Transformers, Phantom Circuits and Repeat Coils


17.3 Transformer Applications to Telephone Circuits

The applications of transformers to telephone circuits are numerous and varied. The reduction of energy losses in alternating-current transmission, as illustrated in Figure 17-6, has an application to telephone transmission but is not so important as other uses. One very general use is to accomplish the result given as b above. In this case, the primary function of the transformer is to transfer energy to another circuit rather than to change the voltage and current values. When so used in telephone work, they are generally called repeating coils rather than transformers because their primary function is to "repeat" the energy into a different circuit rather than to transform it into a different state. There are, however, inequality ratio repeating coils which perform both functions. On the other hand, in connection with telephone repeater circuits and certain other telephone apparatus, input and output coils are used primarily to match impedances to permit maximum energy transfer, as explained in later Chapters.

One of the most common applications of the repeating coil in telephone work is in connection with the common battery cord circuit, as illustrated by Figure 17-7. Here the alternating current flow in one subscriber's line is repeated into the other subscriber's line with little energy loss, and at the same time the windings of the coils afford the proper direct-current connections for each subscriber's station to receive a superimposed d-c current for transmitter supply. Another very general use of repeating coils in the telephone plant is for deriving "phantom" circuits. Here the coils serve a unique purpose which has no counterpart in electric power work, and is not included in the classification of transformer functions given above. We shall therefore need to consider this application more fully. However, it may be noted that the coils, while serving this particular purpose, may also function as impedance matching devices.


17.4 The Phantom Circuit

Figure 17-8 is a simplified diagram of two adjacent and similar telephone circuits arranged for phantom operation. By means of repeating coils installed at the terminals of the wire circuits, a third telephone circuit is obtained. This third circuit is known as the phantom and utilizes the two conductors of each of the two principal, or "side" circuits, as one conductor of the third circuit. The two side circuits and the phantom circuit are together known as a phantom group. These three circuits, employing only four line conductors, can be used simultaneously without interference with each other, or without crosstalk between any combination, provided the four wires have identical electrical characteristics and are properly "transposed" to prevent crosstalk.

The repeating coils employed at the terminals are designed for voice-current and ringing-current frequencies, and do not appreciably impair transmission over the principal or side circuits. The third or phantom circuit is formed by connecting to the middle points of the line sides of the repeating coil windings, as shown in the Figure. Since the two wires of each side circuit are identical, any current set up in the phantom circuit will equally divide at the mid-point of the repeating coil line windings. One part of the current will flow through one-half of the line winding, and the other part of the current will flow in the opposite direction through the other half of the line winding. The inductive effects will be neutralized, and there will be no resultant current set up in the drop or switchboard side of the repeating coil. Since the phantom current divides into two equal parts, the halves will flow in the same direction through the respective conductors of one side circuit, and likewise return in the other side circuit. At any one point along a side circuit, there will be no difference of potential between the two wires due to current in the phantom circuit, and a telephone receiver bridged across them will not detect the phantom conversation.

Since there is no connection, inductive or otherwise, between the two circuits at the terminals, it is equally true that a conversation over a side circuit cannot be heard in th ephantom. This can be understood by imagining a flow in the closed side circuit through the line wires and the windings of the repeating coils at each end. With the side circuit conductors electrically equal, there can be no difference of potential between the mid-point of the repeating coil winding at one end and the mid-point of the repeating coil line winding at the other end because the drops of potential for the two parts of the side circuit are equal and opposite. If the side circuit, therefore, impresses no difference of potential on any part of the phantom circuit, the side circuit conversation cannot be heard over the phantom.

In the theory of the phantom it should not be forgotten that the conductors are assumed to be electrically identical, or in other words, the conductors are perfectly "balanced". The phantom is very sensitive to the slightest upset of this balance, and circuits that are sufficiently balanced to prevent objectionable crosstalk or noise in physical circuit operation, may not be sufficiently balanced for successful phantom operation.

17.5 Standard Repeating Coils

A number of general types of repeating coils are currently standard in the Bell System. One principal type, illustrated by the 62 and 93 series, has four windings, the terminals of which are designated by numbers as shown by Figure 17-9(A). The other type, illustrated by the 173 series, has six windings which may be connected as shown in Figure 17-9(B) with four windings on the line side, or with the 9-10, 11-12 windings not used, depending on the impedance ratio required. In all types, the windings which are used to form the line side are precision manufactured so as to be as nearly identical electrically as possible. This balance is required, as we have already seen, to avoid crosstalk where the coils are used in phantom operation. The drop windings (that is, 1-2 and 5-6) do not need to be so well balanced in normal use.

The 62- and 93-type coils have toroidal cores made of many turns of fine-gage silicon-steel wire sawed through at one point to introduce a gap in the magnetic circuit. In the 93-type coil this gap is filled with compressed powdered iron which, while increasing slightly the reluctance of the core gives it a high degree of magnetic stability, preventing permanent magnetization under abnormal service conditions. In the 62-type coil the gap in the magnetic circuit is unfilled which tends to make the coil even more stable. This coil is especially well adapted for use on circuits composited for d-c telegraph operation. The same feature, however, tends to make the 62 series inefficient at low frequencies and they cannot be used on circuits employing 20-cycle signalling, whereas the 93 series may be used for such purposes. Standard 173-type coils are built with permalloy cores.

TABLE VII
STANDARD REPEATING COILS
IMPEDANCE RATIO
LINE TO DROP 4-3 AND 8-7
TO 2-1 AND 6-5
93-TYPE 62-type
1:1 93-A 62-A
1:1.62 93-B 62-B
1.62:1 93-F 62-C
2.66:1 93-G 62-E
1.24:1 93-H
2.28:1 93-J
1:1.28 62-F
1:2.34 62-G
The types of repeating coils discussed above are manufactured with a number of different turn ratios to provide various impedance matching combinations. Table VII gives the standard impedance ratios for 93- and 62-type coils. The 173-type coils are likewise available in a wide range of impedance ratios. The impedance ratio obtained in their use depends on whether all four of the line windings are used and on how those used are connected. The impedance ratios that can be obtained accordingly do not lend themselves readily to tabular presentation, but various ratios line-to-drop ranging from as low as 0.6:1 to as high as 2.52:1 may be obtained.


Extracted from:
Principles of Electricity
applied to
Telephone and Telegraph Work
A Training Course Text
Prepared for Employees of the
Long Lines Department
AMERICAN TELEPHONE AND TELEGRAPH COMPANY
January, 1953


Western Electric 111C 119C 153A Diagram courtesy of Chuck Leavens