Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TELEPHONE LINE INTERFACE CIRCUIT WITH VOLTAGE CONTROL
Technical Field and Industrial Applicability
This invention relates to line interface circuits for telecomrnunications lines. Line
interf~ce circuits are commonly connected to two-wire telephone subscriber lines at a
5 telephone central office or remote termin~l in order to provide a variety of well-known
desired functions.
Background Art
Various forms of line interface circuit, and va;ious desirable features in line
interface circuits, are known. In particular, it is known for example from Rosenbaum
United States Patent No. 4,484,032 issued November 20, 1984 and entitled "ActiveImpedance Transformer Assisted Line Feed Circuit" to provide a line interface circuit in
which two amplifiers have their outputs coupled to the tip and ring wires of a two-wire
telephone line via feed resistors and primary windings of a very small transformer. In this
arrangement, a secondary winding of the transformer and a resistive network coupled to
15 the feed resistors provide for sensing of a.c. and d.c. conditions on the line.
It is also known from Bolus et al. United States Patent No. 4,431,868 issued
February 14, 1984 and entitled "Solid State Telephone Line Interface Circuit With
Ringing (~apability" to provide a line interface circuit in which a low-level ringing
reference signal can be supplied to the input of high-voltage tip and ring wire amplifiers,
20 to be amplified thereby to produce a desired high voltage ringing signal on the line. The
supply voltage rails of the amplifiers are controlled to follow, with an offset, the
in~t~nt:~ncoUS values of the ringing signal waveform in order to reduce power dissipation.
D.c. levels at the amplifier outputs are determined by d.c. potentials at the amplifier inputs
and the d.c. gain of the amplifiers. This arrangement has the disadvantage that the
25 amplifiers must operate with a substantial gain to produce the large signal amplitudes at
their outputs, and consequently have a relatively narrow bandwidth.
With evolution of telephone systems, it is desirable to provide improved line
interface circuits which in particular have a bandwidth which is sufficiently great to
accommodate ISDN (integrated selvices digital network) services, for example a signal
30 bandwidth of the order of 200 kHz. At the same time, it is desirable to provide
improvements in line interface circuits with respect to such features as their size, cost,
versatility, and operation especially in relation to fault conditions, common mode signal
rejection, and power consumption and dissipation.
Considered generally, there is a need for a line interface circuit which can be used
35 to operate in conjunction with any arbitrary telephone commlmications line to provide
~biLI~ y voice and data communications services as may be desired at any particular time
and which the line is capable of carrying, which selvices can be readily changed under
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software control from a telephone central office processor without requiring any hardware
changes of the line interface circuit.
While the line interface circuit desclibed in United States Patent No. 4,484,032provides considerable advantages over other line interface circuits, in particular in
S allowing precise longihl-lin~1 balance and the use of a very small transformer facilitating
improved noise performance and bandwidth which is independent of the resistance values
used in the resistive network, which accordingly can be high to minimi7~ power
consumption and dissipation, nevertheless some disadvantages remain, in particular
relating to noise susceptibility, a residual non-uniformity in the frequency response, and
10 an inability to monitor common mode current separately from differential or loop current,
as is desirable in a highly versatile line interface circuit. These disadvantages are
discussed further in the det:~iled description in this specification.
In order to provide cancellation of common mode signals, and to provide a
substantially constant threshold for ground fault current limiting, it is known from Rosch
lS et al. United States Patent No. 4,764,956 issued August 16, 1988 and entitled 'iActive
Impedance Line Feed Circuit" to connect a potential d*ider between the tip and ring wire
voltage taps of the resistive network, in a transformerless line interface circuit, and to
connect a d.c. amplifier to the tapping point of the potential divider. The output of the
d.c. amplifier is coupled to the voltage taps via oppositely-poled parallel diodes and
20 resistors, and the voltage taps are also coupled to a control circuit differential amplifier and
to a compensating circuit differential amplifier. While such an arrangement provides some
advantages in operation of the line interface circuit, each of the multiple amplifier stages
contributes a phase shift, resulting in potential instability and limited bandwidth. For a
wideband line interface circuit it is necessary to minimi7e. the number of cascaded
25 amplifier stages which are used.
