Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a telephone line
circuit for terminating a telephone subscriber loop associated
with a digital telephone system.
Digital techniques and systems first appeared in
telephony as a practical trunking means between various telephone
switching offices. More recently, some pulse code modulated (PCM)
telephone switching systems have been developed. These systems are
~nc~
~ ~ used mostly as private automatic bu3iness exchanges (PABX) and
comparatively few PCM telephone switching offices are used as
end offices. One of the factors responsible for this
disproportionate amount of use appears to be the different
environments in which the systems are required to operate. For
example, trunking, toll trunking facilities, and PABX facilities
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tend to be substantially free from serious llghni~ strikes or
utility power line connections. On the other hand, subscriber
loops tend to be far more susceptible to high voltage and
current surges due to such occurrences. Furthermore, a-c high
voltage ringing signals on the subscriber loops are a common
operating occurrence.
PCM telephone systems require interface devices
to convert analogue signals to digital signals and back again.
Such a device is often termed a coder-decoder or codec and is
economically provided by integrated circuit technology. Thus
the operation of a practical codec tends to be limited to the
required operating parameters of typical integrated circuits.
One of the limitations of integrated circuits is the inability
to withstand high voltage or current surges such as are from
time to time experienced on a typical subscriber loop.
A typical subscriber line circuit includes a
repeat coil or transformer having sufficient core material to
essentially remain unsaturated while carrying the direct line
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current of the subscriber loop. The connection with the
switching office battery is typically accomplished through a
pair of balanced windings, as it has been found that this
configuration is preferred for the rejection of crosstalk and
other conversation interference signals that can occur.
A problem arises in the use of the typical line
circuit with the typical integrated circuit codec. As the
repeat coil in the line circuit includes sufficient core
material to be in an unsaturated state while carrying energizing
loop current, it also has the capacity to transmit substantial
amounts of a-c energy in excess of normal voice signals from
the subscriber loop to the codec. Hence the codec in this
application becomes failure-prone during electrical storms and
the like due to random occurrences of extraordinary voltage and
current conditions on the subscriber loop. Surge protection such
as carbon blocks or gas tubes are typically used in combination
with a subscriber loop to limit the amplitude of high voltage
pulses. In addition, the inputs to the codec are usually each
protected by clipping diodes connected between positive and
negative low voltage power sources. However, this protection
has been found insufficient for reliable codec performance in a
central office environment.
Improved protection against electrical storm
induced voltages and the like is obtained if the repeat coil
in the line circuit is reduced in size so that it is unable to
transmit power very much in excess of normal voice signal power.
Both Ernest S. Kelsey in Canadian patent No. 775,709 issued
January 9, 1968 and Max S. Macrander in United States patent
No. 3,714,548 teach direct current compensation circuits which
permit a smaller transformer to be used in a subscriber line
circuit. In Kelsey's circuit a tertiary winding is added to the
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transformer and connected to a battery via a resistor. The
value of the resistor is matched or adjusted in relation to the
resistance of the subscriber loop so that the loop current in
the primary winding is balanced by the current through the resistor
and the tertiary winding. A-C signals induced in the tertiary
winding are bypassed around the resistor by a capacitor connected
in parallel therewith. However, Kelsey's circuit introduces signal
loss via the bypass capacitor and, in each installation of the
line circuit, the value of the resistor must be adjusted for the
particular subscriber loop to which the line circuit is connected.
In the patent to Macrander, the above-mentioned
disadvantages are substantially overcome; however, this circuit
is also unsuitable for interfacing with an integrated circuit
codec.
A typical subscriber loop circuit includes tip and
:~ leaJs
ring ~Lea~4 which can be meters or kilometers in length. Con-
sequently, current leakage from the tip lead and from the ring
lead is seldom if ever equal. Hence the loop current conducted
in the opposite halves of the primary winding in a repeat coil
are different and the amount of this difference varies from
one subscriber loop to the next. Also, in each subscriber loop
this difference changes with aging weather variations. These
changes are not compensated in Macrander's circuit. The
compensation circuit taught by Macrander monitors only the
current from one side of the subscriber loop and compensates
for the total current on the assumption that the tip and ring
currents are balanced. The resulting residual d-c flux in the
transformer core can be accommodated by ensuring that the core
material is of sufficient bulk to prevent saturation. This
solution is unusable to solve applicant's problem since it is
required that only normal a-c voice signals be transmitted
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through the repeat coil in order that the associated codec be
adequately protected.
