US1636146A - Control of electric waves - Google Patents

Description

July 19, 1927. 1,636,146

F. MOHR CONTROL oF ELECTRIC wAvEs Filed July 12. 1922 Ferran/dye fes/fante by VME/N Patented July 19, 1927.

UNITED -VSTATES PATENT OFFICE.

FRANKLIN MOHR, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

CONTROL OF ELECTRIO WAVES.

alipncauoa ined my 12,

The present invention relates to electric wave transmission and control. More specifically it involves provisions for readily and accurately balancing or adjusting wave modifying circuits such as amplifiers, modulators, detectors and the like, especially of the balanced circuit type.

Balanced vacuum discharge tube circuitsi are used'for a variety of purposes such as amplifying waves of different frequencies without combining them, modulating one wave by another while confining one of the Waves to the tube circuit itself, and'in other applications. Here-tofore it has been customary in setting up such circuits to select `tubes as nearly identical in their operating characteristics as practicable and to operate the circuit with the degree .of balance afforded b` these similar tubes. For many vpurposes tliis procedure has proved sufficient, but there are times when a more accurate balance is desired than can be had by merely carefully selecting the tubes.. Also tubes frequently fail in service and need to be replaced. Circumstances do not always allow time for carefully matching tubes and moreover, there may not be available a number of tubes from which to make a careful selection.

It is an object of the invention tol provide for balancing'circuits of this general character regardless of whether the tubes themselves arel identical or not.` It is a further object of the invention to `make readil adjustable provisions for balancing the circuit so that 1n case of failure of a tube a replacement can be made and the circuit can be adjusted to a balanced condition in a very small amount of time.

Specifically the means involved in carrying out the invention comprise resistances, preferably ohmic, inserted in series in the anode circuit of the discharge tubes. Experiment has shown, as will be described more fully hereinafter, that for practical purposes resistances so inserted act within considerable and ractical limits like so much resistance ad ed to the out ut internal impedance path of the tube itself).

in which Fig. 1 is a schematic representation of a balanced amplifier circuit; Fi 2 is a similar representation of a balance modulator circuit; and Fig. 3 shows curves to be 1922. Serial No. 574,374.

explained later in connection with the operation of the circuits. 4

Referring first to Fig. 1 the balanced amplifier comprising the electron discharge tubes 1 and 2 and their associated circuits is inserted between the line sections L1 and L2. These line sections may be used for the transmission of telephone currents or other currents and it will be assumed that currents comprising a considerable range of frequencies are being transmitted from line section L1 to line section Lz.- The amplifier A' is associated with the line L1 by means of the transformer. 3 and with the line L2 by the transformer 4.

The operation of an amplifier of this type is Well understood by those familiar with the art. The Voice currents or other -currentsto be amplified are transmitted through the coil 3 from the line section L1L and produce correspondin potential variations in the grid or input circuits of the tubes l and 2. Corresponding current variations of increased amplitude are transmitted through the condensers 8 and 9 to the transformer 4 by which they are impressed on the outgoing line L2. It is a property of an amplifier of the balanced circuit type that if the circuit is accurately balanced and if a proper steady Ypolarizing potential is maintained on the grids, waves of a number of frequencies may be simultaneously repeated without the production of combination waves due to modulating action of the amplifier tubes. Space current for the tubes -1 and 2. is, in the case of Fig. 1, supplied througli'the choke coils 6 and .7 to the respective anodes.

As stated above, it is important, if the waves of different frequencies are to be repeated without the production of distorting currents due to combinations of the repeated waves, that an accurate balance be maintained between the tubes 1 and 2. Heretofore such balance has been obtained by carefully selecting a pair of tubes having as nearly as possible identical impedance and amplification characteristics. It has been discovered, however, that in case the two tubes do not have identical impedance or Reference will now be had to the drawings lseries in the anode circuit. Such resistances are shown at 2O and 21 and one or preferably both of the resistances is variable to permit of adjustment.

Any one of a number of testing arrangements may be used to determine when the tube circuits are balanced to a high degree of accuracy. For purposes of illustration sources S1 and S2 of waves of dierent frequencies are shown arranged to be connected with the input of the amplifier and a measuring circuit 17, 18 is shown for determining the amount of intermodulation occurring between the waves from these two sources for different values of the resistances 20 and 21. The waves from sources S1 and S2 i should, of course, be applied to the amplifierindependently of each other and in a Way to insure that no intermodulation occurs be-` ier circuit such as would cause distortion.

in the speech transmission, a certain amount of combina-tion current of, for example, 200 cycles will be produced and this can be measured in the circuit 17, 18.

yIn making a test to determine the degree of balance in the amplifier circuit the switches 10 to 15 are thrown to the alter nate positions from those indicated in the drawing. This resultsl in the cutting off of the lines L1 and L2 and in the'substitu-` tion of resistance 16 in place of the line L'1 and the measuring circuit 17, 18 in place of the line L2. Assuming that the sources S2 and S2 have frequencies of 800 and 1000 cycles respectively, if the amplifier produces intermodulation of these tWo waves a component of 200 cycle frequency will appear in the output circuit of the amplifier and will be impressed through the transformer 4 and upon the measuring circuit. The circuit 17 contains a tuning or filtering element which may be made selective to the 200 cycle frequency wave but which will be arranged not to transmit the current of 800 cycles or 1000 cycles or other frequencies. The circuit 17 is also assumed to contain a suitable integrating device, as for example, an ordinary vacuum tube detector to render the 200 cycle wave capable of affecting the meter 18. With the circuits so arranged the resistances 20 and -21 are adjusted by trial so that preferably the amount of external resistance included in one of the anode circuits is a minimum, and so that just suiicient resistance is included in the other anode circuit to give the minimum reading -of the meter 18.

