CA1206648A - Subscriber circuit for a telephone network - Google Patents
Subscriber circuit for a telephone networkInfo
- Publication number
- CA1206648A CA1206648A CA000440700A CA440700A CA1206648A CA 1206648 A CA1206648 A CA 1206648A CA 000440700 A CA000440700 A CA 000440700A CA 440700 A CA440700 A CA 440700A CA 1206648 A CA1206648 A CA 1206648A
- Authority
- CA
- Canada
- Prior art keywords
- subscriber
- transistor
- current path
- circuit
- current
- 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
Abstract
ABSTRACT:
A subscriber circuit for a telephone network which is coupled to earth via a frequency-dependent coupling circuit. As for direct current the coupling circuit has a much lower impedance than for alternating current, the electrolytic corrosion due to leakage current in the subscriber line is counter-acted and no sever requirements need to be imposed on the equality of the impedances in the two wires of the subscriber line.
A subscriber circuit for a telephone network which is coupled to earth via a frequency-dependent coupling circuit. As for direct current the coupling circuit has a much lower impedance than for alternating current, the electrolytic corrosion due to leakage current in the subscriber line is counter-acted and no sever requirements need to be imposed on the equality of the impedances in the two wires of the subscriber line.
Description
r~
1.
PH~ 10.491 l ~.6.19~3 "Subscriber circuit for a telephone network".
The invention relates to a subscriber circuit ~or a telephone network, comprising an in-ternal portion of -the subscriber's current path which portion is conductively coupled to a point of fixed reference potential Such a circuit is disclosed in inter alia the publication "The Proceedings of the International ~onfe-rence on Private Electronic Switching Systems", April 1978, London~ pages 132 to 136, inclusive.
In a telephone network the subscriber circuit forms the interface network between the subscriber line and the telephone exchange. The subscriber circuit provi-des inter alia the transmiss~on of a.c. signals between the subscriber and the exchange and vice versa, and the 15 subscriber line is supplied with direct current from the central power supply battery of the telephone exchange via the subscriber circuit.
Both the direct current and an alternating current superposed on the direct current flow from the 20 subscriber circuit via a wire of the subscriber line, the subscriber set and the other wire of the subscriber line back to the subscriber circuit. It is possible that in the subscriber circuit the direct current component and the alternating current component follow the same path;
25 in non-electronic subscriber circuits it is however customary for these two components to follow different paths within the subscriber circuit. The above-described current loop through which the direct current component and the alternating current component of the subscriher ~0 current, respectively flow is designated -the subscriber's current path.
A generally known problern with subscriber circuits is electrolytic corrosion by leakage to earth of ~2~6~
PH~ 10.491 ~ ~.6.1983 the subscriber line. This problem is counteracted in known manner by connecting the posi-tive pole of the cent.ral power supply battery to earth, as a result of ~hich the entire subscriber cable gets a negative po~ential with respec-t to earth.
Subscriber circuits as described above are generally known. United Sta-tes Patent No. 3,300,~88 dis-closes for example a non-electronic subscriber circuit having an in-ternal subscriber's current path in which the direct current and the alternating current follow di~ferent paths. The subscriber line receives direct current from a central power supply battery whose positive pole is connected to earth. The diract current can be applied to the subscriber line in two different manners.
In the -first manner the ^entral power supply battery is connectsd to the subscriber line via two supply resis-tors and two choke coils. In the second manner the power supply battery is connected to the subscriber line via two supply resistors and two windings of a trans-20 former.
An electronic subscriber circuit is described in the above-mentioned "Proceedings of the International Conference on PESS". In that subscriber circuit the direct current and the alternating current follow the same path 25 and also in this subscriber circuit the positive side of the central supply source is connected to earth.
Each of the said subscriber circuits comp-rises two supply resistors, one arranged bet~een the positive side of the central supply and a wire of the subscriber 30 line and one between the negative side of the central supply and the other wire of -the subscriber line. In the event of a short circuit between the wires of the sub-scriber line -these resistors provide a current limita-tion for the central supply. Wi-th the customary currents and 35 voltages these supply resistors must be capable of dissi-pating a continuous power of the order of magnitude of 3 ~.
Due to external causes voltages which result, in P~l~ 10~91 3 1.671 each of the lvires, in eurrents of mutually equal directions, the what are commonly referred to as common-mode currents may be produced on the subseriber line. These currents may be the result of, for e~ample, lightning induction, induction by electrie vehicles or induction of or shortcireuit ~ith the mains supply Generally -these eommon-mode currents have an alternaiing eurrent charaeter.
In a grounded subscriber circuit these currents flow to the earth point, voltages being generated across the impedances passed through. If these impedances in the -two line wires are not aecurately equal to each other these common-mode currents may produce unwanted differential voltages, l~hich are conveyed by the signal transmission lS system.
In order to prevent these interfering differen-tial voltages from occurring -to a significant e~tent some impedances through which the common-mode eurrents flow must satisfy high requirements as regards aecuraey~
The subscriber eircuit disclosed in the above-mentioned United States Patent 3,3OO,588, in whieh the direct current supply is effeeted via ehoke eoils has for eommon-mode signals two parallel eurrent paths between the subscriber line and ground, each consisting of a series 25 arrangement of a ehoke eoil and a supply resistor~ To prevent dif.erential voltages due to eommon-mode eurrents from oeeurring~ a eustomary requirement as regards the accuracy is that the impedanees of the two series arrange-ments shall not differ for more than 0.1%. Beeause of their 30 eomparatively high priee and heavy weight and large vclume ehoke eoils are generally unwanted eornponents in a sub-seriber circuit, inter alia because of the fact that they eannot be rnanufacturecl in integrated eireuit teehniques.
They are partieularly unwanted if very elose manufaeturing 35 tolerances are requiredO Resistors having the said close manufaeturing tolerances and l~hieh are eapable of dissipa-ting the saicl power are also undesirable in view of their high eost.
