Patent GB-155789-A (GB)
Title: Improved method of and apparatus for converting direct current into alternating current by electrostatically contrilled valve oscillators
IPC: H02M7/533
Filed: 1920-12-23
Abstract: 155,789. Hazeltine, L. A. Nov. 28, 1919, [Convention date]. Electric oscillations, producing.-Relates to thermionic apparatus for producing electric oscillations for wireless transmission and like purposes, and comprises means for improving the efficiency of the apparatus by reducing the internal losses in the valves, and also means for preventing or minimizing harmonics in the main oscillatory circuit. The first object is effected by adjusting the valves- and circuits so that the plate current is large and uniform during that part of the cycle in which the plate voltage is small, and the plate current is small when the plate voltage is high. The plate current thus has a stepped form and includes harmonics, which are eliminated or reduced in the main oscillatory circuit or aerial by the use of a pair of valves and balanced circuits. The power valves may have their grid circuits energized by separate oscillators, or by reactive couplings to the plate circuits. Fig. 2a shows a pair of valves 3, 4 excited bv a separate oscillator 1. The grids are connected to opposite ends of the input secondary coil which has its mid-point connected through a battery Ec to the filaments. The plates are connected to opposite ends of a coil coupled to the aerial 10 and having its mid-point connected through the high potential source to the filaments. The plate-current grid-potential characteristic for each valve has the form shown in Fig. 1<a>, and the normal grid potential Ec is so chosen that the curve deviates symmetrically, on either side of Ec, from the straight-line characteristic shown in dotted lines. A sinusoidal grid potential then produces a plate current in which the odd harmonics are eliminated. The balanced arrangement of the valve circuits eliminates the even harmonics owing to the plate currents being displaced in phase 180 degrees, the aerial current being of sinusoidal form. The correct voltage for the battery Ec and the correct balance of the circuits can be determined experimentally by coupling to the output circuit a heterodyne detector tuned to the various harmonics in turn, and adjusting till the harmonics are a minimum. A single valve with central filament, two grids, and two plates may be used in place of the two valves 3, 4. Approximately similar results may be obtained with an oscillator in which the grids are excited by reactive coupling to the plate circuits. The even harmonics in the plate circuits of the valves 3, 4 may be reduced by inserting an impedance in series with the high potential source. The same purpose is attained by the arrangement shown in Fig. 8, in which two coils 26, 27, which may be coupled, are inserted between the source and the halves of the transformer primary 9. In addition, coils 28, 29 are provided with a condenser 30 between them and are tuned to the fundamental frequency. Another arrangement shown in Fig. 9 comprises coils 28, 29 in the plate leads closely coupled with such relative polarity as to offer high impedance to even harmonics. Odd harmonics are shunted from the coil 9 by a condenser 31. Fig. 9 also shows an impedance coil 32 in series with the grid battery Ec, causing even harmonics to appear in the grid voltage. The coil 32 may be magnetically coupled to the plate coils 28, 29; or the battery Ec and coil 32 may be replaced by a resistance, which may be connected directly between the filaments and the mid-point of the input coil, or may be divided into branches 33<1>, 33<11> connected across the filaments and grids as shown in Fig. 12. If the valves 3, 4 have sufficient ionization, the internal leak between grids and filaments may enable the resistances 33<1>, 33<11> to be dispensed with. Fig. 14 shows a reactivelycoupled oscillator with a coil 25 common to the two plate circuits, and a coil 32 and resistance 33 common to the two grid circuits, the coils 25, 32 being coupled. The coils 25, 32 cause even harmonics to appear in the plate and grid potentials, and the resistance 33 gives the grid a normal potential due to the direct current flowing therein. The normal grid potential thus varies with the load. The circuit arrangement shown in Fig. 14 may be applied to valves having a mercury cathode. Particular harmonics of the odd series may be shunted from the output transformer 9 by placing divided coils in series with the condenser 31 and tuning to the frequency of the harmonic. Fig. 16<a> shows a grid-potential plate-current characteristic suitable for reducing internal losses, and Fig. 16<b> shows the corresponding curves of grid potential eg, plate current ip, and plate voltage ep. obtained or approximated to by the arrangements shown in Figs. 9-14. The plate current is zero during the periods of high plate voltage, and the plate current is uniform during the periods of low plate voltage. In a modified circuit arrangement, closely-coupled coils, functioning similarly to the coils 25, 32, Fig. 14, are placed in the leads adjacent to the plates and grids. The resistance 33 may be shunted by a large capacity. In another modification, the coil 25 in the plate circuit is shunted by a negative resistance such as an arc or a valve. In order to supply three-phase currents to a load circuit, three valve-pairs adjusted to the same frequency and same output voltage may be coupled to three coils connected in series in a closed circuit. The three output leads are connected to points between the coils. The three-phase current may be converted by known methods into any desired polyphase cutrent, and may be used for directive wireless transmission. Reference has been directed by the Comptroller to Specifications 147,851 and 150,037.