In some cases within normal operating conditions,
the subscriber loop is intentionally unbalanced by the addition
of an impedance to ground at the subscriber set. By this means,
individual parties on a party line are identified or the operating
state of a coin telephone is ascertained. Canadian patent
No. 975,092 issued to J.A. McCrudden on September 23, 1975, gives
an example of the application of loop imbalance. Imbalances of
this type are not compensated by the circuits of Kelsey or
Macrander.
The present invention is a telephone line circuit
in which a transformer includes a core of saturable magnetic
material, a first winding for connection to a subscriber loop
and a power source, a second winding for connection to a
switching facility, and a balancing winding. A current sensing
circuit~ is responsive to current flow in the subscriber loop
for generating a signal proportional to the sum of the direct
current components in the tip portion and in the ring portion
of the subscriber loop. A current regulation means is connected
in series with the balancing winding and is responsive to
the signal from the current sensing circuit to pass a current
from the power source via the balancing winding in an amount
sufficient to cancel the direct current flux of the first
winding. Abnormally large pulse signals from the subscriber
loop cause the core material to saturate and thereby impede the
transmission of signals between the first and second windings.
The signal from the current sensing circuit is
also useful for detecting transitions in the state of the
subscriber set connected to the subscriber loop. Hence a dial
tone requirement or a ring trip requirement, as for example
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indicated by the subscriber set going OFF-HOOK, is determined by
additional circuits responsive to the signal from the current
sensing circuit.
An example embodiment of the invention will now
be described w'th reference to the accompanying schematic drawing
of a telephone line circuit.
The telephone line circuit includes a repeat coil
or transformer 10 which is illustrated as being connected between
a subscriber apparatus 5 via terminals 1 and a subscriber loop 9,
and a codec 4 with its associated diode protection network via
terminals 2. The transformer 10 includes a first or primary
winding having first and second half windings 12 and 13. The
winding 12 is connected in series between ground and one of the
terminals 1 via a resistor 17. The winding 13 is connected ~
between a source of d-c power -V and the other terminal 1 via ~ ;
a resistor 18 and the break contact portion of relay transfer
contacts Kl. The resistors 17 and 18 are of similar ohmic value.
The windings 12 and 13 are joined by a capacitor l9a and also
joined by a series combination of a capacitor l9b and a relay
break contact K2. A make relay contact X3 is connected in
parallel with the winding 12. ~ second or secondary winding 14
is connected to the terminals 2. The make contact portion of
the contact Kl is connected between the resistor 18 and a
ringing source 3 via a terminal 8.
The transformer is basically similar in operation
to a typical repeat coil which operates by passing current
through the primary winding between ground and -V via the
subscriber apparatus 5. The subscriber apparatus includes a
switch hook contact 5a and a modulating device 5b, for
example a carbon microphone or the like. Fluctuations imparted
to the loop current flowing through the microphone 5b are passed
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or transmitted via the subscrlber loop, the primary and secondary
windings 12, 13 and 14 to the codec 4. Likewise a-c signals
from the codec 4 are transmitted via the secondary and primary
windings 14, 12 and 13 to the subscriber apparatus 5.
The transformer 10 also includes a balancing
winding 15. All the windings 12, 13, 14 and 15 reside adjacent
one another, on a ferrite pot core 16. The winding 15 is
provided to compensate for direct current in the windings 12 and
13, and is poled relative the primary winding as indicated by
conventional dot notation. Direct current in the balancing
winding is determined by a compensation circuit which includes a
current sensing circuit and a regulation circuit.
The current sensing circuit includes a differential
amplifier 20 having two inputs. One input is connected to the
junction between the resistor 18 and the winding 13 via a
resistor 22 and the other input is connected via a resistor 21
to the junction between the resistor 17 and the winding 12. The
resistors 21 and 22 are similar in ohmic value. A resistor 24
is connected between the output and the one input of the
differential amplifier 20. A resistor 23, similar in ohmic
value to the resistors 21 and 22, is connected between the
other input and -V. The one and the other inputs of the
differential amplifier are connected to ground via resistors 26
and 25 respectively. The resistances 21 - 26 in combination
substantially determine the amplification factor of the
differential amplifier 20.
In operation, the current sensing circuit
produces a voltage signal at the output of the differential
amplifier 20 which is substantially proportional to the sum of
the voltages developed across the resistors 17 and 18.