~ , P11~- 10.491 4 1.~i.1g~3 The subscriber circuit described in the above-mentioned United States Patent 3,300,~c~8, in which the direct current is applied to the subscriber line ~ia the -transformer for signal transmission has for common-mo~e signals also two parallel current paths between the subscriber line and ground, each formed by a series arrangement of a transformer winding and a supply resistor. However~ in said subscriber circui-t the supply resistors are shortcircuited for alternating current by a capacitor, so that the requirements imposed on the accuracy of the supply resistors may be less severe. How-ever, because of the -fact that direct current is used a transformer is required which has considerably larger dimensions than in the event in which the transformer is only utilized for signal transmission.
The electronic subscriber set of the said "Proceedings of the International Conference on PESS"
comprises two line drivers, each driving a wire of the subscrib~r line and each applying a direct current to these ~,rires via a supply resistor arranged in series therewith.
These supply resistors are not only used as a shortcircuit protection but also as pick-off resistors for the signal entering the exchange via the subscriber line. The connecting path for common-mod~ currents between the subscriber line and ground is provided in this electronic subscriber circuit by two current pa-ths. The first current path leads from a wire of the subscriber line via the associated supply resistors and the output stage of the associated line driver to the earth point of the central supply; the second current path leads in a corresponding manner from tha other line wire.
So for the case of "common-mode" currents there occurs in this electronic subscriber circuit also an unwanted differential voltage if the supply resistors are unequal. The consequent requirement that these resistors must then sa-tisfy very close manufacturing tolerances, in combination with the possible po~er dissipation p~l~T 10.491 5 1.7J.1~
of the order of magnitude of 3 1~ renders the above-mentioned manner of grounding the subscriber circuit objectionable.
This objection is even larger for voltages on the sub-scriber line which are still just within the -~esponse margin of the overvoltage protection, In these circum-stances a many times higher power dissipation may occur as a result of ~hich the supply resistors and the subsequent equipment may be damaged irrepairably.
It is an object of the invention to provide 0 a subscriber circuit i.n which electrolytic corrosion is counteracted7 in which the use of choke coils and trans-formers for the supply of the subscriber line is avoided and in which the problem as regards the equality of the supply resistors has been solved.
To this end the subscriber circuit according to the invention is characterized in that the internal sub-scriber's current pa-th portion is coupled to the point of fi~ed reference potential via a two-pole coupling circuit the impedance of which has for direct current a low value as compared with the impedance for alternating current.
The above-mentioned measure renders it possible to make the subscriber line negative for d.c. voltage with respect to earth without th.e need of imposing severe requirements on the impedance symmetry of the subscriber circuit~ The said neasure has the additional advantage that the overvoltage protection can be limited to a pro-tection against very high voltages, such as for example lightning induction on the subscriber line.
The fact -that the entire subscriber's current path including the power supply is coupled in only one position to the point of fi~ed reference voltage, and the very high altern~ting current impedance result in the circuit being passed through by a very low current only in normal operating conditions. It is consequently possible to assemble this circuit from components designed for a limited energy dissipation. According to the inven-tion7 an embodiment of -the subscriber circuit which , ;
~ 2 ~
Pl-1~10.1~9l 6 1.6 1~3 satisfies this is characterized in ~hat the coupling circuit comprises:
- a subscriber's current path terminal for connection to the internal portion of the subscriber's current path;
- a reference terminal for connection to the fi~ecl reference potential;
- and a parallel arrangement provided by a first series arrangement of in succession a first resistor, the main current path of a first transistor and a capacitor on the one hand and a second series arrangement provided by the main current path of a second transistor and a second resistor on the other hand, that - the first resistor is coupled to the collector of the lS second transistor and is connected to the subscriber's current path terminal, - the collector of the first transistor is connected to the base of the second transistor, - the base of the first1ransistor is connected to the 20 reference terminal ?
and that the first and second transistors are of a complementary conductivity type.
This embodiment has the advantage that the coupling circuit can be manufactured in integrated circuit form.
Embodiments of the invention and their advan-tages will now be further described, by way of e~ample, l~ith reference to the Figures, corresponding elements in the different Figures having been given the same reference numerals.
Therein:
Figure 1 shows a subscriber circuit comprising a coupling circuit in accordance with the invention~
Figure 2 shows a basic circuit diagram of the coupling circuit of Figure 1, Figure 3 shows an embodiment of the coupling circuit of Figure 1 in greater detail.
Figure I shows a subscriber's current path ~ 2 ~T ~
P7I.``T 10,!~91 'I .6.1gi33 portion of an electronic subscriber circuit. This subscriber's current path portion corrlprises two line drivers 1 and 2 which each drive a wire of the subscriber line 3 and apply or withdraw, respectively direct current thereto or therefrom. The line drive is effected in anti-phase bet~veen the two line wires because a signal source 33 drives an inverting and a non-inverting input, respectively of each ofthe line drivers; the d.c. voltage setting of each of the driver outputs is effected under the 0 control of two d.c. voltages which are applied to the two other inputs by the d.c. voltage sources 34 and 35.
The setting of the d.c. voltage level of the subscriber line with respect to earth occurs because of the fact that the two d.c. voltage sources 34 and 35 are also 15 conne^ted -to the subscriber's curren-t path terminal 9 of the coupling circuit 7 still to be described hereafter.
The direct current feed of the line drivers 1 and ~ is effectedby a direct current supply source This supply source must be a floating source; it is 20 possible to use for that purpose an embodiment which is known per se and which is specific for each subscriber circuit, such as for e~ample, the secondary side of a supply transformer followed by a rectifier diode switch and a smoothing circuit.
Drive and feeding of the subscriber line 3 is effected ~ia the low-ohmic impedance output of the line drivers and via -two supply resistors 5 and 6 which limit the feed current in the event of shortcircuiting of the subscriber line. These resistors can also function as pick-30 off resistors for -the signal en-tering the e~change via the subscriber line 3.
The subscriber's curren-t path fllrther comprises the subscriber line and the equipment connected thereto, which i-tems are not shown in the Figure~ In addition, a subscriber's current path generally corllprises several components which are related to other functions of the sub-scriber circuit such as the feed of ringing current~ signal 12~
P~l~ 10.49-1 8 i.~ 198, transmission, metering etc. These components are not important for a proper understanding of the invention and are therefore not shown in the ~igure. The portion o~ the subscriber's current path shown in the Figure is -that portion ofthe subscriber's current path which is located wit}lin the subscriber circui-t; it will be denoted as the internal current path portion.