In Rosch et al. United States Patent No. 4,947,427 issued August 7, l990 and
entitled "Protection Arrangement For A Subscriber Line Interface Circuit" there is
described an aurangement for protecting a line interface circuit from faults on the line by
opening contacts of an isolation relay via which the line interface circuit is connected to the
30 line. In this arrangement, current on the line is detected, when the relay contacts are
closed, by a resistive network coupled to the feed resistors, and voltage is detected on the
telephone line side of the relay contacts, so that the line voltage is detected whether or not
the relay contacts are closed. A capacitor is charged in dependence upon the detected line
voltage during positive half-cycles, and in dependence upon the detected line current
35 exceeding a threshold level duling negative half-cycles, of an a.c. waveform on the line,
and the capacitor voltage is compared with a threshold value for controlling the relay.
An object of this invention is to provide an improved line interface circuit.
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Disclosure of the Invention
This invention provides an active impedance line interface circuit for connection to
tip and ring wires of a two-wire telephone line, comprising: tip and ring d.c. amplifiers
having outputs d.c. coupled to resistive feed means for connection to the tip and ring
5 wires respectively, each d.c. amplifier having a gain of approximately unity; means for
monitoling loop culTent and common mode culTent conducted via the resistive feedmeans; means for selectively determining d.c. potentials at inputs of the d.c. amplifiers
thereby to determine d.c. levels at the outputs of the amplifiers; and digital control means
responsive to the monitored cul~ents for controlling the means for selectively determining
10 d.c. potentials.
~ he line interface circuit preferably includes two capacitors for coupling an a.c.
signal wi~ opposite phases from respective low impedance sources to the inputs of the
two d.c. amplifiers, the capacitors also serving to smooth the d.c. potentials at the inputs
of the d.c. amplifiers.
Conveniently the means for monitoring loop culTent and common mode culTent
comprises means, such as an analog-to-digital converter, for supplying digital signals
representing the monitored currents to the digital control means.
Bnef Description of the Drawin~
The invention will be further understood from the following descliption with
20 reference to the accompanying drawings, in which:
Fig. 1 is a block diagram illustrating components of a programmable telephone
line interface circuit; and
Figs. 2 and 3 are circuit diagrams illustrating parts of the line interface circuit of
Fig. 1.
25 Detailed Description
Referring to Fig. 1, there is illustrated a block diagram showing main components
of a programmable telephone line interface circuit, which is assumed in the following
descliption to form part of and be located at a telephone central office (not shown), or CO,
but which may ~lt~rn~tively form part of a remote ter-m--inal which is coupled to a CO via a
30 multiplexed commllnications path.
These main components of the line interface circuit comprise a line drive circuit
10, a sensing network 12 via which the line drive circuit 10 is coupled to the tip wire T
and the ring wire R of a telephone line 14, a tr~n~mi~ n interface 16 which
commllnicates with the rem~in-ler of the CO via tr~n~mit and receive signal paths 20 and
35 22 respectively, and a power and sign:~lling control interface 18 which communicates
signals to and receives progr~mming comm:~ntl~ from other parts of the CO via respective
paths 24 and 26 respectively. As further desclibed below, the sensing network 12supplies sensed current and voltage signals to the interfaces 16 and 18 via paths 28 and 30
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respectively, and the interface 18 supplies control and other signals to the line drive circuit
10 and to the tr~nsmi~sion interface 16 via paths 32 and 34 respectively. In the drawings,
including Fig. 1, where desirable for simplicity and clarity multiple line paths are
represented by a single line with a transverse bar.
