IE921394A1

IE921394A1 - Interface circuit for telephone exchanges

Info

Publication number: IE921394A1
Application number: IE139492A
Authority: IE
Inventors: Franz Hoellrigl
Original assignee: Siemens Ag
Priority date: 1991-04-30
Filing date: 1992-07-01
Publication date: 1992-11-04
Also published as: WO1992020151A1

Abstract

Interface circuit for telephone exchange installations for the d.c. decoupling of two line sections, especially to convert symmetrical to asymmetrical voltage conduction. The ends of these two line sections are connected at the interface to the output of a controllable current (14) or voltage source, said sources being controlled dependently upon the current or voltage at the other side of the interface circuit or upon the differences between the currents or voltages on both sides in such a way that the currents (J1, J2) and voltages (V1, V2) on one side are the same as those on the other side or are in a predetermined mutual relationship. It is thus possible to provide impedance-transparent interface circuits without repeater.

Description

Interface circuit for telephone exchanges.

The invention concerns a current and voltage control circuit for interface circuits used in telephone exchanges for the purpose of decoupling from d.c. two line sections, especially speech conductors.

The speech conductor sections making up a telephone connection are supplied with direct current from the most diverse sources and must therefore be isolated from each according to d.c. The interfaces between successive speech conductor sections are therefore provided with interface circuits which transmit the alternating currents between the adjacent speech conductor sections, but which block the flow of direct currents and voltages between each other. In traditional telephone technology the interface circuits are transformers with protective circuits which in modern technology are replaced by electronic interface circuits in the form of electronic hybrid termination units. The combination of transmitter and hybrid also is used.

The invention concerns an interface circuit in impedance transparent communication technology, in order to avoid an additional ’’four-wire island.

It is the object of the invention therefore to provide a current and voltage control circuit for interface circuits which meet the demands made during the decoupling from d.c. of two line sections in telephone exchanges. According to the invention this is achieved in that for the direct or modified replica of the impedance relationships between the two output sections, the line terminals of these two line sections are connected at the interface to the output of each controllable current or voltage source which in response to the current or voltage in each case are controlled on the other side of the interface circuit or controlled on both sides of the interface circuit in such a way that the currents and voltages on one side are always the same as on the other side, or in a predetermined relationships to each other.

A special embodiment example of the invention is constructed in such a way that the two decoupled line sections are connected in each case to the inverting inputs of an operational amplifier, serving as a controllable current source, whose non-inverting inputs are connected to the output of a third operational amplifier, to the inputs of which is applied a voltage which is proportional to the difference of the current of the two line sections. However, it is also possible to connect one of the decoupled line sections to the inverting input and the other line section to the non-inverting input of an operational amplifier whose output signal directly controls an adjustable current source in the form of an operational amplifier supplying the first-mentioned line section and a second adjustable current source supplying the last-mentioned line section via an interconnected inverting element.

Another variant of the circuit according to the invention is concretely carried out in the manner that one line section is supplied by an adjustable current source constructed in the form of an operational amplifier and is connected via an impedance transformer to the second line section, wherein the impedance transformer contains an ammeter which by means of the adjustable current source adjusts the strength of the current supplied to the first line section to the strength of the current flowing in the second line section.

In some applications a transformation of the impedances according to a given division ratio, which can be measured between the two sides of the interface, is desirable. In the case of the previous circuit, for adjusting the voltage on the first side of the interface it is possible to use a reduced voltage obtained by dividing the voltage on the second side and in the reverse direction the current adjustment must be modified by altering the current strength at the same ratio.

The interface circuit according to the invention is also suitable for converting an asymmetrical voltage conductor (voltage between a line and earth) to a symmetrical voltage conductor (voltages in the two conductors are diametrically opposed) . In this case the symmetrical lines can be supplied via two complimentary transistors, acting as current and voltage sources, which are connected in series and driven in the opposite sense.

In the At-PS 381 430 is described an interface circuit with asymmetrical input and symmetrical output, using a network with a complex capacitive impedance. This makes tolerance demands on the capacitors, which increases the cost of the system. To overcome this, this known circuit uses only one single capacitor whose tolerance has no bearing on the symmetry of the circuit. An equalising circuit in this case additionally compensates the frequency sensitivity of the network. In the case of the invention, on the other hand, matching the impedance between input and output is important, whereas in the known circuits no measures have been taken regarding the impedances.

Embodiment examples of the invention are shown in the drawing. The Figs. 1, 2 and 3 show interfaces constructed as two-terminal networks, whilst the embodiment examples according to Figs. 4 to 7 relate to interface circuits with conversion from symmetrical to asymmetrical voltage distribution. The voltages applied to the different sides of the interface circuits in all examples are denoted by Ul and U2. The resistances existing in the individual examples are provided with their own reference numerals where necessary. In some circuits there are only two types of resistances, namely low ohmic which are commonly referred to with their resistance value Rl and high ohmic resistances which commonly have the same value and reference R2. The references Rl and R2 are generally shown in brackets, as these are not used throughout for designating the position. In all examples the resistance values R2 are substantially bigger than for Rl.

