Patent GB-407933-A (GB)
Title: Improvements in or connected with modulating or frequency changing systems for radiosignalling and like purposes
IPC: H03C1/52
Filed: 1932-09-19
Abstract: 407,933. Modulating and frequencychanging circuits. DEHN, F. B., 103, Kingsway, London.-(Elwell, C. F., and Terman, F. E.; Stanford University, California, U.S.A.) Sept. 19, 1932, No. 26055. [Class 40 (v).] Modulation or change of frequency in radio reception or transmission or for other purposes, is effected by imposing one frequency on a circuit containing a valve, the resistance of which is varied at a second frequency. Sum and difference frequencies are produced in the circuit, and the second frequency may be entirely eliminated. The valve may be a dynatron valve operating as a negative resistance. Fig. 3 illustrates the principle of the invention, one frequency being supplied by a source 1 to a circuit comprising a condenser 3 inductance 5 and variable element 2. The condenser and inductance are tuned to the frequency at which resistance 2 is cyclically varied ; and a choke coil 6 may shunt the condenser 3 to allow passage for currents of lower frequency from the source 1. The output is taken from across the coil 5. If the conductance of the element 2 varies sinusoidally about a mean value, the output current includes the sum and difference frequencies, but does not include any current of the frequency of variation of the element 2. In Fig. 6, the variable element comprises the plate-cathode path of a four-electrode valve 16 arranged as a dynatron. Its outer grid is given a high positive potential, and its plate a lower potential, by a battery 20. The inner grid is connected to a wave source 25. If the source 1 is of low or audio frequency the source 25 may be of carrier frequency, and the modulated wave may be taken off at the terminals 27, 28. The plate-cathode path has a negative resistance characteristic over a certain range of plate voltage, and the valve may be operated on this portion of the range or on the positive resistance portion. The working point is so chosen, that the voltage variations imposed by the source 25 do not affect the plate current, but only affect the slope of the plate-current plate-voltage curve. Instead of placing the dynatron directly in the circuit of the source 1, it may be placed in a coupled circuit. (Fig. 8 not shown). For heterodyne reception the source 1 is replaced by the tuned aerial circuit of a radio receiver, the source 25 is the local oscillator, and the beat or intermediate frequency is taken off by a transformer connected across the series condenser 17. (Fig. 7 not shown). Fig. 9 shows a modified heterodyne receiver, in which a soft triode valve of the kind described in Specification 256,279, [Class 40 (v)], is used as the variable element. The aerial is coupled to a tuned circuit 53, 54 which is connected in series with a condenser 56 and bias battery 57 across the grid and filament of the valve 55. The local oscillation is applied to the plate circuit through the coupling 60, and the beat frequency taken off by a transformer 61 connected across the grid circuit condenser 56. Specifications 184,875, 186,138, [both in Class 40 (v)], 407,934, and 407,935 also are referred to.