The line interface circuit of Fig. 1 is intended to be operable, with appropriate
eomm~n(1s" including down-loaded progr~mming software, supplied to the interface 18
from the CO and used for control purposes and for setting up the tran~mi~ion interface
16 in a desired manner, to provide any of a wide range of eommunieations serviees for
any of a wide range of eommunieations e~uiplllellt (not shown) eoupled to the telephone
10 line 14. Accordingly, this single form of the line interface circuit can be used for
extensive types of services without any hardwa-re changes, simply by approp-riate
software control from the CO. This results in substantial commercial benefits, in that only
a single design of line interface eircuit need be m~n~lf~ctllred and installed to provide
arbitrary eommunieations services, and the line interface cireuit can be easily reeonfigured
15 under software control to change the services which itprovides.
Examples of communications services which can be provided by the single design
of line interfaee eireuit inelude residential telephone services such as customer local area
sign~lling (CLASS) and call management (CMS) services, multi-party services, andfrequency seleetive ringing; business telephone and data services such as individual line,
20 attendant console, message waiting, coin, ISDN U, and switehed voiceband data services;
and switehed special services such as Centrex lines, WATS lines, and direct dialling.
In order to provide such serviees, the line interfaee circuit provides a large number
of features through the design and arrangement of the interfaces 16 and 18, ineluding
ringing, dial pulse digit colleetion, loop or ground start, DC feed with curlent limi~ing,
25 low power standby, battery reversal, modem tr:~nsmi~sion, ABCD bit sign~lling, CLASS
and DTMF tone tr:~n~mi~ion, on-hook and off-hook signalling, and surge proteetion
with automatie reeovery. To this end, the tr~n~mi~ n interface 16 comprises a digital
signal proeessor and eodee (coder-decoder), control software for which is supplied from
the CO via the interface 18 and the paths 34. The interface 18 comprises a processor,
30 memory, communications interfaces, and control circuits for controlling the overall
funetioning of the line interfaee circuit. Except for parts described below with reference to
Fig. 3 of the drawings, these interfaces 16 and 18 are not described further here.
Fig. 2is a eircuit diagram illustrating parts of the sensing network 12, and Fig. 3
is a eireuit diagram illustrating the line drive eireuit 10 and assoeiated parts of the
35 interfaees 16 and 18. Output lines 36 of the line drive circuit 10, shown at the left-hand
side of Fig. 3 and also indicated in Fig. 1, are coupled to the tip and ring wires of the
telephone line 14 via the sensing network 12 as already explained above.
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Referring to Fig. 2, the wires 36 aue coupled to the telephone line tip and ringwires T and R via primary windings, sensed as shown by dots adjacent the windings, of a
transformer 38, matched feed resistors 40, and relay contacts 42. A resistor network 44
is connected to the feed resistors 40 to provide on lines 46 a balanced output dependent
5 upon current flowing through the feed resistors 40 and on the telephone line 14. A
secondary winding of the transformer 38 provides on lines 48 an output dependent upon
the differential alternating current flowing through the primary windings and hence on the
telephone line 14.
The two lines 46 are connected to the inverting inputs of respective differential
10 amplifiers 50, whose non-inverting inputs are supplied with a reference potential Vr. For
convenience, the label Vr is used commonly at various places in Figs. 2 and 3 to denote a
reference potential, for example half the supply voltage, but it should be understood that
dir~~ l actual values of reference potential can be provided at different ones of these
places to provide desired operating conditions. The output of one of the amplifiers 50 as
15 shown (or alternatively the differential outputs of the two amplifiers 50) constitutes a
~signal ID on the line 28. This signal ID represents the differential current on the telephone
line 14, and is supplied to the tr~n~mi~ion interface 16 as already described.
Each of the amplifiers 50 has a d.c. feedback path from its output to its inverting
input via two series-connected resistors 52 and 54, and an a.c. feedback path from its
20 output to its inverting input via a selies-connected resistor 56 and capacitor 58 connected
to the junction between the resistors 52 and 54. Each line 48 from the secondary winding
of the transformer 38 is connected to a respective junction between the resistor 56 and
capacitor 58.
