US1857830A - Transmission circuit - Google Patents

Description

May 10, 1932.V H. wHlTTLE TRANSMISSION CIRCUIT Filed Aug. 14, 1930 INVENTOR ff. W/-l/TT/.E By

A TTORNE V Patented May 10, 1932 UNETED STATES PATENT OFFICE HORACE WHITTLE, OF MAPLEWOOD, NEW JERSEY, yASSIG-NOR TO BELL TELEPHONE LABORATORIES, INCORPORATED, OF NEW YORK,v N. Y., .A CORPORATION 0F NEW YORK TRANSMISSION CIRCUIT This invention relates to transmission circuits and pertains more especially to the reduction of crosstallr in speech transmission circuits in which repeating coils are employed for providing phantom circuits.

It is an object of this invention to provide means and methods for eliminating the crosstalk in phantom circuits in which repeating coils are employed. l

The repeating coils or transformers employed for this purpose should have their windings as symmetrical with respect to the tapping point as is possible in order to reduce crosstalk to a minimum. A

Among the factors contributing to crosstalk between the phantom andk side circuits are inductance unbalance between the two halves of the winding to which the ,phantom circuit is physically connected, resistance unbalance between the two halves of the winding to which the phantom circuit is physically connected, distributed capacity unbalance between the two halves of the winding to which the phantom circuit is physically connected, mutual impedance imbalance between the windings of the repeating coil and admittance unbalance between the windings of the repeating coil.

The first four causes of crosstalk referred to above can be eliminated by methods well known in the prior art, such as twisted pair windings andbalancing resistances and cavpacitances.

The fifth cause of crosstalk referred to above is due to the capacity which exists between the primary or drop winding and the winding which forms a part of the phantom lcircuit or line winding. In a coil of this type there is a capacity between one-half of the line winding and one-half o f the primary or drop winding, a. second capacity between the same half of the line winding and the other half of the primary or kdrop winding, a third capacity between the other half of the line winding and the iirst mentioned half of the pl'lnary'or drop winding, and a fourth capacity between the last mentioned half of the line winding andthe second mentioned half of the primary or drop winding. Since the two halves ofthe line windingare formed by a twisted pair, the lirst and third' capaci i ities mentioned will be practically equal and the second and fourth rcapacities mentioned will be practically equal. However, since the two halves of the primary or drop winding are wound on opposite halves of the core, the sum of the first and third capacities will not necessarily be equal to the sum of the second and fourth capacities. When the sum of the first and third capacities is different from the sum of the second and fourth capacities, the result is an effective residual capacity, equal to the diference'between the two sums, acting vfrom the primary or drop winding to the line winding.

This effective residual capacity will in effect introduce a current in the phantom circuit when a currentA flows in the side circuit. The effective current flowing in the phantom circuit will be proportional to the magnitude of the residual capacity and the difference of potential between its terminals. All the effective current flowing through residual capacity will not iow into the phantom circuit through the physical connection to the side circuit, but this current will be divided between the three'conductors leading from the line Winding of the repeating coil'. The amount of the effective current iiowing to the side circuit and the amount flowing to the phantom circuit will depend upon the relative impedance characteristics of the different paths.

It isan 'object of this' invention to eliminate the crosstalk in phantom circuits due to the admittance imbalance between the windingsof the repeating coil.

If a small variable condenser of any suitl able typeV is introduced between two points that have a difference in potential opposite to that of the residual capacity and the capacity of this condenser is adjusted, the crosstalk can be be balanced out or eliminated.

A feature ofthe invention is, therefore, the v tralize the admittance unha-lance between the i the windings of each individual repeat-ing coil, the repeating coil is preferably connected in a bridge or measuring circuit having balanced ratio arms and with the ratio arms ungrounded the coil is kadjusted for self and mutual impedance and resistance unbalance. Then the current has been reduced to a minimum in a receiver connected from the tapping point, or point of physical lcon- 10 i nection of the'phantoin circuit-to the point of connection of balanced ratio arms, it indicates that these unbalances are eliminated. lVith the coil connected in the bridge circuit in this manner no admittance currents can flow due to the eiective residualcapacity between the windings as there is .no return path for these currents. The .mid-point of the .balanced yratio arm is then grounded which provides a return path to the grounded source causing current to. flow through the effective residual capacity between therwi-ndings. rI`he small variable condenser isthen introduced between .two points in the coil which have a diiierence of potential opposite to that of the capacity unbalanceand this condenser is adjusted un- `til the -crosstalk isbalanced out or eliminated.

As this condenser can in most cases be very small, it is preferably `formed by twisting two insulated conductors together, or by winding bare wire around an insulated conductor and soldering Athe turns of the bare wire together :to form a sheath, althoughany lsuitable variable capacitance will be satisfactory for vthis purpose. T he capacity can be varied .changing the .length of Wire twisted together .or the length of :the sheath `formed bythe bare wire. The advantage in `employing this type of condenser is .that it can be firmly secured to the coil .after adjustment.

