US2555147A

US2555147A - Cathode-ray tube magnetic focusing

Info

Publication number: US2555147A
Application number: US616374A
Authority: US
Inventors: Ralph E Meagher
Original assignee: Individual
Current assignee: Individual
Priority date: 1945-09-14
Filing date: 1945-09-14
Publication date: 1951-05-29
Anticipated expiration: 1968-05-29

Description

y 1951 R. E. MEAGHER 2,555,147

CATHODE-RAY TUBE MAGNETIC xocusmc Filed Sept. 14, 1945 I I I3 ACCELERATING ANODE VOLTAGE J FOOUSING COIL 6 8 7 /REGULAT ED VOLTAGE 4 5 ATTORNEY.

Patented May 29, 1951 CATHODE-RAY TUBE MAGNETIC FOCUSING Ralph E. Meagher, Watertown, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application September 14, 194.5, Serial No. 616,374

4 Claims.

The invention relates to an electrical circuit for keeping the electron beam of a cathode ray tube in focus, substantially independently of variations in potential on the accelerating anode of the tube caused for instance by variations in the line voltage or of variations in the resistance of the focusing coil due to temperature changes.

It has been found that, in cathode ray tubes using magnetic type focusing and deflection apparatus for focusing and deflecting the electron beam, images that have been expanded for projection become seriously distorted, due to defocusing, with even a small deviation from the proper current in the focusing coil. Even the change in resistance due to temperature change of the coil is sufficient to cause defocusing. The critical state of the focus makes it necessary for the focus control knob to be readjusted continually to compensate for small unavoidable changes in the A.-C. voltage from which the high positive potential for the accelerating anode is obtained. It is also necessary to make adjustments to compensate for the change in resistance of the focusing coil as the apparatus warms up. Constant manual readjustment is undesirable and, in addition, is impractical. It is therefore an object of this invention to provide a circuit which compensates for voltage variations and temperature changes and which eliminates the need for readjustment of the focus knob once it is set initially.

With cathode ray tubes used in radio detecting and ranging equipment it is essential that the pattern on the screen be clear and undistorted since measurements are made from the screen and any distortion or lack of clarity would introduce inaccuracies. It is also important, for obvious reasons, in cathode ray tubes used for television to keep the image clear and undistorted.

The problem does not arise in cathode ray tubes using electrostatic type focusing for the electron beam, since the voltage required for focusing varies directly with the voltage on the accelerating anode and therefore small changes in line voltage will not cause deiocusing.

In a cathode ray tube using magnetic type deflection and focusing but with electrostatic acceleration, current in the focusing coil should vary as the square root of the potential on the accelerating anode if the beam is to be kept in proper focus. The reason for this is that the effect of magnetic flux on the electron beam depends directly on the flux density which in turn depends directly on the current through the coil. The effect of the accelerating potential on the electron beam varies inversely as the square root of the potential. If the focusing coil is connected to a source of potential which varies in the same manner as the accelerating potential, the current through the coil, neglecting the change of resistance in the coil due to temperature variation, will vary directly with the accelerating potential and defocusing will take place.

Another cause of defocusing is the variation of the resistance of the focusing coil with changes in the temperature of the coil due to changes in ambient temperature and the warming up of the apparatus in which the cathode ray tube is installed. It is therefore a further object of the invention to provide a circuit which will cause the current through the focusing coil to vary as the square root of the accelerating potential despite variations in the resistance of the coil.

A further object is to provide a circuit that will keep the fiux density of the focusing field of a cathode ray tube proportional to the square root of the accelerating potential.

Other objects and advantages will become apparent from the hereinafter described specification.

The current for the focusing coil is supplied through a pentode type vacuum tube. The voltage supply for the screen is obtained from a regulated source of positive potential. The cathode of the tube is connected to ground through an adjustable resistor. The grid bias for the tube is obtained from a source of positive potential which varies substantially as the square root of the accelerating anode potential. The current through a pentode type vacuum tube depends almost entirely on the grid bias, over a wide range of anode potential, if the screen is held at a constant potential. ihe current through the focusing coil will therefore be substantially independent of variations in focusing coil resistance and in the focusing coil voltage supply and will depend on the bias on the grid of the tube. The bias on the grid of the tube varies as the square root of the accelerating voltage and the focusing coil current will vary in the same manner, keeping the focus independent of small variations in line voltage and of variation in the resistance of the focusing coil.