In Fig. 2 the balanced modulator is shown arranged `for modulating a carrier wave from a source C S in accordance with currents received from the line L3 and for impressing the modulated wave on the outgoing vline section L4. With the carrier wave source C S connected as shown in common to both grids of the modulator tubes it is a property of this circuit, when accurately balanced, to suppress transmission of the unmodulated carrier wave to the line L4 but to permit transmission of the modulated wave components. Any unbalance in the modulator circuit results in the transmission to the line L4 of some of the unmodulated waves from the source C S. The output side of the tube circuit of Fig. 2 has a different arrangement from that of Fig. 1, although either type of anode circuit is suitable for either the amplifier or the modulator. In Fig. 2 the plate current is supplied from the battery indicated, through a single choke impedance 22 to the midpoint of the primary winding of the output transformer.

In order to adjust the circuit of Fig. 2 for an accurate balance the switches 10, 11, 14 and 15 are thrown to the alternate positions from those in which they are shown in the drawing. This substitutes resistance 16 for line Ls and the measuring circuit for the line L4. The measuring circuit comprises the thermo-couple TC and a meter 18 which may be any suitable type of current meter. vWith the connections so made and the/source CSvoperating, the resistances 20 and 21 areI adjusted to give a minimum reading on meter 18 and as in the above described case the resistances are preferably so adjusted that one of them is practically all cut out of the anode circuit.

Fig. 3 shows experimental curves obtained with a typical balanced tube circuit. In this figure the abscissae represent the ratio of resistance 20 or 21 that is being used r to the nominal anode-cathode impedance of one of the tubes, expressed in per cent, while the ordinates represent the 'percentage of efficiency or distortion of the circuit as the case may be, based on the etiiciency or distortion of the circuit when resistances 20, 21 are` at zero value. With both resistances 20 and 21 at zero the circuit is in the unbalanced condition and the distortion 'is for that case, assuming that the tubes are not identical. As either resistance is ins erted in the anode circuit to reduce the amount of distortion the eiiciency of the circuit as a whole is also reduced, but from the relation of the curves, it will be seen that only a slight sacrifice in efficiency is necessary in order to reduce the distortion very greatly. i

The invention is not to be construed as limited to the particular forms of embodiment that have been shown and described, since it is capable of use generally. Its scope is defined in the claims.

What is claimed is: k l

l. The method of purifying the output wave of a wave combining circuit employing a pair of balanced space discharge tubes, which comprises controlling the amount of ohmic resistance in the output path of said wave 4in relation to the internal impedance of said tubes to neutralize in said output path any wave components present due to slight unbalance of said tubes.

2. The method of combining waves of a plurality of frequencies in a modulator circuit employing a pair of space discharge tubes connected in balanced circuit relation which comprises varying the ohmic resistance in series in the circuits external to said tubes in relation to the eectiveA internal impedances of said tubes -to suppress in the outgoingv circuit undesired components of said Waves due to slight differences in said eii'ective internal impedances of said tubes.

3. In a wave modifying circuit employing a pair of space discharge tubes connected in balanced circuit relation, means for supplying waves of different frequencies to said circuit, anode-cathode branches tor each of said tubes, an outgoing circuit connected to said anode-cathode branches, and means for controlling the ohmic resistance in said cathode-anode branches to suppress in said outgoing circuit components of said Waves due to slight differences in the impedance characteristics of said tubes.

4:. A wave modifying circuit comprising a pair of space discharg'etubes, each having cathode, anode and grid elements, con-- trol circuits for said grids, a cathode-anode branch for each tube, an outgoing circuit connected to said cathode-anode branches, means for supplying waves of different frequencies to the grid circuits of said tubes, and regulating means to Suppress in said outgoing circuit components of said waves due to slight differences in the eiective impedances of said tubes comprising an ohmic resistance in the anode circuit of one of said tubes.

5. A'wave modifying circuit comprising incoming and outgoing circuits, means for supplying waves or' a plurality of frequencies to the incoming circuit, a differential circuit inserted between said incoming and outgoing circuits comprising a, pair ofelectron discharge devices, each having cathode, anode and grid elements, said cathode and grid elements being associated with said incoming circuit, anode branches including a connection from each anode to the outgoing circuit and to said cathodes, and variable ohmic resistance means in said outgoing-circuit i'or varying the total impedance of one anode branch with respect to the .other to neutralize in said outgoing circuit undesired wave components due to dierences in the eti'ective internal impedances of said tubes.

6. A balanced modulator circuit comprising `a pair of electron discharge tubes, each having a cathode, a grid and an anode, means for simultaneously supplying waves of different frequencies to the grid and cathode elements of said tubes, an outgoing cirsis'tance means in said outgoing circuit for suppressing in the outgoing circ-uit unde-` sired -unmodulated components of said waves resulting from lack of complete balance of said tubes.

7. A balanced modulator circuit comprising a pair of electron-discharge tubes having cathode, anode and grid elements, means for simultaneously supplying waves of different frequencies to said grid and cathode elements of said tubes, one of said waves being impressed on said grid elements in the same sense, anode-cathode branches, an outgoing circuit connected to said anode-cathode branches, and means for suppressing in said outgoing circuit unmodulatedl components of the wave impressed on said grid elements in the same sense due to unbalance between said tubes comprising Variable ohmic resistance in series in said anode-cathode branches. n

In witness whereof, I hereunto subscribe my name this 10th day of July A. D.,1922.

- FRANKLIN MOHR.

'cuit for saidA tubes, and variable ohmic re-

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