US2230111A - Deflecting device for cathode ray tubes - Google Patents

Description

Jan. 28, 1941, J. GUNTHER DEFLECTING DEVICE FOR CATHODERAY TUBES Filed NOV. 22, 1959 FIG. I.

INVENTOF? ATTORNEY Patented Jan. 28, 1941 UNITED STATES DEFLECT'ING DEVICE FOR CATHODE RAY TUBES Johannes Giinther, Berlinezehlendorf, Germany,

assignor to Fernseh Aktiengesellschaft, Berlin- Zehlendorf, Germany Application November 22, 1939, Serial No. 305,696 In Germany November 30, 1938 3 Claims.

The present invention relates to deflecting means of the electromagnetic type for cathode ray tubes. Cathode ray tubes are used for television purposes as well as oscillograph purposes and in both cases it .is' desired to trace upon the screen a scanning pattern of substantially rectangular form. Tubes of this type, particularly those having a wide angle of deflection and relatively short length, show distortions at the edges of the scanned area consisting for instance in a pincushion or barrel shaped outline of the area swept out on the screen. This happens particularly in case the radius of curvature of the screen surface is larger than the length of the deflected ray.

It is an object of the present invention to avoid distortions of this type in cathode ray tubes. It is a further object to provide a deflectingdevice consisting of two deflecting systems for two coordinates of the scanned pattern, one of said systems having the form of coils arranged on both sides of the neck of the tube and having small dimensions, in particular a short length.

According to the invention the two deflecting systems are so arranged with regard to one another that the cathode ray deflected by the first system passes near the front leads of the coils of the second deflecting system when the edge portions of the screen are scanned. The front conductors of the coil of the second deflecting system produce a magnetic field which counteracts to a certain degree the main field of the coils. This oppositely directed field is made use of to produce a compensation of the distortions. The

" cathode ray is deflected by the first system in such a manner that it passes through the magnetic field set up by the front conductors of the coil on the side of the coils which is directed towards the screen so that the deflection is more or less weakened as the ray enters the counterfleld to a greater or smaller degree.

The drawing shows an embodiment of the invention by way of example.

Fig. 1 is a side view of a cathode ray tube carrying two deflecting coils and a deflecting yoke.

Fig, 2 is a view of the coils with a representation of the lines of force.

Fig. 3 is a cross section through the tube in the plane of the line A-A.

Fig. 1 shows the cathode ray tube l with a screen 2 and a neck 3. Deflecting coils 4 are arranged upon the neck of the tube. Another deflecting system 5 consisting of an iron yoke is arranged in such a manner that the field set up between the poles of the yoke lies rectangular to the field produced by the coils.

The field produced by the coils 4 is represented in Fig; 2. This figure shows the field distribution in a plane being through the central axis of the tube. The main deflecting field is indicated by arrows 6 and the lines of force are closed around the side conductors of the coils as indicated in Fig. 3. The front conductors l and the rear conductors 8 of the coils produce also a magnetic field as indicated by arrows 9. This magnetic field increases the main field on the inner side of the coil but it is opposed in direction on the outer side of the coil. The lines of force shown in the drawing exist for a particular value of deflecting current and are reversed when the current is reversed in direction.

The cathode ray indicated by line passes through the central axis of the tube and is deflected in such a manner that it leaves the space between the coils with an angle depending upon? the current flowing through the coils. The first deflecting system consisting of the yoke is so arranged with regard to the coils that the ray does not pass through the field set up by the rear conductors of the coil but through the stray field of the front conductors. The lines of force lying on the outside of the coil act upon the deflected cathode ray more strongly than the lines of force lying on the inside so that the deflection is weakened in those parts where the cathode ray approaches most closely the front conductor. This can be seen from Fig. 3 in which the area swept by the cathode ray in the plane of the front conductors is indicated by dotted lines II. If the distortion produced by the particular shape of the screen would result in a pin-cushion outline of the scanned area it can be seen from this figure that the deflection is weakened near the corners of the rectangle H so that the pincushion distortion is compensated. The strength of this compensating effect can be varied in dependency upon how far the cathode ray enters the region of the stray field, and how strong the inhomogeneous stray field is. It has been found that the compensating effect increases when the curvature of the deflecting coil is increased, i. e. when the coils extend farther around the tube.

The same basic idea can be used for compensating a barrel shaped distortion. In this case the coils must be curved in the opposite direction, for instance, convexly to the cathode ray tube.

The dimensions, particularly the length of the coil, is a means for obtaining the desired strength of the counterfield. It is possible for instance by means of particularly short coils to obtain a strong influence upon the cathode ray because the front conductors produce in this case a counterfield, which is comparatively strong in comparison to the main field. It is preferable in this case to make the coils only a little longer than the diameter of the neck of the tube, for instance, 1.4 to 1.8 of the tube diameter.

Another feature of the invention is to arrange the coils 4 closely to the point where the funnel shaped part of the tube begins and to use a tube of wide opening so that the ray can be deflected very strongly without touching the wall of the tube.

The first deflecting system may have a construction of any suitable kind and may consist also of coils instead of the yoke mentioned above or may be an electrostatic deflecting system. The invention may be used for compensating distortion in one or two directionsof deflecting.

The invention is suited particularly for television transmission and receiving tubes but it can also be used in connection with other types of cathode ray tubes showing not only pin-cushion or barrel shaped distortions but edge distortions of any kind. It is also possible to compensate distortions which are not due to the curvature of the screen of the tube but which are caused by an optical system used in connection with the cathode ray tube, for instance, for projection purposes.

What I claim is:

1. In combination with a cathode ray tube having a screen adapted to be scanned by a beam of electrons, a first deflecting system for developing a first magnetic deflecting field of varying intensity and polarity in a first direction perpendicular to the axis of said tube and for simultaneously developing magnetic counterfields varying in accordance with said first field in radial directions with respect to said tube axis, and a second 'deflecting system for developing a second magnetic deflecting field in a second direction perpendicular to the axis of said tube of varying polarity and such intensity periodically to deflect said beam of electrons into one of said counterfields at extreme angles of deflection, thereby to influence by said counterfields the beam deflection effected by said second field.

2. In combination with a cathode ray tube having a screen adapted to be scanned by a beam of electrons, a first deflecting system including side conductors for developing a first magnetic deflecting field of varying intensity and polarity in a first direction perpendicular to the axis of said tube and including end conductors for developing magnetic counterfields varying in accordance with said first field in radial directions with respect to the axis of said tube, and a second deflecting system for developing a second magnetic deflecting field in a second direction perpendicular to the axis of said tube of varying polarity and such intensity periodically to deflect said beam of electrons into one of said counterfields at extreme angles of deflection, thereby to influence by said counterfields the beam deflection effected by said second field.

3. In combination with a cathode ray tube having a neck portion, a bulb portion, and a screen positioned in said bulb portion adapted to be scanned by a beam of electrons, a first deflecting system comprising coils having side conductors for developing a first magnetic deflecting field of varying intensity and polarity in a first direction perpendicular to the axis of said tube and having front end conductors facing said screen and rear end conductors remote from said screen for simultaneously developing magnetic counterfields varying in accordance-with said first field in radial directions with respect to said tube axis, and a second deflecting system for developing a second magnetic deflecting field in a second direction perpendicular to the axis of said tube of varying polarity and such intensity periodically to deflect said beam of electrons into one of said counterfields developed by said frontend conductors at extreme angles of deflection,

thereby to influence by said counterfields the beam deflection effected by said second field.

JOHANNES GiiNTHER.

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