IE45750B1 - Circuit arrangement for an electronic subscriber feeding system - Google Patents

Abstract

The circuit arrangement for an electronic subscriber feed with loop current monitoring, ringing tone connection and hybrid arrangement for the transfer from two-wire to four-wire operation with current generators disposed in a balanced relationship with one another uses operational amplifiers (J1, J2) purely as isolation amplifiers for the current generators. The modulation of the loop current is controlled via the inverted inputs of the operational amplifiers (J1, J2) in such a way that, when the amplification is increased in the modulation arrangement, the dynamic self-balancing of the current generators is increased. When the subscriber loop is open, a differential amplifier (J3) is controlled to achieve a high hybrid loss in such a way that no AC voltage reaches the output (G) on the four-wire side.

Description

The invention relates to a circuit arrangement for an electronic subscriber feeding system having loop current monitoring, call access and a terminating set for changing from two-wire to four-wire working with current generators arranged symmetrically with one another for telecommunications and more particularly telephone private branch exchanges.

Known feeding arrangements for telephone subscribers' stations having a central current source consist of feed coils, transformers and relay arrangements which, in telephone installations whioh each have a feed device leading to the subscriber's station, both take up a considerable amount of space and involve high outlay, and are conducive i^o crosstalk owing to the fact that the feed coils and transformers are disposed close together.

For reducing the mechanical outlay, it is known, from DT-AS 1,199,827, to use current generators.

Other feeding arrangements employ solid-state component elements, whereby feed coils and transformers are rendered un-necessary and crosstalk is substantiallyeliminated (DT-OS 2,020,527).

In German Patent P 25 56 157 such feeding arrangements are combined with call access and a terminating set and, by means of a loop-current monitoring means, not only are the dialling impulses evaluated for the establishment of a connection, but also call back is started by ground key actuation at the subscriber's 4575 ο - 3 station connecting line a to ground, the current stabilization of the feeding arrangement being ensured during the actuation of the ground key. This arrangement for subscriber feeding is primarily suitable for use in private installations, because it does not fully meet the CCITT recommendations for the transmission loss at the terminating set. The present invention seeks to provide an electronic subscriber's feeding system, including a terminating set, which makes it possible to use long subscriber's lines and in which the influence of tolerances of component parts is low.

According to the present invention there is provided a circuit arrangement for an electronic subscriber 1s feeding system for telecommunications, the arrangement having loop-current monitoring, call access and a terminating set for the change-over from two-wire working to four-wire working: the feeding system including two current-generators arranged symmetrically in relation to one another and each incorporating an operational amplifier which operates as a voltage follower for voltages at one input and having modulation of the loop current effected through the other input whereby the dynamic self-balancing of the currentgenerators is increased by amplification of the modulation voltage; the terminating set including a differential amplifier for passing signals from the subscriber side to the terminating set output with the circuit arrangement being arranged such that in operation, with the subscriber's loop open, a loop signal so controls the differential amplifier that it produces a transmission loss such as to effectively block the passage of such signals to the output; and the terminating set further including either a real input resistance and a first complex artificial line, or a complex internal resistance and second complex artificial line.

The invention will hereinafter be more particularly described with reference to the circuit arrangement of the accompanying drawing, which arrangement shows the features of significance to the invention.

The feeding arrangement shown in the drawing is composed of two current generators arranged symmetrically with one another, for which generators operational amplifiers Jl, J2 are employed each in combination with one transistor. The advantage of the operational amplifiers Jl, J2 instead of transistors, for example in Darlington arrangement, resides in the fact that the loop current is independent of transistor parameters such as, for example, the base-emitter voltage, which may fluctuate considerably. The loop current is thus determined solely by the voltages across the resistors Rl, R2, which voltages pass through the operational amplifiers Jl, J2 to the emitter-side terminal connecting points of the loop resistors R3 and R4 respectively. The operational amplifiers Jl, J2 operate as pure voltage followers (common collector circuits) so that the potentials at the positive inputs of the amplifiers Jl, J2 and the potentials across the loop resistors R3, R4 are of substantially equal value. The balance of the arrangement is ensured by the balancing resistors R5, R6, which are directly connected to the speech wires A, B without the interposition of diodes. Also, due • s to the direct coAnection, the tolerances of the protective resistors R7, R8 are balanced out, the said protective resistors serving to protect the feed arrangement - 5 45750 from high voltages occurring on the wires of the subscribers' lines.

