CA1177985A - Electronic terminator circuit - Google Patents
Electronic terminator circuitInfo
- Publication number
- CA1177985A CA1177985A CA000408607A CA408607A CA1177985A CA 1177985 A CA1177985 A CA 1177985A CA 000408607 A CA000408607 A CA 000408607A CA 408607 A CA408607 A CA 408607A CA 1177985 A CA1177985 A CA 1177985A
- Authority
- CA
- Canada
- Prior art keywords
- current
- slic
- circuit means
- terminator
- resistor
- 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
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/38—Transceivers, i.e. devices in which transmitter and receiver form a structural unit and in which at least one part is used for functions of transmitting and receiving
- H04B1/40—Circuits
- H04B1/54—Circuits using the same frequency for two directions of communication
- H04B1/58—Hybrid arrangements, i.e. arrangements for transition from single-path two-direction transmission to single-direction transmission on each of two paths or vice versa
-
- 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
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- Engineering & Computer Science (AREA)
- Signal Processing (AREA)
- Computer Networks & Wireless Communication (AREA)
- Power Engineering (AREA)
- Interface Circuits In Exchanges (AREA)
- Devices For Supply Of Signal Current (AREA)
Abstract
ELECTRONIC TERMINATOR CIRCUIT
Abstract A subscriber loop interface circuit (SLIC) is used as a terminator circuit which is ground referenced for providing signal conversion between a two-wire bidirec-tional transmission signal path and a pair of two-wire unidirectional transmission signal paths. The SLIC and associated circuitry allows the use of an all electronic circuit to replace bulky transformers generally utilized in telecommunication systems.
Abstract A subscriber loop interface circuit (SLIC) is used as a terminator circuit which is ground referenced for providing signal conversion between a two-wire bidirec-tional transmission signal path and a pair of two-wire unidirectional transmission signal paths. The SLIC and associated circuitry allows the use of an all electronic circuit to replace bulky transformers generally utilized in telecommunication systems.
Description
~7~ S
ELECTRONIC TERMINATOR CIRCUIT
Background of the Invention Field of the Invention:
This invention relates to terminating circuits for providing signal conversion between a bidirectional two-wire signal path and a pair of two-wire unidirectional signal paths. More particularly, the invention concerns the replacement of bulky hybrid transformers typically used in conventional telecommunication systems such as the telephone system, for example, for providing signal conversion by an electronic terminator circuit that is referenced to signal ground.
Description of the Prior Art;
_____ __._ In most, if not all, contemporary telecommunication systems using telephone central office exchanges, hybrid transformers are utilized to provide signal conversion from the balanced, bidirectional subscriber end loop signal path to a pair of unidirectional signal paths. These trans-formers must be well matched with the hybrid transformers located at the telephone central office equipment to insure good AC impedance matching between the termination signal paths and the bidirectional transmission path in order to reduce and/or eliminate signal echo returns. These transformers are generally very bulky and expensive. In addition, impedance matching, as aforedescribed, is not easily adjusted uslng conventional transformers.
Therefore, communications systems utilizing such transformers utilized in a modem application may have degraded performance.
Thus, there exists a need for an electronic terminator circuit that is ground referenced at the terminating end of the subscriber loop which can be used in typical modem 9 ~ 5
ELECTRONIC TERMINATOR CIRCUIT
Background of the Invention Field of the Invention:
This invention relates to terminating circuits for providing signal conversion between a bidirectional two-wire signal path and a pair of two-wire unidirectional signal paths. More particularly, the invention concerns the replacement of bulky hybrid transformers typically used in conventional telecommunication systems such as the telephone system, for example, for providing signal conversion by an electronic terminator circuit that is referenced to signal ground.
Description of the Prior Art;
_____ __._ In most, if not all, contemporary telecommunication systems using telephone central office exchanges, hybrid transformers are utilized to provide signal conversion from the balanced, bidirectional subscriber end loop signal path to a pair of unidirectional signal paths. These trans-formers must be well matched with the hybrid transformers located at the telephone central office equipment to insure good AC impedance matching between the termination signal paths and the bidirectional transmission path in order to reduce and/or eliminate signal echo returns. These transformers are generally very bulky and expensive. In addition, impedance matching, as aforedescribed, is not easily adjusted uslng conventional transformers.
Therefore, communications systems utilizing such transformers utilized in a modem application may have degraded performance.
Thus, there exists a need for an electronic terminator circuit that is ground referenced at the terminating end of the subscriber loop which can be used in typical modem 9 ~ 5
-2-appl~cations to replace bulky tansformers now being used as above described.
