Patent GB-503893-A (GB)
Title: System employing cathode ray oscillographs
IPC: G01R13/20
Filed: 1938-07-05
Abstract: 503,893. Cathode - ray oscillographs; inductance, capacitance, and frequency measurements. STANDARD TELE - PHONES & CABLES, Ltd. July 6, 1838, No. 19902. Convention date, July 5, 1937. [Class 37] [Also in Group XXXV] Apparatus for testing the frequency characteristics of a circuit comprises a cathode-ray tube, means for producing a time-proportional deflection of the cathode-ray along one coordinate axis, means for applying to the circuit to be tested oscillations produced by a thermionic valve oscillator and modulated by means of a potential derived from that used to produce the time deflection and means for causing a deflection of the cathode-ray along a second co-ordinate axis, by the output of the circuit to be tested. As shown in Fig. 1, a cathode-ray oscillograph 1 is connected to a supply circuit comprising a transformer 2 and a rectifier valve 3, a condenser 4 being connected across the rectifier 3 and the secondary winding 12 of the transformer 2. The condenser 4 is charged at each cycle and discharges approximately linearly, giving a saw-tooth potential curve which is applied to the deflector plate 14 to give the time-proportional deflection. A resistance 17 is connected in series with the secondary winding 12 and condenser 4 and produces a potential drop which is applied between the modulation grid 18 and cathode 57 in order to extinguish the spot during the return period. A valve 23 generates relaxation oscillations rich in harmonics, the frequency being modulated by applying to the grid circuit a fraction of the potential at the point 13 through a potentiometer 30 so that for every position of the cathode spot on the screen there is a corresponding determined frequency. A fraction of the potential of the oscillator 23 is applied to the apparatus, such as a radio receiver, to be tested by means of a potentiometer 32, the output potential of the receiver being applied through a condenser 33 to an amplifier 35 and thence to the deflector plate 42 of the cathode-ray tube 1. The spot traces on the screen a curve relating to the selectivity of the receiver to the frequency. The curve of impedance of a circuit may be obtained by connecting the impedance in the plate circuit of an amplifier and applying the terminal voltage to a detector and thence to the amplifier 35. The frequency modulation may alternatively be effected by electrically variable capacities such as the reflected capacity at the terminals of an amplifier valve, or by varying the permeability of a choking coil by a current. Fig. 4 shows a modified arrangement in which the potentials for operation of the cathode-ray tube 1 are obtained direct from the alternating supply without rectification. The ends 64, 65 of the secondary winding 12 of the transformer 2 are connected to the anode 56 and cathode 57 of the cathode-ray tube 1, the cathode connection being through a resistance 17 and potentiometer 10. The modulation electrode 18 is connected through a resistance 11 to the potentiometer 10. The operating potential for the concentration electrode 53 is tapped from a potentiometer 8. The time-proportional deflection is derived from the junction 13 of a resistance 5 and condenser 4 connected between the points 64, 65, a theoretical explanation being given in the Specification. The potential to be observed is applied to the deflector plate 42 through a condenser 33, a resistance 34 fixing the mean potential of this plate relatively to the anode. Capacitance may be measured by connecting across an inductance coil of known value and determining the resonance frequency.