US1744214A - Modulation system - Google Patents

Description

Filed Aug. 6, 1925 5 m a m m m .m A

S a S.) m

Patented Jan. 21,1930;

,UNITED STATES PATENT OFFICE ROBERT L. DAVIS, 01 WILKINSBURG PENNSYLVANIA, ASSIGNOB TO WESTINGHOUSE I v ELECTRIC 8o MANUFACTURING COMPANY, A CORPORATION OFPENNSYLVANIA MODULATION SYSTEM Application filed August 6, 1925 i Serial No. 48,433.

My invention relates to radio sending apparatus and particularly to modulation systems therefor.

radio sending system adapted to be moduing system may be obtained by lated in accordance with audio-frequency vibrations and in which improved quality and more nearly complete modulation is obtained.

Another object of my invention is to pro- 1o vide a radio sending apparatus with a balanced modulator system.

, Another object of my invention is to provide a radio sending system in which the oscillation generator and the modulator may be independently supplied with energy.

Modulation of a triode oscillation generatthe apparatus shown in Patent -#1,442,147 to Heising which discloses the so-called constant current system. This system, however, is lim ited by the fact that it is not possible to obtain complete modulation of the oscillation generator without producing at the same time very serious distortion of the sound frequency modulation. This is by reason of the fact that the resistance of the modulation triode changes in strict linear proportionality to the voltage impressed on the rid only over a relatively narrow range 01: change in grid voltage. For this reason, neither the maximum current carrying capacity of the modulator triode nor zero current capacity is available to change the current through the oscillation generator. The conventional modulator triode system is thus seriously overloaded and complete modulation of the oscillation generator is not obtained, thereby sacrificing the distance range and the quality of the modulation of the system.

My inventionprovides means whereby two modulator triodes may be made to operate with a single oscillation generating triode and whereby the deviations of one tube from linear reproduction are compensated'by devi ations in'the opposite direction in another tube.

Other objects and structural details of my invention will be apparent from the following description when read in connection with the accompanying drawings, wherein Fig. 1 is a diagrammatic representation of form of my invention. An object of my invention is to prov1de a c in the electrical supply sources from influencing the oscillator triode, and for preventmg the presence in the core of the modulation transformer, oi a flux produced by the direct current source of plate potential.

Referring to the drawing, an oscillation radiating system "1 is provided having the customary aerial, counterpoise, and inductance elements. The radiating system 1 is connected to a local oscillating circuit 2 which is energized by the triode oscillator 3; Modulator triodes 4 and 5 are provided, the grids of which are connected to opposite terminals of the secondary coil 6 of the modulation transformer 7. The primary winding of the transformer 7 is connected to a microphone 8 and a battery 9.

The respective plate circuits of the triodes 4 and 5 are connected to opposite terminals of the primary coil 11 of an output transformer 12. The mid-point of the coil 11 is L connected to a source of electrical energy.

The cathodes of the respective triodes are en-' ergized by a generator 14.

In the embodiment of my invention shown in Fig. 1, a singlehigh voltage source 15 is provided for energizing both the modulator triodes and the oscillator triode, connection being made from it through the secondary coil 16 of the transformer 12 to the plate circuit of the triode 3 and in parallel therewith to the mid-point of the primary winding 11 of the transformer 12 and through the wmding to the respective plates of the triodes 4 and 5. A by-pass condenser 17 is provided to bypass the audio-frequency component of the plate current to the triode 3, and is connected in shunt to the generator 15, thereby allowing the audio-frequency vibratory component to flow through a circuit other than the highly inductive windings of the generator 15 and also tending to remove any current fluctuations produced by the generator 15.

In the operation of this embodiment of my invention, current is supplied from the generator 15 through the secondary coil 16 to the oscillator generator 3. Current is simultaneously supplied through the primary coil 11 to the late circuits of the triodes 4 and 5. The mlcrophone 8 produces a vibratory current under the influence of sound, which current causes the develo ment of potentials in the current coils 6, wliich potentials are of opposite polarity and are impressed upon the respective grids of the triodes 4 and 5. The otentials cause a diminution in current t rough one of the triodes 5 and 6, and

an increase in current in the other.

The resulting current changes in the halves of the coil 11 produced by the change in impedance of the respective triodes 4 and 5 causes an integrated change in the induction of the coil 11. This generates a potential in coil 16 which is twice the potential developed by the same current change through a single one of the triodes. The current changes in the triodes 4 and 5 are integrated even though they are in opposite directions- Thepotential generated in coil 16 changes the current flow through the triode 3 and thereby changes the amplitude of the oscillations generated by it in the well-known manner. The vibratory component of the current flowing in this triode is by-passed through the condenser 17. A substantially constant current flows to the tap of coil 11 at all times and fluctuations in its value are without eflect upon the coil 16 since the direction of current through the respective coils causes the neutralization of the magnetizin effect of the steady current.