Summary of the Invention Accordingly, lt is an object of the present invent~on to provide an ~mproved term~nator c~rcuit for use ~n 00dem appl~cations to provlde s19nal convers1On between ~
b~direct~onal transm1ss1On slgnal path and a palr of un1d1rectional transm~sslon slgnal paths.
It ~s another ob~ect of the present ~nvent1On to prov~de an electron~c term~nator clrcu~t ~h~ch is ground referenced and that is compatible w~th typical telephon~c communication systems.
In accordance with the above and other ob~ects there ~s provided an electronlc terminator clrcult ~hlch ~s ground referenced at the subscr~ber term~nating loop for provid~ng s~gnal separat~on for two-wlre d~fferent~al to four-wire s1ngle-ended conversions and suppress~ons of lonsitudinal signals at the two-w~re lnput thereto,as ~ell as echo returns at the four-wlre s~de. The electron~c terminator c~rcu1t compr~ses a subscr~ber loop ~nterf~ce c1rcu1t (SLIC) 1n co~blnat~on w~th assoclated c~rcu1try coupled between the two-wlre 1nput and the SLIC for ~solat1ng the AC and DC feedback appèarlng at the two-~re ~nput.
More particularly, there is provided:
Sn a telephony syate~ including clrcult ~eans for providing ~ignal conver~ion between a pair of unidirec-tional signal paths and a bidirectional signal path, ater~ination circuit comprising:
transformerless hybrid circuit means for providing signal conversion bet~een the bidirectional ~ignal path and a pair of unidirectional signal paths vhile providing a DC current feedback path to th- telephony .
~ ~y~te~
1 1 ~7 9 85 -2a-ir~t clrcu$t ~eans ooupled between aid transformerless hybrid circuit ~eans ~nd the bidirectional signal path or i~ol~ting the DC ~ignal path from the AC
~ignal path while cau6ing the DC current feedback from 6aid hybrid circuit oean6 to be substanelally equal in ~a~nitude to the DC current source thereto from the telephony sy~tem ~o as to cau~e the DC qu~escent operating m~dpoint voltage of the bidlrectional ~ignal path ~nd ~aid hybrid circuit ~e~ns to be ~ub6tantially equal; ~nd second clrcult means coupled between said hybrld circult mean~ ~nd said pair of unidirectional ~ignal paths for provlding the proper AC loop termination impedance to the telephone ~y~te~.
. .
Br1 ef Descr~ption of the Draw1ngs~
FIG. 1 ~llustrates a subscrlber loop 1nterface clrcuit .(SLIC) known to those skilled in the art; and ; FIG. 2 illustrates the electron1c termlnator c1rcult of the present 1nvent10n wh1ch comprlses the SLIC of FIG. 1.
~.t ~77985 'Deta11ed Descrlptlon of the Invention FIG. 1 shows a subscriber loop lnterface clrcult ~SLIC) I0 that ls well-known to those skilled 1n the art 5' for providlng slgnal converslon between two-wlre dlfferentlal slgnals and a pa~r of two-w~re slngle-ended slgnal paths. In general, SLIC lO 1s deslgned to replace the hybrld transformer clrcult ln Central Telephone Offices, PABX, and Subscrlber carrler equlpment provldlng two-way telecommunlcatlon. The operatlon and functlon of SLIC 10 ls well-known ln the art ~nd ls descrlbed ln U.S.
Patents 4,004,109 and 4,300,02l SLIC 10 ~s - . ~ - ;
sulted for fabricatlon 1n monollthlc lntegrated form and ls manufaetured by Motorola, Inc., for lnst'ance, under the part number MC3419.
SLIC 10 1s typ1cally located at the telephone central offl'ce and ls adapted to be coupled at Tlp and R1ng termlnals 12 and 14 vla telephone llnes to a subscrlber loop. The subscrlber loop generally comprlses a telephone for provlding bldlrectlonal slgnal transmlsslon. .Thus, lncomlng slgnals appear~ng at VRX (for example, slgnals transmltted through the central exchange from a calllng party) are recelved by SLIC 10 and converted to ~, -- 25 proport10nal dlfferentlal slgnals at termlnals 12 and 14.