A potential divider is formed by two resistors 60 connected in series between the
25 outputs of the amplifiers 50, the junction between these resistors being connected to the
non-inverting input of an amplifier 62 whose inverting input is supplied with a reference
potential Vr. The output of the amplifier 62 is coupled via respective resistors 64 to the
two lines 46.
Another potential divider is formed by two resistors 66 connected in series
30 between the tip and ring wires T and R of the telephone line 14, on the line side of the
relay contacts 42. The junction between these resistors 66 is connected to a ~ulllllling
node 68, to which the output of the amplifier 62 is connected via a resistor 70. The
~ullllning node 68 is connected to the input of a buffer amplifier 72, whose output can
constitute a common mode voltage signal VCM on a line 74 which constitutes one of the
35 paths 30 in Fig. 1.
The line 74 is also connected via a resistor 76 to the inverting input of a differential
amplifier 78, whose non-inverting input is supplied with a reference potential Vr and
whose output, which is connected via a feedback resistor 80 to the inverting input, is a
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common mode current signal ICM on a line 82 constituting another of the paths 30. A
further one of the paths 30 is constituted by a line 84 carrying a loop current signal IL
from the output of a buffer amplifier 86. The input of the buffer amplifier 86 is connected
via a resistor 88 to the line 28 and via a capacitor 90 to ground, the resistor 88 and
5 capacitor 90 together forming a low pass filter with a corner frequency of about 200 Hz.
The arrangement of the transformer 38, feed resistors 40, and resistor network 44
as described above and illustrated in Fig. 2 is known for example from United States
Patent No. 4,484,032 already referred to. This arrangement allows precise longitudinal
balance of the circuit to be achieved, using a very small transformer as the transformer 38
10 for a.c. sensing, and using resistors matched to ordinary tolerances (for example 1%) for
the feed resistors 40 and resistor network 44. High resistance values used in the resistor
network 44 reduce power consumption during idle conditions (on-hook state of a
telephone connected to the line 14), and the transformer facilitates good noise perfolmance
and a bandwidth independent of the resistor values used.
As described and illustrated in Patent No. 4,484,032, one terminal of the
secondary winding of the transformer is grounded, and the other termin~l is capacitively
coupled to the inverting input of a differential amplifier having a.c. and d.c. feedback
paths. The non-inverting input of the differential amplifier forms a ground reference for
the amplifier, whose gain is determined by the ratio of an a.c. feedback resistance (96 in
20 Fig. 3 of the patent) to the resistance of the secondary winding of the transformer. As the
latter is desirably small, this gain is relatively large. Any potential difference between the
grounded terminal of the transformer secondary winding and the ground reference at the
non-inverting input of the amplifier constitutes a noise source, which is amplified by this
large gain. Especially where the amplifier is implemented in an integrated circuit, as is
25 desirable for economy and small size, such a potential difference and consequent noise is
difficult to avoid.
This disadvantage is avoided by the use in the circuit of Fig. 2 of the two
amplifiers 50 in a balanced configuration. The amplifiers 50 operate with opposite phase
symmetrically about the reference potenhal Vr supplied to their non-inverting inputs,
30 which determines the potential of the lines 46, so that susceptibility to differential ground
noise as in the prior art is eliminz~e~
This arrangement has the advantage, compared with the multi-stage cascaded
amplifiers of United States Patent No. 4,764,956 already referred to, that only a single
stage of amplification (split between the two balanced amplifiers 50) is used to provide a
35 differential mode output signal, and no compensation scheme is necessary as in that patent
because of the balanced or symmehical anangement of the amplifiers 50. This
arrangement thus provides less phase shift, and hence more stable operation over a greater
bandwidth, than this prior art.
