CA1233283A - Subscriber line interface circuit with longitudinal current suppression - Google Patents
Subscriber line interface circuit with longitudinal current suppressionInfo
- Publication number
- CA1233283A CA1233283A CA000490970A CA490970A CA1233283A CA 1233283 A CA1233283 A CA 1233283A CA 000490970 A CA000490970 A CA 000490970A CA 490970 A CA490970 A CA 490970A CA 1233283 A CA1233283 A CA 1233283A
- Authority
- CA
- Canada
- Prior art keywords
- resistors
- coupled
- subscriber line
- current
- inverting input
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M19/00—Current supply arrangements for telephone systems
- H04M19/001—Current supply source at the exchanger providing current to substations
- H04M19/005—Feeding arrangements without the use of line transformers
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M3/00—Automatic or semi-automatic exchanges
- H04M3/005—Interface circuits for subscriber lines
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- Engineering & Computer Science (AREA)
- Signal Processing (AREA)
- Power Engineering (AREA)
- Interface Circuits In Exchanges (AREA)
Abstract
SUBSCRIBER LINE INTERFACE CIRCUIT WITH
LONGITUDINAL CURRENT SUPPRESSION
.
Abstract of the Disclosure A SLIC includes controlled current circuits (CCCs) which simulate feed resistors for controlling d.c. feed to a two-wire telephone subscriber line which is coupled to two terminals, control being achieved via a feedback loop. A differential amplifier has an inverting input coupled to the tapping point of a potential divider connected between the terminals, and a non-inverting input connected to a reference potential of half of the supply battery voltage. For suppressing longitudinal currents on the line, the amplifier output is connected to each terminal via a d.c. blocking capacitor in series with a resistor, the resistors having closely matched resistances.
For maintaining d.c. balance, the output of the amplifier is coupled via a low pass filter to control a current splitter included in the feedback loop, to control the relative proportions of control currents for the CCCs.
-i-
LONGITUDINAL CURRENT SUPPRESSION
.
Abstract of the Disclosure A SLIC includes controlled current circuits (CCCs) which simulate feed resistors for controlling d.c. feed to a two-wire telephone subscriber line which is coupled to two terminals, control being achieved via a feedback loop. A differential amplifier has an inverting input coupled to the tapping point of a potential divider connected between the terminals, and a non-inverting input connected to a reference potential of half of the supply battery voltage. For suppressing longitudinal currents on the line, the amplifier output is connected to each terminal via a d.c. blocking capacitor in series with a resistor, the resistors having closely matched resistances.
For maintaining d.c. balance, the output of the amplifier is coupled via a low pass filter to control a current splitter included in the feedback loop, to control the relative proportions of control currents for the CCCs.
-i-
Description
issues SUBSCRIBER LINE INTERFACE CIRCUIT WITH
LONGITUDINAL CURRENT SUPPRESSION
This invention relates to subscriber line interface circuits (Slits), which provide for coupling to two-wire telephone subscriber lines.
Reference is directed to my cop ending patent application entitled "Subscriber Line Interface Circuit with Improved DO
Balance" and filed simultaneously herewith, the claims of which are directed to a SLIT including features an embodiment of which is also described herein.
Many proposals exist for providing a SLIT in the form of an integrated circuit and for coupling this to a two-wire telephone subscriber line, preferably using direct coupling, i.e. without an intervening transformer. One such proposal, in which the SLIT
comprises controlled current circuits (also referred to as current mirror circuits, current sources, and current sinks), is described in Kelly et at. US. Patent No. 4,300,023 issued November 10, 1981 and entitled "Hybrid Circuit". In this known SLICK controlled current circuits are coupled to the tip and ring wires of the telephone subscriber line and are controlled to provide a desired do feed resistance.
It is also known that longitudinal or common mode currents, which are alternating currents which are typically induced on two-wire telephone subscriber lines from adjacent power lines and/or as a result of cross-talk from other telephone lines, must be suppressed. In the SLIT described in the above patent, longitudinal currents are suppressed by additional controlled current circuits which are coupled to the tip and ring wires of the subscriber line.
However, in order to provide effective suppression of longitudinal currents, such circuits must be very closely matched, for example to about 0.1%, whereas in practice in integrated circuits matching of such circuits to better than about 1% is difficult or impossible to achieve.
