US2281020A - Modulation - Google Patents

Description

Patented Apr. 28, 1942 MODULATION Lothar Bl'iick, Berlin,

7, assignor to Tele- German runken Gesellschaft fiir Drahtlose Telegraphic, Berlin, Zehlendorf, Germany, a corporation of Germany Application 3 Claims.

This application concerns a modulation circuit organization for radio-frequency transmitters comprising a rectifier. The present invention is an improvement of the system disclosed in Brucks application #336,546 filed May 22, 1940 now United States Patent No. 2,269,140, dated January 6, 1942.

A modulation circuit arrangement for radiotrequency transmittershas been disclosed in the said Bruck application which follows a straightline law up to very high modulation percentages. The disclosure of the said application is predicated upon a modulation scheme known'in the prior art in which a rectifier, for example, a diode, is connected'in series with a load resistance and the series combination connected to an oscillatory circuit included in the plate circuit of the characteristic of-the rectifier, direct current as a function of the direct-current potential for various radio-frequency potentials fed to the preceding tube, is covered by the modulating audio frequency as symmetrically as possible. (3) The modulated radio-frequency potential is taken 03 from the oscillation circuit which is included in the plate circuit of the tube having a high internal resistance.

Now, the present invention is concerned with the problem to stabilize the modulation percentage, in a circuit arrangement of the kind disclosed in the said Bruck application, in spite of variations of the operating potentials, that is to say, the direct-current voltages. It will be remembered that such fluctuations of the working voltage, in the first place. cause a change in the radio. frequency amplitude, and, in the presence of unvaried modulation potential it change in the per centage of modulation, and this is undesirable,

March 4, 1941, Serial No. 381,624 GermanyNovember 8, 1939 quency in a second rectifier and second load resistance.

Figs. 1 and 2 illustrate exemplified embodiments of my improved modulation system and Fig. 3- illustrates by graphs practical results of the invention.

Referring to Fig. 1, the amplifier tube V, the

" same as in the parent application, is fed with I or rated working voltages the direct-current volt- 0 ages across resistance RI and R2 are of like size,

the unmodulated radio-frequency potential Ulfl. The carrier is amplified in tube V and supplied to the secondary winding S oi. a transformer T and thence to output means. The diode i has two electrodes 6 and it connected in shunt to the tuned circuit Sch. Rectification in the diode i sets up current in the load resistance Bi. Now, this load resistance RI, through the transformer Ti, is fed with the modulation frequency UNF from a source II. The modulated radio-frequency potential is as stated above taken from the oscillation circuit Sch.

To stabilize the modulation percentage irrespective and independently of fluctuations in the working voltage, according to an object of the invention, a second rectifier 2 having electrodes I4 and II connected with loadresistance R2 is provided. This rectifier 2 is led with the audio frequency by way of the transformer T2. The

condenser 02 serves for audio-frequency bridaing or by-passing oi the resistance R2, while the condenser Cl serves for the radio-frequency bypassing of the resistance RI. The direct-current voltage which is set up across the resistance R2 as a result of the rectification oi the audio frequency is applied by way of the wire I8 to the left hand end of the resistance RI. The circuit is so balanced that in the presence of normal with the result that no direct current flows through the wire l8.

But, ii, for some reason or other; the audiofrequency potential or the radio-frequency potential changes, then a direct current willflow through the wire it. According to its direction this direct current will either act in a damping or a boosting manner upon the radio-frequency circuit. As a consequence, the radio-frequency amplitude is always maintained at a certain ratio to the audio-frequency amplitude so that the modulation percentage is approximately stabi- Suppose the audio-frequency potential is constant, while the radio-frequency p tential ditained by rectification of the modulation frell minishes.

As a result, the direct-current potenaasnoso tial across RI below the value of that acting across resistance R2. The further result is that a current will be caused to flow through wire it tending to equalize the two potentials. The said direct current acts the very same way as if the left hand diode were impressed with a negative biasing potential. As, a result, the damping. efiect produced by the diode upon the radio-frequency circuit is lessened and the radiofrequency potential, acting at the circuit, rises so that the original modulation percentage is restored. Y 1

Fig. 2 illustrates an exemplified embodiment in which the need of audio-frequency transformers is eliminated. The input resistance consists here of the resistance R3. From a tap on the resistance R3 by lead 20 is supplied to the diode I at the same time the audio frequency as well as the direct current voltage resulting from the rectification of the audio frequency to the resistance RI By shifting the tap or slider along-R3 it is possi-- ble to adjust working conditions to any desired modulation percentage. The load resistance R: is high compared with the resistance R3 to the end that, when making adjustments, the directcurrent voltage may change as little as feasible,

and that the modulating audio frequency may V not be distorted.

Fig. 3 shows in what 'way the modulation percentage m is changed upon a variation of the plate direct current voltage Ua of tube V, without application of the idea underlying the invention (Graph'l) and with application thereof (Graph 2). The curvature of the graphs is due to limited measuring accuracy. It will be noticed that the amplitude of said voltage, a source of modulating potentials, a second rectifier coupled to said source of modulating potentials to rectify said modulating potentials to derive a second current the intensity of which corresponds to the amplitude of aid-mama; potentials, means for opposing said currents. to derive resultant current when the said two currents are unequal, the direction of fiow of which depends on the relative intensities of said first two currents, means-for impressing said resultant current on said first rectifier to control the bias thereof, and a connection between said source of modulating potentials and said first rectifier for controlling the instantaneous current therethrough in accord-- ance with the modulating potentials.

2. In a modulation system, a parallel tuned circuit wherein high frequency wave energy to be modulated is caused to ilow,-a'first rectifier and a first resistance in series connected in shunt ing said second resistance in parallel with said I current is greater, said potential being applied to said first rectifier as a biasing potential, and means for impressing modulating potentials on one of said resistances.

3. In a modulation system, a. parallel circuit wherein wave energy of carrier wave frequency is caused to flow, a rectifier and a load resistance for said rectifier in series connected in shunt to said parallel tuned circuit, a source of modulating potentials, means for impressing said modulating potentials on said load resistance, and means for rectifying said modulating potentials to derive direct current voltage which corresponds to the envelope of the modulating potentials and impressing the same on said resistance to thereby stabilize the percentage modulation in the presence of. fluctuations in the wave energy of carrier wave frequency.

.LOTHAR BRiicK.

tuned

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