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As in the prior art represented in Fig. 3 of United States Patent No. 4,484,032,the a.c. and d.c. feedback paths of the sensing amplifiers 50 are arranged to provide the
same gain for a.c. signals as at d.c. In the prior art, the secondary winding of the
transformer is coupled to the inverting input of the sensing amplifier via only a coupling
capacitor which forms part of the a.c. feedback path of the amplifier. However, because
the transformer secondary winding has a finite resistance, there is a voltage drop across
the coupling capacitor which in turn causes a variation in gain and phase in the transition
between d.c. and a.c. signals. This can give rise to distortion of low frequency a.c. or
pulsed d.c. signals, such as dial pulses, or to instability when the output of the amplifier
10 is used in a feedback loop for d.c. control purposes.
This problem of the plior art is avoided in the circuit arrangement of Fig. 2 by the
provision of the resistors 54, one for each of the amplifiers 50 in view of tlle balanced
arrangement. To understand this, it can be seen that the outputs of the amplifiers 50
swing in a balanced manner and with opposite phase about the reference potential Vr
15 applied to the non-inverting inputs, so that the mid-point of the transformer secondary
winding is at this reference potential and hence at the same potential as the lines 46. For
each side of the balanced arrangement, the coupling capacitor 58 is connected between the
tapping points of two potential dividers between the output of the amplifier 50 and the
reference potential Vr, one formed by half the resistance of the secondary winding in
20 series witll the resistor 56, and the other formed by the resistors 54 and 52. By making
the ratio of these two potential dividers equal, there is no net voltage across the capacitor
58, resulting in a more uniform amplifier response in the transition from d.c. to a.c.
signals.
In other words, expressing the resistances of the resistors 52, 54, and 56 and the
25 transformer secondary winding as R52, R54, R56, and Rw respectively, the resistance
values are selected so that R54 = Rw x R52 / 2 x R56. However, it should also beappreciated that this relationship may be modified if desired to control the transition of the
amplifier response between d.c. and a.c. signals, for example by over-compensating for
the transfolmer winding resistance to provide, with a desired capacitance of the capacitor
30 58, an attenuation at a frequency of 60 Hz.
The amplifier 62 selves to reduce common mode signals on the lines 46, through
the coupling of its output to these lines via the resistors 64, in a similar manner to that of
the prior art except that, in view of the balanced arrangement of the circuit, the non-
inverting input of the amplifier 62is connected to the tapping point of the potential divider
35 formed by the resistors 60 connected between the outputs of the amplifiers 50, and the
inverting input of the amplifier 62is connected to a reference potential Vr.
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In order to determine all of the currents on the telephone line 14, it is necessary to
monitor the common mode current on the telephone line as well as the differential current,
represented by the signal ID, or the loop current, represented by the signal IL. The loop
current signal IL is produced on the line 84 from the differential current signal ID by
5 filtering in the low-pass filter con~titlltecl by the resistor 88 and capacitor 90 and buffering
in the buffer ampli~ler 86. The low-pass filtering attenuates frequency components above
about 200 Hz, so that the signal IL represents the d.c. loop current. The currents on the
tip and ring wires of the telephone line are respectively the sum and difference of the
common mode current and the loop current.
The output voltage of the amplifier 62 is (--VT - VR + Icm x R40) x R64 / R44,
where VT and VR are the voltages of respectively the tip and ring wires T and R of the
telephone line 14, Icm is the common mode current through the feed resislors 40, R40
and R64 are the resistances of each of the resistors 40 and 64 respectively, and R44 is the
resistance of each of the resistors in the resistor network 44. At the summing node 68,
this output voltage is summed with a voltage derived from the potential divider formed by
the resistors 66 and which is dependent upon the common mode voltage and not on the
cornmon mode current. This voltage is given by (VT + VR) x R70 / R66, where R70 and
R66 are the resistances of the resistor 70 and each resistance 66, respectively. The
resistance values are selected so that R66 / R70 = R44 / R64, whereby at the Sllmmi ng
node 68 the sllmm~l voltage is independent of the common mode voltage component
VT + VR, and represents only the common mode current. This summed voltage at thenode 68 is buffered by the buffer amplifier 72 and amplified by the amplifier 78, whose
gain is determinecl by the resistors 76 and 80, to provide the signal ICM on the line 82
representing, with a desired sensitivity, the common mode current on the line 14.