The internal current path portion is connected to a point of fixed reference potential via a coupling circuit 7, still -to be described, comprising reference terminal 8 and subscriber's current path terminal 9. In the Figure the point of fixed reference potential is indicated by means of the symbol for an earth connection.
It should be noted that other, fixed potentials are 15 alternatively possible.
In order to counteract electrolytic corrosion due to leakage to earth of the subscriber line the sub-scriber's current path must for direct current be connected to ground via an impedance of a lowest possible 20 value. The requirement that tha subscriber's current path must be coupled to earth via the lowest possiblelmpedance is conflicting with the requirement that the subscriber's current path must preferably float electrically. This latter requirement results from the fact that the so-25 called common-mode currents to earth produce unwanted differential voltages when the impedances of the wires of the subscriber line are not precisely the same. With a subscriber's current path which with respect to the signal-pick-off resistors floats or is highly resistive relative 30 to earth these common-mode currents are no-t or hardly present, Since the common-mode interferences have substantially a]ways an alternating current character the above-mentioned two requirements are satisfied by 35 connecting the subscriber's current path to earth via a coupling circuit 7, which exhibits between its -terminals 8 and 9 a consiclerably lower impedance for direc-t current than fora~ternating current.
~ZG~ 8 PMN 10.'191 9 1,6.19~3 Figure 2 shows -the basic circuit diagram of an embodiment of the coupling circuit 7 for one current direction. This circuit comprises a subscriber's current path terminal 9, a reference terminal 8 and a supply terminal 13. Two series circuits are connected between the subscriber's current path terminal 9 and the supply terminal 13. The first series circuit is provided by an impedance 10, the main current path of a PNP-transistor 11 and a capaci-tor 12. Impedance 10 is connected between the subscriber's current path terminal 9 and the emitter of transistor 11. Capacitor 12 is connected be-tween the collec--tor of transistor 11 and the supply terminal 13. The second series arrangement is provided by the resistor 15, which is connected to the supply terminal 13, and the main current 15 path of an NPN-transistor 14, whose collector is connec-ted to the subscriber's c-urrent path terminal 9. The collector of transis~r 11 is connected to -the base of tran-sistor 140 The base of transistor 11 is connected to a point of a fixed reference potential. The voltage sources 20 15 and 17 are a symbolic representation of the setting vol-tages present in the coupling circuit 7, Figure 3 showing how they are constructed.
Impedance 10 may be in the form of a resistor having a high value, for example 100 kOhm; this impedance 25 10 may also be in the form of a complex network but then a network which conducts direct current, for example by an accelerated increase of the impedance versus the frequency, as will be described in greater detail with reference to Figure 30 The transistors 11 and 14 are of a mutually complementary conductivity type. Wi-thout detracting from the operating principle of the present circuit the two conductivity types of these transistors can be inter-changed provided -the polarity of the setting voltages 16 35 and 17 is also interchanged.
The capacitor 12 partly determines the impedance of the coupling circuit 7. In the frequency range which is PII~ l0.~91 10 1.&.19~3 customary for subscriber lines this impedance of capacitor 12 must be much smaller than the in-ternal resistance of the base of transistor 14. Assuming a resistance of, for example, l kOhm for resistor l~ and a current gain of 3 for example, 100 x for transistor 14, it has been found that a value of, for example, 1/uF for capacitor 12 satisfies the above requirement.
The coupling circuit of ~igure 2 operates as follows. A d.c. voltage variation of, for example, + 0.1 V at subscriber's current path terminal 9 causes a variation of, for example, 1/uA in the direct current through -the impedance 10 and through the main current path of transistor 1l. Use is then made of the fac-t that the internal resistance of the emitter of transistor 11 is so small as to be disregarded with respect to the resistance of impedance 10. As capacitor 12 blocks direct current, the variation in the base current of transistor 14 is also 1 /uA, in response to which the collector current of transis-tor 14 changes by, for example, 0.1 mA. This means -that 20 for slo~Y voltage variations the impedance of coupling circui-t 7, seen from the subscriber's current path terminal 9 is approximately equal to the ratio of the resistance of impedance 10 and the current gain factor of transistor 14 This last-mentioned current gain factor may 25 easily be increased, for example by replacing transistor 14 by several transistors in a Darlington configuration.
A fast positive voltage variation at sub-scriber's current path terminal 9 also produces a current variation in transistor 11. However, this current varia-30 tion is substantially completely offset by capacitor -l2, an a.c. voltage which is so low as to be disregarded then being present across capacitor 12. It is now easy to calculate tha-t the alternating curren-t impedance seen froM subscriber~s current path terminal 9 is equal to the 35 product of the resistor lO and resistor 15, divided by the impedance of capacitor l2.
With the above-mentioned values of 100 kOhm for PIiN 10.491 11 1 5.1983 resistor lO, 1 kOhm for resistor 15, 1/uF for capacitor 12 and furthermore a current gain factor of 100 for tran-sistor 14 and a frequency of 15 Hz, the direct current resistance of this coupling circuit 7 is found to be approximately 1 kOhm, while the alternating current impedance is approxima-tely 10 kOhm. These figures clearly illus-trate the difference between the alternating current behaviour and the direct current beha-viour of the coupling circuit.
The basic circuit dia~ram of the coupling circuit in accordance witll the invention as shown in Figure 2 functions only for voltages on the subscriber's current path terminal 9 which are higher than ground po-tential.
An embodiment of a coupling circuit which functions for voltages both above and below the fixed reference potential is shown in Fig~lre 3. The circuit sho~n in this Figure is based on the principle of Figure 2.
It is, however, of a dual construction, and comprises some components for setting the fi~ed reference potential and has a complex impedance for a steeper variation of the impedance as a function of the frequency~
The circuit shown in Figure 3 is the circuit of Figure 2 to which a number of further elements have been addedO A series arrangement provided by in succession the main current path of an NPN-transistor 18, a capacitor 19, a resistor 21 and the main current path of a PNP-transistor 20 is connected between the emitter of transistor 11 and the anode of diode 28. The emitter of transistor 18 is connected to the emitter of transistor 11 and the collector of transistor 20 is coupled to the collector of transistor l4. In addition, the collector of transistor 18 is connected to the base of transistor 20.