Therefore, transhybrld recept10n ls reallzed by convertlng the AC coupled rece~ved slgnals through capacltor 16 through reslstor 18 to a current whlch 1s processed vla SLIC lO to drlvè'the two-wlre output line co4pled at terminals 12 and 14 vla PNP and NPN Darllngton translstors ~' 20, 22 and reslstors 24, 26 with a balansed AC current propo'rtlonal to the recelve 1nput voltage. Slmilarly, balance~ AC vo!tage slgnals appearlng at terminals 12 and 14, as supplled by the called party vla the telephone ~ 9 ~ S
coupled therewith, are converted to a single~ended output signal by SLIC 10 to appear at output Tx thereof. The conversion produces an output signal AC coupled through resistor 28, capacitor 30 and operational amplifier 32 to the calling party at output VTx. Additionally, resistors 28 and 34, as understood, in combination with resistors 36 and 38 determine the AC termination impedance of SLIC 10. Further, transhybrid rejection is achieved by taking advantage of the 180 phase reversal of the current appearing at Tx output with respect to the VRX input.
Resistor 40, a balance resistor, is placed between VRx input and the virtual ground point between capacitor 30 and resistor 28. By selecting the value of resistor 40, as understood, the return current from Tx output generated by reflected current from the loop impedance to the four-wire output is cancelled by the current flowing through resistor 40 from VRX.
The VAG input of SLIC 10 provides an analog ground reference to isolate the four-wire receive and transmit signal paths from noise on the system power ground by establishing a common AC signal reference. A hook status output, HS0, that among other functions, provides a digital signal derived in response to the telephone being off-hook after a substantial period to supply an input signal at the PDI input to shut down SLIC 10 in order to conserve power.
In addition to the above and other AC characteristics SLIC 10 has several significant DC characteristics germane to the present invention. Thus, when the subscriber loop coupled to terminals 12 and 14 is open-circuited (as when the telephone is on-hook) SLIC 10 is in a quiescent state.
In this condition current is supplied only through resistors 36 and 3~ and power dissipation is limited primarily to leakage currents. With a load applied across terminals 12 and 14 (telephone being off-hook for example) SLIC 10 in its normal application provides a DC current to S
the line, i.e., a current is sourced through transistor 20, and resistor 24. This current is returned through the subscriber loop to SLIC 10 via resistor 26 and transistor 22. Additionally, the DC current flowing through the subscriber loop produces a voltage across Tip and Ring terminals 12 and 14 which is sensed and converted to a current flow through resistor 36, the TSI input, and resistor 38 through the RSI input. If the current sourced from terminal 12 is not equal to the current sinked at terminal 14, a high gain compensation circuit internal to SLIC 10 commonly referred to as the "C-circuit", which is operatively coupled to the Tip and Ring current sense inputs TSI and RSI as well as terminal CC and transistors 20, 22, causes the bias supplied to transistors 20, 22 to be varied such that the DC midpoint voltage of the subscriber loop resistive load is maintained midway between ground potential and VEE.
Turning now to FIG. 2 there is illustrated electronic terminator circuit 100 which allows the use of SLIC 10 to be used as a terminator which is ground referenced to provide signal conversion from the two-wire Tip and Ring terminals 102 and 104 to four-wire unidirectional inputs at TX and Rx inputs 106 and 108. Thus, a bidirectional differential signal appearing across hybrid transformer 114 is supplied across terminals 102 and 104 through the line resistance symbolized by resistors 110 and 112. This slgnal is then converted by SLIC 10 to appear as a proportional, single-ended signal at the Tx output of SLIC 10 to be received by an utilizatlon means coupled thereat. Similarly, as described above, a unidirectional AC signal applied at VTX ls coupled to the Rx input of SLIC 10 to appear as a proportional, differential AC
signal at terminals 102 and 104. (Components in FIG. 2 corresponding to like components in FIG~ 1 are referenced by the same numerals.) Although not shown, it is to be understood that each of the two unidirectional signal 1~77~8~i paths, VTx, VRX comprise a two-wire pair with one wire each being at AC ground reference (VAG).
Terminator circuit 100 can be used in many modem applications. For example, a speaker-phone could be coupled to terminals 106 and 108 such that the speaker would be coupled to terminal 108. The speaker-phone microphone would then be coupled to terminal 106 whereby two-way voice communication could be conducted through terminator 100 and the telephone system. Teletype equipment could be coupled at terminals 106 and 108 as another example of an application of terminator 100.