The invention will vbe better understood by reference .tothe accompanying drawings and detailed description thereof, in which:

Fig. lis a-schematic `drawing or a phantom repeating coil;

Fig. 2 isa schematic drawing of a phantom circuit employing repeating coils of the type shown in Fig. 1; and

c Fig.r 3 is a schematic drawing of a testing ofcore structures, as for example a toroidal core without an airgap and the so-called shell type core with or without an airgap. TheV line winding 13, 14, 17, 18, is preferably wound adjacent the core and the primary or drop winding 11, 12, 15, 16 over the line winding. The line winding 13, 14, 17, 18 comprises two windings 13, 14 and 17, 18 connected in series aiding. The winding 13, 14 comprises windings 13, 7 and 8, 14 connected in series aiding, and the winding 17, 18 comprises two windings 17, 9 and 10, 18 connected in series aiding. findings 13, 7 and 17 9 comprise a .twisted pair of conductors wound on one-half of the vcore 2() and windings 8, 14 and 17 9 comprise a twisted pair of conductors wound on the other half of the core 20. The primary or drop winding 11, 16 comprises two windings 11, 12 and 15, 16-ccnnected in series aiding. The winding 11, 12 is wound over the windings 13, 7 and 17 9. The winding 15, 16 is wound over the windings 14, 8 and 18, 10. A variable condenser 5 is connected from terminal 17', 14 to the terminal 11, to accomplish capacity balance in accordance with the invention.

Referring to Fig. 2, a phantom repeating coil 24, similar in construction to that shown in Fig. 1 and described above has its primary or drop winding 11, 12, 15, 16, connected to a side circuit 30, 31 and its line winding 13, 1.4, 17, 18 connected to line wires 32 and 33. A phantom repeating coil 25, similar to the repeating coil shown in Fig. 1, connects the other end of line wires 32 and 33 to a side circuit at 34, '35. A phantom repeating coil 26, similar to the repeating coil shown in Fig. 1, has its primary or drop winding connected to a side circuit at 3:6, 37 and its line winding connectedto line wires 38 and 39. A phantom repeating coil 27, similar to the repeating coil shown in Fig. 1, connects the other end of the line wires 38 and 39 to another side circuit at 40, 41.

The primary winding of a repeating coil 28 is connected to a phantom circuit at 42, 43. The secondary winding of the repeating coil 28 isfconnected between the mid-point of the line winding 1.3, 14, 17, 18 of the phantom repeating coil 24 at 14, 17 and the mid-point l of the line winding of the phantom repeating coil 26. The repeating coil 29 connects the mid-point of the line windings of the phantom repeating coils 25 and 27 to a phantom l circuit at 44, 45.

Referring to the phantom repeating coil 24, of Fig. 2, constructed as shown in Fig. 1, it will be seen that there are eiiective capacities 1, 2, 3 and 4 between the respective winding sections as shown.

Since the two halves 13, 7, 8, 14 and 17 9, 10, 18 of the line winding 13, 14, 17 18 are formed by a .twisted pair of conductors, the capacity 1 and the capacity 3 will be equal to each other and the capacity 2 and the capacity 4 will be equal to each other. However, since the two halves 11, 12, and 15, 16

of the primary or drop winding 11, 12, 15, 16

.are wound on opposite'sidesof the core 20 the sum of the capacities 1 and 3 will not neceslim() 'iso sarily equal the suni of the capacities 2 and 4. When the sum of the capacities 1 and 3 differs from the sum of the capacities 2 and 4 the result is an effective residual capacity, equal to the difference of the two sums acting from the primary or drop winding 11, 12, 15, 16 to the line winding 13, 14, 17, 18.

There will then be either more or less current flowing from the side circuit through capacities 3 and 4 to the line winding than through capacities l and 2. All of the current flowing from the side circuit through the Vcapacities 1 to 4 inclusive will not tend to fiow in the phantom circuit through the physical connection of the repeating coil 28 t0 the line winding 13, 14, 17 18, but will tend to divide between three conductors, the one conductor to the line winding 13, 14, 17 18 at l14, 17 and the other two conductors 32 and 33 leading from the line Winding. The amount of current tending to flow to the phantom circuit repeating coil 28 will'depend upon the relative impedancel characteristics of the three conductor paths. If capacities 1 and 2 tend to cause a current L 2 to fiow in the phantom circuit and the capacities 3 and 4 tend to cause a current I3.,E to flow in the phantom circuit, the residual current Ig flowing in the phantom circuit will be equal to lpg-134. The variable capacity element 5 is provided by the present invention and is connected from terminals 14, 17 to terminal l1 to produce a current I5 equal and opposite to the current IX.

