Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
S~BSCRIBE~ LINE INTERFACE CIRCUIT W[TH IMPROVED D.C. ~ALANCE
. .
This invention relates to subscriber line interface circuits
(SLICs), which provide for coupling to two-wire telephone
subscriber lines.
Reference is directed to my copending pa-tent application
entitled "Subscriber Lirle Inter-Face Circuit with Longitudinal Current
Suppression" an~l filed simultaneously herewith, the claims of which
are directed to a SLIC including features an embodiment of which is
also described herein.
Many proposals exist for providing a SLIC in the form o-f
an integrated circuit and -for coupliny this to a two-wire telephone
subscriber line, preferably using direct coupling, i.e. without an
intervening transformer. One such proposal, in which the SLIC
comprises controlled current circuits (also referred to as current
mirror circuits~ current sources, and current sinks), is described in
Kelley et al. U.S. Patent No. 4,300,023 issued November 10, l9Sl, and
entitled "Hybrid Circuit". In this known SLIC, controlled current
circuits are coupled to the tip and ring wires of the telephone
subscriber line and are controlled to provide a desired d.c. -feed
resistance.
It is also well known that d.c. balance of two-wire telephone
subscriber lines must be closely maintained. In the SLIC described
in the above patent, it is assumed that d.c. balance will be achieved
by virtue of the fact that the controlled current circuits, which
serve to simulate d.c. feed resistors, are coupled to the tip and
ring wires using a generally symmetrical or balanced arrangement and
are formed in the same integrated circuitO This assumption fails in
practice, however, because matching of the controlled current
circuits to better than about 1% is difficult or impossible to
achieve.
Accordingly, an object of this invention is to provide
an improved SLIC which enables d.c. balance of a two-wire line, which
is coupled to the SLIC in opera-tion, to be enhanced.
According to this invention -there is provided a subscriber
line interface circuit (SLIC) comprising: two terminals for coupling
to a two-wire telephone subscriber line; two controlled current
circuits (CCCs) for respectively supplying direct current to and
"~
sinking direct curren-t from the line via the terminals; a feedback
loop responsive to direct current on the line for controlling the
CCCs; and d.c. balance control means responsive to a d.c. imbalance
of potentials at the terminals for controlling the feedback loop to
modify control of the CCCs to reduce the d.c. imbalance~ the d.c.
balance control means comprising a poten-tial divider coupled between
the terMinals and having d tapping point; and a differential
amplifier having a first input coupled to the tapping point, a second
input coupled to a point of reference potential, and an output
coupled to an ou-tput of the d.c. balance control means.
Thus in a SLIC in accordance with this invention the feedback
control of the CCCs which serve to simulate doc~ feed resistors is
modified to maintain d.c. balance.
In an embodiment of the invention the feedback loop comprises
means for producing a sum current dependent upon a sum of direct
currents flowing via the terminals; current splitting means for
splitting the sum current into two control currents; and means for
controlling the two CCCs each in dependence upon a respective one of
the two control currents; wherein the current splitting means is
responsive to the output of the d.c. balance control means to control
the relative proportions of the two control currents.
Conveniently the current splitting means comprises two
transistors having bases coupled respectively to the output of the
d.c. balance control means and to a point of reference potential, and
having emitters coupled together and supplied with the sum current,
and means for deriving the two control currents from collectors of
the transistors.
The d.c. balance control means preferably further comprises
low pass filter means coupled between the output of the differential
amplifier and the output of the d.c. balance control means.
This arrangement also enables longitudinal currents, which
are common mode alternating currents typically induced on the
two-wire telephone subscriber line from adjacent power lines and/or
as a result of cross-talk from other telephone lines, to be
suppressed in a particularly convenient manner. To this end
preferably the first input of the differential amplifier is
an inverting input, and the SLIC further comprises a resistor and a
~2~ 3
capacitor coupled in series between each o-f -the terminals and -the
output of the di-Fferential amplifier.
The invention will be -further understood from the following
description with re~erence to -the accompanying drawings, in which:
Fig. 1 schema-tically illustrdtes parts of a subscriber line
interface circuit (SLIC) in accordance with this inven-tion; and
Figs. 2 to 7 schelnatically illus-trate the particular forms of
controlled current circuits (CCCs) used in the SLIC o-f Fig. 1.
Referring to Fig. 1, only those parts of a SLIC which are
relevant to this invention are shown, other parts, for example -for
handling voice frequency signals and for applying ringing signals to
a telephone subscriber line 10 to which the SLIC is connected, are
not shown but are provided in known rnanner, for example in the manner
described in Kelley et al. U.S. Patent No. 4,300,023 already referred
to.