The junc-tion between capacitor 19 and resistor 21 is connected to the positive side of the supply voltage which is to be connected to supply terminal 26.
The base bias voltages of -the -transis-tors 11 and 18 are supplied by the biasing circuit 17 which in P~ .T 10.491 1 ' 1 .~. 1983 Figure 2 is shown symbolically as a voltage sourcs.
This biasing circuit 17 is here pro~ed by a series arrangement of a diode 25 and a resistor 23 which is connected to the negative side of the supply voltage.
The base of transistor 18 is connected to the anode of diode 25, the base of transistor 11 is connected to the cathode of diode 25, The anode of diode 25 is connected to the reference terminal 8.
A diode 27 is provided in series with the collec-tor of transistor 20 while a diode 28 is provided in series ~vith the collector of transistor 14. These two diodes conduct the current in the same direction as the transistors 14 and 20. A resistor ~9 is arranged between impedance 10 and subscriber's current path terminal 9. This resistor and also the diodes 27 and 28 have a current and voltage-limiting function to protect the present coupling circuit. They are not essential to the functioning of the coupling circuit.
As regards negative voltages on subscriber's 20 current path terminal 9 the circuit formed by the elements 10, 18, 199 20 and 21 operates in a similar way as the circuit of Figure 2 does with respect to positive voltages, also because ccrresponding elements have mutually the same values.
A current flows from the fixed reference potential tothe negative supply side via the biasing circuit 17. This current produces a voltage drop - the diode forward voltage - across diode 25~ which voltage drop is only sensitive to a limited e~tent to variations in 30 the said current. As a result thereof the base of transis-tor 18 is biased with a value which is equal to the fi~ed reference voltage, which is symbolically shown as the earthpoint in Figure 3; the base of transistor 11 is biased to the negative diode forward voltage with respect to the fi~ed reference voltage. Since the voltage drop across the base-emitter diode of transistor -l1 is also equal to the diode forward voltage this emitter is properly PHN -IO.!I91 13 1 .k. 19~3 biased to the same po-ten-~ial as the reference terminal 3.
The impedance 10 is formed by a series arrangement of two resistors 31 and 32, connected to a capacitor 30 which is connected between a point o~ fi~ed potential (for e~ample earth) and the junction of the resistors 31 and 32. This circuit 10 has for its function to further increase the alternating current resis-tance of the coupling circuit of Figure 3 with respect to the circuit shown in Figure 2. The effect of this circuit 10 can be derived by assuming that o~ing to the voltage division by the resistor 31 and the capacitor 30 the a.c. voltage signal across capacitor 30 is reduced with respect to the signal at subscriber's current path terminal 9 by a given - high - factor. The a.c. voltage signal on the base of the lS transistors 14 and 20 is then reduced by the same factor~
causing the current through the transistors 14 and 20 to be reduced by the same factor too~ Seen from subscriber's current path terminal 9 the impedance of the coupling circuit~ being the ratio of the voltage variation to this 20 current varia-tion has then increased by the same factor.
1.
PH~ 10.491 l ~.6.19~3 "Subscriber circuit for a telephone network".
The invention relates to a subscriber circuit ~or a telephone network, comprising an in-ternal portion of -the subscriber's current path which portion is conductively coupled to a point of fixed reference potential Such a circuit is disclosed in inter alia the publication "The Proceedings of the International ~onfe-rence on Private Electronic Switching Systems", April 1978, London~ pages 132 to 136, inclusive.
In a telephone network the subscriber circuit forms the interface network between the subscriber line and the telephone exchange. The subscriber circuit provi-des inter alia the transmiss~on of a.c. signals between the subscriber and the exchange and vice versa, and the 15 subscriber line is supplied with direct current from the central power supply battery of the telephone exchange via the subscriber circuit.
Both the direct current and an alternating current superposed on the direct current flow from the 20 subscriber circuit via a wire of the subscriber line, the subscriber set and the other wire of the subscriber line back to the subscriber circuit. It is possible that in the subscriber circuit the direct current component and the alternating current component follow the same path;
25 in non-electronic subscriber circuits it is however customary for these two components to follow different paths within the subscriber circuit. The above-described current loop through which the direct current component and the alternating current component of the subscriher ~0 current, respectively flow is designated -the subscriber's current path.
A generally known problern with subscriber circuits is electrolytic corrosion by leakage to earth of ~2~6~
PH~ 10.491 ~ ~.6.1983 the subscriber line. This problem is counteracted in known manner by connecting the posi-tive pole of the cent.ral power supply battery to earth, as a result of ~hich the entire subscriber cable gets a negative po~ential with respec-t to earth.
Subscriber circuits as described above are generally known. United Sta-tes Patent No. 3,300,~88 dis-closes for example a non-electronic subscriber circuit having an in-ternal subscriber's current path in which the direct current and the alternating current follow di~ferent paths. The subscriber line receives direct current from a central power supply battery whose positive pole is connected to earth. The diract current can be applied to the subscriber line in two different manners.
In the -first manner the ^entral power supply battery is connectsd to the subscriber line via two supply resis-tors and two choke coils. In the second manner the power supply battery is connected to the subscriber line via two supply resistors and two windings of a trans-20 former.
An electronic subscriber circuit is described in the above-mentioned "Proceedings of the International Conference on PESS". In that subscriber circuit the direct current and the alternating current follow the same path 25 and also in this subscriber circuit the positive side of the central supply source is connected to earth.
Each of the said subscriber circuits comp-rises two supply resistors, one arranged bet~een the positive side of the central supply and a wire of the subscriber 30 line and one between the negative side of the central supply and the other wire of -the subscriber line. In the event of a short circuit between the wires of the sub-scriber line -these resistors provide a current limita-tion for the central supply. Wi-th the customary currents and 35 voltages these supply resistors must be capable of dissi-pating a continuous power of the order of magnitude of 3 ~.
