Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to interface circuits for
communication lines and particularly to a line circuit for supplying
energizing current and a.c. signals to a pair of leads in a communication
line.
In telephony, a subscriber loop has typically been provided
by a communication line having a pair of leads for connecting a remote
station set to the telephone network. The communication line is
terminated at d line circuit which traditionally includes a transformer
having windings for coupling a.c. signals between the communication line
and a port associated with a telephone switching facility in the telephone
network. One of the windings in the transformer is connected to the leads
and usually provides d d.c. path for supplying energizing current to -the
communication line. Supervisory circuitry usually associated with the
line circuit is responsive to variations in the flow of energizing current
to indicate ON HOOK and OFF HOOK conditions at the station set, and to
detect d1al pulses transmitted by the station set.
As the transforlner tends ~o be the most expensive element
in a line circuit, it has been a 10ng stand1ng objective of telephony
circuit designers to either reduce the size of the transformer or
eliminate it altogether. Various trans-Formerless line circuits have been
developed. These transformerless line circuits may be categorized in two
groups in accordance with some of their common characteristics. In one
group, a.c~ signals and d.c. energizing current are supplied by a pair of
very closely matched current regulating circuits sometimes referred to as
current mirrors. Some examples of circuits in this cate~ory are described
by Frank S. Boxall in United States Patent Nos. 4,004,109 and 4,203,012
issued on January 18, 1977 and on May 13, 1980 respectively and in a
description of a circuit designated as XC3419, the circuit description
having been publ1shed by Motorola Semiconduc~or, eox 20912, Phoenix,
Arizona 85036. In the other category, high impedance electrodes of
complementary transistors are connected to provide a current source and a
current sink for connection at opposite leads in the communication line.
Current regulation is obtained by voltages applied at associated base
electrodes in conjunction with matched emitter resistors. Some examples
of circuits in this category are described by Earl Thomas Cowden in United
States Patent No. 4,041,252 issued on August 99 l977 and by Graham D.
Tattersall in United States Patent No. 4,203,009, issued on May 13, 1980.
Line circuits having a pair of current regulators in series
with the tip and ring lead of a communication line are capable of
supplying a preferred constant current for energizing the line. However
if there is less current available at one of the current regulators than
is normally regulated, the regulator tends to saturate, becoming
dynamically non-functional. Additional circuitry is required to reduce
the probability of either of the current regulators becoming saturated.
Saturation tends to occur because of current imbalance as between the tip
and ring leads of the operating communication line. Such imbalance can
occur through a leakaqe to ground somewhere along the communication line
and also through intentional imbalance introduced in a remote station set
associated with the communication line. Imbalance can also occur directly
in the line circuit because of deviation from necessarily narrow
tolerances in circuit components. Of course when either of the current
regulators becomes saturated, its d.C. irnpedance becomes virtually nil,
obviating further use of the communication line until repair or
replacement of the line circuit is effected. Line circuits in these
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categories have not yet found general acceptance in operating telephone
companies as compared to line circuits of the transformer type.
In accordance with the present invention the problem of
'!; saturation in one or the other of the current regu1ators feed paths in a
transformerless line circuit is overcome by providing a pair of resistive
feed elements for supplying some of the energizing current to the
communication line. Current regulators are provided by current source and
sink paths. These current paths conduct at least sufficient current for
a.c. signal modulation to provide for transmitting a.c. signals on the
operating communication line. The resistive feed elements substantially
accommodate differences in current flow as between one and the other of
the conductors in the communication line, and/or as between the current
source and the current sink paths and thereby maintain sufficient
operating voltage across the current regulators to prevent saturation.
The invention provides a line circuit for coupling a.c.
signals between a communication line and an a.c. signal path and for
supplying the communication line with energizing current from a d.c. power
supply. The line circuit includes a pair of line terminals for connection
; to a pair of leads in the communication line, a pair of battery terminals
for connection across the d.c. power supply, a fixed impedance means, a
variable current feed means, and a control circuit for controlling
operation of the variable current feed means. The fixed impedance means
includes a pair of resistances of predetermined value, each of the
resistances being connected in series between separate ones of the line
and battery terminals. The variable current feed means includes source
and sink current paths, the source current path being connected in series
between one of the line terminals and one of the battery terminals, and
the sink current path being connected in series between the other of the
line terminals and the other of the battery terminals. The source and
sink current paths each exhibit a.c. impedance characteristics at the line
terminals which are substantially higher than the impedance
characteristics of the fixed impedance means. The source and sink current
paths also include control inputs and ar~ responsive to control signals at
their respective inputs for conducting direct current between the battery
terminals and the line terminals. The control circuit includes first and
second outputs at which it generates the control signals such that similar
flows of current are caused to flow in the source and sink current paths.