When the relay contacts 42 are opened for example for protection of the line
interface circuit, the common mode current through the feed resistors 40 becomes zero.
The output voltage of the ampli~ler 62 then represents only the sum of the drive voltage
levels provided by the line drive circuit 10 as described further below, while the common
mode voltage on the telephone line 14 continues to be monitored via the resistors 66.
Accordingly, the s- mming node 68 in this situation is at a summed voltage representing
the common mode voltage on the line 14 referenced to the sum of the drive voltage levels.
This sllmm~l voltage, buffered by the buffer amplifier 72, constitutes the common mode
voltage signal VCM on the line 74. Thus common mode voltages on the line 14 are
monitored when the relay contacts 42 are open.
Referring now to Fig. 3, there is illustrated a circuit diagram of the line drive
circuit 10 together with a signal coupling circuit 100, which can be considered to form
part of the tr~n~mi~ion interface 16 in Fig. 1, and control circuitry 102, which can be
considered to be part of the power and ~ign~lling control interface 18 in Fig. 1.
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The signal coupling circuit 100 comprises two differential amplifiers 104 and 106,
resistors evenly numbered from 108 to 116, an impedance (Z) 118, and a capacitor 120.
The impedance 118 can complise fixed or switched reactive components as is known in
the prior art, but for full versatility of the line interface circuit is preferably a
programmable impedance.
The differential current signal ID on the line 28, coupled via the capacitor 120,
constitutes a transmit path signal Tx and is also fed back to the inverting input of the
ampli~ler 104 via the impedance 118 and the resistor 110. A receive path signal Rx is
coupled to the inverting input of the amplifier 104 via the resistor 112. The amplifier 104
10 has its non-inverting input coupled to a reference potential Vr, and has a feedback path
including the resistor 108 from its output to its inverting input. The resistor 108
determines, with the resistors 112 and 110, the gain of the amplifier 104 respectively for
the receive path signal Rx and for the signal fed back via the impedance 118.
The output of the amplifier 104 is supplied on a line 128 to the line drive circuit
10. The resistors 114 and 116 and the amplifier 106 together form a unity gain inverter
which inverts this output and produces a complementary output on a line 130 to the line
drive CilCUit. The lines 128 and 130 are also referenced in Fig. 1.
The d.c. blocking of the signal ID by the capacitor 120 results in the impedance118 and the amplifiers 104 and 106 having to accommodate only the relatively low voltage
signals being tr~n~mittecl and received, and not larger d.c. components which may exist
on the telephone line 14. Consequently, the amplifiers 104 and 106 can be designed to
have a large bandwidth as is desired, with the amplifier 104 still providing a significant
gain.
The line drive circuit 10 comprises two d.c. buffer amplifiers 132 and 134 whoseoutputs are connected to the wires 36 on respectively tip wire and ring wire sides of the
line drive circuit. The outputs of the amplifiers 132 and 134 are also connected directly to
the inverting inputs of the amplifiers, respectively, to provide unity gain so that these
amplifiers have a m~illlulll bandwidth and a minimllm output impedance. The line 128 is
a.c. coupled to the non-inverting input of the amplifier 132 via a capacitor 136, and the
line 130 is a.c. coupled to the non-inverting input of the amplifier 134 via a capacitor 138.
It is observed that the amplifiers 132 and 134 need not necessalily have a gain of
exactly unity as described here and as illustrated in Fig. 3. They may alternatively provide
a desired gain, providing a proportional relationship between the d.c. potentials at their
inputs and outputs. However, as any such gain decreases the bandwidth and increases
the output impedance of the amplifiers, it is desirably kept small and approximates to a
gain of unity.