The modulation of the steady loop current takes place by way of the negative inputs of the amplifiers Jl, J2. The associated resistors R9, Rll, and RIO, R12, respectively, determine the ratio of modulation voltage to modulation current. With equal resistors R9 to R12, the modulation voltage at the capacitors C2, C3 is transmitted with an amplification factor of one to the loop resistors R3, R4. As the amplification of the modulation arrangement increases due to its design, the alternating-voltage amplification of the positive inputs of the amplifiers Jl, J2, and hence also the dynamic selfbalancing, is improved.

For the loop evaluation, the resistors R13, R14 are connected to the outputs of the amplifiers Jl, J2. Without these resistors R13, R14, the same voltage would be present at the loop resistors R3, R4, both with the subscriber's loop open and with it closed, and the said loop resistors may prevent a loop evaluation. As the ratio of the resistance values of the resistors R13, R14 to the loop resistors R3, R4 increases, the evaluating threshold for the loop signals also increases.

The amplifier J6 has an amplification factor of one and serves to invert the signals coming from the preceding amplifiers J4 and J5, respectively, for the modulation of the speech wire B.

In order to obtain high transmission loss at the terminating set when the subscriber's loop is open, the differential amplifier J3 is driven to the limit by the loop signal at the point S by way of the - 6 transistor T, so that effectively no alternating voltage can pass to the' point G of the outgoing branch of the terminating set.

The terminating set is shown in two different 5 circuit arrangements, both of which will hereinafter be particularly described.

The first circuitry variant comprises a real input resistance and hence a real generator internal resistance and a complex artificial line, whereby measured trans10 mission values are produced, which correspond to those of conventional inductive terminating sets. The second circuitry variant is provided with complex input resistance and hence with complex generator internal resistance and with a complex artificial line. The designs of the artificial lines depend upon the particular applications and also upon the type of cable in the subscriber1s line and upon the internal resistance of the subscriber's station.

In the circuit construction comprising a real input resistance R15, the latter is connected between the points 1-1. The connection between the points 2-3 is broken and the connection between the points 2-4 is established, the amplifier J5, the artificial line N2, as well as the resistor (not designated) and the capacitor (not designated) being omitted in the practical circuit construction. The voltage present at the real input resistor R15 is transmitted by way of the differential amplifier J3 to the outgoing branch of the terminating set, point G.

The voltage present in the incoming direction at the point K passes through the modulation circuit to the speech wires A, B and thus also to the inputs of - 7 the differential amplifier J3. In order to obtain high transmission damping at the terminating set, a countervoltage is applied by the amplifier J51 to the inverting input of the differential amplifier J3, so that no voltage is set up at the point G.

Under service conditions, i.e. when the voltage present at the point K generates a frequency-dependent voltage along the speech wires A, B, the frequency response of the artificial line N must correspond to that of the external termination of the speech wires A, B. Since the input resistance R15 is also frequency-dependent to a particular extent in combination with the capacitor Cl, an R-C element is connected in parallel with the artificial line N, of which R-C element the electrical values correspond to those of the input resistor R15 and of the capacitor Cl.