The current regulation circuit includes a
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differential amplifier 30 having a non-inverting input connected
to the output of the differential amplifier 20 via a resistor 27
and connected to ground via a capacitor 28. The output of the
differential amplifier 30 is connected to the base electrode of
a transistor 31. The balancing winding 15 is connected between
-V and the collector electrode of the transistor 31. The emitter
electrode of the transistor 31 is connected to ground via a
t~
resistor 32 and~the inverting input of the differential
amplifier 30. A diode 33 is connected across the base and
emitter electrodes of the transistor 31. A diode 35 and a
zener diode 36 are connected in series across the balancing
winding 15. A diode 37 is connected from the collector electrode
of the transistor 31 to ground.
In operation, the current regulation circuit
responds to the average signal voltage appearing at the
output of the differential amplifier 20. Short term fluctuations
having frequencies in the voice band are not responded to. The
resistor 27 and the capacitor 28 act as a low pass filter having
a time constant of about 50 ms. The voltage appearing at the
non-inverting input of the differential amplifier 30 causes the
amplifier to drive the transistor 31 to conduct current in
sufficient quantity to develop a corresponding voltage across
the resistor 32. Hence the ohmic value of the resistor 32 and
the number of turns in the balancing winding 15 determine the
proportional relationship between the signal from the current
sensing circuit and the resulting ampere turns of compensation.
The diode 35 and the zener diode 34 are used to short circuit
any unduly large negative potentials which might be induced
from the subscriber loop into the balancing winding 15. Positive
potentials induced into the balancing winding 15 are conducted
via the diode 37.
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In one practical embodiment of the line circuit
an optimum degree of compensation is obtained by providing the
resistive components as thick film resistive components on a
substrate. After the circuit is assembled, it is operated with
a simulated subscriber loop circuit to facilitate a final tuning
or optimization of its operating characteristics. Laser trimming,
well known and practiced as a means by which the values of thick
film components are adjusted, is used to arrive at the optimum
values for loop direct current compensation and common mode
rejection in the current sensing circuit. Optimum common mode
rejection by the differential amplifier 20 is preferred in the
current sensing circuit. This is to ensure that longitudinally
induced voltages have no significant effect upon current
compensation in the transformer 10.
The line circuit also includes detection and
ringing circuits for transferring an indication of OFF-HOOK and
dial pulse signals to the switching facility and for supplying
ringing signals to the subscriber loop. A diffexential
amplifier 40 includes inverting and non-inverting inputs, the
inverting input being connected to the emitter electrode of the
transistor 31 via a resistor 38. A capacitor 41 is connected
between the inverting input and the output of the amplifier 40.
Diodes 42 and 46, with their respective cathodes connected in
common,are connected in series-opposing relationship between
the output of the differential amplifier 40 and the base
electrode of a transistor 47. Resistors 43 and 49 are connected
in series between the junction of the diodes 42 and 46f and
the non-inverting input of the differential amplifier 40.
The junction between the resistors 43 and 49 is connected to a
ring control circuit 60 in the switching office via a terminal 61.
The non-inverting input of the differential amplifier 40 is also
connected to the power supply -V and to ground via resistors 44
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and 45 respectively. The emitter electrode of the transistor 47
is connected to ground. A relay K is connected between -V and
the collector electrode of the transistor 47. A diode 48 is
connected in parallel with the relay K. The relay con~acts Kl,
K2 and K3 are associated with and operated by the relay K.
A differential amplifier 50 has a non-inverting
input connected to -V and ground via resistors 52 and 51
respectively, and an inverting input connected directly to the
output of the differential amplifier 20. The output of the
differential amplifier 50 is connected to an output terminal 55
via a resistor 53. The anode of a diode 54 is connected to the
terminal 55 and the cathode of the diode 54 is connected to the
output of the differential amplifier 40.
The differential amplifier 50 operates in response
to the signal from the current sensing circuit to indicate at
the terminal 55, the "hook" state of the subscriber apparatus 5.
It also follows dial pulse signals from the subscriber apparatus 5.
The output state of the differential amplifier 50 changes as
the signal traverses the reference potential derived by the
resistors 51 and 52/ as for example in the case of a transition
of the subscriber apparatus from the ON-HOOK to the OFF-HOOK state.
The differential amplifier 40 responds more slowly to variations
in the potential at the emitter electrode of the transistor 31.
