US1790576A - Modulating system - Google Patents

Description

Jan. 27, 1931. c. A. CULVER 7 MODULATING SYSTEM Filed July 16, 1926 I N VEN TOR.

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Patented Jan. 27, 1931 UNITED STATES igaasit PATENT :oFFic CHARLES A. CULVER, or NORTHFIELD, MINNESOTA, ASSIGNOR, BY MESNE nssien mnnrrs, TO FEDERAL TELEGRAPH CQMPANY, A CORPORATION or CALIFORNIA MODULATING SYSTEM Application filed July 16,

lation circuits for the output of a high frequency electrical oscillator, and more particularly to a system of modulation for use in connection with radio broadcasting.

An object of this invention is to effect an improvement in the means whereby voice generated potentials may be made to control the high frequency output of a radio transmitter, and more particularly to provide a more flexible system whereby the potential on the oscillator and modulatortubes may be independently controlled.

Another object is to reduce the amplitude of the non-signalling carrier wave.

Still another object of the invention is to reduce the non-signalling plate current in the transformer supplying plate potential to the power tubes to a minimum, thus avoiding distortion due to high flux density.

My invention will be more fully understood from the specification hereinafter following by reference to the accompanying diagram which shows the arrangement of a a radio transmitter employing the prlnclples 5 the output of which is applied to the plate circuit of the oscillator above indicated through the transformer T This modulator organization consists of tubes VT and VT together with the usual accessory parts as shown. The condenser C is of low impedance to currents of audio frequency and serves to insulate the grid of the power tube VT from the constant plate potential but does, however, permit the passage of any currents of audio frequency which may obtain in the secondary of the transformer T A common source of positive constant potential B supplies both the plates of the modulator tubes and the power tube. However, it will be understood that the power tube and 1926. Serial No. 122,985.

the modulator tubes may have independent potential supplies if so desired.

Voice generated potentials are impressed on the grid of the modulator tubes VT and VT by means of the microphone M actingin the usual manner in the primary of the transformer T or through suitable audio frequency amplifiers. The variations in the grid potential of the tubes VT and VT; thusefiected cause avariation in the plate current I through said tubes and hence through the primary P of the transformer T The ratio of the turns in the primary P and secondary S of the transformer T are so chosen that, because of the variations in the primary current I there will be developed an alternating potential of audio frequency in the secondary of said transformer of suitable valve topro-perly operate the power tube VT Under'certain circumstances it may be found advisable to apply to the plate of the power tube VT a constant potential of suflicient magnitude to cause feeble oscillations to obtain. The exact value of this constant potential is in any given case controlled by the series reistance lks shown in the drawings the high frequency power tube has impressedupon it a relatively low direct current potential from source '13, this direct current potential being of a value just great enough to maintain the tube in an oscillating condition. The working potential is supplied by the transformer T The input side of T constitutes the output circuitof a balanced amplifienbut the balanced feature is not essential. ever, possesses certain advantages, particularly'in that it makes possible the. working of the iron at a relatively low non-signaling 7 flux density value. The chief point is that the working anode potential is not supplied from a battery or direct current generator but from a source of potential which gives an alternating wave form corresponding to the modulating sound wave, and that said potential is produced bymeans of aitube or tubes acting as Variable resistances in the primary of said transformers.

As a result of the operation of the various current and audio frequency alternating current components of the plate currentof the power tube may be adjusted independently of one anot ier. i

Another advantage of my invention is that,

: because of the balanced condition in the prihigh flux density will be absent.

mary of the transformer T and the very low or zero direct current value in the secondary,

the iron of the transformer T will be'worked at a relatively low permanent flux density and hence distortion of the wave form due to In previous attempts to design an arrangement whereby the plate of the power tube might be supplied with an audio frequencypotential, there has existed a direct current component in the primary of the transformer supplying that potential. q

Still another advantage of this system of modulation is that, because of the very low constant plate current component in the power tube the amplitude of the carrier wave is maintained at an extremely low value, and in fact may, if desired, bereduced to zero. As a result of this condition the general interference caused by the non-signalling radiation is substantially reduced, and in addition the over-all efliciency of the transmitting organization is increased. I

Having described my invention in one of its preferred embodiments, I desire that it be understood that modifications may be made and that no limitations upon the invention are intended other than are imposed by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of therUnited States is as follows: V

1. A modulation system comprisinga plurality of electron tubes each having grid,;iilament andplate electrodes, input and output circuits interconnecting said electrodes with the circuits of one of said electron tubes mutually coupled and connected with a source of potential for the generation of high frequency oscillations, a transformer having primary and secondary windings with the secondary winding thereof connected in circuit with; said first mentioned electron tube, one of the terminals of said secondary winding being connected directly to the plate electrode of said first mentioned electron tube, a microphone circuit, a pair of said electron tubes being connected in balanced relationship and disposed between said microphone circuit and the primary winding of said transformer, whereby variable audio frequency currents may be impressed upon said first mentioned electron tube and superimposed upon the potential supplied by said source for controlling the amplitude of the high frequency currents generated in the circuits thereof, and means for controlling said source of potential independently of said variable'audio frequency currents.

2. A modulation system comprising a plurality of'electron tubes each having grid, filament and plate electrodes, input and output circuits interconnecting said electrodes with the circuits of one of said electrontubes mutually coupled for the generation of high frequency oscillations, a transformer having pri- 'mary and secondary windings, with the sec,

said source of potential being adjustably controlled independent'of current in said primary winding, said source connected wlth said mid-tap in said primary winding and with the output circuit of said first men-v tioned electron tube and operating to sustain oscillations at a predetermined amplitude whereby variable alternating current may be impressed upon said first mentionedelectron tube and superimposed upon the oscillations sustained therein for controlling the amplitude of said oscillations in accordance with the operation of said microphone'circuit.

3. Asystem of modulation comprising a plurality of electron tubes each having'grid, filament andplate electrodes, input and-output circuits interconnecting said electrodes, with the circuits of one of said tubes mutual 1y coupled for the generation of high frequency oscillations, a transformer having pri mary and secondary windings, with the secondary winding thereof connected in circuit with the first mentioned electron tube, a mi lie crophone circuit, a pair of electron tubes concontrolling the value of the potential impressed upon said plate circuit independently of controlling the intensity of the Variations in audio frequency current supplied to said first mentioned electron tube by virtueof the operation of said microphone circuit.

In testimony whereof I alfix my signature.

CHARLES A. CULVER.

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