Accordingly, an object of this invention is to provide an improved SLIT which facilitates suppression of longitudinal currents.
; According to this invention there is provided a subscriber line interface circuit (SLIT) including: two terminals for coupling .
I.
I,..
~.Z33283 to a two-wire telephone subscriber line; two impedance means each coupled between a respective one of the terminals and a junction point, each impedance means comprising means for providing an arc.
impedance and means for blocking do a potential divider coupled between the terminals and having a tapping point and a controlled voltage source responsive to the voltage at the tapping point for supplying a controlled voltage to the junction point.
Thus in a SLIT in accordance with this invention the controlled voltage source and two impedance means serve for the suppression of longitudinal currents. The impedance means conveniently each comprise a resistor and a capacitor coupled in series, the capacitors serving to block do so that do conditions of the SLIT and subscriber line are not affected by the longitudinal current suppression. The resistors are conveniently thick film resistors which are external to an integrated circuit including the controlled voltage source whereby they can have resistances which are closely matched for example within a tolerance of 0.1%. The two impedance means may each further include other impedance elements, for example an inductance, for providing a particular desired complex impedance for matching the impedance of the two-wire line.
The controlled voltage source conveniently comprises a differential amplifier having an inverting input coupled to the tapping point, a non-inverting input coupled to a point of reference potential, and an output coupled to the junction point.
The invention will be further understood from the following description with reference to the accompanying drawings, in which:
Fig. 1 schematically illustrates parts of a subscriber line interface circuit (SLIT) in accordance with this invention; and Figs. 2 to 7 schematically illustrate the particular forms of controlled current circuits (CCCs) used in the SLIT of Fig. 1.
Referring to Fig. 1, only those parts of a SLIT 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 SLIT is connected, are not shown but are provided in known manner, for example in the manner described in Kelly et at. US. Patent No. 4,300,023 already referred to.
. ....
i Z I 3 3 The parts of the SLIT 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 COO
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 with 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 COO 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 proportionality 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 illustrated in Figs. 2 to 7, each COO comprises three NUN
or PUP (depending on the desired current direction) transistors, two of which have their bases interconnected and have their emitters interconnected via respective emitter resistors, resistance values for which are given in ohms in Figs. 2 to 7. The ratio of the resistances of the emitter resistors in each COO, which is made the same as the emitter area ratio of the transistors, determines the gain of the COO. The third transistor of each COO has its base connected to the controlling current path and its emitter connected to the bases of the other two transistors.
In addition to the CCCs 2 to 7, as illustrated in Fig. 1 the SLIT also includes a differential amplifier 12, a further differential amplifier 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 SLIT which are not shown.
The SLIT is supplied with power from a 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 ", ~;Z;33'~83 +15 volts, both with respect to ground or 0 volts.
The CCCs 3 and 6 serve to supply do loop current I to terminals T 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 US. 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 calms each. The resistors 20 to 23 are also conveniently thick film resistors.
The part of the loop current which flows via the resistor 20 is mirrored by the COO 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 a control current for the COO 7. The controlled path current of the COO 7, which flows via the emitters of the transistors 14 and 16 and their common emitter resistor 28, is initially 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 that the CCCs 3 and 6 serve to simulate feed resistors for supplying the loop current I to the subscriber line 10. For the same reasons that such feed resistors must be closely matched to provide a do balanced arrangement (for example this is needed for some automatic number identification detectors), so the CCCs 3 and 6 should be closely matched for do balance purposes. However, perfect matching of these CCCs in the SLIT integrated circuit is not possible, resulting in some degree of do imbalance. In the SLIT as illustrated in Fig.
1, do 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 calms each which are matched to Q.1% to 2% depending on the integrated circuit process which is used. In a state of do balance, the do potential at the . , ``:
~Z~3~83 junction between 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 do imbalance results in the amplifier 12 producing an output signal with a do component which differs from VB/2, the difference constituting 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 calms, 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 do 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 current from the COO 7 between the transistors 14 and 16. In consequence, the CCCs 3 and 6 are controlled to correct the do imbalance.