Due to external causes voltages which result, in P~l~ 10~91 3 1.671 each of the lvires, in eurrents of mutually equal directions, the what are commonly referred to as common-mode currents may be produced on the subseriber line. These currents may be the result of, for e~ample, lightning induction, induction by electrie vehicles or induction of or shortcireuit ~ith the mains supply Generally -these eommon-mode currents have an alternaiing eurrent charaeter.
In a grounded subscriber circuit these currents flow to the earth point, voltages being generated across the impedances passed through. If these impedances in the -two line wires are not aecurately equal to each other these common-mode currents may produce unwanted differential voltages, l~hich are conveyed by the signal transmission lS system.
In order to prevent these interfering differen-tial voltages from occurring -to a significant e~tent some impedances through which the common-mode eurrents flow must satisfy high requirements as regards aecuraey~
The subscriber eircuit disclosed in the above-mentioned United States Patent 3,3OO,588, in whieh the direct current supply is effeeted via ehoke eoils has for eommon-mode signals two parallel eurrent paths between the subscriber line and ground, each consisting of a series 25 arrangement of a ehoke eoil and a supply resistor~ To prevent dif.erential voltages due to eommon-mode eurrents from oeeurring~ a eustomary requirement as regards the accuracy is that the impedanees of the two series arrange-ments shall not differ for more than 0.1%. Beeause of their 30 eomparatively high priee and heavy weight and large vclume ehoke eoils are generally unwanted eornponents in a sub-seriber circuit, inter alia because of the fact that they eannot be rnanufacturecl in integrated eireuit teehniques.
They are partieularly unwanted if very elose manufaeturing 35 tolerances are requiredO Resistors having the said close manufaeturing tolerances and l~hieh are eapable of dissipa-ting the saicl power are also undesirable in view of their high eost.
~ , P11~- 10.491 4 1.~i.1g~3 The subscriber circuit described in the above-mentioned United States Patent 3,300,~c~8, in which the direct current is applied to the subscriber line ~ia the -transformer for signal transmission has for common-mo~e signals also two parallel current paths between the subscriber line and ground, each formed by a series arrangement of a transformer winding and a supply resistor. However~ in said subscriber circui-t the supply resistors are shortcircuited for alternating current by a capacitor, so that the requirements imposed on the accuracy of the supply resistors may be less severe. How-ever, because of the -fact that direct current is used a transformer is required which has considerably larger dimensions than in the event in which the transformer is only utilized for signal transmission.
The electronic subscriber set of the said "Proceedings of the International Conference on PESS"
comprises two line drivers, each driving a wire of the subscrib~r line and each applying a direct current to these ~,rires via a supply resistor arranged in series therewith.
These supply resistors are not only used as a shortcircuit protection but also as pick-off resistors for the signal entering the exchange via the subscriber line. The connecting path for common-mod~ currents between the subscriber line and ground is provided in this electronic subscriber circuit by two current pa-ths. The first current path leads from a wire of the subscriber line via the associated supply resistors and the output stage of the associated line driver to the earth point of the central supply; the second current path leads in a corresponding manner from tha other line wire.
So for the case of "common-mode" currents there occurs in this electronic subscriber circuit also an unwanted differential voltage if the supply resistors are unequal. The consequent requirement that these resistors must then sa-tisfy very close manufacturing tolerances, in combination with the possible po~er dissipation p~l~T 10.491 5 1.7J.1~
of the order of magnitude of 3 1~ renders the above-mentioned manner of grounding the subscriber circuit objectionable.
This objection is even larger for voltages on the sub-scriber line which are still just within the -~esponse margin of the overvoltage protection, In these circum-stances a many times higher power dissipation may occur as a result of ~hich the supply resistors and the subsequent equipment may be damaged irrepairably.
It is an object of the invention to provide 0 a subscriber circuit i.n which electrolytic corrosion is counteracted7 in which the use of choke coils and trans-formers for the supply of the subscriber line is avoided and in which the problem as regards the equality of the supply resistors has been solved.
To this end the subscriber circuit according to the invention is characterized in that the internal sub-scriber's current pa-th portion is coupled to the point of fi~ed reference potential via a two-pole coupling circuit the impedance of which has for direct current a low value as compared with the impedance for alternating current.
The above-mentioned measure renders it possible to make the subscriber line negative for d.c. voltage with respect to earth without th.e need of imposing severe requirements on the impedance symmetry of the subscriber circuit~ The said neasure has the additional advantage that the overvoltage protection can be limited to a pro-tection against very high voltages, such as for example lightning induction on the subscriber line.
The fact -that the entire subscriber's current path including the power supply is coupled in only one position to the point of fi~ed reference voltage, and the very high altern~ting current impedance result in the circuit being passed through by a very low current only in normal operating conditions. It is consequently possible to assemble this circuit from components designed for a limited energy dissipation. According to the inven-tion7 an embodiment of -the subscriber circuit which , ;
~ 2 ~
Pl-1~10.1~9l 6 1.6 1~3 satisfies this is characterized in ~hat the coupling circuit comprises:
- a subscriber's current path terminal for connection to the internal portion of the subscriber's current path;
- a reference terminal for connection to the fi~ecl reference potential;
- and a parallel arrangement provided by a first series arrangement of in succession a first resistor, the main current path of a first transistor and a capacitor on the one hand and a second series arrangement provided by the main current path of a second transistor and a second resistor on the other hand, that - the first resistor is coupled to the collector of the lS second transistor and is connected to the subscriber's current path terminal, - the collector of the first transistor is connected to the base of the second transistor, - the base of the first1ransistor is connected to the 20 reference terminal ?
and that the first and second transistors are of a complementary conductivity type.
This embodiment has the advantage that the coupling circuit can be manufactured in integrated circuit form.
Embodiments of the invention and their advan-tages will now be further described, by way of e~ample, l~ith reference to the Figures, corresponding elements in the different Figures having been given the same reference numerals.
Therein:
Figure 1 shows a subscriber circuit comprising a coupling circuit in accordance with the invention~
Figure 2 shows a basic circuit diagram of the coupling circuit of Figure 1, Figure 3 shows an embodiment of the coupling circuit of Figure 1 in greater detail.