The parts of the SLIC which are illustrated in Fig. 1 include
six CCCs (controlled current circuits) 2 to 7 each of which is
represented in Fig. 1 by a circle having three connections. Each CCC
has a controlled current path, for which the direction of current is
shown in Fig. 1 by an arrow within the respective circuit and aligned
wi-th the connections to this path, and a controlling current path for
which the direction of current is shown on a side connection to the
respective circle. Thus each CCC is a current mirror circuit in
which the current in the controlled path is directly proportional to
the current in the controlling path, with a propor-tionality factor
which is referred to as being the gain of the circuit.
Such CCCs are generally known and can take various forms.
For the sake of completeness of this description, Figs. 2 to 7
illustrate particular forms which the CCCs 2 to 7 respectively may
have in one particular embodiment of the invention, it being
understood that other forms of these circuits may be adopted. The
positions and orientations of Figs. 2 to 7 correspond to those of the
respective CCCs 2 to 7 in Fig. 1.
As illustra-ted in Figs. 2 to 7, each CCC comprises three NPN
or PNP (depending on the desired current direction) transistors, two
of which have their bases interconnected and have their emitters
interconnected Vid respective emitter resistors, resistance values
:~3~
for which are given in nhrns in Figs. 2 to 7. The ra-tio of the
resistances of the emitter resistors in each CCC, which is made the
same dS the ernitter ared ra-tio of the transistors, determines the
gain of the CCC. The -third transistor of each CCC has its base
connected to the controlling current pa-th and its emi-tter connected
to the bases of the other two transistors.
In addition to the CCCs 2 to 7, as illustrated in Fig. 1 the
SLIC also includes a differential amplifier 12, a further
differential ampli-Fier constituted by two transistors 14 and 16,
resistors 18 to 28, and capacitors 30 to 32. The components 2 to 7,
12, 14, 16, and 24 to 28 are conveniently formed in an integrated
circuit together with other parts of the SLIC which are not shown.
The SLIC is supplied with power from d negative supply voltage VB,
which is nominally ~48 volts and is typically derived from a
telephone central office battery, and a positive supply voltage of
+15 volts, both with respect to ground or 0 volts.
The CCCs 3 and 6 serve to supply (d.c.~ loop current I to
terminals -r and R of the integrated circuit, and thence to the tip
and ring wires of the subscriber line 10 via resistors 18 and 19,
which have closely matched resistances of for example 200 ohms each.
As described in Jakab U.S. Patent No. 4,467,310 issued August 21,
1984, these resistors can comprise thick film resistors connected in
series with switching type PTC resistors, all mounted on a common
substrate for thermal coupling. The loop current is controlled in
dependence upon the resistances of, and is partly also supplied via,
the resistors 20 and 21; for example these resistors have matched
resistances of 22 kilohms each. The resistors 20 to 23 are also
conveniently thick film resistors.
The ,oart of the loop current which flows via the resistor 20
is mirrored by the CCC 2, and the resultant current is summed at a
junction point 34 with the part of the loop current which flows via
the resistor 21 to produce d control current for the CCC 7. The
controlled path current of the CCC 7, which flows via the emitters of
the transistors 14 and 16 and their common emitter resistor 28, is
initidlly assumed to be equally shared between these two transistors
to provide equal control currents for the CCCs 4 and 5. The
controlled currents produced by these CCCs in turn produce equal
control currents for the CCCs 3 and 6.
From the above description, it should be appreciated tha-t the
CCCs 3 and 6 serve to simulate feed resis-tors for supplying the loop
current I to the subscriber line lO. For the same reasons that such
feed resistors must be c-losely matched to provide a d.c. balanced
arrangement (For example, this is needed For some automatic number
identification detectors), so the CCCs 3 and 6 should be closely
matclled for cl.c. balance purposes. Ilowever, perfect matching o-f
these CCCs in the SLIC integrated circuit is not possible, resulting
in some degree o-f d.c. imbalance. In the SLIC as illustrated in Fig.
1, d.c. imbalance is corrected in the manner described below.
The resistors 24 and 25 form a potential divider between the
terminals T and R, and have resistances of 100 kilohms each which are
matched to 0.1% to 2% depending on the integra-ted circuit process
which is used. In a state of d.c. balance, the d.c. potential at
the junction be-tween these resistors is equal to half the supply
voltage VB. The differential amplifier 12 has its inverting input
connected to this junction and its non-inverting input supplied (for
example from a potential divider which is not illustrated) with the
voltage VB/2. Consequently, any d.c. imbalance results in the
amplifier 12 producing an output signal with a d.c. component which
differs from VB/2, the difference consti-tuting an amplified error
signal.
