US1565847A - Circuits for the production of pure modulated carrier waves - Google Patents

Description

Dec. 15, 1925- ,565,3 7

R. E. CORAM CIRCUITS FOR THE PRODUCTION OF PURE IODULATED CARRIER WAVES Filed Sept. 18. 1923 Patented Dec. 15, 1925.

UNITED STATE s PATENT OFFICE.

ROY E. CORAM, 0F NEWARK, NEW JERSEY, AB SIGNOB TO WESTERN ELECTRIC CO!- IPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION 01' NEW YORK CIRCUITS FOR THE PRODUCTION 0! Application illed September 2 '0 all whom it may concern Be it known that I, Roy E. Comm, a citizen of the United States, residing at Newark, in the county of Essex, State of New Jersey, have invented certain new and useful Improvements in Circuits for the Production of Pure Modulated Carrier Waves, of which the following is a full, clear, concise, and exact description.

This invention relates to modulating circuits and more particularly to circuits for the production of pure modulated carrier waves.

The ordinary circuit for modulatin a carrier wave in accordance with a Sigma or other control wave produces a complex resultant wave comprising a pure modulated wave and an unmodulated component of the carrier wave frequency. It is frequently desirable to produce a modulated "carrier wave having no unmodulated carrier frequency component. An. example of a circuit for accomplishing this purpose is the balanced modulator of J. R. Carson Patent No. 1,449,882, March 27, 1923. It is an object of the present invention to provide a circuit for producing pure modulated waves with the simplest possible apparatus.

According to the present invention, signal -or other modulating waves are impressed upon the input circuit of an electron discharge device of a well-known three-element type, preferably highly evacuated. A source of carrier waves is connected to the discharge electrodes and to a path including an equivalent impedance in such manner as to supply alternating currents in par-' allel to the variable impedance between the discharge electrodes and to a constant impedance. A signal transmission circuit is differentially associated with the alternating current paths including the discharge electrodes and the fixed impedance element and accordingly receives no current as long as the impedance balance is maintained. When,

however, the impedance of the discharge device is varied by incoming signals, the

balance is disturbed and carrier waves'corresponding to the resulting difierence currents in the two alternating current paths,

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as to the details of its organization and its mode of operation will be best understood by reference to the following description taken in connection with the accompanyin drawing in which Fig. 1 illustrates a re. io telephone system embodyin the invention, and Fig. 2 illustrates a modi cation of the modulator circuit of Fig. 1.

Referring to Fig. 1 a source of speech or other control currents represented by the microphone 1, in series with a battery 2, is associated by means of the transformer 3 with the input circuit of a ,three-element electric discharge device 4 of well-known type. Connected in series in the input circult of the grid or impedance control element 5 and the cathode 6, are the secondary of transformer 3, a choke coil 7' and a polarizing battery, 8 for maintaining the norimpedance 15. Each of these circuits includes a large blocking capacity element 16.

The .two alternating current circuits or branches are connected by means of threewinding transformers 17 to a transmission circuit 18, in such manner as to induce opposing electromotive forces therein. The circuit 18, which is coupled to a transmission circuit 19 by transformer 20, includes a band filter 21 which passes with low and substantially uniform attenuation, fre uencies corresponding to one side band 0 the modulated carrier wave and substantially extinguishes currents outside this frequency range.

In an actual operation of this circuit, the device 4 consisted of a standard tele hone repeater tube. This tube was operate with a negative grid potential impressed by means of source 8, so that its alternatnig current impedance between the cathode and anode was of approximately 6000 ohms. The condenser 16 in the alternating current path between the cathode and anode had a capac- LOO ity of one microfarad, and the condenser 16 in the other branch was of like ca acity.. It

- was found in operation of this evice, that without the inductance device 7, there was atendency for an alternating electromotive force of the carrier frequency to be induced in the grid circuit leads, and to be impressed upon the input elements of the discharge device 4, thus giving rise of an amplified carrier frequency current in the alternating current plate circuit path, in addition to that directly impressed on this path by source 12. To prevent this effect, the element 7 having inductance of approximately 10 henrys, was inserted in t 1e grid circuit. The. potentiometer 13 enables the alternating electromotive force supplied to the discharge device to be made such that the device is not overloaded, thus making it ssible to eliminate, to a large degree, a istortion giving rise to resulting undesired frequency components which will not be neutralized by the current through fixed impedance.

The band filter 21 may be designed in accordance with well-known principles, as, outlined in United States patent to G. A.

-Cam bell,.No. 1,227,113, May 22,1917. The

circult 19, although illustrated as an antenna, may obviously be a transmission conductor of any sort, and the transformer 20 may be replaced by any suitable coupling.

