US1730878A - Modulation system - Google Patents

Description

OC. 8, 1929. R, L, DAVlS MODULATION SYSTEM Filed Aug. 6, 1925 INVENTOR Rober l.. Dov/'S l Il 5 wlTNEssEs';

ATTOR NEY Patented Oct. 8, 1929 UNTED STATES PATENT OFFICE ROBERT L. DAVIS, OF WILKINSBURG, FENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACYEURING COMPANY, A CORPORATION OF PENNSYLVANIA MODULATION SYSTEM Application filed August G, 1925. Serial No. 48,435.

My invention relates to radio sending apparatus and particularly to y modulation methods for impressing audio-frequency vibrations upon the radiated. output.

An object of my invention is to provide a radio sending apparatus and modulating means therefor', whereby complete mochilation of the oscillation generator may be obtained without distortion.

Another object of my invention is to provide means whereby a modulator triode may be effectively worked at a higher rate of energy input than the associated oscillator. Another object of my invention is to provide a voltage transformation means between an oscillator triode and a modulator triode.

Modulation of the radio frequency out-` plied passes through the oscillator triode.

Fluctuat-ions impressed upon the grid of the modulator' triode then cause the development of corresponding fluctuations in the voltage impressed upon the oscillator triode and in the amplitude of its oscillations.

This system has been found incapable of providing complete modulation of the out-- put of the oscillator without the introduction of distortion, because the modulator triode current does not follow in strict proportion the changes in voltage imprcsseduponits grid when the current values are near the maXimum or the minimum value.

Under' usual operating conditions when no modulation is being effected, the two triodes take integrated amounts of current proportional to the average grid bias as eifected by the oscillator.

NWhen trie modulator triode operated and the current Ain it changed, causing it to take the minimum instantaneous amount of current, it does not take zero current, and the oscillator triode docs not take the total current supplied to the system. Likewise, at the other extreme, the modulator' triode does not become'a perfect conductor and, therefore, the current value through the oscillator is never reduced to zero. Because of this distribution. of current in the respective triodes, the voltage upon the oscillator never reaches either zero or twice the supply voltage and therefore complete modulation is not obtained.

In the operation of the conventional systems, the current distribution between the two triodes is not equal. With the same bias upon the grids of the modulator and oscillator triodes, when the grids are biased to the midpoint of the characteristic curves, with the generator triode in oscillation, less than half of the total current is taken by the oscillator and the modulator triode takes more than half. The total energy absorbed by the modulator and available for modulation in the oscillator is therefore more than half of the energy supplied, but since the modulation of the oscillator is controlled by the voltage produced by the modulator no greater modulation is obtained than would be obtained if equal currents were taken by both triodes.

My invention provides means comprising ka voltage transformation device between the modulator and yoscillator' triodes whereby advantage is taken of the excess energy absorbed by the modulator' triode to obtain more nearly complete modulation of the oscillation generator. By this means, I amy enabledto obtain more nearly complete modulation without the production of distortion.

Other objects and structural details of my invention will be apparent from the following description when read in connection with the accompanying drawing, wherein,

Fig. l is a diagrammatic representation of circuits and apparatus embodying a preferred form ci my invention.

Figi 5.2, shows curves of the oscillator voltage kice with and without the device et my invention, anc

3 is an alternative en'ibodiment ot my invention.

The iirst einliiodiment et my invention provides an oscillation radiating system l oonnected to a local oscillation circuit 2 which is energized by an oscillator triode 3. A modulater triode el is provided and the grid circuit thereole is connected through a transformer 5 to a microphone S and local battery 7. The cathodes ot the triodes 3 and l are energized by a current source 8.

An anode current source 9 is provided and its negative terminal is connected to the cathode circuit. The positive terminal of the generator 9 is connected to one terminal of an autoftranstorr er ll, the other terminal of which is connected to the plate circuit ot the triode 3. A. tap l0 connects the auto-transformer ll to the anode of t-riode The gen era-tor 9 is shunted by an audio-frequency condenser l2 connected across its terminals.

It the tap upon the auto-transformer ll were shitted to the terminals to which the oscillator anode circuit is connected the device would correspond to the system in common use and the trL nstorme-r ll would tunetion simply as a steadying inductance. Under these conditions, the system would open ate in the known manner and the Voltage applied to the oscillator triode would correspond to the curve a orn Fig. 2. ylhis curve which is incomplete, represents the. maximum modulation which can be obtained from such system. The curve a represents the change in Voltage in the conventional systems, with change in grid potential ot the modulator both of the modulator anode circuit and the oscillator anode circuit.

By positioning the tap upon the autotranstormer coil of the transformer ll at a point whereby two-thirds to three-fourths of the inductance is included in the modulator circuit and all ot the inductance is in the oscillator circuit, a transformer action is obtained.

Under this condition curve a of F ig. 2 still represents the change in TVoltage upon the modulator triode but the transformer action et the transformer ll increases the ettectiye Voltage applied to the oscillator 3 to produce a voltage curve corresponding to curvo o ot Fig. 2, which indicates t-he Voltage applied to the oscillator when the voltage change is that ot curve (z. By this means, the eXcess current carrying capacity ot the modulator triode over that of the oscillator triode is made to produce greater modulation or the oscillation generator. rlhis Vsystem of connections causes the production ot a small audio-frequency vibra-tory current component in the generator circuit, which is Vby-passed by the condenser l2.

Similar results may be obtained by the em- 42 veigy tiansiei bodiment shown in Fig. 3 in which parts corresponding to those ci' F ig. l bear similar numerals. ln this embodiment, the modulator triode l is supplied through a separate filter inductance l5 and the audio-frequency component is conveyed to the auto-transformer 14- and the condenser l2. The presence ot this audio-frequency component in a portion ot t ie win dings ot the transformer la results in an increased modulation voltage upon the triode 3 in a manner similar to that produced by the embodiment of Fig. l.

lhile l have shown and described but two embodiments of my invent-ion, it is capable.

of other modilications and it is desired theretore tnat only such limitations shall be imposed thereon as are imposed by the prior art as indicated. in the appended claims.

l claim as my invention :v

l. A. radio signalling system comprising an oscillation generator tube, a modulator tube, und connecting means therebetween having a roltage ratio other than unity, said modulator tube having a current capacity different trom the current capacity ot' the generator tube by an amount proportional to the voltn ratio oi said connecting means.

2. A radio signalling system comprising an esci ation generator tube, a modulator tube ot 'eater current capacity, and electric enmeans therebetween having a volta ge ratio other than unity the modulator side ir said transfer means having the larger current, whereby complete modulation of said oscillatoris obtained without such great change ot current in said modulator as to depart from the linear portion of its characteristie curve.

3. lin a radio transmitting system, a Vacuum-tube oscillator, a Vacuum-tube modulator, a common source ot plate potential for said ifacuum tubes, an inductor included between the positive terminal of said potential source and the plate ot said oscillator tube, and a connection from the plate ot said modulator tube to an intermediate point on said Inductor, whereby potential Variations across said inductor arising from changes in the impedance or' said modulator tube are stepped up and applied to the plate ot said oscillator tube.

ln testimony whereof, l have hereunto subscribed my name this 11th day ot July, 1925.

ROBERT L. BAVIS.

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