It was discovered that SLIC 10 described above could not be simply connected as a terminator circuit with the central office equipment, if the central office equipment in turn utilized a subscriber interface circuit. If such was the case it was determined that in most instances the DC characteristics of SLIC 10 caused either the central office SLIC or the terminator SLIC to become latched to one of the two power supply rails, thus, assuming for discussion purposes that resistors 36 and 38 of terminator 100 were directly coupled to Ring and Tip terminals as shown in FIG. 2, and that SLIC 10 was connected to the Tip and Ring terminals at the central office as shown in FIG. 1, then a DC current would be sourced to Tip terminal 102 to resistor 38 and transistor 22 of terminator 100 and terminator 100 would source a DC current to Ring terminal 104 to the SLIC at the central office. Ideally, the current sourced from the central office to terminator 100 would be equal to the current sourced from terminator 100 to the central office equipment with the DC voltage across Tip and Ring terminals being substantially equal to (VCc-VEE)/2. However, in reality, because of varying ground potentials to which the Central Office SLIC
and terminator 100 are referenced, unbalanced subscriber line impedances and other factors, the voltages at which the Central Office SLIC and terminator 100 attempts to 1~77~35 center their respective Tip and Ring terminals at may be different. This difference in DC voltage would then be applied across the line impedances that could result in large DC longitudinal currents which are in excess of the SLIC and terminator 100 C-circuit compensating capabilities. Thus, either one or both the Central Office SLIC or terminaotr 100 may be driven to the opposite power supply rail which would deleteriously effect the operation thereof.
In view of the above, it was determined to isolate the DC feedback of terminator 100 from the AC feedback path.
Hence, AC coupling capacitors 116 and 118 are connected between Ring and Tip terminals 104 and 102 to respective sense resistors 36 and 38. Therefore, SLIC 10 of terminator 100 will respond to the AC differential signals appearing at Tip and Ring terminals to produce a single-ended AC signal at Tx as previously described. However, the DC feedback path has been changed to prevent the aforementioned offset problem from occurring. As illustrated, the emitter of transistor 22 is now coupled through current sensing resistor 120 to the EN terminal of SLIC 10 whereas before, as shown in FIG. 1, the emitter was directly returned to the EN terminal. Thus, the DC current sourced from the c~ntral office equipment to Tip terminal 102 is sourced through transistor 22 through resistor 120.
The current through resistor 120 establishes a voltage at the emitter of transistor 122. Translstor 122, the current flow therethrough being determined by resistor 124, has its collector returned to its base through resistor 126 and functions as a diode of a well known current mirror circuit such that the voltage appearing at node 128 is a function of the current through resistor 120. Resistor 130 coupled between node 128 and the base of transistor 132 (transistor 132 being matched to transistor 122) being of equal value as resistor 126 causes the voltage at the emitter of transistor 132 to be equal to the voltage at the emitter of 77~85 transistor 122. Thus, the current through transistor 132 is made a function of the value of resistor 134 which is coupled between the emitter of transistor 132 and EN. By adjusting the size of resistor 134 the collector current sourced through transistor 132 is made proportional to the current sourced to Tip terminal 102. This forms a current source at the TSI terminal of SLIC 10. As aforementioned, a high gain internal C-circuit of SLIC 10 is coupled to the RSI and TSI terminals which causes the bias on transistors 20 and 22 to be varied lf the TSI and RSI sensed currents are not equal. Thus, the current sourced by transistor 132 causes the transistors 20 and 22 to adjust to an equili-brium wherein the currents through resistors 124 and 136 are equal; these resistors being of equal value with resistor 136 being coupled between ground and the RSI input of SLIC 10 of terminator 100. This condition forces the DC
current sourced from transistor 20 to Ring terminal 104 to be set substantially equal to the DC current sourced to Tip terminal from the Central Office equipment which causes SLIC 10 of terminator 100 to float to the DC balance point of SLIC 10 in the Central Office. Capacitor 138 and resistor 130 provide compensation to the DC feedback loop.
In addition, resistor 40 of FIG. 1 has been substituted by serial connected resistors 140, 142 and capacitor 144 which is coupled between analog ground VAG and the intersectlng node between these two res~stors to provide ln combination with reslstors 36 and 38 the proper AC loop terminatlon impedance.
If, for instance, the Motorola, Inc. MC3419 is used as SLIC 10 forming terminator lOO, the gain of the internal C-circuit thereof is approximately 95 wherein resistor 134 is made 95 times greater than resistor 120. In fact, an electronic terminator circuit as described above has been built and tested utilizing an MC3419 subscriber loop interface circuit having the following component values:
~.~ 77~1S
g Transistor 20 MJE271 Transistor 22 MJE270 Resistor 36 17.4K ohms Resistor 38 17.4K ohms S Resistor 120 20 ohms Resistor 124 180K ohms Resistor 126 20K ohms Resistor 130 20K ohms Resistor 134 l.9K ohms Resistor 136 180K ohms Capacitor 116 1.0 ~F
Capacitor 118 1.0 ~F
Capacitor 138 2.0 ~F
Summary of the Invention Accordingly, lt is an object of the present invent~on to provide an ~mproved term~nator c~rcuit for use ~n 00dem appl~cations to provlde s19nal convers1On between ~
b~direct~onal transm1ss1On slgnal path and a palr of un1d1rectional transm~sslon slgnal paths.