In the circuit construction with complex input resistance, the complex input resistor Nl is connected between the points 1-1. The connection exists between the points 2-3 and that between the points 2-4 is broken, the amplifier J51, the complex artificial line N with its R-C element and the resistor (not particularly designated) being omitted in the practical construction of the circuit. The circuit arrangement is so designed that the voltage along the speech wires A, B passes by way of the differential amplifier J3 as an unbalanced voltage to the point G of the outgoing branch of the terminating set. An unbalanced voltage at the point K of the incoming branch passes as a balanced voltage to the speech wires A, B. The frequency response both of the complex input resistance Nl and of the artificial line N2 for the complex generator 457S0 - 8 internal resistance corresponds to the load at the speech wires Ά, B.

The current flowing in the artificial line N2 is converted by the amplifier J5 into a control voltage for the current generators. The current supplied by the current generators to the parallel connection of the complex input resistor Nl and the subscriber1s station thus exhibits the same frequency response as the current in the artificial line N2. By appropriate choice of the amplification of the operational amplifiers Jl, J2 and J5, the voltage at the speech wires A, B is made exactly equal to that at the point K. The equality of these voltages is utilised for the two-way (terminating set) function: with equal two-way (terminating set) resistors R16 to R21, the voltage at the point K is applied by way of the amplifier J4 to the resistor R17 and acts with double amplitude in opposition to the voltage from the speech wire A, which is thereby quenched. Together with the voltage of the speech wire B, therefore, there is applied to the inputs of the differential amplifier J3 an in-phase signal which remains disregarded.

The advantage of the last-described circuitry variant resides in that the input resistance is adapted to the resistance of the subscriber's line. A telephone system built up in this way is very well adapted to a line network. If, in addition, the input resistance and the internal resistance of the subscriber1s station are adapted to the line resistance, a good terminating set transition damping or sidetone damping can be achieved even in the case of very short subscriber1s lines. 45?s0 - 9 The use of operational amplifiers makes it possible to construct the artificial line N2 with component parts of small dimensions, the impedance of which is a number of times higher than that of conventional artificial lines.

Claims (9)

1. CLAIMS i 1. A circuit arrangement for an electronic subscriber 's feeding system for telecommunications, the arrangement having loop-current monitoring, call access and a terminating set for the change-over from two-wire working to four-wire working; the feeding system including two current-generators arranged symmetrically in relation to one another and each incorporating an operational amplifier which operates as a voltage follower for voltages at one input and having modulation of the loop current effected through the other input whereby the dynamic self-balancing of the current generators is increased by amplification of the modulation voltage; the terminating set including a differential amplifier l \ for passing signals from the subscriber 1 s side to the terminating set output with the circuit arrangement being arranged such that in operation, with the subscriber's loop open, a loop signal so controls the differential amplifier that it produces a transmission loss such as to effectively block the passage of such signals to the output; and the terminating set further including either a real input resistance and a first complex artificial line, or a complex internal resistance and second complex artificial line.

2. Circuit arrangement according to claim 1, characterised in that, for achieving a high transition damping at the terminating set between the incoming and outgoing wires of the two-wire side, with the subscriber's loop closed, a counter-voltage is applied to the inverting input of said differential amplifier.

3. Circuit arrangement according to claims 1 and 2, characterised in that the frequency response of -lithe first complex artificial line corresponds to that of the termination of the subscriber's side speech wires.

4. Circuit arrangement according to claims 1 to 3, characterised in that an RC element is connected in parallel with the first complex artificial line.

5. Circuit arrangement according to claim 1, characterised in that the frequency response of the complex input resistance and of said complex artificial line for the complex generator internal resistance corresponds to the load at the subscriber's side speech wires.

6. Circuit arrangement according to claims 1 and 5, characterised in that an unbalanced input voltage at the terminating set line branch passes as balanced voltage to the subscriber's side speech wires.

7. Circuit arrangement according to claim 1, characterised in that, for the loop evaluation, two resistors are connected one to each of the outputs of the two operational amplifiers.

8. Circuit arrangements according to claim 1, characterised in that associated balancing resistors are directly connected to the subscriber's side speech wires.

9. Circuit arrangement for an electronic subscriber feeding system, substantially as hereinbefore described with reference to the accompanying drawing.