GB392053A - An improved method of and means for measuring the modulation of electric currents - Google Patents

Abstract

392,053. Measuring modulation. DEHN, F. B., 103, Kingsway, London.- (Elwell, C. F. and Terman, F. E.; Stanford University, California, U.S.A.) Sept. 19, 1932, No. 26059. [Class 40 (v).] Direct currents are established, proportional respectively to the crest voltage and the average voltage of a modulated wave, and the degree of, modulation is deduced from a comparison of the values of the two currents. The high-frequency currents are impressed on grids 26, 38 of thermionic detectors 27, 37 across an inductance 1. The grid-leak and condenser 28, 30 of the detector 27 have a time constant so large that the peak voltage is maintained on the grid 26. The grid-leak and condenser 40, 41 of the detector 37 have a time constant sufficiently small to enable the voltage on the grid to follow the modulating currents. The anode currents from the valves 27, 37 are passed through differential coils 34, 43 respectively. of an indicating-instrument 35. The inertia of the indicator is sufficient to integrate the effect of the varying current from the detector 37, and the instrument thus indicates the difference between the peak voltage and the average voltage of the high-frequency wave, and can be calibrated to read percentage modulation. The steadv anode current from the detectors is balanced out from the coils 34, 43 by potentiometers 47, 48. According-to modifications : crystal rectifiers are used instead of the valve detectors, Fig. 1 (not shown), the current ratio in the differential coils is made equal to the degree of modulation by reversing one of the detectors, Fig. 3 (not shown), or by winding the coil 34 in two equal and opposite sections, one of which is in series with the coil 43, while the other carries the current from the detector 27, Fig. 4 (not shown).