The output of the amplifier 12 is connected via a low-pass
filter, constituted by the resistors 26 and 27 and the capacitor 32,
to the base of the transistor 14. The resistors 26 and 27 each have
a resistance of 100 kilohms, and the capacitor 32 has a capacitance
of 330nF, so that the low-pass filter has a cut-off frequency well
below voice-band and power line frequencies. Consequently only the
d.c. component of the signal at the output of the amplifier 12 is
applied to the base of the transistor 14. The voltage VB/2 is
applied to the base of the transistor 16, so that the differential
amplifier formed by these two transistors is supplied with a
differential input signal constituted by the amplified error signal,
which thereby controls the sharing of the controlled curren-t from the
CCC 7 between the transistors 14 and 16. In consequence, the CCCs 3
and 6 are controlled to correct the d.c. imbalance.
For example, iF the e-F~ective impedance of the CCC 3 is less
than tha-t of the CCC 6, then a d.c. imbalance tends to arise causing
-the signal at the inver-ting input of -the amplifier 12 to have a d.c.
component more positive than V3/2~ In consequence, the d.c.
component o-F the s-ignal at the output of the amplifier 12 is more
negative than VB/2, and the transistor 14 conducts less than half,
and the transistor 16 conducts more than hdlf, o-F the total
(constant) curren-t passed by -the CCC 7. Consequently the CCCs ~ and
3 pass less current, with a corresponding increase in ef-fective
impedance, and the CCCs 5 and 6 pass more current, with a
corresponding decrease in effective impedance, thereby substantia11y
correcting the d.c. imbalanceO
In addition to serving in maintaining d.c. balance in
the manner described above, -the resistors 24 and 25 and the amplifier
12 also serve wi-th the resistors 22 and 23 and the capacitors 30 and
31 to compensate for longitudinal currents on the subscriber line
10. Longitudinal currents may for example arise as a result
of induction from power lines and/or crosstalk from other subscriber
lines, and comprise alternating currents, represented as i in Fig. 1,
which flow in the same direction on both of the tip and ring wires of
the subscriber line 10.
It is known to compensate for longitudinal currents by
providing two commonly controlled current sinks which would be
coupled to the terminals T and R. For longitudinal current balance
it is necessary for such current sinks to be closely matched, for
example within about 0.1~, and this is very difficul-t or impossible
to achieve if the current sinks are formed in an integrated circuit
for which matching is not normally better than 1%. This problem is
avoided by the circuit illustrated in Fig. 1.
As illustrated in Fig. 1, the terminal T is coupled via the
resistor 22 and the capacitor 30 to a junction point 36, and the
terminal R is similarly coupled to this point 36 via the resistor 23
and the capacitor 31. The resistors 22 and 23 are for example thick
film resistors each having a resistance of 500 ohms, the resistances
35 being closely matched to within 0.1X. The capacitors 30 and 31 serve
to block d.c. so that d.c. line curren-ts are not affected by the
presence of the resis-tors 22 and 23, and for example have
capaci-tances of 33 microfarads and a tolerance oF 5%. The junction
point 36 is connected to the outpu-t of the differential amplifier 12,
which acts as a controlled voltage source for longi-tudinal currents.
Thus in the SLIC illustrated in Fig. 1, -the controlled current sinks
of the prior art are replaced by the closely matched resistors 22 and
23 toyether wi-th a controlled voltage source.
If~ for exanlple, the (a.c.) longi-tudinal current i increases,
tending to make the po-tentials at the terminals T and R more
positive, -then via the resis-tors 24 and 25 the potential at the
inverting input of the amplifier also tends to become rnore positive.
The fixed potential V~/2 at the non-inverting input of the amplifier
12 constitutes an a.c~ ground. In consequence, the potential at the
output of the ampli-fier 12, and hence at the junction point 36, tends
to become more negative, increasing the (a.c.) potential difference
across the resistors 22 and 23 and thereby passing more current
to compensate for the increased longitudinal current i. In
consequence, the terminals T and R are virtual ground points
for longitudinal al-ternating currents. The potentials at these
terminals can change differentially, however, without affecting the
potential at the inverting input of the amplifier 12, so that -the
desired transmission of differential voice frequency signals on the
subscriber line 10 is not adversely affected.
It should be appreciated that numerous modi-fications,
variations, and adaptations may be made to the particular embodiment
of the invention described above without departing from the scope of
the invention as defined in the claims.
