Patent GB-1238368-A (GB)
IPC: H01J29/52
Filed: 1968-11-13
Abstract: 1,238,368. Cathode-ray tube circuits. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 13 Nov., 1968 [16 Nov., 1967], No. 53872/68. Heading H4T. In an arrangement for blanking the beam of a cathode-ray tube oscilloscope (e.g. during flyback periods) by the application of blanking pulses to the control grid which, like the cathode, is maintained at a high negative potential with respect to earth, the blanking pulses which are applied to the control grid via a first capacitor passing the high-frequency components, are also fed to a "chopper" producing a train of voltage peaks amplitude modulated by the blanking pulses, the modulated train being fed to the control grid via the series arrangement of a second capacitor and a grid resistor the junction between the second capacitor and grid resistor being connected to the cathode by means of a clamping diode shunted by a resistor which, with the second capacitor, constitutes an R.C. network having a long time constant relative to the recurrence period of the voltage peaks so that the second capacitor passes the D.C. and low-frequency components of the blanking pulses to the grid at the mean level of the cathode voltage. This arrangement allows a tube without a separate blanking electrode (which is cheaper and shorter) to be employed and obviates the need for a "floating" high voltage source for supplying the cathode. Additionally, although the two capacitors must withstand the cathode voltage only comparatively low capacity valves are required. Fig. 3, shows the general arrangement in which the blanking pulses, comprising rectangular pulses derived from a source SV and having the form shown by the lower full line waveform of Fig. 4a, are supplied to the tube grid via (the first capacitor 4, which passes the high-frequency components, and are also fed to a "chopper" CH producing the voltage peak waveform amplitude modulated by the blanking pulses as shown in Fig. 4a and having a D.C. voltage level indicated by line n. The chopper output is then fed via (the second) capacitor 13 and grid resistor 7 to the tube grid so that the latter follows the variations in the D.C. and low-frequency components of the blanking pulses with respect to a mean level approximating to the cathode potential produced by the high voltage source 2 and shown at k in Fig. 4b, which is established at point D by the clamping action of the diode 14 (which passes only the tops of the voltage peaks produced by the chopper) in conjunction with capacitor 13 and shunting resistor 15. Fig. 5, shows an embodiment in which the "chopper" comprises a transistor 20 supplied with short positive pulses derived from a D.C. converter 0 (e.g. supplying the D.C. voltage for the oscilloscope) operating at about 20 Kc/s. Because of this comparatively low frequency the capacitors 4 and 13 must have a comparatively large capacity (e.g. of the orders of 4700 pF) and therefore the voltage produced by the chopper across capacitor 13 is very high and because of this a voltage limiter comprising a glow-discharge tube 29 is connected across the diode 14 to prevent damage to the latter should the high-voltage source 2 be switched-off. In a further embodiment the "chopper" is made self-generating by an arrangement comprising two transistors connected as an astable multivibrator (Fig. 6, not shown).