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
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.