The non-inverting input of the amplifier 132 is also coupled via a resistor 140 to a
supply voltage +V, and via a controlled current source 142 to a supply voltage -V. It is
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also coupled to the supply voltage +V via a switched current source 144, in parallel with
the resistor 140. Conversely, the non-inverting input of the amplifier 134 is also coupled
via a resistor 146 to the supply voltage--V, and via a controlled current source 148 to the
supply voltage +V. It is also coupled to the supply voltage--V via a switched current
source 150, in parallel with the resistor 146. The supply voltages +V and -V mayconveniently be the CO battery voltages of ground and -48 volts respectively, and this is
assumed in the following description, but other voltages may be used if desired.Currents passed by the current sources 142, 144, 148, and 150 are controlled by a
digital control circuit 152 which forms part of the control circuitry 102. The switched
10 cunent sources 144 and 150 are controlled via lines 154 and 156 respectively simply to be
on or off as further described below; these sources can alternatively be replaced by
switched resistors. The controlled culTent sources 142 and 148 are precisely controlled
by eight-bit digital signals supplied by the digital control circuit 152 to digital-to-analog
converters 158 and 160 respectively, and thence by stepped analog signals on lines 162
15 and 164 respectively. The lines 154, 156, 162, and 164 constitute the paths 34 in Fig. 1.
The signals IL, ICM, and VCM on the paths 30 are supplied to inputs of a
multiplexed analog-to-digital converter 166, whose digital outputs are supplied to the
digital control circuit 152. The digital control circuit is also supplied, as already explained
with reference to Fig. 1, with comm~n(ls in accordance with which it determines
20 appropriate operating conditions for the rem~in~ler of the line interface circuit and provides
control signals accordingly. As part of this control, the digital control circuit 152, which
for example is con~itllte~l by a digital state machine, monitors the digitized signals IL,
ICM, and VCM to detect (possibly faulty or undesired) operating conditions some of
which are discussed further below, and instigates action to adapt the line interface circuit
25 in relation to such conditions. This action may in particular include control of the current
sources as desclibed below, and control of the relay contacts 42 for protection of the line
interface circuit.
By way of example, it is observed that the resistors 140 and 146 may have
resistances of about 320 kQ, the switched current sources 144 and 150 may pass a current
30 of either 0 or 10 mA, the controlled current sources 142 and 148 may pass a precisely
controlled current in a range from 0 to 150 ,uA, and the capacitors 136 and 138 may have
capacitances of about 0.22 ,uF.
Because the capacitor 136 blocks d.c., the d.c. voltage level at the non-inverting
input of the amplifier 132 is determined by the supply voltages +V and -V, the resistance
35 of the resistor 140, and the controlled currents passed by the current sources 142 and 144.
Similarly, the d.c. voltage level at the non-inverting input of the amplifier 134 is
determined by the supply voltages +V and-V, the resistance of the resistor 146, and the
controlled currents passed by the current sources 148 and 150. The amplifiers 132 and
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134 are d.c. amplifiers having a desired large bandwidth, unity gain as described above,
and a large d.c. voltage range, so that the d.c. voltage levels at their non-invelting inputs
determine the d.c. voltage levels at their outputs and on the wires 36, and hence on the tip
and ring wires T and R of the telephone line 14.
Consiclçring the tip side of the line drive circuit, the current source 144 is normally
off so that it passes no current. The controlled current source 142 is controlled, for
example in 256 discrete steps determined by the 8-bit output from the digital control circuit
152 to the digital-to-analog converter 158, to pass any desired current in the range from 0
to 150 IlA. This current flows through the resistor 140, which accordingly drops a
10 voltage dependent upon the current and which can be from zero to the difference between
the supply voltages +V and -V. Accordingly, the control of the controlled current sowce
142 enables the d.c. voltage at the non-inverting input of the amplifier 132, and hence on
the tip wire T, to be steered to any desired level from +V to -V. With the resistor 140
having a resistance of 320 kQ and the supply voltages +V and -V being ground (0 volts)
15 and -48 volts as indicated above, the tip wire T can be set to any desired voltage from 0 to
~8 volts.