In the design according to Fig. 1 these voltages Ul and U2 are applied to the inverting inputs of two operational amplifiers 1 and 2. These are each provided with a feedback resistance 3 or 4 (current measuring resistances) which both have the resistance value Rl and which connect the outputs of the respective operational amplifiers 1 or 2 to their inverting inputs. The four resistances 5, 6, 7, and 8, which together have the same resistance value as R2, make up two current dividers whose centre tap is connected to the inputs of a third operational amplifier 9.

Between the centre taps of the two voltage dividers 5, 6 and 7, 8 there is produced a voltage U3 which is proportional to the difference between the two voltages across the feedback resistances 3 and 4. Since the voltages J1.R1 or J2.R1 appearing across the feedback resistances 3 and 4 are proportional to currents JI or J2 flowing in or out on both sides of the interface circuit, this current U3 is also proportional to the difference of these currents. In other words the voltage U3 representing the current difference J2 - JI is applied to the inputs of the operational amplifier 9 whose output voltage is connected to the non-inverting inputs of the operational amplifier 1 and 2, causing the output currents Jl or J2 of the operational amplifiers 1 or 2 to be adjusted to the same values. This however also ensures that the input voltage Ul and the output voltage U2 of the interface circuit always has the same value U1=U2 irrespective of the particular load on the input or output side.

Fig. 2 shows in simplified form an interface circuit provided with two controllable current sources 10 and 11. The input current Jl enters via the current source 10, whilst the output current J2 is supplied by the current source 11. The input voltage Ul acts upon the non-inverting input of an operational amplifier 12, whilst its inverting input is applied to the output voltage U2. The output voltage of the operational amplifier 12, which is proportional to the difference U2-U1 between the input and output voltage, on the one hand acts directly on the control input of the current source 11 and on the other hand in the opposite sense, via a inverting element 13, on the control input of the current source 10. This control arrangement has the effect that each voltage difference appearing between the voltages Ul and U2 causes a voltage to appear at the output of the operational amplifier 12 which drives the current sources 11 and 12 in the opposite sense and in such a way that in response to the particular load on the inside or outside such changes are effected to the currents Jl or J2 that the difference U1-U2 between the input and output voltage disappears and the impedance transparency of the interface circuit is achieved.

In the embodiment according to Fig. 3, the first line section, which carries the voltage Ul, is supplied with a current Jl from an adjustable current source 14. The first line section is furthermore connected to the non-inverting input of an operational amplifier 15 acting as an impedance transformer. To the output of the operational amplifier 15 via a current measuring device 16 is connected the second line section whose voltage U2 through a feedback-controlled operational amplifier 15 is adjusted to the value of the voltage Ul. The condition U1=U2 is therefore maintained. The measurand output of the ammeter 16 is connected to the adjusting input of the current source 16 and with this controls the output current Jl of the current source 16 to the value of the current J2 flowing through the second line section. This therefore ensures the maintenance of the condition J1=J2 which together with the forced equalisation of the voltages Ul and U2 guarantees the impedance transparency of the interface circuit.

Fig. 4 shows an impedance transparent interface circuit used in telecommunications as the separating point between the speech conductors a, b, which are symmetrical with respect to earth potential, leading to a user point and the line leading to the switching matrix which has a single earth polarity. The circuit shown also has the ability of transforming the impedance at a ratio of 1:4. This example also shows the resistances denoted with their resistance values. The voltage U2 applied to the side of the switching matrix is split in a voltage divider with the partial resistances 3R2 and R2 at a ratio 1:4 and the quartered voltage is supplied to the non-inverting input of ann operational amplifier 17 which, together with a second operational amplifier 18 operating in push-pull mode, produces the current Jl flowing to the user point. The symmetrical portion of the current flowing on the user side is supplied between the inputs of a current source with differential inputs 19. This symmetrical portion is produced with the aid of a resistance bridge consisting of four resistances of values R2 which are supplied by the voltage drop of two output resistances of value Rl which are situated in the output current circuits of the operational amplifiers 17 and 18. The current source 19 supplies the switching matrix current J2 which is controlled with the aid of an output resistance and with the aid of a positive feedback circuit and a negative feedback circuit in the sense that the impedances on the user side are transformed back in the same 4:1 ratio.

The use of the differential current source 19 allows for the compensation of asymmetrical interference signals which are fed into the user lines, as they cancel out as negative signals as a result of the differential forming at the amplifier input.