It ~s another ob~ect of the present ~nvent1On to prov~de an electron~c term~nator clrcu~t ~h~ch is ground referenced and that is compatible w~th typical telephon~c communication systems.
In accordance with the above and other ob~ects there ~s provided an electronlc terminator clrcult ~hlch ~s ground referenced at the subscr~ber term~nating loop for provid~ng s~gnal separat~on for two-wlre d~fferent~al to four-wire s1ngle-ended conversions and suppress~ons of lonsitudinal signals at the two-w~re lnput thereto,as ~ell as echo returns at the four-wlre s~de. The electron~c terminator c~rcu1t compr~ses a subscr~ber loop ~nterf~ce c1rcu1t (SLIC) 1n co~blnat~on w~th assoclated c~rcu1try coupled between the two-wlre 1nput and the SLIC for ~solat1ng the AC and DC feedback appèarlng at the two-~re ~nput.
More particularly, there is provided:
Sn a telephony syate~ including clrcult ~eans for providing ~ignal conver~ion between a pair of unidirec-tional signal paths and a bidirectional signal path, ater~ination circuit comprising:
transformerless hybrid circuit means for providing signal conversion bet~een the bidirectional ~ignal path and a pair of unidirectional signal paths vhile providing a DC current feedback path to th- telephony .
~ ~y~te~
1 1 ~7 9 85 -2a-ir~t clrcu$t ~eans ooupled between aid transformerless hybrid circuit ~eans ~nd the bidirectional signal path or i~ol~ting the DC ~ignal path from the AC
~ignal path while cau6ing the DC current feedback from 6aid hybrid circuit oean6 to be substanelally equal in ~a~nitude to the DC current source thereto from the telephony sy~tem ~o as to cau~e the DC qu~escent operating m~dpoint voltage of the bidlrectional ~ignal path ~nd ~aid hybrid circuit ~e~ns to be ~ub6tantially equal; ~nd second clrcult means coupled between said hybrld circult mean~ ~nd said pair of unidirectional ~ignal paths for provlding the proper AC loop termination impedance to the telephone ~y~te~.
. .
Br1 ef Descr~ption of the Draw1ngs~
FIG. 1 ~llustrates a subscrlber loop 1nterface clrcuit .(SLIC) known to those skilled in the art; and ; FIG. 2 illustrates the electron1c termlnator c1rcult of the present 1nvent10n wh1ch comprlses the SLIC of FIG. 1.
~.t ~77985 'Deta11ed Descrlptlon of the Invention FIG. 1 shows a subscriber loop lnterface clrcult ~SLIC) I0 that ls well-known to those skilled 1n the art 5' for providlng slgnal converslon between two-wlre dlfferentlal slgnals and a pa~r of two-w~re slngle-ended slgnal paths. In general, SLIC lO 1s deslgned to replace the hybrld transformer clrcult ln Central Telephone Offices, PABX, and Subscrlber carrler equlpment provldlng two-way telecommunlcatlon. The operatlon and functlon of SLIC 10 ls well-known ln the art ~nd ls descrlbed ln U.S.
Patents 4,004,109 and 4,300,02l SLIC 10 ~s - . ~ - ;
sulted for fabricatlon 1n monollthlc lntegrated form and ls manufaetured by Motorola, Inc., for lnst'ance, under the part number MC3419.
SLIC 10 1s typ1cally located at the telephone central offl'ce and ls adapted to be coupled at Tlp and R1ng termlnals 12 and 14 vla telephone llnes to a subscrlber loop. The subscrlber loop generally comprlses a telephone for provlding bldlrectlonal slgnal transmlsslon. .Thus, lncomlng slgnals appear~ng at VRX (for example, slgnals transmltted through the central exchange from a calllng party) are recelved by SLIC 10 and converted to ~, -- 25 proport10nal dlfferentlal slgnals at termlnals 12 and 14.