The act that the grids of the two triodes 4 and 5 are changed in potential in opposite directions results in a compensation of any deviation from linear reproduction which may occur since one triode may be approaching the toe of its characteristic curve while the other triode is approaching the knee of its characteristic curve and a lessened change of conductivity of one triode is compensated b the increased change in conductivity of tlie other. Likewise, the output of both triodes is available for modulating the ouput of the oscillator 3 because a current chan e of only half the total capacity of the individual triodes 4 and 5 will result in the same degree of modulation of the oscillator 3 as is produced by system of the Heising Patent.

The halves of the coil 11 may be of the same size as coil 16, in which case, the above statement is correct, or the halves of coil 11 may be larger than coil 16, in which case a greater amount of modulation may be obtained.

The embodiment of Fig. 2 contains one generator 18 to supply the plate current of the oscillator 3 and a second generator 19 to supply the modulator triodes 4 and 5 by the system of connections shown. By this means, it is possible to employ triodes having different characteristics for modulators from those employed as oscillators and to supply them with a suitable potential. An additional filter inductance 21 is provided in the circuit of the generator 18 and the triode 3 as a means for steadyin'g the generator output. A condenser 22, correspondin denser 17 of Fig. 1, is inserted between the filter inductance coil 21 and the cathode circuit. An ammeter 23 may be-inserted in series with the condenser 22 and, when properly calibrated, will indicate the percentage of modulation of the generator.

The operaton of the embodiment of Fig. 2 is substantially similar to that of Fig. 1 exto the con- I cept that two separate current sources are 7 provided.

The embodiment of Fig. 3 shows a construction adapted to embody a single plate current generator for both modulator and oscillator with means for removing any generator fiuctuations from the plate current of the oscillator triode 3. In this embodiment, a terminal of the secondary coil 16 is connected through a condenser 24 directly to the cathode circuit of the oscillator 3. The generator 25 is then connected between the cathode circuit and the mid-tap of coil 11 directly, and the generator 25 is also connected through a filter choke inductance 26 to the plate circuit of the oscillator 3.

In the operation of the embodiment shown in Fig. 3, the modulator portion functionsin a manner substantially similar to that of Fig. 1, and the vibratory energy induced in the coil 16 is impressed upon'the plate circuit of the triode 3 through the condenser 24. The plate circuit is then supplied with a steady current through the filter choke inductance 26. This construction prevents the presence in the core of the transformer 12 of any flux produced by the direct current source of potential for the plate of oscillator triode 3, as none of the oscillation direct current su ply passes through the coil 16 and the stea y currents in the one half of coil 11 neutralizes that of the other half. 7

By the means of construction shown, I am enabled to provide in a radio sending system modulating means whereby complete modulation ofthe oscillation generator is obtained without overloading the oscillator triodes and whereby deviations of the characteristics of the modulators are compensated for, and whereby a plurality of plate energy sources may be employed.

While I have shown but three embodiments of my invention, it is capable of other modifications without departing from the spirit thereof and it is, therefore, desired that only such limitations be imposed thereon as are imposed by the prior art or indicated in the appended claims.

I c aim as my invention:

1. In a radio system, an oscillator tube,

two modulator tubes, a source of late current, a transformer, connections .rom said source to the respective plates of said modulator tubes in parallel, said connections including a mid-tap to the primary of said transformer, and connections for the plate of said oscillator tube includin said source and the secondary of said trans ormer in parallel, the branch of said last-named connection including said secondary also including means for preventing the passage of direct current.

2. In a radio system, an oscillator tube, two modulator tubes, 3. source of plate current for said modulator tubes, a transformer having a primary winding provided with a center tap, a secondary winding, said secondary wlnding being included serially with a block ing condenser between the plate and the cathode of said oscillator tube, and a connection between the positive terminal of said source and said center tap, whereby said transformer is prevented from saturating.

3. In a radio system, an oscillation generator, a plurality of modulator devices, connec o tions whereby said devices influence said 0scillation generator in push-pull relation, said connections including a three-winding transformer, and means for preventing the flow of direct current in the secondary winding of as said transformer, whereby saturation thereof is prevented.

In testimony whereof, I have hereunto subgcribed my name this 11th day of July, 192

4o ROBERT L. DAVIS.

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