The control circuit is also responsive to d.C. signals being received from
the a.c. signal path to generate corresponding variations in the control
signals whereby signals corresponding to the a.c. signals are transmitted
along the communication line.
The invention is also a method of supplying energizing
current and a.c. signals at one end of a two wire conductor communication
line. The method includes the steps of conducting current from a d.c.
power source via a pair of fixed resistances and the communication line,
conducting current from the d.c. power source to one of the conductors in
the communication line via a current source and conducting a similar
current from the other conductor in the communication line via a current
sink, and causing variations in the currents being conducted in the
previous step in response to a.c. signals being received from a
unidirectional a.c. signal path to transmit corresponding a.c. signals
along the communication line.
In operation current imbalances are accommodated by the
fixed resistances with the result that in normal OFF HOOK operating
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conditions the variable current feed means is always maintained out of
saturdtion, and the terminating impedance -for the comlnunication line is
substantially determined by the ohmic value of the fixed resistances.
An embodiment of a line circuit in accordance with the
invention is described with reference to the accompan,ying schematic
drawing which is identified as figure 1.
In figure 1 a pair of line terminals 10 provide for
connection of the line circuit to TIP and RING leads o~ a cornmunication
line. Ancillary to the line circuit a protector circuit 11 is connected
in series with resistors 12 and 13 across the line terminals 10.
Protector circuits for telephone lines are generally well known to persons
skilled in the telephony art and may be conveniently provided for example,
by various networks including diodes~ varistors, or other solid state
transient voltage suppressors. The protector circuit 11 in combination
with the resistors 12 and 13 each having a value of about ten to forty
ohms is intended to prevent darnage to the line circuit as might otherwise
occur when the TIP or RING lead is subjected to induced or conducted line
surges. Ringing signal is supplied to the RING lead through a ringing
terminal 1~, a ringing feed resistor 15, a make portion of a relay
transfer contact 16 and so on by way of' the resistor 12. The transfer
contact 16 is associated with a relay operate coil (not shown) which when
at rest effects a connection of the TIP and RING leads to the remainder of
the line circuit and to a hybrid circuit 17. Many examples of electronic
hybrid circuits adaptable for this application have been published.
Basically the hybrid clrcuit provides a well known 2-wiYe/4-wire
.~ conversion fuckion for voice signals between the TIP and RING leads and IN
and OUT leads 18 and 1g of a voice signal path~ The hybrid circuit 17
includes a unidirectional a.c. signal output 20 which provides for d~C.
signal injection onto the TIP and RIN& leads in accordance with a.c.
signals received at the IN lead 18. The hybrid circuit receives a.cO
signals from the TIP and RING leads via leads 21 and 22 and in combination
with a.c. signals on a lead 23 generates d.C. signals at the OUT lead 19.
A fixed irnpedance feed means is provided b~ a pair oF
resistors 24 and 25 of similar ohmic value. The resistor 24 is connected
between a positive battery terminal 26 and the leaci 22. The resistor 25
is connected between a negative battery terminal 27 and the lead 21.
A source current path is provided by a PNP transistor 28
having a base electrode, a collector electrode connected to the lead 22
and arl emitter electrode connected in series with a resistor 29 to the
positive battery terminal 26. A sink current path is provided by a NPN
transistor 30 having a base electrode~ a collector electrode connected to
the lead 21 and an emitter electrode connected in series with a resistor
31 to the negative bat~ery terminal 27.
A control circuit includes a first voltage divider~
having resistors 32 and 33 connected in series with the positive battery
terminal 26. A second voltage divider includes resistors 34 and 35
connected in series with the ne~ative battery terminal 27. Junctions of
the resistors 32 and 33, and 34 and 35, provide voltage taps which are
connected to the base electrodes of the transistors 28 and 30
respectively. A third voltage divider includes resistors 36 and 37
connected in series between the positive and negative battery terminals 26
and 27. A differential amplifier 38 includes a non-inverting input
connected to the junction of the resistors 36 and 37, and an inverting
input connected to the lead 20 via a resistor 39. A transistor 40
includes a base electrode connected in common with the lead 23 and an
output of the dif-ferential amplifier 38. The transîstor 40 also includes
an emitter electrode connected to the resistor 35 and a collector
electrode connected to the resistor 33. A feedback resistor 41 is
connected between the emitter electrode of the transistor 40 and the
inverting input of the differential amplifier 38. A ~hreshold detector 42
includes an input connected to the lead 21 and an output connected to an
input of a buffer amplifier 43. An output of the buffer ampllifier 43
provides a path for transmission of supervisory signals to associated
telephony equipment~ not shown. A diode 44 includes an anode connected to
the output of the threshold detector 42 and a cathode connected to the
junction of the resistor 39, 4l and the inverting input of the
differential amplifier 38.