Therefore, transhybrld recept10n ls reallzed by convertlng the AC coupled rece~ved slgnals through capacltor 16 through reslstor 18 to a current whlch 1s processed vla SLIC lO to drlvè'the two-wlre output line co4pled at terminals 12 and 14 vla PNP and NPN Darllngton translstors ~' 20, 22 and reslstors 24, 26 with a balansed AC current propo'rtlonal to the recelve 1nput voltage. Slmilarly, balance~ AC vo!tage slgnals appearlng at terminals 12 and 14, as supplled by the called party vla the telephone ~ 9 ~ S
coupled therewith, are converted to a single~ended output signal by SLIC 10 to appear at output Tx thereof. The conversion produces an output signal AC coupled through resistor 28, capacitor 30 and operational amplifier 32 to the calling party at output VTx. Additionally, resistors 28 and 34, as understood, in combination with resistors 36 and 38 determine the AC termination impedance of SLIC 10. Further, transhybrid rejection is achieved by taking advantage of the 180 phase reversal of the current appearing at Tx output with respect to the VRX input.
Resistor 40, a balance resistor, is placed between VRx input and the virtual ground point between capacitor 30 and resistor 28. By selecting the value of resistor 40, as understood, the return current from Tx output generated by reflected current from the loop impedance to the four-wire output is cancelled by the current flowing through resistor 40 from VRX.
The VAG input of SLIC 10 provides an analog ground reference to isolate the four-wire receive and transmit signal paths from noise on the system power ground by establishing a common AC signal reference. A hook status output, HS0, that among other functions, provides a digital signal derived in response to the telephone being off-hook after a substantial period to supply an input signal at the PDI input to shut down SLIC 10 in order to conserve power.
In addition to the above and other AC characteristics SLIC 10 has several significant DC characteristics germane to the present invention. Thus, when the subscriber loop coupled to terminals 12 and 14 is open-circuited (as when the telephone is on-hook) SLIC 10 is in a quiescent state.
In this condition current is supplied only through resistors 36 and 3~ and power dissipation is limited primarily to leakage currents. With a load applied across terminals 12 and 14 (telephone being off-hook for example) SLIC 10 in its normal application provides a DC current to S
the line, i.e., a current is sourced through transistor 20, and resistor 24. This current is returned through the subscriber loop to SLIC 10 via resistor 26 and transistor 22. Additionally, the DC current flowing through the subscriber loop produces a voltage across Tip and Ring terminals 12 and 14 which is sensed and converted to a current flow through resistor 36, the TSI input, and resistor 38 through the RSI input. If the current sourced from terminal 12 is not equal to the current sinked at terminal 14, a high gain compensation circuit internal to SLIC 10 commonly referred to as the "C-circuit", which is operatively coupled to the Tip and Ring current sense inputs TSI and RSI as well as terminal CC and transistors 20, 22, causes the bias supplied to transistors 20, 22 to be varied such that the DC midpoint voltage of the subscriber loop resistive load is maintained midway between ground potential and VEE.
Turning now to FIG. 2 there is illustrated electronic terminator circuit 100 which allows the use of SLIC 10 to be used as a terminator which is ground referenced to provide signal conversion from the two-wire Tip and Ring terminals 102 and 104 to four-wire unidirectional inputs at TX and Rx inputs 106 and 108. Thus, a bidirectional differential signal appearing across hybrid transformer 114 is supplied across terminals 102 and 104 through the line resistance symbolized by resistors 110 and 112. This slgnal is then converted by SLIC 10 to appear as a proportional, single-ended signal at the Tx output of SLIC 10 to be received by an utilizatlon means coupled thereat. Similarly, as described above, a unidirectional AC signal applied at VTX ls coupled to the Rx input of SLIC 10 to appear as a proportional, differential AC
signal at terminals 102 and 104. (Components in FIG. 2 corresponding to like components in FIG~ 1 are referenced by the same numerals.) Although not shown, it is to be understood that each of the two unidirectional signal 1~77~8~i paths, VTx, VRX comprise a two-wire pair with one wire each being at AC ground reference (VAG).
Terminator circuit 100 can be used in many modem applications. For example, a speaker-phone could be coupled to terminals 106 and 108 such that the speaker would be coupled to terminal 108. The speaker-phone microphone would then be coupled to terminal 106 whereby two-way voice communication could be conducted through terminator 100 and the telephone system. Teletype equipment could be coupled at terminals 106 and 108 as another example of an application of terminator 100.
