Patent GB-1403061-A (GB)
Title: Method of guiding the beam of charged particles of a luminescent  screen tube
IPC: H01J29/34
Filed: 1972-10-12
Abstract: 1403061 Cathode-ray tubes J LEJON 12 Oct 1972 [21 Oct 1971] 47156/72 Heading H1D [Also in Division H4] In a cathode-ray display tube such as an oscilloscope tube or a colour television tube, guidance means are provided at the beginning of each line of the scan to direct the beam to the correct position, and along each line to maintain the beam on the line. In Fig. 1 the guidance means comprises pairs of electrodes 2, 3, and 4 with enlarged ends at the input zone A; the electrodes are connected to respective leads 11 and 12 which are connected to the input of an integrating circuit which supplies the vertical deflection coil (Fig. 3, not shown). In operation, if the beam impinges at 33 the rate of integration is increased so that the beam is deflected along the path shown to 41 and is thereafter maintained between the electrodes 2a and 2b. A colour television tube may include four coplanar beams (Fig. 4, not shown) each provided with magnetic convergence means (Fig. 5, not shown) which are adjusted so that the beams are parallel and close to the axis at the deflection means. One beam is a pilot beam which scans the guidance means, whilst the other three beams scan respective lines of red, green and blue phosphors arranged between the guidance strips. The guidance strips may be conductive, secondary emissive, or fluorescent. Each strip may be in the form of an interrupted line or of a line of variable thickness, the interruptions or indentations of opposite strips of a pair being at different pitches (Figs. 6 and 7, not shown); the index signals then comprise different frequencies which are preferably outside the video band, and a single pick-up means can detect both signals which are fed to respective tuned circuits and rectifiers, the outputs of which are subtracted to provide the error signal (Figs. 8-11, not shown). One of the signals may be compared in frequency with a reference signal to derive an error signal which is used to maintain the horizontal scanning at a constant velocity (Fig. 10, not shown). In a tube containing only two beams, the pilot beam scans the guidance strips whilst the display beam is switched to the three different lines of phosphors in synchronism with the switching of the luminance signal at a frequency of 30 Mc/s under the control of a ternay counter (Figs. 12 and 13, not shown). Interlacing.-In one arrangement, (Fig. 14, not shown) a red and a green phosphor line are arranged above, and blue red and green lines are arranged below each pair of guidance strips. In one field the red and green phosphor above the guidance strips are energized, and in the next field those below; the same blue phosphor strip and guidance strips are scanned in both fields. In another arrangement, Fig. 15, the high frequency guidance strip 2a lies above the low frequency strip 2b in one line but in the succeeding line 3 this is reversed. The error signals are switched by a field identification signal so that on one field a pilot beam landing at 192 is directed upwards to line 2, and on the next field it is directed downwards to line 3. In this arrangement one set of three colour strips such as 182 is provided between each pair of guidance strips; in another arrangement (Fig. 17, not shown) two sets of colour strips are located between each pair of guidance strips and the pilot beam is shifted vertically with respect to the other beams by the width of a set at field frequency.