The invention will be best understood by reference to the single figure of the drawing which shows one embodiment thereof.

Point A is connected to the same source of high positive potential as the accelerating anode of the cathode ray tube. Resistors 2 and 3 are connected between point [and ground. These resistors are so proportioned that the potential at point 4 is approximatel the same as that of the source of regulated voltage (not shown) connected to point 5. Resistors 6 and l are of equal value and are connected in series between points :3 and 5. At the common junction of the two resistors (point B) one terminal of a potentiometer is connected with the other terminal connected to ground. The movable contact of the potentiometer is connected to the control grid of the vacuum tube 10. The screen grid of the tube is connected to point (the source of regulated potential). The cathode of tube III is connected to ground through an adjustable resistor l I. The anode of the tube is connected through the focusing coil I2 to an unregulated source of positive potential (not shown) connected to terminal I3.

Point I will be at the same potential as the accelerating anode and will vary in the same manner. Point 4 will be at a lower potential, due to the voltage divider action of resistors .2 and 3, but will vary in the same manner as the accelerating anode potential. Point 8, the midpoint of the voltage divider connecting points 5 and 8, will have a percentage variation equal to one half the percentage variation at point t.

The potential at point 5 is kept constant by any well known means, such as a voltage regulator tube. If the percentage variation at point 8 is one half the percentage variation at point t, the

potential at point 8 will vary substantially as the square root of the potential at point i, if the variations are not too large. The screen grid of the tube It is kept at a constant potential. The control grid is connected to the potentiometer 9 and the anode current of the tube and consequently the focusing coil current, will vary in accordance with the variations in potential at point 8'. The initial focus adjustment may be made by adjusting the potentiometer 9. Adjustment could also be made by means of the adjustable cathode resistor l I.

It can be seen that the above described circuit will cause the current through the focusing coil to vary substantially as the square root of the accelerating anode potential regardless of variations in the resistance of the focusing coil.

It will be apparent that there may be deviations from the invention, as described, which still fall fairly with-in the spirit and scope of the invention. For example, any other type of circuit may be used to provide a potential proportional to the square root of the accelerating anode potential.

Accordingly I claim all such deviations which fall fairly within the spirit and scope of the invention as identified in the hereinafter appended claims.

What is claimed is:

1. In an electrical circuit, a cathode ray tube having an accelerating anode and a magnetic focusing coil, a first source of positive potential connected to said anode, a first resistance means connecting said first source of positive potential to a first point of positive potential, a second source of reference potential, a second resistance means connected between a tap on said first resistance means and said second source of positive potential, a vacuum tube having at least three grids including a screen grid, an anode and a cathode, means electrically connecting the screen grid of said tube to said second source of potential, a potentiometer connecting the midpoint of said second resistance means to said point of reference potential, means electrically connect i'ng a control grid of said tube to said potentiometer, a third source of positive potential, and means including said focusing coil electrically connecting the anode of said vacuum tube to said third source of positive potential.

2. In an electrical apparatus including a source of high potential, a source of regulated potential, a source of unregulated potential, and a cathode ray tube having an electron-emitting cathode, an anode connected to said high potential source for accelerating electrons emitted from said cathode, and a focusing coil for said beam; a circuit for supplying a current to said focusing coil which varies as a square root of said high potential, for maintaining the focus of said beam in spite of fluctuations of said high potential and resistance changes of said focusing coil; comprising a first resistor, having a tap thereon, connected between said high potential source and ground; a second resistor connected between said regulated potential source and said tap on said first resistor, said second resistor having a midpoint tap thereon; a third resistor having an adjustable tap thereon connected between said midpointof said second resistor and ground; means for supplying a current to said coil through a constant current impedance means; and means connected to said movable tap for controlling the current through said impedance in accordance with the square root of said high potential.

3 A circuit according to claim 2, wherein said constant current impedance comprises a pentodetype vacuum tube having its screen grid connected to said regulated potential source, its plate connected through said focusing coil to said source of unregulated potential, and wherein its control grid connected to said movable tap comprises said controlling means.

4. A circuit according to claim 3, further including a cathode bias control for said pentode for regulating the current through said focusing coil.

RALPH E. MEAGl-IER.

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

UNITED STATES PATENTS Number Name Date 2,255,485 Dome Sept. 9, l9ll 2,291,682 Blumlein et al. Aug. l, 1942