It was discovered that SLIC 10 described above could not be simply connected as a terminator circuit with the central office equipment, if the central office equipment in turn utilized a subscriber interface circuit. If such was the case it was determined that in most instances the DC characteristics of SLIC 10 caused either the central office SLIC or the terminator SLIC to become latched to one of the two power supply rails, thus, assuming for discussion purposes that resistors 36 and 38 of terminator 100 were directly coupled to Ring and Tip terminals as shown in FIG. 2, and that SLIC 10 was connected to the Tip and Ring terminals at the central office as shown in FIG. 1, then a DC current would be sourced to Tip terminal 102 to resistor 38 and transistor 22 of terminator 100 and terminator 100 would source a DC current to Ring terminal 104 to the SLIC at the central office. Ideally, the current sourced from the central office to terminator 100 would be equal to the current sourced from terminator 100 to the central office equipment with the DC voltage across Tip and Ring terminals being substantially equal to (VCc-VEE)/2. However, in reality, because of varying ground potentials to which the Central Office SLIC
and terminator 100 are referenced, unbalanced subscriber line impedances and other factors, the voltages at which the Central Office SLIC and terminator 100 attempts to 1~77~35 center their respective Tip and Ring terminals at may be different. This difference in DC voltage would then be applied across the line impedances that could result in large DC longitudinal currents which are in excess of the SLIC and terminator 100 C-circuit compensating capabilities. Thus, either one or both the Central Office SLIC or terminaotr 100 may be driven to the opposite power supply rail which would deleteriously effect the operation thereof.
In view of the above, it was determined to isolate the DC feedback of terminator 100 from the AC feedback path.
Hence, AC coupling capacitors 116 and 118 are connected between Ring and Tip terminals 104 and 102 to respective sense resistors 36 and 38. Therefore, SLIC 10 of terminator 100 will respond to the AC differential signals appearing at Tip and Ring terminals to produce a single-ended AC signal at Tx as previously described. However, the DC feedback path has been changed to prevent the aforementioned offset problem from occurring. As illustrated, the emitter of transistor 22 is now coupled through current sensing resistor 120 to the EN terminal of SLIC 10 whereas before, as shown in FIG. 1, the emitter was directly returned to the EN terminal. Thus, the DC current sourced from the c~ntral office equipment to Tip terminal 102 is sourced through transistor 22 through resistor 120.
The current through resistor 120 establishes a voltage at the emitter of transistor 122. Translstor 122, the current flow therethrough being determined by resistor 124, has its collector returned to its base through resistor 126 and functions as a diode of a well known current mirror circuit such that the voltage appearing at node 128 is a function of the current through resistor 120. Resistor 130 coupled between node 128 and the base of transistor 132 (transistor 132 being matched to transistor 122) being of equal value as resistor 126 causes the voltage at the emitter of transistor 132 to be equal to the voltage at the emitter of 77~85 transistor 122. Thus, the current through transistor 132 is made a function of the value of resistor 134 which is coupled between the emitter of transistor 132 and EN. By adjusting the size of resistor 134 the collector current sourced through transistor 132 is made proportional to the current sourced to Tip terminal 102. This forms a current source at the TSI terminal of SLIC 10. As aforementioned, a high gain internal C-circuit of SLIC 10 is coupled to the RSI and TSI terminals which causes the bias on transistors 20 and 22 to be varied lf the TSI and RSI sensed currents are not equal. Thus, the current sourced by transistor 132 causes the transistors 20 and 22 to adjust to an equili-brium wherein the currents through resistors 124 and 136 are equal; these resistors being of equal value with resistor 136 being coupled between ground and the RSI input of SLIC 10 of terminator 100. This condition forces the DC
current sourced from transistor 20 to Ring terminal 104 to be set substantially equal to the DC current sourced to Tip terminal from the Central Office equipment which causes SLIC 10 of terminator 100 to float to the DC balance point of SLIC 10 in the Central Office. Capacitor 138 and resistor 130 provide compensation to the DC feedback loop.
In addition, resistor 40 of FIG. 1 has been substituted by serial connected resistors 140, 142 and capacitor 144 which is coupled between analog ground VAG and the intersectlng node between these two res~stors to provide ln combination with reslstors 36 and 38 the proper AC loop terminatlon impedance.
If, for instance, the Motorola, Inc. MC3419 is used as SLIC 10 forming terminator lOO, the gain of the internal C-circuit thereof is approximately 95 wherein resistor 134 is made 95 times greater than resistor 120. In fact, an electronic terminator circuit as described above has been built and tested utilizing an MC3419 subscriber loop interface circuit having the following component values:
~.~ 77~1S
g Transistor 20 MJE271 Transistor 22 MJE270 Resistor 36 17.4K ohms Resistor 38 17.4K ohms S Resistor 120 20 ohms Resistor 124 180K ohms Resistor 126 20K ohms Resistor 130 20K ohms Resistor 134 l.9K ohms Resistor 136 180K ohms Capacitor 116 1.0 ~F
Capacitor 118 1.0 ~F
Capacitor 138 2.0 ~F
Claims (2)
1. In a telephony system including circuit means for providing signal conversion between a pair of unidirec-tional signal paths and a bidirectional signal path, a termination circuit comprising:
transformerless hybrid circuit means for providing signal conversion between the bidirectional signal path and a pair of unidirectional signal paths while providing a DC current feedback path to the telephony system;
first circuit means coupled between said transformerless hybrid circuit means and the bidirectional signal path for isolating the DC signal path from the AC
signal path while causing the DC current feedback from said hybrid circuit means to be substantially equal in magnitude to the DC current source thereto from the telephony system so as to cause the DC quiescent operating midpoint voltage of the bidirectional signal path and said hybrid circuit means to be substantially equal; and second circuit means coupled between said hybrid circuit means and said pair of unidirectional signal paths for providing the proper AC loop termination impedance to the telephone system.
