitance network 26, 27. The resistance 26 may be, for example, between 5 and megohms. Whenever the beam current increases or decreases, there is a corresponding change in the voltage drop across the resistance 26, which varies the target-tocathode potential as a function of the level of illumination on the photocathode. As in the previously described circuits, beam current response should be faster than targetto-cathode potential response. Hence the R-C time constant of the cathode circuit should be relatively long with respect to that of the grid circuit. By making the cathode resistor a non-linear current sensitive (or voltage sensitive) type, the range of operation and stability of the circuit can be improved.

Once the basic concept of the invention has been made known to those skilled in the art, various types of circuitry for accomplishing the desired result other than the circuits of FIGS. 2 to 5, inclusive, will become apparent. Hence the inclusion of these circuits should be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a cathode ray beam video pickup tube system ineluding an evacuated envelope having therein a photocathode, a target and an electron beam gun provided with a cathode and beam control means; means for selecting and automatically maintaining a value of target-to-cathode potential for each photocathode light level such that maximum video signal output and substantially complete target discharge or bleach occur at the same beam electron density, and means for feeding a selected value of the video signal output back to said beam control means.

2. In a cathode ray beam video pickup tube system including an evacuated envelope having therein a photocathode, a target and an electron gun provided with a cathode and beam control means, a video output dynode, and a source of potential for said photocathode; a target feedback circuit adapted to impress a potential on said target varying inversely with variations in the response of said photocathode to variations in light level, means for adjusting the value of said feedback potential to select a target-to-cathode potential of a value such that maximum video signal output and substantially complete target discharge or bleach occur at the same beam density for all photocathode light levels, and an automatic feedback control circuit connecting said beam control means with the video output dynode.

3. In a cathode ray beam pickup tube system including an evacuated envelope having therein a photocathode, a target and an electron gun provided with a cathode and beam control means, a video output dynode, and a source of potential for said photocathode; an automatic target feedback control circuit having its output connected to said target and its input connected to a source of potential varying with variations in photocathode potential, means in said feedback circuit for converting variations in the input potential to a reference level of target feedback potential, means for setting the target-to-cathode potential at a value such that maximum video signal output and substantially complete target discharge or bleach occur at the same beam density for all light levels, and another automatic feedback control circuit interconnecting said beam control means with the video output dynode.

4. In a cathode ray beam pickup tube system including an evacuated envelope having therein a photocathode, a target and an electron gun provided with a cathode and beam control means, a video output dynode, and a source of potential for said photocathode; a supply circuit connecting the photocathode with its source of potential and having therein means for creating a potential drop proportional to variations in photocathode illumination, a target feedback circuit having means therein for converting said varying potential to a target reference voltage, a target feedback control for setting the target-to-cathode potential at a value such that maximum video signal output and substantially complete target discharge or bleach occur at the same beam density for all light levels, and an automatic feedback control circuit interconnecting said beam control means with the video output dynode.

5. In a cathode ray beam pickup tube system of the return beam type including an exacuated envelope having a photocathode, a target and an electron gun provided with a cathode and beam control means, a video signal output dynode, and a source of potential for said photocathode; a target feedback circuit having its output connected to said target and its input connected to the video signal output dynode, amplifying and integrating means in said feedback circuit for converting the pulsating video output into a steady target potential of the desired magnitude, means for setting the target feedback voltage at a value such that maximum video signal output and substantially complete discharge or bleach occur at the same beam density, and another feedback circuit interconnecting said beam control means and said output dynode.

6. In a system according to claim 5 wherein the time constant of said target feedback circuit is relatively long with respect to that of the beam control feedback circuit to cause the beam to respond to a change in video signal at a rate faster than the target responds to such changes.

7. In a cathode ray beam pickup tube system of the return beam type including an evacuated envelope having therein a photocathode, a target, an electron gun provided with a cathode and a control electrode, a video output dynode and a source of potential for said photocathode; an automatic feedback control circuit for establishing and maintaining a selected target-to-cathode potential having its input connected to said dynode and its output connected to said control electrode, amplifying and integrating network in said circuit for converting the pulsating video output to a steady target potential of the desired magnitude, means for setting the target feedback voltage at a value such that maximum video signal output and substantially complete discharge or bleach occur at the same beam electron density, and another automatic gain control feedback circuit interconnecting said dynode and cathode.

8. In a system according to claim 7 wherein the time constant of said control electrode feedback circuit is short compared to said cathode feedback circuit to cause the beam to respond to changes in video signal at a rate faster than the target responds to such changes.

9. In a cathode ray beam pickup tube system including an evacuated envelope having therein a photocathode, a target and an electron gun provided with a cathode and control electrode, a video output dynode and a source of potential for said photocathode and target; an automatic feedback control circuit for establishing and maintaining a selected target-to-cathode potential, said circuit having its input connected to said dynode and its output connected to said control electrode, amplifying and integrating network in said circuit operative to convert the pulsating video output to a steady'target potential of the required magnitude, and a cathode supply circuit provided with an R-C network having a long time constant relatively to that of the control electrode feedback circuit to render beam current response faster than target-to-cathode potential response.

Thalner Oct. 19, 1948 Kell M31. 1, 1949

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