In operation the line circuit performs two basic functions,
that of supplying the TIP and RING leads in the communication line with
energizing current and that of transmitting a.c. signals along the
communication line. A primary function of the transistors 28, 30 and 40
and the differential amplifier 38 is that of transmitting the a.c.
signals. However this function also involves supplying a portion of the
energizing current to the comrrlunication line. Assuming that the
communication line is remotely terminated by a communication device
providing a direct current path between the TIP and RING leads, for
example a telephone in an OFF HOOK condition, the transistor 40 is in a
conducting state. Current ~low through the transistor 40 is via the first
and second voltage dividers. The voltage at the base electrode of the
transistor 40 is reproduced, less one base emitter junction drop, at the
emitter electrode of the transistor 40. The amount of current flow is
substantially determined by the sunl ohlnic value of the resistors 34 and 35.
The resistors 35 and 33 are chosen to be of similar value and likewise the
resistors 32 and 34 are chosen to be o-F similar value. Hence the voltage
developed across the resistor 32 is similar to the voltage developed
across the resistor 34. The values of the resistors 29 and 31 are also
chosen to be similar one with respect to khe other so that currents
conducted by the transistor 28 and 30 are likewise similar one with
respect to the other. Ideally the respective resistors 24 and 25 32 and
34 and 33 and 35 should be as identical as is practical. However in the
present line circuit this matching ot` component values is not essential.
Variation of up to 5% in resistance values are tolerable. Likewise it is
not essential to have the operating characteristics of the transistors 28
and 30 closely matched.
Variations in the current conducted by the transistors 28
and 30 follow variations in the current conducted by the transistor 40.
The transistor 40 is in turn controlled by the a.c. signals being received
on the lead 20 from the hybrid circui~ 17 and amplified by the
differential amplifier 38. The average current conducted by the
transistors 28 and 30 is determined by the vol-tage at the junct-ion of the
resistors 36 and 37. The dif-ferential amplifier 38 -transfers this voltage
value acting as a voltage follower to the emitter electrode of -the
transistor 40. The differential amplifier also acts as an inverting
amplifier for signals appearing at its inverting input and with a gain as
determined by the ratio of values of the feedback resistor 41 and the
resistor 39.
In summary of the operation as so far described7 the current
source~ transistor 28~ and the current sink transistor 30r are operated to
exhibit similar d.c. resistances and thus conduct similar direct currents
to and from the RI~G and TIP leads respectivelyO However this comprises
only part of the energizing current. Alternating current signal
transmission is accomplished by varying the direct currents in accordance
with the a.c. signals being received on the lead 20. The function of
supplying the energizing current is also accomplished by the resistors 24
and 25. These resistors are chosen to be of similar value typically in a
range of about 200 to 5QO ohms for transmission of voice band signals
For transmission of higher frequency signals, for example digital pulse
signals, the value of the resistors 24 and 25 are lower, typically in a
range of 50 to 200 ohms. These resistors provide a fixed resistance feed
for the communication line. Any difference as between the currents
conducted by the transistors 28 and 30 is accommodated by the resistances
24 and 25. Likewise any imbalance along the communication line or in the
remote communication device is compensated for by the resistors 24 and 25.
This compensating function effectively prevents either of the transistors
28 or 30 from becoming saturated during normal OFF HOOK operation and
hence the terminating a.c. impedance for the communication line is
suhstantially determined by the values oF the resistors 24 and 25. In the
line circuit as illustrated in figure 1, the source and sink current paths
are shown as being connected in parallel with the resistors 24 and 25
respectively. In an alternative arrangement, the source current path is
connected between the positive battery terminal 26 and the lead 21 and the
sink current path is connected between the negative battery terminal 27
and the lead 22. In either case the energizing current -For the tip and
ring leads is the algebraic sum of the current in the feed elements
associated with each lead respectively.
The supervisory state of the communication line is detected
by the threshold detector 42. During the OFF HOOK condition the lead 21
is of a substantial potential difference with respect to the negative
battery terminal. This results in a signal at about the negative battery
terminal potential being generated by the threshold detector 42. During
the ON HOOK condition, there being substantially no energizing current
flow, the potential on the lead 21 approaches that of the negative battery
terminal. In this instance the threshold detector 42 generates a more
positive signal. This signal is gated by the diode 44 to the inverting
input of the differential amplifier 38 which in turn causes its output to
become sufficiently negative to turn OFF the transistor 40 and thereby
prevent power dissipation in the resistors 32 through 35. As
communication lines of this type are active for only a small fraction of
time, the reduced power dissipation in a group of these line circuits is a
significant saving.
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