Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to tëlephone apparatus generally and
more particularly to a central office interface circuit for connecting
subscriber apparatus to a subscriber loop.
Background of the Invention
A central office interface circuit is usually located on the
subscriber's premises and is used to connect the subscriber loop to one or
more subscriber apparatus units. The central office interface is
generally designed to appear electrically to the central office, as a
typical telephone set. It is also designed to isolate the DC component of
the signal from the AC component of the signal at the subscriber's end.
Since such a circuit is located at the subscriber's end of the loop there
is not a battery feed provided at the interface circuit. A typical central
office interface circuit particularly adapted for use with electronic key
telephone apparatus is described in Canadian patent application Serial No.
350,902 filed 30 April, 1980 and assigned to the instant assignee.
The isolation of the DC component on the subscriber loop
from the subscriber apparatus is most conveniently achieved by using an
isolation transformer. However, such a transformer has to carry
substantial current of up to approximately 120 milliamps on a short
subscriber loop to the central office. In order to prevent saturation,
the transformer has to be provided with a relatively expensive and fairly
bulky làminated core with an airgap. In addition, since such a core
generates significant fringing magnetic fields, the transformer must be
shielded.
The solution to the problem of course lies in the provision
of a circuit wherein the DC flux of the transformer is balanced to
approximately a zero value. Flux balancing or cancellation is generally
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well known in the art for example as disclosed in United States patent No.
3,384,810, issued May 21, 1968 to E.S. Kelsey and assigned to the assignee
of the instant invention. Other flux cancellation circuits are described
in United States patent No. 3,9~9,718 and United States patent No.
~,103,112. These patents describe various serial and parallel transformer
configurations which provide balance windings to substantially cancel the
DC flux generated in the primary windings of the transformer in
combination with means for connecting a source of power or battery feed to
the subscriber loop. Basically, all the known prior art circuits operate
by varying the DC current through the balance winding(s) to compensate for
the ~C flux generated by the current flowing through the primary winding.
The Invention
In accordance with the invention, there is provided a flux
cancellation circuit for use in a central office interface circuit. The
isolation transformer is provided with a balance winding connected to a
constant current source supplying it with a predetermined amount of
current. A current sink circuit is connected serially with the split
primary winding of the transformer to sink the current flowing
therethrough to an amount proportional to that flowing through the balance
winding, whereby the DC flux generated by the current flowing through the
primary winding substantially cancels the DC flux generated by the current
flowing through the balance winding.
Whereas, in the prior art, the flux cancellation current is
varied proportionally to the current causing the flux, the flux
cancellation current in this circuit is held substantially constant. In
other words, the current causing the necessity for flux cancellation is
regulated to a predetermined level by a current sink circuit and the flux
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cancellation current is maintained at a predetermined level by a constant
current source. This results in a relatively simple and economical
circuit.
An example embodiment of the invention will now be described
in conjunction with the drawing which is a circuit diagram of a central
office interface circuit having a flux cancelling transformer arrangement
in accordance with the invention. So as not to detract from the
invention, only that portion of the interface circuit necessary to the
understanding of the invention is illustrated.
The diagram shows a transformer generally at 10 having a
secondary winding S for connection to subscriber apparatus as for example
an electronic key telephone unit, and a split primary winding Nl, N2 for
connection to the tip and ring leads of the subscriber loop to the central
office via a conventional diode bridge 11. The primary winding Nl, N2 is
also connected serially to a current sink circuit 12. It should be
realized that the diode bridge may encompass other voltage control
circuitry which is not described herein since such circuitry is not
relevant to the invention.
The transformer 10 is also shown as having a balance winding
N3 connected to a source of power and to a constant current source 13
which may, for example, comprise a series-connected transistor 14 and
resistor 15. The windings of the transformer 10 are poled as indicated
with the conventional symbol.
Also connected serially with the split primary windings Nl,
N2 of the transformer 10 is a network RlCl which functions to quench the
effect of a switch 16 as for example during dialling. The switch 16 may
be a relay or an electronic switch controlled by the subscriber apparatus.
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A capacitor C2 is connected across the current sink circuit 12 to reduce
the AC impedance thereof.
The current sink circuit 12 comprises a transistor 17,
resistor 18 and an adjustable current device 19 which may con~eniently be
an off-the-shelf component, for example, component number LM334 available
from National Semiconductor.
The current flowing through the primary winding Nl~ N2 flows
through transistor 17 and resistor 18 which generates the control voltage
for the device 19 which in turn controls the base current of transistor
17. Therefore, the current through the primary winding Nl, N2 and the
subscriber loop is regulated to a predetermined level, This current
should be adjusted to be within the normal operating range for a
subscriber loop. Ideally, the current level should be adjusted to be at
least slightly larger than the current flowing in the longest permissible
subscriber loop (e.g. about 30 milliamps),
In operation, the line circuit power source at the central
office causes current to flow through the tip and ring leads which in turn
causes a DC current 1l to flow through the primary winding Nl, N2 and the
current sink circuit 12 which regulates that current to a predetermined
value. The constant current source 13 causes a predetermined current I2
to flow through the balance winding N3. If the current through the
primary winding Nl, N2 is regulated such that Il (Nn + N2) = I2 N3, then
effective flux cancellation takes place and is maintained at that level
due to the current regulation function of the current sink circuit 12.