Figure I shows a subscriber's current path ~ 2 ~T ~
P7I.``T 10,!~91 'I .6.1gi33 portion of an electronic subscriber circuit. This subscriber's current path portion corrlprises two line drivers 1 and 2 which each drive a wire of the subscriber line 3 and apply or withdraw, respectively direct current thereto or therefrom. The line drive is effected in anti-phase bet~veen the two line wires because a signal source 33 drives an inverting and a non-inverting input, respectively of each ofthe line drivers; the d.c. voltage setting of each of the driver outputs is effected under the 0 control of two d.c. voltages which are applied to the two other inputs by the d.c. voltage sources 34 and 35.
The setting of the d.c. voltage level of the subscriber line with respect to earth occurs because of the fact that the two d.c. voltage sources 34 and 35 are also 15 conne^ted -to the subscriber's curren-t path terminal 9 of the coupling circuit 7 still to be described hereafter.
The direct current feed of the line drivers 1 and ~ is effectedby a direct current supply source This supply source must be a floating source; it is 20 possible to use for that purpose an embodiment which is known per se and which is specific for each subscriber circuit, such as for e~ample, the secondary side of a supply transformer followed by a rectifier diode switch and a smoothing circuit.
Drive and feeding of the subscriber line 3 is effected ~ia the low-ohmic impedance output of the line drivers and via -two supply resistors 5 and 6 which limit the feed current in the event of shortcircuiting of the subscriber line. These resistors can also function as pick-30 off resistors for -the signal en-tering the e~change via the subscriber line 3.
The subscriber's curren-t path fllrther comprises the subscriber line and the equipment connected thereto, which i-tems are not shown in the Figure~ In addition, a subscriber's current path generally corllprises several components which are related to other functions of the sub-scriber circuit such as the feed of ringing current~ signal 12~
P~l~ 10.49-1 8 i.~ 198, transmission, metering etc. These components are not important for a proper understanding of the invention and are therefore not shown in the ~igure. The portion o~ the subscriber's current path shown in the Figure is -that portion ofthe subscriber's current path which is located wit}lin the subscriber circui-t; it will be denoted as the internal current path portion.
The internal current path portion is connected to a point of fixed reference potential via a coupling circuit 7, still -to be described, comprising reference terminal 8 and subscriber's current path terminal 9. In the Figure the point of fixed reference potential is indicated by means of the symbol for an earth connection.
It should be noted that other, fixed potentials are 15 alternatively possible.
In order to counteract electrolytic corrosion due to leakage to earth of the subscriber line the sub-scriber's current path must for direct current be connected to ground via an impedance of a lowest possible 20 value. The requirement that tha subscriber's current path must be coupled to earth via the lowest possiblelmpedance is conflicting with the requirement that the subscriber's current path must preferably float electrically. This latter requirement results from the fact that the so-25 called common-mode currents to earth produce unwanted differential voltages when the impedances of the wires of the subscriber line are not precisely the same. With a subscriber's current path which with respect to the signal-pick-off resistors floats or is highly resistive relative 30 to earth these common-mode currents are no-t or hardly present, Since the common-mode interferences have substantially a]ways an alternating current character the above-mentioned two requirements are satisfied by 35 connecting the subscriber's current path to earth via a coupling circuit 7, which exhibits between its -terminals 8 and 9 a consiclerably lower impedance for direc-t current than fora~ternating current.
~ZG~ 8 PMN 10.'191 9 1,6.19~3 Figure 2 shows -the basic circuit diagram of an embodiment of the coupling circuit 7 for one current direction. This circuit comprises a subscriber's current path terminal 9, a reference terminal 8 and a supply terminal 13. Two series circuits are connected between the subscriber's current path terminal 9 and the supply terminal 13. The first series circuit is provided by an impedance 10, the main current path of a PNP-transistor 11 and a capaci-tor 12. Impedance 10 is connected between the subscriber's current path terminal 9 and the emitter of transistor 11. Capacitor 12 is connected be-tween the collec--tor of transistor 11 and the supply terminal 13. The second series arrangement is provided by the resistor 15, which is connected to the supply terminal 13, and the main current 15 path of an NPN-transistor 14, whose collector is connec-ted to the subscriber's c-urrent path terminal 9. The collector of transis~r 11 is connected to -the base of tran-sistor 140 The base of transistor 11 is connected to a point of a fixed reference potential. The voltage sources 20 15 and 17 are a symbolic representation of the setting vol-tages present in the coupling circuit 7, Figure 3 showing how they are constructed.
Impedance 10 may be in the form of a resistor having a high value, for example 100 kOhm; this impedance 25 10 may also be in the form of a complex network but then a network which conducts direct current, for example by an accelerated increase of the impedance versus the frequency, as will be described in greater detail with reference to Figure 30 The transistors 11 and 14 are of a mutually complementary conductivity type. Wi-thout detracting from the operating principle of the present circuit the two conductivity types of these transistors can be inter-changed provided -the polarity of the setting voltages 16 35 and 17 is also interchanged.
The capacitor 12 partly determines the impedance of the coupling circuit 7. In the frequency range which is PII~ l0.~91 10 1.&.19~3 customary for subscriber lines this impedance of capacitor 12 must be much smaller than the in-ternal resistance of the base of transistor 14. Assuming a resistance of, for example, l kOhm for resistor l~ and a current gain of 3 for example, 100 x for transistor 14, it has been found that a value of, for example, 1/uF for capacitor 12 satisfies the above requirement.
The coupling circuit of ~igure 2 operates as follows. A d.c. voltage variation of, for example, + 0.1 V at subscriber's current path terminal 9 causes a variation of, for example, 1/uA in the direct current through -the impedance 10 and through the main current path of transistor 1l. Use is then made of the fac-t that the internal resistance of the emitter of transistor 11 is so small as to be disregarded with respect to the resistance of impedance 10. As capacitor 12 blocks direct current, the variation in the base current of transistor 14 is also 1 /uA, in response to which the collector current of transis-tor 14 changes by, for example, 0.1 mA. This means -that 20 for slo~Y voltage variations the impedance of coupling circui-t 7, seen from the subscriber's current path terminal 9 is approximately equal to the ratio of the resistance of impedance 10 and the current gain factor of transistor 14 This last-mentioned current gain factor may 25 easily be increased, for example by replacing transistor 14 by several transistors in a Darlington configuration.