Referring to Fig. 3, a phantom repeating coil constructed as shown in F ig. 1 and described above has its primary winding 11, 12, 15, 16 connected to a source of constant frequency 46 through a repeating coil 47, the mid-point of its secondary winding being connected to ground. Two resistances 48 and 49 having the same impedance characteristic are connected in series at 50 and across the line winding 13, 14, 17, 18. A variable resistance 51 and a receiver 52 are connected in series from the connection 50 of i'esistaiices 48 and 49 to the connection 14, 17 of the line winding13, 14, 17, 18. A switch 53 is connected from the connection 50 of resistances 48, 49 and 51 to ground at 54.

The operation of this circuit to test the coil, for instance, for unbalance and to adjust it so as to neutralize the current flowing in the phantom circuit due to the effective residual capacity is as follows: l/Vith the switch 53 open the phantom repeating coil is first adj usted for self and mutual impedances and resistance imbalance in the manner welllniown in the art and which is indicated by minimum current flow inthe receiver 52.

With the phantom repeating coil connected in the circuit as shown and with the switch 53 open, no admittance current can iiow from the winding 11, 12, 15, 16 to the winding 13,

5 14, 17 18 due to the effective residual capacity between the windings as there is no return path for these currents. Switch 53 is closed, providing a returnv path to the. grounded source 47, which causes a current Ix to flow from the grounded frequency source through i The balancing capacity provided byfthe present invention is then connected from connectionf14, 17 on the line winding to a point on the priinaryor drop winding 11, 12, 15, 16 and is made to have such value as will produce acurrent L between the connection 14, 17 on the line winding and ground which is equal and opposite to the current Ix. (The condenser 5 is preferably connected from 14, 17 on the line winding to either 11l or`16 on the drop winding.) The capacity of condenser 5 is then varied untilno current flows in the receiver 52. This-completes the adjustment of the phantom repeating coil for the elimination of admittance imbalancel stalled in its permanent position in the phantom circuit and the frequency source 46 and repeating coil 47 located in the central cnice,

in which case any capacity unbalanceto ground `from the Ytwo sides of the line connecting thercentral office to the winding 11, 12, 15, 16 willbe compensated for as will the residual capacitybetween the windings of the coil.

It is to be understood that the invention is not limited to the'specilic embodimentV that has been shown and described ink detail, but only by the scope of the appended claims.

lVhatis claimed is: f Y

1. r1`he method of eliminating admittance uiibalance due to the residual capacity unbalanee between one winding of a repeating `coil and another winding of said repeatingu coil symmetrical to a center tap, which comvided inner winding on said core wound in a plurality of layers, a divided outer winding wound in a plurality of layers over said inner winding, said transformer having natural capacities between each half of the outer winding and each of the halves ofthe inner winding, which capacities are unsymmetrically located with respect to the common terminals of the halves of the inner winding, and means comprising aicapacity ele-ment connected between said common terminal of'the khalves of the innerl winding,

andsaid vouter winding for compensating for said unsymmetrical location of said capacities from the halves of the outer winding with respect to the common terminal of the halves of the inner winding.

3. In combination, side and phantom circuits,a repeating coil in one of said side circuits having .a divided primary and a divided secondary winding, said windings so related that there eXists natural .capacities between each half of the primary winding and each of the halves of the secondary winding, and that the capacit-ies betweeneach halof the primary and the two halves of the secondary are equal, but the sum of the capacities between the one half of the primary and the two halves of the secondary diHers from the sum of the capacities between the other half of the primary and thetwo halves of the secondary so as to produce an admittance imbalance current between said windings, and a capacity element connected between said primary and secondary winding which neut-ralizes said admittance imbalance current.

4.. In combination, a transformer having primary and secondary windings and natural capacities between said windings, said secondary winding having a center tap, a source ofcurrent connected to said primary winding, an electrical conductor, one end of which is connected to said center tap, an electrical path from the other end of said electrical conductor to said source of current, and means to prevent admittance currents from being transmitted from said primary winding to said source of current over an electrical path comprising said natural capacities, said electrical conductor, and said first mentioned electrical path, vsaid means comprising a capacity element connected between said windings.

'5. In combination, two lines, a phantom circuit, a repeating coil associated with each of said lines and having a primary winding and a center tapped secondary winding to mary winding to saidscurce of current over an electrical path comprising said natural capacities, said electrical conductor, and said Vfirst mentioned electrical path, said means comprising a capacity element connected between points in said windings which have a potential opposite to that of the eective residual capacity due to said natural capacities between said primary and secondary windings.

In witness whereof I hereunto subscribe` my name this 13th day of August, 1930.

` HORACE WHITTLE.

which tap the phantom circuit is physically i connected, and means for balancing out the admittance currents in each of said repeating coils due to the eifective residual capacity vunbalance between the windings thereof, said means comprising an auxiliary capacity element connected between points in lsaid windings which have a potential opposite to that across said residual capacity.

6. The method of eliminating admittance unbalance currents from a repeating coil having two windings, at least one of which windings is symmetrical to a center tap, which comprises adjusting said coil for self and mutual impedance and resistance unbalance, testing the coil for admittance currents due to residual capacity unbalance between the windings, adding a capacity element between points in said windings which have .a potential opposite to that across said re-

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