In operation, the alternating currents induced in the two paths traversing space discharge device 4 and the impedance element 15 are normally equal and in phase. In consequence of the differential connection of the transmission circuit 18, the electromotive forces induced therein are oppositely directed and neutralized. When speech or other signal control currents are impressed by means of the transformer 3 upon the input circuit of device 4, thus varying its impedance, the current in this branch alternately increases and diminishes,

thus destroying the balanced condition with respect to circuit 18. There will accordingly be impressed in circuit 18, a carrier wave of the frequency of source 12, the amplitude of which is'proportionate to the variation currents impressed upon the input circuit of device 4. The modulated carrier .wave supplied in the transmission circuit 18 com rises, asis well known, an upper side ban and a lower side band, equally removed in frequency, from the frequency of the carrier source 12. In order to improve the quality of signals received at the receiving stationv and to economize in the frequency range of the waves transmitted,

the filter 21 is provided to transmit one of the side bands and to eliminate waves of other frequencies including the other side hand. To an extent, this fi ter 21 may serve to suppress any unmodulated component of the carrier frequency, should the impedance 15 fail to match the normal impedance of device 4. However, the percentage frequency difierence between the carrier frequency and the contiguous frequenciesfof the side band to be transmitted, is, in general, small, and it is therefore very desirable to make the balance as accurate as possible. For this purpose, the resistance 15 may preferably be variable.

Fig. 2'shows a similar arrangement with elements similarly designated. A coupling circuit between the signal source and the inputcircuit of'tube 4 includes a potentiometer 31 to regulate the magnitude of the modulating current impressed thereon. To generate cagrier current, there is provided a vacuum tube oscillator comprising a space discharge device 32 inthe plate filament circuit of which there is-included a closed oscillatory circuit 33, tuned'to the desired carrier wave frequency and coupled by transformer 34 in feed-back relation-to its input circuit. vThe cathode of oscillator 32, preferably heated from the source 35, is in series with the cathode of modulating device 4. Space current is supplied to the discharge devices 4 and 32 from the common source 9, througllli the choke coils 10 and36 respectively, w 'chv prevent by-passing of carrier. frequency current to thefspace current source 9. A resistance 37 in the alternating current plate circuit path of the oscillator is provided to stabilize the oscillations of the oscillator and prevent overloading. A potentiometer resistance 38 is connected between the cathode of discharge device 4 and the midpoint of the windings of transformer 17 in the same relative position as the secondary winding of transformer 14 of Fig. 1. The oscillator 32 impresses an alternating electromotive force between the common cathode connection and the contact of a variable tap 39 on the potentiometer resistance 38. A blocking condenser 40 of large capacityin series with the tap 39 prevents transmission of direct current through the resistance element 38. This element is preferably of large impedance as, for example, 40,000 ohms. The operation of this "connected thereto, one path 0 discharge impedance element, a constant impedance of substantially equivalent magnitude, and two circuits differentially connecting said source to said work circuit and including respectively said variable electric discharge impedance and said constant impedance, whereby alternatin current is supplied to said work circuit on y during and in proportion to variations in said impedance.

2. A modulating system comprising a source of carricr waves, a twoath circuit which includes a variable thermionic impedance, and the second path of which includes a constant impedance simulating in magnitude the nor-' ma impedance of said first-mentioned path, and means for deriving from said circuit a current proportionate to the difference between the currents in the two paths.

3. In combination, a source of alternating current, a three-element electric discharge device having discharge electrodes and an impedance control element, a metallic circuit having an impedance comparable in magnitude to that between said discharge electrodes, means for im ressing an alternating electromotive force ii'om said source u u said discharge electrodes and a like e ectromotive force upon said circuit, a work circuit, and means for impressing upon said work circuit an electromotive force corresponding to the difference between the alternating current in said impedance circuit and that'between said discharge electrodes. k

4. In combination, an electric discharge device having discharge electrodes and means for controlling the im ance therebetween, a closed metallic pat of predetermined impedance, a source of alternating current associated with said discharge device and said path to sup 1y current thereto in parallel, and a wor circuit'difierentially connected to said device and saidpath whereby difierences of current traversing said device and said path produce a resultant current in said work circuit.

, 5. A system comprising an electric discharge device having discharge electrodes and means for controlling the impedance therebetween, a source of space current for said device, a two-branch circuit including said discharge device in one branch and a constant impedance equivalent to the normal impedance of said device in the other branch, a common source for supplying alternating current to both of said branches, and a work circuit to which said branches are connected in' opposition whereby current is supplied to said work. circuit only when the impedance controlling means changes the impedance of said device from its normal value. a

6. A signal system comprising a source of signaling currents, a source of carrier currents, an electric discharge device having an anode, a cathode, andan impedance control element, an input circuit associated with said source of signaling current and connected to said cathode andi'mpedance control element, ,a second circuit connected with said cathode and-anode and associated with ROY E. CORAM.

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