Improvements in apparatus for measuring the variation of electrical impedance with frequency

Abstract

Abstract 431,731. Inductance and capacitance measurements; oscillographs. GENERAL ELECTRIC CO., Ltd., Magnet House, Kingsway, London, and LOVELOCK, R. T., c/o General Electric Co., Coventry. March 27, 1934, Nos. 9573/34 and 752/35. [Class 37] Apparatus for measuring the variation of the impedance of an element of an alternating-current circuit with the frequency is shown diagrammatically in Fig. 1. The output of a variable-frequency oscillator 1 is applied to two paths each consisting of an amplifier 2 or 2<1>, an impedance T or S, and a rectifier 3 or 3<1>. These paths are connected alternately to one pair of plates of a cathoderay oscillograph 4 by a switch 5 coupled with the movable frequency-varying member of the oscillator 1. A time-base device 6 also operated from the switch 5 supplies a potential difference to the other pair of plates of the oscillograph. The switch 5 connects the upper path containing the impedance T under test, to the oscillograph while the frequency of the oscillator changes regularly between its two limits, the time-base device 6 being also connected so that a constantly increasing or decreasing potential is applied to the other pair of plates. The switch 5 then substitutes the lower path, containing a standard impedance S for the upper path, at the same time again starting the time-base potential. Two lines are thus produced on the oscillograph screen, one determined by the variation of the impedance T and the other by the variation of the standard S. Coincidence of these lines indicates similarity of the impedances T, S. The means for varying the .frequency of the oscillator may consist of a rotary variometer, for example one in which a copper ring is rotated about a diameter. In the apparatus shown in Fig. 2 three leads 1 apply the oscillations from the variable oscillator continuously to the standard element S and two test elements T1, T2. The oscillator and amplifiers are not shown, but correspond to those shown at 1, 2, 2<1> in Fig. 1. The output from each of the elements S, T1, T2 is rectified by a diode 2, corresponding to 3, 3<1>, Fig. 1. That from the standard S is applied to the grids 6 of the triodes 7, whilst that from the test impedances T1, T2 is applied to the grids 6<1> of the triodes 7<1>. The rotating commutator 8, connected mechanically to the variable element of the oscillator, connects the grids 6 and the grids 6<1> alternately to earth and to a negative bias through grid leaks 9, 9<1>, thus rendering the valves 7, 7<1> alternately conducting. When the valve 7 is conducting, the output of the standard S is applied, after amplification, to a cathoderay oscillograph by means of a lead 10. When the valve 7<1> is conducting, the outputs of the impedances T1, T2 under test are similarly applied. The two leads 10 may lead to the same oscillograph, or each test impedance may have its own oscillograph. The time base may be operated from the commutator 8 by additional contacts (not shown). If each test impedance has its own oscillograph, the time base may be common to all. The voltage applied to the time-base plates of the oscillograph may be arranged to vary directly with the frequency of the oscillator instead of being proportional to time. This may be effected as shown in Fig. 3, in which the output of the variable oscillator is led to input terminals 11 and thence between earth and the grid of an amplifier 12, the anode circuit of which contains a high non-reactive resistance 13. A condenser 14 is shunted between the anode and the cathode, the voltage across this decreasing as the frequency increases. This voltage is applied to the grid of a second amplifier 16 through a potentiometer 15. The output of the amplifier 16 is rectified by a diode 17 biassed by means of a potentiometer 18 so that little current flows in its anode circuit when the frequency is a maximum. The anode current increases with decrease of frequency, the output being applied as a grid bias to a directcurrent amplifier 19, the output of which is applied to one plate of the oscillograph by a lead 20, the other plate being earthed. The condenser 14 may be replaced by an inductance in series with the resistance 13, the voltage across this inductance being applied to the potentiometer 15.

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