GB407935A - Improvements in or connected with variable reactances and circuits including variable reactance - Google Patents

Abstract

407,935. Valve modulating circuits. DEHN, F. B., 103, Kingsway, London.- (Elwell, C. F., and Terman, F. E. ; Stanford University, California, U.S.A.) Sept. 19, 1932, No. 26057. [Class 40 (v).] The input reactance of a thermionic valve is varied in a cyclic manner by imposing similar variations on a positive or negative resistance connected in its plate circuit. The resistance may take the forms described in Specifications 407,933 and 407,934 and it may be varied at a carrier frequency. If a modulating frequency is applied to the valve input, side-frequencies without the carrier may be picked up from the input circuit. Fig. 1 shows the principle of the invention, the grid and plate of a valve 5 being coupled by a condenser 7, and the plate voltage being applied through a choke 10. The variable resistance 11 is connected across the plate and filament in series with a blocking condenser 12. As the resistance 11 varies, the effective input reactance between the terminals 1, 2 is varied ; and if a signal voltage is applied at the terminals 1, 2 and the resistance 11 is varied cyclically at carrier frequency, side-bands or side-frequencies will be produced in the input circuit. In Fig. 2, the grid and anode of the valve 5<1> are connected by an inductance 15 in series with a condenser 16. The variable resistance in the plate circuit of valve 5<1> comprises the plate-filament path of a dynatron valve 17. This valve has an outer grid 22 maintained positive by connexion to a battery 8<1>, and the plate voltage has a mean zero valve maintained by the coil 19. The carrier wave is applied to its control grid 25, thus varying the slope-resistance of the plate-filament path, and so varying the reactance across the terminals 1<1>, 2<1>. A block condenser 20 may be connected between the plates of the valves 5<1>, 17, or they may be directly connected so as to operate at the same battery voltage. The specification states that if the dynatron is adjusted to give a negative resistance in its plate-filament path, then the reactance at the terminals 1<1>, 2<1> will be negative, and will be either inductive or capacitative according as the coupling between the grid and plate of the valve 5<1> is predominantly inductive or capacitative. These negative reactances may be such as to balance any positive reactance in the input circuit, and thus give the input circuit zero reactance over a range of frequencies.