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 aforedescri bed, is not easily adjusted using conventional transformers. Therefore, commun cations 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

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applications to replace bulky tansformers now being used as above described.

Summary of the Invention

Accordingly, it is an object of the present invention to provide an improved terminator circuit for use in modem applications to provide signal conversion between a bidirectional transmission signal path and a pair of unidirectional transmission signal paths.

It is another object of the present invention to provide an electronic terminator circuit which is ground referenced and that is compatible with typical telephonic communication systems. In accordance with the above and other objects there is provided an electronic terminator circuit which is ground referenced at the subscriber terminating loop for providing signal separation for two-wire differential to four-wire single-ended conversions and suppressions of longitudinal signals at the two-wire input thereto as well as echo returns at the four-wire side. The electronic terminator circuit comprises a subscriber loop interface circuit (SLIC) in combination with associated circuitry coupled between the two-wire input and the SLIC for isolating the AC and DC feedback appearing at the two-wire i nput.

Brief Description of the Drawings

FIG. 1 illustrates a subscriber loop interface circuit (SLIC) known to those skilled in the art; and

FIG. 2 illustrates the electronic terminator circuit of the present invention which comprises the SLIC of FIG. 1.

Detailed Description of the Invention

FIG. 1 shows a subscriber loop interface circuit (SLIC) 10 that is well-known to those skilled in the art for providing signal conversion between two-wire differential signals and a pair of two-wire single-ended signal paths. In general, SLIC 10 is designed to replace the hybrid transformer circuit in Central Telephone Offices, PABX, and Subscriber carrier equipment providing two-way telecommunication. The operation and function of SLIC 10 is well-known in the art and is described in U.S. Patents 4,004,109 and 4,300,023 the teachings of which are incorporated herein by reference made thereto. SLIC 10 is suited for fabrication in monolithic integrated form and is manufactured by Motorola, Inc., for instance, under the part number MC3419.

SLIC 10 is typically located at the telephone central office and is adapted to be coupled at Tip and Ring terminals 12 and 14 via telephone lines to a subscriber loop. The subscriber loop generally comprises a telephone for providing bidirectional signal transmission. Thus, incoming signals appearing at VRX (for example, signals transmitted through the central exchange from a calling party) are received by SLIC 10 and converted to proportional differential signals at terminals 12 and 14. Therefore, transhybrid reception is realized by converting the AC coupled received signals through capacitor 16 through resistor 18 to a current which is processed via SLIC 10 to drive the two-wire output line coupled at terminals 12 and 14 via PNP and NPN Darlington transistors 20, 22 and resistors 24, 26 with a balanced AC current proportional to the receive input voltage. Similarly, balance AC voltage signals appearing at terminals 12 and 14, as supplied by the called party via the telephone

coupled therewith, are converted to a single-ended output signal by SLIC 10 to appear at output Tχ thereof. The conversion produces an output signal AC coupled through resistor 28, capacitor 30 and operational amplifier 32 to the calling party at output V ^* rχ. 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 Tχ 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 Tχ 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-c rcuited (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 38 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

CV.

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 V _j r _j r.

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 Tχ and Rχ 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 signal is then converted by SLIC 10 to appear as a proportional, single-ended signal at the Tχ output of SLIC 10 to be received by an utilization means coupled thereat. Similarly, as described above, a unidirectional AC signal applied at Vγχ is coupled to the Rχ 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

paths, VJX, 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 (VQC-VEE)/ . 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

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 del eteriously 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 Tχ 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 central 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. Transistor 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

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 if 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 intersecting node between these two resistors to provide in combination with resistors 36 and 38 the proper AC loop termination impedance.

If, for instance, the Motorola, Inc. MC3419 is used as SLIC 10 forming terminator 100, 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:

Transistor 20 MJE271 Transistor 22 MJE270 Resistor 36 17.4 ohms Resistor 38 17.4 ohms Resistor 120 20 ohms Resistor 124 180K ohms Resistor 126 20K ohms Resistor 130 20K ohms Resistor 134 1.9 ohms

10 Resistor 136 180K ohms Capacitor 116 1.0 yF Capacitor 118 1.0 yF Capacitor 138 2.0 yF