For example, if the effective impedance of the COO 3 is less than that of the COO 6, then a do imbalance tends to arise causing the signal at the inverting input of the amplifier 12 to have a do component more positive than VB/2. In consequence, the do component of the signal 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 half, of the total (constant) current passed by the COO 7. Consequently the CCCs 4 and 3 pass less current, with a corresponding increase in effective impedance, and the CCCs S and 6 pass more current, with a corresponding decrease in effective impedance, thereby substantially correcting the do imbalance.
In addition to serving in maintaining do balance in the manner described above, the resistors 24 and 25 and the amplifier 12 also serve with the resistors 22 and 23 and the capacitors 30 and 31 to compensate for longitudinal currents on the subscriber line ., ~233Z~33 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 difficult 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 being closely matched to within 0.1%. The capacitors 30 and 31 serve to block do so that do line currents are not affected by the presence of the resistors 22 and 23, and for example have capacitances of 33 micro farads and a tolerance of 5%. The junction point 36 is connected to the output of the differential amplifier 12, which acts as a controlled voltage source for longitudinal currents.
Thus in the SLIT illustrated in Fig. 1, the controlled current sinks of the prior art are replaced by the closely matched resistors 22 and 23 together with a controlled voltage source.
If, for example, the (arc.) longitudinal current i increases, tending to make the potentials at the terminals T and R more positive, then via the resistors 24 and 25 the potential at the inverting input of the amplifier also tends to become more positive.
The fixed potential VB/2 at the non-inverting input of the amplifier 12 constitutes an arc. ground. In consequence, the potential at the output of the amplifier 12, and hence at the junction point 36, tends ; 35 to become more negative, increasing the (arc.) potential difference across the resistors 22 and 23 and thereby passing more current to compensate for the increased longitudinal current i. In Jo z33283 consequence, the terminals T and R are virtual ground points for longitudinal alternating 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 modifications, 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.
, , --. .,,,~
LONGITUDINAL CURRENT SUPPRESSION
This invention relates to subscriber line interface circuits (Slits), which provide for coupling to two-wire telephone subscriber lines.
Reference is directed to my cop ending patent application entitled "Subscriber Line Interface Circuit with Improved DO
Balance" and filed simultaneously herewith, the claims of which are directed to a SLIT including features an embodiment of which is also described herein.
Many proposals exist for providing a SLIT in the form of an integrated circuit and for coupling this to a two-wire telephone subscriber line, preferably using direct coupling, i.e. without an intervening transformer. One such proposal, in which the SLIT
comprises controlled current circuits (also referred to as current mirror circuits, current sources, and current sinks), is described in Kelly et at. US. Patent No. 4,300,023 issued November 10, 1981 and entitled "Hybrid Circuit". In this known SLICK controlled current circuits are coupled to the tip and ring wires of the telephone subscriber line and are controlled to provide a desired do feed resistance.
It is also known that longitudinal or common mode currents, which are alternating currents which are typically induced on two-wire telephone subscriber lines from adjacent power lines and/or as a result of cross-talk from other telephone lines, must be suppressed. In the SLIT described in the above patent, longitudinal currents are suppressed by additional controlled current circuits which are coupled to the tip and ring wires of the subscriber line.
However, in order to provide effective suppression of longitudinal currents, such circuits must be very closely matched, for example to about 0.1%, whereas in practice in integrated circuits matching of such circuits to better than about 1% is difficult or impossible to achieve.
Accordingly, an object of this invention is to provide an improved SLIT which facilitates suppression of longitudinal currents.
; According to this invention there is provided a subscriber line interface circuit (SLIT) including: two terminals for coupling .
I.
I,..
~.Z33283 to a two-wire telephone subscriber line; two impedance means each coupled between a respective one of the terminals and a junction point, each impedance means comprising means for providing an arc.
impedance and means for blocking do a potential divider coupled between the terminals and having a tapping point and a controlled voltage source responsive to the voltage at the tapping point for supplying a controlled voltage to the junction point.
Thus in a SLIT in accordance with this invention the controlled voltage source and two impedance means serve for the suppression of longitudinal currents. The impedance means conveniently each comprise a resistor and a capacitor coupled in series, the capacitors serving to block do so that do conditions of the SLIT and subscriber line are not affected by the longitudinal current suppression. The resistors are conveniently thick film resistors which are external to an integrated circuit including the controlled voltage source whereby they can have resistances which are closely matched for example within a tolerance of 0.1%. The two impedance means may each further include other impedance elements, for example an inductance, for providing a particular desired complex impedance for matching the impedance of the two-wire line.