A fast positive voltage variation at sub-scriber's current path terminal 9 also produces a current variation in transistor 11. However, this current varia-30 tion is substantially completely offset by capacitor -l2, an a.c. voltage which is so low as to be disregarded then being present across capacitor 12. It is now easy to calculate tha-t the alternating curren-t impedance seen froM subscriber~s current path terminal 9 is equal to the 35 product of the resistor lO and resistor 15, divided by the impedance of capacitor l2.
With the above-mentioned values of 100 kOhm for PIiN 10.491 11 1 5.1983 resistor lO, 1 kOhm for resistor 15, 1/uF for capacitor 12 and furthermore a current gain factor of 100 for tran-sistor 14 and a frequency of 15 Hz, the direct current resistance of this coupling circuit 7 is found to be approximately 1 kOhm, while the alternating current impedance is approxima-tely 10 kOhm. These figures clearly illus-trate the difference between the alternating current behaviour and the direct current beha-viour of the coupling circuit.
The basic circuit dia~ram of the coupling circuit in accordance witll the invention as shown in Figure 2 functions only for voltages on the subscriber's current path terminal 9 which are higher than ground po-tential.
An embodiment of a coupling circuit which functions for voltages both above and below the fixed reference potential is shown in Fig~lre 3. The circuit sho~n in this Figure is based on the principle of Figure 2.
It is, however, of a dual construction, and comprises some components for setting the fi~ed reference potential and has a complex impedance for a steeper variation of the impedance as a function of the frequency~
The circuit shown in Figure 3 is the circuit of Figure 2 to which a number of further elements have been addedO A series arrangement provided by in succession the main current path of an NPN-transistor 18, a capacitor 19, a resistor 21 and the main current path of a PNP-transistor 20 is connected between the emitter of transistor 11 and the anode of diode 28. The emitter of transistor 18 is connected to the emitter of transistor 11 and the collector of transistor 20 is coupled to the collector of transistor l4. In addition, the collector of transistor 18 is connected to the base of transistor 20.
The junc-tion between capacitor 19 and resistor 21 is connected to the positive side of the supply voltage which is to be connected to supply terminal 26.
The base bias voltages of -the -transis-tors 11 and 18 are supplied by the biasing circuit 17 which in P~ .T 10.491 1 ' 1 .~. 1983 Figure 2 is shown symbolically as a voltage sourcs.
This biasing circuit 17 is here pro~ed by a series arrangement of a diode 25 and a resistor 23 which is connected to the negative side of the supply voltage.
The base of transistor 18 is connected to the anode of diode 25, the base of transistor 11 is connected to the cathode of diode 25, The anode of diode 25 is connected to the reference terminal 8.
A diode 27 is provided in series with the collec-tor of transistor 20 while a diode 28 is provided in series ~vith the collector of transistor 14. These two diodes conduct the current in the same direction as the transistors 14 and 20. A resistor ~9 is arranged between impedance 10 and subscriber's current path terminal 9. This resistor and also the diodes 27 and 28 have a current and voltage-limiting function to protect the present coupling circuit. They are not essential to the functioning of the coupling circuit.
As regards negative voltages on subscriber's 20 current path terminal 9 the circuit formed by the elements 10, 18, 199 20 and 21 operates in a similar way as the circuit of Figure 2 does with respect to positive voltages, also because ccrresponding elements have mutually the same values.
A current flows from the fixed reference potential tothe negative supply side via the biasing circuit 17. This current produces a voltage drop - the diode forward voltage - across diode 25~ which voltage drop is only sensitive to a limited e~tent to variations in 30 the said current. As a result thereof the base of transis-tor 18 is biased with a value which is equal to the fi~ed reference voltage, which is symbolically shown as the earthpoint in Figure 3; the base of transistor 11 is biased to the negative diode forward voltage with respect to the fi~ed reference voltage. Since the voltage drop across the base-emitter diode of transistor -l1 is also equal to the diode forward voltage this emitter is properly PHN -IO.!I91 13 1 .k. 19~3 biased to the same po-ten-~ial as the reference terminal 3.
The impedance 10 is formed by a series arrangement of two resistors 31 and 32, connected to a capacitor 30 which is connected between a point o~ fi~ed potential (for e~ample earth) and the junction of the resistors 31 and 32. This circuit 10 has for its function to further increase the alternating current resis-tance of the coupling circuit of Figure 3 with respect to the circuit shown in Figure 2. The effect of this circuit 10 can be derived by assuming that o~ing to the voltage division by the resistor 31 and the capacitor 30 the a.c. voltage signal across capacitor 30 is reduced with respect to the signal at subscriber's current path terminal 9 by a given - high - factor. The a.c. voltage signal on the base of the lS transistors 14 and 20 is then reduced by the same factor~
causing the current through the transistors 14 and 20 to be reduced by the same factor too~ Seen from subscriber's current path terminal 9 the impedance of the coupling circuit~ being the ratio of the voltage variation to this 20 current varia-tion has then increased by the same factor.
Claims (3)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A subscriber circuit for a telephone network, comprising an internal portion of the subscriber's current path which portion is conductively coupled to a point of fixed reference potential, characterized in that the internal subscriber's current path portion is coupled to the point of fixed reference potential via a two-pole coupling circuit the impedance of which has for direct current a low value as compared with the impedance for alternating current.
2. A subscriber circuit as claimed in Claim 1, characterized in that the coupling circuit comprises:
- a subscriber's current path terminal for connection to the internal portion of the subscriber's current path, - a reference terminal for connection to the fixed reference potantial, - and a parallel arrangement provided by a first series arrangement of in succession a first resistor, the main current path of a first transistor and a capacitor on the one hand and a second series arrangement provided by the main current path of a second transistor and a second resistor on the other hand, that - the first resistor is coupled to the collector of the second transistor and is connected tothe subscriber's current path terminal, - the collector of the first transistor is connected to the base of the second transistor, - the base of the first transistor is connected to the reference terminal, and that the first and the second transistor are of a complementary conductivity type.