Fig. 5 shows an interface circuit which is connected between an exchange via the connections A, B and a switching matrix via the terminal K. An operational amplifier 20 serves as current measuring device on the side of the switching matrix. A further operational amplifier 21 in conjunction with a transistor 22 controls the current Jl in the direction towards the exchange with the aid of a direct current supplied via the terminals A and B. Reference numeral 23 denotes an operational amplifier acting as an inverting element which in push-pull arrangement to the output voltage of the operational amplifier 20 together with this voltage supplies the input signal for the operational amplifier 21, whereby the symmetrical control is improved in the direction of the exchange and less demands are made here on suppressing the in-phase rejection of the current source. A differential amplifier 38 measures the a.c. voltage at the A/B conductors and delivers it in the direction of switching matrix.

In Fig. 6 is shown concretely an embodiment example of an interface circuit, including details of all the components, for connecting the user point speech conductors a, b to a terminal K leading to an associated switching matrix. Connected in series are the transistors 24 and 25 which serve as current sources for supplying the speech conductors a, b and which are driven via associated operational amplifiers 26 and 27. The voltage information is decoupled from the speech conductors a, b to the switching matrix terminal K by means of an operational amplifier 28 which serves as a voltmeter. An operational amplifier 29 is provided for measuring the current on the side of the switching matrix. A further operational amplifier 30 serves as a control for stabilising the current sources 24 and 25.

Finally, Fig. 7 shows an embodiment example with solid data which also represents an interface circuit for connecting an electrically symmetrical pair of user speech lines a, b with an electrically asymmetrical switching matrix line K. The speech conductors a, b contain two complimentary transistors 31 and 32, serving as voltage sources, of which the latter is directly driven by the switching matrix line K via a repeater 33. An inverting amplifier 34 transforms into an inverting signal the signal supplied to the transistor 32 for driving the transistor 31.

The transmission of the alternating speech currents in the speech conductors a, b is effected by two resistances 35 and 26 connected in the collector circuits of the transistors 31 and 32. The voltages produced across the terminals of these resistances 35 and 36 are supplied via associated measuring resistors and capacitors to the inputs of a current source with differential inputs 37, to the output of which is connected the switching matrix line K.

Claims

1. Patent Claims:

1. The invention concerns a current and voltage control circuit for interface circuits used in telephone exchanges for the purpose of decoupling from d.c. two line sections, especially speech conductors, characterised in that for the direct or modified replica of the impedance relationships between the two output sections, the line terminals of these two line sections are connected at the interface to the output of each controllable current or voltage source (1,2 in Fig.l; 10,11 in Fig. 2; 14,16 in Fig.3; 17,18,19 in Fig. 4; 22 in Fig. 5; 24,25 in Fig. 6; 31,32,37 in Fig. 7) which in response to the current or voltage in each case are controlled on the other side of the interface circuit or controlled on both sides of the interface circuit in such a way that the currents and voltages on one side are always the same as on the other side, or in a predetermined relationships to each other.

2. Circuit according to claim 1, characterised in that the two decoupled line sections are connected in each case to the inverting inputs of an operational amplifier (1,2), serving as a controllable current source, whose noninverting inputs are connected to the output of a third operational amplifier (9) to the inputs of which is applied a voltage which is proportional to the difference of the current of the two line sections. (Fig. 1)

3. Circuit according to claim 1, characterised in that of the two decoupled line sections one is connected to the inverting input and the other line section is connected to the non-inverting input of an operational amplifier (12) whose output signal directly controls an adjustable current source (11) in the form of an operational amplifier supplying the first-mentioned line section and a second adjustable current source (10) supplying the last-mentioned line section via an interconnected inverting element (13). (Fig. 2)

4. Circuit according to claim 1, characterised in that one of the line sections is supplied by an adjustable current source (14) constructed in the form of an operational amplifier and is connected via an impedance transformer (15) to the second line section, wherein the impedance transformer (15) contains an ammeter (16) which by means of the adjustable current source (14) adjusts the strength of the current (Jl) supplied to the first line section to the strength of the current (J2) flowing in the second line section. (Fig. 3).

5. Circuit according to claim 1, with additional transformation of impedances measurable between the two sides of the interface according to a given ratio of division, characterised in that for controlling the voltage (Ul) on the first side of the interface, a reduced voltage obtained by dividing the voltage (U2) on the second side is used and that in the reverse direction the adjustment of the current is subjected to a division in the same way. (Fig. 4)

6. Circuit according to claim 1, with interface for transforming asymmetrical voltage (voltage against earth) to symmetrical voltage (voltages in the two conductors are of opposite polarity), characterised in that the symmetrical lines (a,b) can be supplied via two complimentary transistors (24,25 or 31,32) acting as current and voltage sources, which are connected in series and driven in the opposite sense. (Fig. 6,7)

7. A circuit according to claim 1, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.