The controlled voltage source conveniently comprises a differential amplifier having an inverting input coupled to the tapping point, a non-inverting input coupled to a point of reference potential, and an output coupled to the junction point.
The invention will be further understood from the following description with reference to the accompanying drawings, in which:
Fig. 1 schematically illustrates parts of a subscriber line interface circuit (SLIT) in accordance with this invention; and Figs. 2 to 7 schematically illustrate the particular forms of controlled current circuits (CCCs) used in the SLIT of Fig. 1.
Referring to Fig. 1, only those parts of a SLIT 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 SLIT is connected, are not shown but are provided in known manner, for example in the manner described in Kelly et at. US. Patent No. 4,300,023 already referred to.
. ....
i Z I 3 3 The parts of the SLIT 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 COO
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 with 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 COO 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 proportionality 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 illustrated in Figs. 2 to 7, each COO comprises three NUN
or PUP (depending on the desired current direction) transistors, two of which have their bases interconnected and have their emitters interconnected via respective emitter resistors, resistance values for which are given in ohms in Figs. 2 to 7. The ratio of the resistances of the emitter resistors in each COO, which is made the same as the emitter area ratio of the transistors, determines the gain of the COO. The third transistor of each COO has its base connected to the controlling current path and its emitter connected to the bases of the other two transistors.
In addition to the CCCs 2 to 7, as illustrated in Fig. 1 the SLIT also includes a differential amplifier 12, a further differential amplifier 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 SLIT which are not shown.
The SLIT is supplied with power from a 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 ", ~;Z;33'~83 +15 volts, both with respect to ground or 0 volts.
The CCCs 3 and 6 serve to supply do loop current I to terminals T 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 US. 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 calms each. The resistors 20 to 23 are also conveniently thick film resistors.
The part of the loop current which flows via the resistor 20 is mirrored by the COO 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 a control current for the COO 7. The controlled path current of the COO 7, which flows via the emitters of the transistors 14 and 16 and their common emitter resistor 28, is initially 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 that the CCCs 3 and 6 serve to simulate feed resistors for supplying the loop current I to the subscriber line 10. For the same reasons that such feed resistors must be closely matched to provide a do balanced arrangement (for example this is needed for some automatic number identification detectors), so the CCCs 3 and 6 should be closely matched for do balance purposes. However, perfect matching of these CCCs in the SLIT integrated circuit is not possible, resulting in some degree of do imbalance. In the SLIT as illustrated in Fig.
1, do 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 calms each which are matched to Q.1% to 2% depending on the integrated circuit process which is used. In a state of do balance, the do potential at the . , ``:
~Z~3~83 junction between 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 do imbalance results in the amplifier 12 producing an output signal with a do component which differs from VB/2, the difference constituting 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 calms, 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 do 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 current from the COO 7 between the transistors 14 and 16. In consequence, the CCCs 3 and 6 are controlled to correct the do imbalance.
For example, if the effective impedance of the COO 3 is less than that of the COO 6, then a do imbalance tends to arise causing the signal at the inverting input of the amplifier 12 to have a do component more positive than VB/2. In consequence, the do component of the signal 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 half, of the total (constant) current passed by the COO 7. Consequently the CCCs 4 and 3 pass less current, with a corresponding increase in effective impedance, and the CCCs S and 6 pass more current, with a corresponding decrease in effective impedance, thereby substantially correcting the do imbalance.
In addition to serving in maintaining do balance in the manner described above, the resistors 24 and 25 and the amplifier 12 also serve with the resistors 22 and 23 and the capacitors 30 and 31 to compensate for longitudinal currents on the subscriber line ., ~233Z~33 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 difficult 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 being closely matched to within 0.1%. The capacitors 30 and 31 serve to block do so that do line currents are not affected by the presence of the resistors 22 and 23, and for example have capacitances of 33 micro farads and a tolerance of 5%. The junction point 36 is connected to the output of the differential amplifier 12, which acts as a controlled voltage source for longitudinal currents.
Thus in the SLIT illustrated in Fig. 1, the controlled current sinks of the prior art are replaced by the closely matched resistors 22 and 23 together with a controlled voltage source.