- a subscriber's current path terminal for connection to the internal portion of the subscriber's current path, - a reference terminal for connection to the fixed reference potantial, - and a parallel arrangement provided by a first series arrangement of in succession a first resistor, the main current path of a first transistor and a capacitor on the one hand and a second series arrangement provided by the main current path of a second transistor and a second resistor on the other hand, that - the first resistor is coupled to the collector of the second transistor and is connected tothe subscriber's current path terminal, - the collector of the first transistor is connected to the base of the second transistor, - the base of the first transistor is connected to the reference terminal, and that the first and the second transistor are of a complementary conductivity type.
3. A subscriber circuit as claimed in Claim 2, characterized in that the coupling circuit further comprises, connected in parallel with the first resistor, a third series arrangement of in succession the main current path of a third transistor, a second capacitor, a third resistor and the main current path of a fourth transistor, that the collector of the fourth transistor is coupled to the subscriber's current path terminal, that the collector of the third transistor is connected to the second capacitor and to the base of the fourth transistor, and that the first and the third transistor are of a complementary conductivity type, which also applies to the third and fourth transistors.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NL8204377A NL8204377A (en) | 1982-11-12 | 1982-11-12 | SUBSCRIPTION CIRCUIT FOR A TELEPHONE NETWORK. |
| NL8204377 | 1982-11-12 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1206648A true CA1206648A (en) | 1986-06-24 |
Family
ID=19840578
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000440700A Expired CA1206648A (en) | 1982-11-12 | 1983-11-08 | Subscriber circuit for a telephone network |
Country Status (8)
| Country | Link |
|---|---|
| US (1) | US4608461A (en) |
| EP (1) | EP0109705B1 (en) |
| JP (1) | JPS59101955A (en) |
| AU (1) | AU563742B2 (en) |
| CA (1) | CA1206648A (en) |
| DE (1) | DE3370737D1 (en) |
| DK (1) | DK512583A (en) |
| NL (1) | NL8204377A (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4551670A (en) * | 1984-04-19 | 1985-11-05 | At&T Bell Laboratories | Circuit for maintaining the DC voltage on an electrically isolated telecommunication line at a reference level |
| US4679232A (en) * | 1985-08-02 | 1987-07-07 | American Telephone And Telegraph Company, At&T Bell Laboratories | Method and apparatus for providing a ground reference for telephone customer special circuits powered from a floating battery feed |
| US7869179B2 (en) * | 2007-03-14 | 2011-01-11 | Panduit Corp. | Protection patch panel |
Family Cites Families (13)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3223781A (en) * | 1962-02-13 | 1965-12-14 | Itt | Constant voltage device |
| FR2254168A1 (en) * | 1973-12-11 | 1975-07-04 | Ass Ouvriers Instr Precision | Telephone line supply device - is an active electronic device acting as resistance for direct current and high impedance for A.C. |
| FR2315811A1 (en) * | 1975-06-27 | 1977-01-21 | Labo Cent Telecommunicat | ELECTRONIC DIPOLE FOR LOOPING A TELEPHONE LINE |
| GB1563802A (en) * | 1975-11-10 | 1980-04-02 | Post Office | Electronic current feed circuit |
| FR2373927A1 (en) * | 1976-12-10 | 1978-07-07 | Trt Telecom Radio Electr | ELECTRICAL NETWORK INTENDED TO BE INSERTED BY A PAIR OF TERMINALS IN A TELEPHONE SUBSCRIBER LOOP |
| FR2382810A1 (en) * | 1977-03-02 | 1978-09-29 | Labo Cent Telecommunicat | TELEPHONE LINE POWER CIRCUIT WITH PROTECTION |
| FR2403690A1 (en) * | 1977-09-14 | 1979-04-13 | Labo Cent Telecommunicat | Active dipole gyrator for telephone line - uses transistor with base coupled to output of operational amplifier |
| US4272656A (en) * | 1979-04-05 | 1981-06-09 | Precision Monolithics, Inc. | Quasi-resistive battery feed for telephone circuits |
| NL7903662A (en) * | 1979-05-10 | 1980-11-12 | Philips Nv | LINE CIRCUIT. |
| NL7904159A (en) * | 1979-05-28 | 1980-12-02 | Philips Nv | DEVICE FOR SUPPLYING SIGNALS TO A TELEPHONE LINE. |
| US4476351A (en) * | 1982-03-25 | 1984-10-09 | Rockwell International Corporation | Subscriber loop current regulator |
| US4440979A (en) * | 1982-05-03 | 1984-04-03 | Gte Automatic Electric Incorporated | Current sink for DPT channel unit |
| US4511763A (en) * | 1982-11-01 | 1985-04-16 | Gte Communication Systems Corporation | Floating dc current source for a subscriber loop circuit |
-
1982
- 1982-11-12 NL NL8204377A patent/NL8204377A/en not_active Application Discontinuation
-
1983
- 1983-01-21 US US06/459,858 patent/US4608461A/en not_active Expired - Fee Related
- 1983-11-08 CA CA000440700A patent/CA1206648A/en not_active Expired
- 1983-11-09 DK DK512583A patent/DK512583A/en not_active Application Discontinuation
- 1983-11-10 EP EP83201608A patent/EP0109705B1/en not_active Expired
- 1983-11-10 AU AU21144/83A patent/AU563742B2/en not_active Ceased
- 1983-11-10 DE DE8383201608T patent/DE3370737D1/en not_active Expired
- 1983-11-11 JP JP58211124A patent/JPS59101955A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| DE3370737D1 (en) | 1987-05-07 |
| DK512583A (en) | 1984-05-13 |
| AU563742B2 (en) | 1987-07-23 |
| DK512583D0 (en) | 1983-11-09 |
| NL8204377A (en) | 1984-06-01 |
| EP0109705B1 (en) | 1987-04-01 |
| EP0109705A1 (en) | 1984-05-30 |
| US4608461A (en) | 1986-08-26 |
| AU2114483A (en) | 1984-05-17 |
| JPS59101955A (en) | 1984-06-12 |
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