In the ON-HOOK state, the emitter electrode of the transistor 31 -
is usually near or at ground potential as there is no loop current
intended. However leakage current on one side of the loop and
possibly via the subscriber apparatus may exist and must be ~-
compensated for. Such leakage current causes the transistor 31
to conduct a small amount of current. The inverting input of
the differential amplifier 40 is at a lower potential than the non-
inverting input which is biased positively from the ringing control
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clrcult 60 via the terminal 61 and the resistor ~. Thus the
output of the differential amplifier 40 is positive, preventing
current flow at the base electrode of the transistor 47 to
maintain the K relay in its rest state. The power source -V
is thus connected to the subscriber loop 9 via the break portion
of the transfer contact Kl and the windings 12 and 13. The
capacitor l9b is connected between the windings 12 and 13 via
the break contact K2 to provide a low a-c impedance shunt across
the resistors 17 and 18.
In the event that the subscriber apparatus 5 goes
OFF-HOOK, the potential at the emitter electrode of the transistor
31 becomes more negative and the relay K remains at rest. The
output of the differential amplifier 20 crosses the threshold
voltage determined by the resistors 51 and 52 causing the output
of the differential amplifier 50 to become positive. This positive
voltage is transmitted via the terminal 55 to the switching office.
Subsequently dial pulses are typically relayed from the subscriber
loop in like manner via the differential amplifiers 20 and 50.
When the subscriber apparatus 5 returns to the ON-HOOK state, the
20 output of the differential amplifier 50 is restored to the '
previous negative state. During dialling, the slow response ,~
time of the differential amplifier 40 prevents any significant
fluctuations in the state of the positive output of this
amplifier.
In the case of a ringing requirement, the ringing
control circuit 60 in the switching facility applies a negative
potential, for example -6 volts, to the junction of the resistors
43 and 49 in accordance with the required ringing interval. This ,
causes the base emitter junction of the transistor 47 to become
30 forward-biased, thus switching the transistor ON and operating ~ ,
the contacts Kl,K2 and X3 associated with the relay K. The ~
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transfer contacts Kl disconnect -V and connect the ringing source 3
to the line circuit to supply ringing signals to subscriber
apparatus 5 via the subscriber loop 9. The ringing signal is
typically an alternating current signal of about 80 to 120 volts
in a frequency range of between 15 and 60 HZ, imposed upon a
d-c voltage similar to the voltage from -V. The break contact K2
disconnects the capacitor l9b from the circuit and the make
contact K3 short circuits the winding 12. Hence the impedance
between the ringing source and ground is substantially increased
and the a-c impedance of the winding 13 is reduced to almost
zero. At the same time, the threshold voltage at the non-inverting
input of the differential amplifier 40 becomes more negative
as a result of the -6V potential from the ringing control
circuit 60 applied via the resistor 43. As the input potential
at the inverting input of the differential amplifier 40 is at
about ground, the output becomes negative. This clamps the
potential at the terminal 55 at a negative value via the diode 54, -
and so inhibits the output of the differential amplifier 50.
As direct current continues to be supplied to the subscriber
loop from -V during the dormant portion of the ringing int~rval,
the circuit performs as previously described when the subscriber
apparatus goes OFF-HOOK during the dormant period. When the
subscriber apparatus goes OFF-HOOK during an active portion of ~ -
the ringing interval the differential amplifier 30 in the current ~ ~-
regulation circuit responds to the output of the differential
amplifier 20 in the current sensing circuit to permit current to
flow in the third winding 15 via the transistor 31. As the
voltage at the emitter electrode of the transistor 31 becomes
more negative, the potential at the inverting input of the
differential amplifier 40 crosses the threshold potential at the
non-inverting input causing the output to become positive. This
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has the effect of turning the transistor 47 OFF via the diodes 42
and 46 and unclamping the output of the differential amplifier 50
via the diode 54. Hence the relay K returns to its rest state,
disconnecting the ringing source 3 and reconnecting -V to the
loop 9, and an OFF-HOOK indication is presented to the switching
office via the terminal 55.
Other embodiments of the invention are envisaged
in which for example the currents in the tip and ring leads are
respectively sensed by current transducer means incorporating
for example Hall effect devices or magnetoresistive elements.
However, in every case the sum of the currents in the tip and
ring leads is determined and used to control the amount of
compensating current in the third winding 15. This allows the
subscriber loop energizing current to be substantially compensated -
in the transformer 10 in spite of tip and ring current imbalances,
intentional or otherwise.
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