transformerless hybrid circuit means for providing signal conversion between the bidirectional signal path and a pair of unidirectional signal paths while providing a DC current feedback path to the telephony system;
first circuit means coupled between said transformerless hybrid circuit means and the bidirectional signal path for isolating the DC signal path from the AC
signal path while causing the DC current feedback from said hybrid circuit means to be substantially equal in magnitude to the DC current source thereto from the telephony system so as to cause the DC quiescent operating midpoint voltage of the bidirectional signal path and said hybrid circuit means to be substantially equal; and second circuit means coupled between said hybrid circuit means and said pair of unidirectional signal paths for providing the proper AC loop termination impedance to the telephone system.
2. The termination circuit of claim 1 wherein said first circuit means includes:
current sensing means for sensing said DC current source thereto from the telephony system to establish a DC
voltage that is proportional thereto;
current mirror means coupled to said current sensing means for producing a current that is proportional to said sensed DC current; and said hybrid circuit means being responsive to said current provided by said current mirror means for sourcing said DC current feedback to said bidirectional signal path of said telephone system.
current sensing means for sensing said DC current source thereto from the telephony system to establish a DC
voltage that is proportional thereto;
current mirror means coupled to said current sensing means for producing a current that is proportional to said sensed DC current; and said hybrid circuit means being responsive to said current provided by said current mirror means for sourcing said DC current feedback to said bidirectional signal path of said telephone system.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US30501281A | 1981-09-24 | 1981-09-24 | |
| US305,012 | 1981-09-24 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1177985A true CA1177985A (en) | 1984-11-13 |
Family
ID=23178923
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000408607A Expired CA1177985A (en) | 1981-09-24 | 1982-08-03 | Electronic terminator circuit |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0089350A4 (en) |
| JP (1) | JPS58501529A (en) |
| CA (1) | CA1177985A (en) |
| WO (1) | WO1983001162A1 (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4727574A (en) * | 1985-09-24 | 1988-02-23 | Northern Telecom Limited | Subscriber line interface circuit with improved d.c. balance |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| NL7208148A (en) * | 1972-06-15 | 1973-12-18 | ||
| US3899646A (en) * | 1974-05-28 | 1975-08-12 | Bell Telephone Labor Inc | Telephone set speech network |
| US4004109A (en) * | 1975-05-09 | 1977-01-18 | Boxall Frank S | Hybrid circuit |
| US4041252A (en) * | 1976-06-07 | 1977-08-09 | North Electric Company | Transformerless two-wire/four-wire hybrid with DC sourcing capability |
| US4203012A (en) * | 1977-07-14 | 1980-05-13 | Boxall Frank S | Hybrid circuit using current mirror circuits |
| FR2437757A2 (en) * | 1978-06-26 | 1980-04-25 | Trt Telecom Radio Electr | SUBSCRIBER TRUNK FOR ENSURING TWO-WIRE OR FOUR-WIRE COUPLING WITH A TELEPHONE CENTRAL |
| US4300023A (en) * | 1979-08-13 | 1981-11-10 | Motorola, Inc. | Hybrid circuit |
-
1982
- 1982-08-03 CA CA000408607A patent/CA1177985A/en not_active Expired
- 1982-08-06 JP JP50274782A patent/JPS58501529A/en active Pending
- 1982-08-06 WO PCT/US1982/001068 patent/WO1983001162A1/en not_active Application Discontinuation
- 1982-08-06 EP EP19820902779 patent/EP0089350A4/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| EP0089350A4 (en) | 1985-06-06 |
| WO1983001162A1 (en) | 1983-03-31 |
| EP0089350A1 (en) | 1983-09-28 |
| JPS58501529A (en) | 1983-09-08 |
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|---|---|---|---|
| MKEC | Expiry (correction) | ||
| MKEX | Expiry |