Patent GB-589427-A (GB)
Title: Improvements in and relating to oscilloscopes
IPC: G01R13/32
Filed: 1945-03-22
Abstract: 589,427. Cathode-ray oscillographs. BRITISH THOMSON-HOUSTON CO., Ltd. (General Electric Co.). March 22, 1945, No. 7318. [Class 37] An apparatus for enlarging a portion of an oscillograph trace comprises means for applying a wave of greater value than is required for full scale deflection to one of the deflection plates, means for superimposing a voltage on this wave for adjusting the selected portion into the centre of the screen and means for cutting off the beam when the sweep deviates beyond the screen. A sawtooth wave is applied to an amplifier, the last two stages of which comprise a double triode 46, Fig. 4. The tube 46 has a pair of anodes 48, 49 to which a source of voltage 50 is connected through resistors 51, 52, the mid-points of which are grounded through ripple absorbing condensers. The grid 54 is capacitively coupled to the output of the previous stage. The amplifier output is applied through a conductor 29 to one of the horizontal deflection plates 19 of the cathode-ray tube 20. A voltage obtained from between the points 26, 27 of the potentiometer 24 connected across a battery 25 is superimposed on the amplifier output so that a selected portion of the trace can be adjusted into the centre of the screen. The amplified sweep wave voltage is also applied to the grid 61 of a triode 47 having a sharp bend characteristic so that whilst the sweep wave voltage is below the value required to deflect the beam beyond the edge of the screen, the anode current is negligible and a positive voltage is maintained on the grid 58 of the cathode-ray tube. When the sweep wave voltage rises above this value, a heavy discharge current flows through the tube 47 causing a large voltage drop in the anode resistor 66, depressing the voltage of the grid 58 below that of the cathode 65 and cutting off the cathode beam. When the sweep wave voltage falls more than is sufficient to maintain the selected portion on the screen, a triode 47<1> acts in a similar manner to the triode 47 and stops the cathode beam. To obtain this effect an inversion circuit or phase inverter 30 is interposed between the anode 49 of the amplifier and the grid of the tube 47<1>. In a modification, Fig. 9, the final stage of the amplifier consists of a push-pull arrangement comprising triodes 46A, 46B. Anodes 91, 92 are connected through condensers 93, 94 to the horizontal sweep plates 19A, 19B. Resistors 95, 96 are connected in series between the plates 19A, 19B and have a common earth terminal 97 so that variations in the voltage output of the amplifier produces opposite variations in potential with respect to earth on the horizontal sweep plates 19A, 19B. To change the effective centre of the sweep wave and thereby select the portion of the wave which is to be examined, potentiometers 98, 99 are connected in parallel across a battery 25 and the potentials of the taps 100, 102 are varied in opposite directions. If the voltage on either of the sweep plates 19A, 19B, which are connected to the grids 106, 107 of the tube 103 respectively, exceeds the value required to deflect the beam to the edge of the screen, one section of a double triode 103 becomes strongly conductive, the potentials of the anodes 104, 105 and the electrode 58 are reduced and the beam is cut-off. Adequate strength of beam during enlargement of the signal wave and cutoff during the remainder of the wave is obtained by adjustment of the rheostat 67 and cathode bias potentiometer 115.