Patent GB-152811-A (GB)
Title: Modulating and transmitting systems for electric signalling
IPC: H03C1/20
Filed: 1919-08-07
Abstract: 152,811. Western Electric Co., (Western Electric Co.). Aug. 7, 1919. Wireless telegraphy and telephony; electric oscillations, producing.-In a wireless transmitting system employing a thermionic oscillation generator and a thermionic modulator for modulating the oscillations in accordance with low-frequency signal fluctuations, the three electrodes of the generator are connected to points of different potential in the aerial circuit so that the frequency generated is primarily determined by the constants of that circuit. Fig. 1 shows an arrangement for telephony, and Fig. 3 an arrangement for telegraphy. The thermionic generator O has its plate connected to an adjustable point on the aerial inductance 3, its grid connected through a block condenser 7 to another adjustable point of the aerial inductance 3, and its filament earthed, a series condenser 4 being interposed between the aerial inductance and earth. The plate battery Ep is earthed at one side, and is connected to the plate through iron-cored inductance 11, high-frequency choke-coil 12, and aerial inductance 3. The coil 12 has little self-capacity, and therefore prevents the oscillations from being shunted through the battery Ep, the coil 11 being ineffective for this purpose owing to its self-capacity. The grid is earthed through a high-resistance leak path 8. The microphone circuit is connected through a transformer 18 to the grid-filament circuit of the modulator M. The transformer secondary is connected across a resistance 19, and a condenser 20 is interposed between the transformer secondary and the earthed side of the filament. The normal potential of the modulator grid is determined by a connexion to a mid-point of the grid leak 8. The plate and filament of the modulator are connected across the battery Ep and the inductance 11. The coil 11 tends to maintain constant current through itself, and variations of internal impedance in the modulator M cause variations of potential across the generator O, thus modulating the oscillations. In a modification, the plate of the generator O is connected directly to the chokecoil 12, and a condenser is interposed between the plate and the coil 3, thus preventing the battery Ep from being short-circuited if the aerial 2 should become accidentally earthed. This condenser also helps to prevent the electromotive force of speech frequency from being shunted through the coil 3 and condenser 4, whether these speech frequencies are high or low. In adapting the system for telegraphy, Fig. 3, the low-frequency modulations are supplied by a thermionic generator O<1>, and are applied to the grids of the modulators M. The plates and filaments of the modulators M are connected across the battev En and iron-cored inductance 11 supplying current to the plate circuit of the generator 0, as previously described. An oscillatory circuit comprising inductance coils 25, 28 and condensers 24, 27 is connected across the plate and filament of the low-frequency generator O<1>, and the grid of this generator is connected through a high resistance 28 and inductance 25 to the filament. The battery Ep is connected through a switch S and resistance 30 to the plate and filament of the generator O<1>. The grids of the modulators M are given a suitable normal potential by connecting them to a variable point on the grid leak 28, and a condenser 29 is interposed between the grids and the coil 25. For signalling, a key K<1> may be arranged to short-circuit a part 26 of the inductance of the low-frequency oscillatory circuit, thus altering the frequency of the modulations. An alternative method of signalling is by varying the frequency of the carrier oscillations, a key K being arranged to short-circuit a part 4<b> of the series aerial capacity. An adjustable condenser 31 may be connected across the aerial inductance 3 and capacities 4<a>, 4<b>; and a condenser 32 of large capacity may be interposed between the aerial earth G and the negative side of filaments and batteries. A blocking circuit 23 tuned to the modulating frequency may be used to prevent oscillations of this frequency reaching the grids of tubes 0 through the coil 3 and condenser 7. When signalling by the key K the modulations may be operative or inoperative, and reception is effected by heterodyning on the high frequency, giving an audible beat frequency when the key is depressed and an inaudible one when the key is raised. When signalling by the key K<1>, the modulated oscillations are first rectified. thus deriving the modulating frequency, which is then combined with a suitable locally-generated frequency.