Patent GB-984496-A (GB)
Title: Circuit arrangement for stabilizing the beam current of an image-orthicon pickup tube
IPC: H04N5/228
Filed: 1963-10-03
Abstract: 984,496. Television; cathode-ray tube circuits. FERNSEH G.m.b.H. Oct. 3, 1963 [Oct. 3, 1962], No. 38944/63. Headings H4F and H4T. A circuit arrangement for stabilizing the beam current of an image-orthicon pick-up tube so that a stable referred level signal is obtained, comprises means for suppressing the beam current during the fly-back intervals of the sunning process, means connected in the beam current circuit for developing voltage impulses as the beam is suppressed and for applying them to a time-constant circuit by which they are integrated to yield an output signal comprising only the field blanking impulses and means for deriving from said field blanking impulses a direct voltage which is applied to an electrode influencing the magnitude of the beam current in such a manner as to tend to reduce any fluctuations in the beam current due to changes in operating conditions, e.g. fluctuations in the heater voltage. As shown, image-orthicon pick-up tube 1 includes an anode 2, a control electrode 3 and a thermionic cathode 4 and by way of a capacitor 37 horizontal and vertical blanking impulses are applied to the control electrode 3 periodically driving it negative so that the electron beam is suppressed. A high voltage B, which is applied to the anode 2 produces a train of positive-going impulses of both horizontal and vertical frequency. The horizontal-frequency impulses are partly suppressed by capacitor 10. The signal is applied by way of capacitor 11 to the grid of valve R 1 , this grid being returned by way of a leak resistor 12 to the tapping of a voltage divider 14, 15 connected across an anode voltage supply 13 2 , to which the anode of valve R 1  is connected directly. Load resistor 13 and capacitor 16 provide an integrating timeconstant which suppresses the residual horizontal-frequency impulses. The positive-going signal which appears across cathode resistor 13 comprising vertical-frequency impulses is fed by way of a capacitor 17 to the grid of valve R 2  at which the direct-current component of the impulse train is restored by means of diode D 1  shunted by resistor 18 and biased by,potential divider 19, 20 connected across anode supply B 2 . Across load resistor 21 a negative-going pulse signal corresponding to the verticalfrequency impulses is developed and a fraction only of this signal, e.g. 10%, is passed by diode D 2 . This signal fraction is adjusted by means of variable resistor 27 and applied to the grid resistor 38 of pentode R 3  and gives rise to an alternating voltage across the anode load resistor 32. This signal is applied by way of capacitor 40 to the junction of diedes D 3 , D 4  which form part of a peak rectifier circuit yielding across resistor 34 a direct voltage proportional to the peak value of the applied signal. This direct voltage is applied by way of resistor 36 to charge capacitor 37, diodes D 3  and D 4  being so poled that an increase in the amplitude of the positive-going impulse at the anode 2 gives rise to an increasingly negative voltage across capacitor 37, and therefore as the voltage across capacitor 37 is applied directly to the control electrode 3 the fluctuations in the beam current are reduced. The values of the components used in Fig. 3 are given in the Specification. The circuit to the left of broken line 16 is usually disposed in the camera head and that on the right is disposed in the camera amplifier.