Similarly, on the ring side of the line drive CilCUit the current source 150 normally
passes no current, and the controlled current source 148 can be controlled via the
convelter 160 to pass any current from 0 to 150 IlA to drop a voltage from 0 to 48 volts
20 across the resistor 146, so that the non-inverting input of the amplifier 134, and hence the
ring wire R, can be steered to any voltage from -48 to 0 volts. It should be noted that the
sefflngs of the d.c. voltage level of the tip and ring wires T and R are independent of one
another, so that each wire can be set to any voltage in the desired range between the
supply voltages +V and -V. It should also be noted that the connections of the resistors
25 140 and 146 to the supply voltages +V and-V respectively enables the most common
situation, of these voltages being applied to the tip and ring wires respectively, to be
provided with the minimllm power consumption.
The current sources have a high impedance, so that they do not adversely affect
the capacitive coupling of a.c. signals from the outputs of the amplifiers 104 and 106 to
30 the amplifiers 132 and 134 respectively. The amplifiers 104 and 106 have low output
impedances, so that the resistor 140 and capacitor 136, and likewise the resistor 146 and
capacitor 138, form low pass filters each with a pole at a sufficiently low frequency, for
example about 3.5 Hz, to filter out any residual a.c. noise on the d.c. control signal lines
162 and 164. This low pass filteling action is also of particular benefit in the normal
35 course of operation in changing the d.c. voltage levels, because it smooths or filters out
the discrete steps as the d.c. voltage level is changed so that there is no degradation of
tr~n~mi~ion performance.
CA 0220~320 1997-0~-14
As a result of the filtering action, when the current passed by the controlled current
source 142 is reduced, the tip voltage set at the non-inverting input of the ampli~ler 132
will become more positive at a relatively slow rate determined by the time constant (about
70 ms) of the resistor 140 and the capacitor 136. While such a slow rate of change is
desirable for some operating situations of the line interface circuit, a faster rate of change
is desirable in other situations. Where a fast rate of change is desired, the switched
current source 144 is turned on, under the control of the digital control circuit 152 via the
line 154, for a short period det.Ql~ine-l by the digital control circuit, to discharge the
capacitor 136 at the higher current, for example 10 mA, passed by the current source 144.
10 Similarly, the switched current source lS0 is turned on by the digital control circuit 152
via the line 156 to make the ring voltage level more negative at a faster rate than is
e~ by the time constant of the resistor 146 and capacitor 138.
The independent control of the d.c. voltage levels on the tip and ling wires as
described above is useful for a variety of purposes, during normal operation, for
lS protection against faults, and for self-testing of the line interface circuit. For example, in
normal operation this independent control enables battery reversal conditions, for example
for ~ipn~lling pulposes, to be set up without using a relay. In addition, the d.c. voltage
applied to the telephone line can be set in dependence upon the loop current signal IL
supplied to the digital control circuit 152 in order to limit the loop current to a desirable
20 low level, whereby power consumption is reduced and the line interface circuit is
adaptively adjusted for different loop resistances. For protection purposes, for example in
the case of a ground fault (ling wire shorted to ground) the voltage level of the ring wire
can be changed towards 0 volts to limit current during the fault.
It should be appreciated in this respect that the provision of both the loop current
25 signal IL and the common mode current signal ICM to the digital control circuit 152
enables this circuit to determine both the tip wire current (common mode current + loop
current) and the ring wire current (common mode current - loop current), so that the d.c.
voltage levels supplied to the tip and ring wires can be individually determined in relation
to the tip and ring wire currents.
Although a particular embodiment of the invention has been described in detail, it
should be appreciated that numerous modifications, variations, and adaptations may be
made within the scope of the claims.