If, for example, the (arc.) longitudinal current i increases, tending to make the potentials at the terminals T and R more positive, then via the resistors 24 and 25 the potential at the inverting input of the amplifier also tends to become more positive.
The fixed potential VB/2 at the non-inverting input of the amplifier 12 constitutes an arc. ground. In consequence, the potential at the output of the amplifier 12, and hence at the junction point 36, tends ; 35 to become more negative, increasing the (arc.) potential difference across the resistors 22 and 23 and thereby passing more current to compensate for the increased longitudinal current i. In Jo z33283 consequence, the terminals T and R are virtual ground points for longitudinal alternating 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 modifications, 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.
, , --. .,,,~
Claims (7)
1. A subscriber line interface circuit (SLIC) including:
two terminals for coupling to a two-wire telephone subscriber line;
two impedance means each coupled between a respective one of the terminals and a junction point, each impedance means comprising means for providing an a.c. impedance and means for blocking d.c.;
a potential divider coupled between the terminals and having a tapping point; and a controlled voltage source responsive to the voltage at the tapping point for supplying a controlled voltage to the junction point.
two terminals for coupling to a two-wire telephone subscriber line;
two impedance means each coupled between a respective one of the terminals and a junction point, each impedance means comprising means for providing an a.c. impedance and means for blocking d.c.;
a potential divider coupled between the terminals and having a tapping point; and a controlled voltage source responsive to the voltage at the tapping point for supplying a controlled voltage to the junction point.
2. A SLIC as claimed in claim 1 wherein each impedance means comprises a resistor and a capacitor coupled in series between the respective terminal and the junction point.
3. A SLIC as claimed in claim 2 wherein the resistances of the resistors in the impedance means are closely matched.
4. A SLIC as claimed in claim 3 wherein the controlled voltage source is part of an integrated circuit and the resistors are external to the integrated circuit.
5. A SLIC as claimed in claim 4 wherein the resistors are thick film resistors.
6. A SLIC as claimed in claim 1, 2, or 3 wherein the controlled voltage source comprises a differential amplifier having an inverting input coupled to the tapping point, a non-inverting input coupled to a point of reference potential, and an output coupled to the junction point.
7. A SLIC as claimed in claim 4 or 5 wherein the controlled voltage source comprises a differential amplifier having an inverting input coupled to the tapping point, a non-inverting input coupled to a point of reference potential, and an output coupled to the junction point.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000490970A CA1233283A (en) | 1985-09-18 | 1985-09-18 | Subscriber line interface circuit with longitudinal current suppression |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000490970A CA1233283A (en) | 1985-09-18 | 1985-09-18 | Subscriber line interface circuit with longitudinal current suppression |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1233283A true CA1233283A (en) | 1988-02-23 |
Family
ID=4131419
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000490970A Expired CA1233283A (en) | 1985-09-18 | 1985-09-18 | Subscriber line interface circuit with longitudinal current suppression |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1233283A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0517321A2 (en) * | 1991-06-06 | 1992-12-09 | Koninklijke Philips Electronics N.V. | Telecommunication system comprising a supply circuit for a telecommunication line, and a supply circuit for a telecommunication line suitable for use in the telecommunication system |
| CN109817648A (en) * | 2017-11-21 | 2019-05-28 | 湖北东光电子股份有限公司 | A kind of thick film hybrid integrated circuit and its batch production control method |
-
1985
- 1985-09-18 CA CA000490970A patent/CA1233283A/en not_active Expired
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0517321A2 (en) * | 1991-06-06 | 1992-12-09 | Koninklijke Philips Electronics N.V. | Telecommunication system comprising a supply circuit for a telecommunication line, and a supply circuit for a telecommunication line suitable for use in the telecommunication system |
| EP0517321A3 (en) * | 1991-06-06 | 1993-06-16 | Koninkl Philips Electronics Nv | Telecommunication system comprising a supply circuit for a telecommunication line, and a supply circuit for a telecommunication line suitable for use in the telecommunication system |
| CN109817648A (en) * | 2017-11-21 | 2019-05-28 | 湖北东光电子股份有限公司 | A kind of thick film hybrid integrated circuit and its batch production control method |
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| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |