US1795714A - Absorption control modulating system - Google Patents

Description

March 10, 1931. DELQRAINE 1,795,714

ABSORPTION CONTROL MODULATING SYSTEM Filed Sept. 20', 1926 lm/exrfor v Edmond M D/Uf'fl/ll Patented Mar. 10, 1931 UNITED STATES PATENT OFFICE A EDMOND M. DELORAINE, OF BLACKHEATH, LONDON, ENGLAND, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, 015 NEW YORK, N. Y., A. CORPORATION OF NEW YORK ABSORPTION CONTROL MODULATING SYSTEM Application filed September 20, 1926, Serial No. 136,502, and in Great Britain September 21, 1925.

This invention relates to modulating systems and more particularly to a modulating system of the type delivering a carrier wave of variable amplitude.

A method of modulation heretofore employed has consisted of an oscillator tube delivering a constant power output, of which, a variable fraction being absorbed, a variable amount of energy has been delivered to a load circuit. The system employing such a method is known as an absorption modulator. The efficiency of such a system has been found to be low, being of the order of {-6 for 100% of sinusoidal modulation of the carrier wave,

where e is the eificiency of the oscillator.

According to the present invention the said absorbed energy is caused to react upon the source of oscillations to augment modulation, this reaction effect preferably taking a different specific form of the method of modulation described in British Patent No. 133,366, or British Patent No. 168,056, corresponding respectively to the United States patents to Heising Nos. 1,442,147 and 1,537 ,941, and known as plate modulation.

According to a feature of this invention the space current to a power amplifier may be varied rather than the space current to the oscillator.

A more pronounced modulating effect may be obtained by applying modulation to the oscillator only or to both oscillator and power amplifier, but the preferred arrangement has the advantage of stabilizing the performance of the oscillator where such is of the thermionic type while still admitting of a high efficiency of modulation.

A feature of the system disclosed by this invention is, therefore, an overall efficiency 40 which is above that attained by any modulating system heretofore disclosed, the system having also the further feature that the modulating variations may be directed to a part of the system which leaves the osc llator free from interference due to modulatlon so that the carrier frequency is kept constant.

The reaction of the said absorbed energy may be effected either through a transformer 5 or by means of a condenser. In the latter case during absorption modulation the absorbed energy is admitted to a condenser of large capacity through valves controlled by the modulating wave, the said condenser bemg charged during one-half-cycle of the modulating wave. In the next half-cycle the absorbed energy is released and carries on the modulation of the carrier on the plate modulation principle, augmenting the space current to the oscillator or power amplifier or both as the case may be.

One form of the invention which is described hereafter involves the use of a transformer, the inductance of which tends to make up for any discrepancy between modulation during one half-cycle of the modulatmg wave and the next half-cycle, thus rovidmg acheaper and better means of e ecting the result described in connection with the condenser.

The system will be better understood from oscillator or preferably a high frequencyann plifier tube 3, driven from a separate oscillator 2. The amplifier tube 3 delivers high frequency power to the load circuit 7. Part of this high frequency energy may be taken from the load circuit through a coupling 4 to a rectifying device 5. This latter rectifier is controlled at audio frequency by the microphone 9 acting through an amplifier 8. The rectifier 5 introduces an impedance in the load circuit which follows the variations of amplified speech currents, and consequently draws high' frequency energy at speech frequency. The rectified speech currents are applied through a speech frequency transformer 6 to the plate of the high frequency amplifier 3, the windings being connected in the correct direction in order to produce further modulation of the radio frequency wave already modulated by absorp- Inwl CPI

tion of energy in the output of the high frequency amplifier.

in Fig. 2 the driving oscillator tube 2 which may be of the well-known Colpitts type is coupled through the transformer 11 to the grid of the amplifier tube Plate voltage is supplied to tube 3 from the D. C. generator 1 through one winding of transformer 6 and a high frequency choke coil 10. The output circuit H is coupled to the antenna system 157, and also through inductance 4 to the rectifier tube 5. This tube is of the three-electrode type and its grid is normally kept highly negative by the cells 13. Voice frequency curents generated at the microphone 9 are amplified through the amplifier tube 8, and impressed through transforn er 12 on the grid of rectifier 5.

In the operation of the system it will be seen that if we assume that the amplified speech currei'its from the microphone 9 at a certain instant produce a decrease of the impedance of rectifier 5, a current would flow in the primary of transformer 6 due to the absorption of power through coil 4, magnetizing the iron core and producing as it builds up an elcctromotive force in the secondary circuit of transformer (i, which is in opposition with the electromotive force from the source 1.

At the next instant speech currents produce an increase of the rectifier impedance. The rectifier action ceases and the current in the primary of the transformer (S then dies out. The core of the transformer 6 becomes (lo-magnetized and the variation of flux produces an clectromotive force in the secondary circuit which is in series with the electromotive source from the source 1. 111 other words, at the same time that the amplitude of the carrier is increased in the process of modulation, the voltage supplied by the source 1 is applied to the plate of the tube 3 in series with the voltage across the secondary of transformer (3. At the same time the rectifier 5 presents a high impedance in respect to the load circuit l fe-l5. and draws only a small amount of energy through coil f'. The amplitude of the carrier wave is decreased by modulation when the impedance of the rectifier is low, and a large part of the high frequency energy delivered in the output of the high frequency amplifier 3 thus goes into the rectifier, producing a voltage across the secondary of transformer (S which opposes the voltage supplied from source 1. The modulating eifects of-transformers 4 and 6 thus add to each other, as they control in cumulative manner the space current input and the output of the high frequency amplifier.

It is clear from the above that the invention is not confined to the circuit arrangement disclosed in the figures but may be embodied in systems and in different forms varying considerably from that here given by way of illustration, the scope of the inventiou being only limited by the appended claims.

\Vhat is claimed is: Y

1. The method of modulating a high frequency wave in accordance with a wave of lower frequency which comprises absorbing at least a part of the energy of alternate half cycles of the said high frequency wave in accordance with the lower frequency wave, and manipulating the said absorbed energy to augment modulation during such absorption and to effect a variation of amplitude of said high frequency wave in an opposite sense in the remaining half cycles thereof.

2. The method of modulating a high frequency wave produced by a thermionic oscillator and amplified by a. space discharge tube which comprises variably diverting the energy from the output circuit of said amplifier tube in accordance with a low frequency modulating wave, variably absorbing said high frequency energy from the output of said amplifier tube in accordance with the modulating wave and impressing the said absorbed energy on the space current circuit of said amplifier thereby to effect plate modulation.

3. The method of modulating a high frequency wave produced by a thern'iionic oscillator and amplified by a space discharge tube which comprises variably diverting the energy from the output circuit of said amplifier tube in accordance with a low frequency modulating wave, rectifying said diverted high frequency energy, and thereafter causing the rectified energy to add to and substract from the space voltage applied to the said amplifier at the modulating frequency.

l. A modulating system comprising a source of high frequency oscillations delivering high frequency power to a load circuit. a source of low frequency current, an absorbing circuit associated with the said load circuit and controlled by the last mentioned source, a transformer fed from said absorblng circuit and means associating said transformer with the first mentioned source in such a manner that cumulative modulation of the said power to the load circuit takes place due to the said absorption and reactionf 5. A modulating system according to claim 4- characterized by the provision of a threeelectrode rectifier tube of which the controlling circuit is associated with the said source of low frequency current and the anodeeathode circuit is associated with both the said absorbing circuit and the prime energy source of said high frequency source of oscillations.

6. A modulating system comprising a power oscillator tube delivering high frequency power through a power amplifier tube to a load circuit, a low frequency wave source, modulating means comprising in part an absorbing circuit associated with the space current source of said amplifier tube to va the amplitude of the power delivered to the load circuit in accordance with the low frequency wave, a three-electrode rectifier tube of which the controlling circuit is associated with the said source of low frequency current and the anode-cathode circuit is associated with the said absorbing circuit and the space current source of the amplifier, the control electrode of said rectifier being so polarized that the impedance of the anode-cathode circuit is raised to a maximum value at one peak value of the low frequency cycle and decreased to the corresponding minimum at the next low frequency peak value of opposite sign.

7. The method of modulating a high frequency wave in accordance with a lower frequency wave which comprises absorbing at least a portion of alternate half cycles of the high frequency energy in accordance with said lower frequency wave, reproducing the lower frequency wave from the resultant modulated absorbed wave, and using said reproduced wave, independently of said absorption process, but cumulatively with respect to it, to modulate each half cycle of said high frequency wave.

In witness whereof, I hereunto subscribe my name this 1st day of September A. D.,

EDMOND M. DELORAINE.

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