Patent GB-634783-A (GB)
Title: Improvements in and relating to cathode ray oscilloscopes
IPC: G01R13/28
Filed: 1946-10-10
Abstract: 634,783. Cathode-ray oscillographs. BRITISH THOMSON-HOUSTON CO., Ltd. (General Electric Co.). Oct. 10, 1946, No. 30289. [Class 37] An oscilloscope comprises a cathode-ray tube having a pair of deflecting plates, two separate deflection wave sources connected to energize these plates, means for rendering the sources operative alternately, and means for neutralizing the effect of leakage from the sources to the plates so as to prevent interference with the wave from the operative source due to leakage through the inoperative source. The operating potentials for the cathode-ray tube 10 are derived from a voltage dividing resistor 21 supplied from a battery 20. Voltage dividers 22, 23 are connected between the resistance 21 and the pairs of deflecting plates 16, 17 for centring the beam. Deflection voltages for determining the path of the beam in the absence of a signal are supplied from vertical and horizontal deflection voltage amplifiers 4, 25 which are supplied with sine waves from generators 26, 27, the frequency of the generator 27 being higher than that of the generator 26. In order to produce a circular trace, the sine waves impressed on the vertical and horizontal deflecting plates are caused to be 90 degrees out of phase by supplying sine waves from the generators 26, 27 directly to the amplifier 24 and through 90 degree phase shift circuits 28, 29 to the amplifier 25. A signal from an external source 30 is coupled through a condenser 31 and resistance 32 to a signal point 33 in the centre of the fluorescent screen 15 and deflects the circular beam. The vertical deflection amplifier 24 comprises two triodes 35, 36 having their grids coupled to the generators 26, 27 and normally biassed by batteries 37, 38. A battery 39 is arranged to be connected to either grid by a switch 40 and biasses the corresponding triode 35 or 36 beyond cut-off, so that in the position shown the valve 36 is inoperative and the wave from the generator 26 is amplified by the valve 35 and impressed across the deflection plates 16 through a coupling condenser 41 and resistance 42. The amplifier 25 similarly comprises triodes 45, 46, normal biassing batteries 47, 48, and a cut-off biassing battery 49 arranged to be connected to either grid by a switch 50 interlocked with the switch 40 so that the amplifiers 24, 25 are always connected to a selected one of the generators 26, 27. The output voltages from the anodes of the valves 45, 46 are coupled to the horizontal deflection plates 17 through a condenser 51 and resistance 52. In the position shown, the generator 26 is connected to the amplifier valve 45 through the phase shift circuit 29, the valve 46 being biassed beyond cut-off by the battery 49. When the switches 40, 50 are moved together to their upper positions the valves 35, 45 are biassed beyond cut-off and the highfrequency deflection wave from the generator 27 is coupled to the valve 36 directly and to the valve 46 through the 90 degree phase shift circuit 29. By changing the position of the switches 40, 50 the deflection wave is thus changed from the low-frequency of the generator 26 which gives a trace 54, Fig. 2, to the higher frequency of the generator 27 which gives a trace 55 rotating once for each indicated scale division of the trace 54 and providing a vernier reading. A pulse impressed on the point 33 by the signal source 30 appears at 58 on the trace 54 and at 59 on the trace 55, its position on the trace 55 indicating on a larger scale the position between adjacent markers on the trace 54. The switches 40, 50 are preferably electronic in order to give a sufficiently high rate of operation to enable the traces 54, 55 to appear simultaneously by persistence of vision. Interference between the deflection voltages of the generators 26, 27 through the anode to grid capacitances of the inoperative valves is prevented by means of variable condensers 60, 61 between the grids of the valves 35, 36, and the lower vertical plate 16 and variable condensers 62, 63 between the grids of the valves 45, 46 and the left-hand horizontal plate 17. These variable condensers are adjusted so that their capacitances are equal to the total anode to grid capacitance of the corresponding valves. Resistors 65, 66 byepassed by variable condensers 67, 68 connect the lower potential plates of each pair 16, 17 to earth. Condensers 69, 70, 71 byepass the resistors 22, 23, 21 'so that the resistors 65, 66 are both connected to earth for alternating currents. The resistance 65 is made equal to the resistance of the resistors 42, 46 in parallel so that the resistance between each of the pair of plates 16 and earth is the same. The condenser 67 is adjusted so that the capacitance of the lower plate to earth is equal to that of the upper plate of the pair 16. Similar adjustments are made to the resistor 66 and condenser 68. The separate plates of each of the pairs 16, 17 are then equally and oppositely energized by leakage through the grid to anode capacitances of the amplifier valves and their neutralizing condensers, so that the cathode-ray beam is not deflected.