GB773368A - Improvements in or relating to electrostatic deflecting means for cathode ray tubes - Google Patents

Abstract

773,368. Cathode-ray tubes. TELEFUNKEN GES. July 16, 1954 [July 18, 1953], No. 20734/54. Class 39(1). In a cathode ray tube preferably a receiving tube, having electrostatic deflecting plates, the plates are so constructed as to provide a flat deflecting field, i.e. their width is greater than their separation which tends to impart astigmatism to the beam and a localised field having a diminishing strength from the axis outwards in a plane perpendicular to the plane of deflection at a point between the beam entry and exit portions in such a manner that astigmatism of the beam during deflection is substantially reduced or eliminated. The desired reduction in field strength in a plane perpendicular to the plane of deflection is obtained by channel shaped depressions 5, Figs. 1, 3 in the deflecting plates 3, 4 or by apertures backed by troughs, Fig. 4 (not shown). The troughs act as screens from other electrodes e.g. an accelerating anode. Both the width of the plates and the spacing between them increase in the direction of travel of the beam. In another embodiment, Fig. 5, the desired reduction in field strength is obtained by making portions 10, 11 narrower than in Figs. 1-4. The narrowest part 10 of the plates is smaller than the spacing between the plates. Extra screens which may be similar to those of Fig. 4 near part 10 may be required toprevent fields of other electrodes from penetrating the deflection field. It is explained that the astigmatism causes the spot to become elliptical the major axis of the ellipse lying in the direction of deflection so that a reduction of field in a perpendicular direction as above described compensates for this, provided that symmetrical deflection potentials are used. For asymmetrical deflection potentials however, the astigmatism is over-compensated due to field distortion produced by the conventional screen electrode between the two pairs of deflecting plates, so causing the beam cross section to be elliptical with a major axis perpendicular to the direction of deflection. This is compensated by cutting away a small part of both plates at 7, Fig. 5, which makes the field at the exit end increase away from the axis perpendicular to the plane of deflection.. The deflecting plates which may be the pair nearer to the cathode, may be secured by arms to insulating rods which also support the cathode and any further intermediate electrodes.