GB2042848A

GB2042848A - Line current feed and hybrid arrangement (system X)

Info

Publication number: GB2042848A
Application number: GB7850384A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1978-12-30
Filing date: 1978-12-30
Publication date: 1980-09-24
Also published as: IE792528L; DE2952259A1; AU5323179A; NZ192462A; FR2445666B1; ZA796287B; IE48895B1; GB2042848B; NL7909148A; FR2445666A1; JPS5593363A; BR7907891A; SE7910670L

Abstract

In a 2/4 wire converter the 2 wire line is isolated from the electronic circuits by means of a transformer T, which also performs the function of a hybrid circuit. The 2 wire terminating impedance Zt also serves as the line current feed. In order to avoid shunting this impedance by the amplifiers A1 and A2, it is necessary to make the input impedance of A1 and the output impedance of A2 relatively high. A conventional way of achieving a high output impedance for A2 is to arrange the internal feedback circuits to give a current output rather than the more usual voltage output. <IMAGE>

Description

SPECIFICATION Line current feed circuits for telecommunication exchanges The present invention relates to line current feed circuits for use with 2/4 wire interfaces in telecommunication exchange line circuits.

With the increased application of digital switching techniques to telephone exchange switching systems, it is necessary to consider how the functions performed by the well known electromagnetic relay line circuit can be interpreted in electronic integrated circuit form.

The requirements for the interface include a) line current feed, b) line termination, c) iongitudinal balance, d) 2/4 wire splitting and e) protection and/or isolation of adverse line conditions. The latter requirement involves the isolation of high voltage transients and the a.c.

mains which may be induced into the subscribers line from the integrated circuit components.

According to the invention there is provided a telecommunications two-to-four wire line circuit comprising line current feed resistors providing line current to the two wire circuit, a line isolation transformer connected on one side to the two wire circuit and on the other to go and return amplifiers of the four wire circuit in which the return amplifier is arranged to have a high output impedance to provide a current output rather than a voltage output and the go amplifier is arranged to have a high input impedance so that the line current feed resistors are also used to provide the terminating impedance for the line.

Preferably the line isolation transformer is also used as the hybrid transformer coupling the two wire circuit to the four wire circuit. Further the return amplifier is arranged to exhibit a negative resistance characteristic to compensate for the line current feed resistors having a value less than the total line impedance.

This latter feature is required to reconcile the apparent incompatability between the values required for line termination, of the order of 600 ohms, and the line feed current for a standard telephone instrument which requires to be fed from a 200 ohm plus 200 ohm source from a 50 volt supply.

The invention together with its various features will be more readily understood from the following description which should be read in conjunction with the accompanying drawing which shows a 2to-4 wire telecommunications line circuit in broad outline.

The line circuit comprises a two-wire side A and a four-wire side B. The two-wire side comprises two conductors + and - which are connected over the subscribers telephone lines to the subscribers premises and telephone instrument. The four-wire side of the line circuit comprises a go-path GO and a return-path RET and these paths will provide input and output connections to the codec equipment of the digital exchange. Such codec equipment is not shown in the drawing since it plays no part in this invention.

Typically the codec equipment will be used to convert speech currents received from the subscribers line and routed to the go path GO into digital samples (an analogue-to-digital conversion function) and to convert digital samples into speech currents (a digital-to-analogue conversion function) for passage over the return path RET.

The adverse subscribers line condition (such as induced a.c. mains and high voltage transients) are isolated from the go and return path amplifiers Al and A2 respectively by the transformer T which also performs the function of a hybrid circuit. The line circuit further includes the line current feed resistors R1 and R2 which in addition comprise the line terminating impedance.

In practice a standard telephone instrument is designed to operate within its specification when fed from a 200 + 200 ohm source from a 50 volt supply. To provide an optimum line termination the line matching impedance Zt should be 600 ohms, although this varies with line length. By increasing the supply voltage to the order of 60 volts and by using a 300 + 300 ohm source, the current feed is sufficiently close to the existing standard not to effect the performance of the telephone instrument. In order to further improve the line termination match a capacitor C1, shown dotted in the drawing, may be connected across the two wire line A.

Detection of on-hook and off-hook and other line signals may be achieved either with voltage sensing devices (not shown) connected across the two wire line A or by using current sensing devices (also not shown) connected in series with resistors R1 and R2. Typically opto-coupler devices may be connected in series with resistors R1 and R2 to provide full isolation between the line conditions and the line condition signalling equipment.

In operation speech signal currents flowing in the two-wire line A are developed across the line termination impedance Zt and coupled by windings W3 and W4 to winding W2 and into amplifier Al with the line balancing impedance Zb providing a line matching function. Signals at the output of amplifier A2 however are shared between the reflected impedance of the line and the line termination seen across winding W1 and the balance impedance Zb. The signal developed across W1 also appears across the identical winding W2 but in anti-phase with the signal across Zb. The the two signals cancel at the input to amplifiers Al.

As mentioned above the terminating impedance at Zt also serves as the line current feed. In order to avoid shunting this terminating impedance by the amplifiers Al and A2, it is necessary to make the input impedance of amplifier Al and the output impedance of amplifier A2 relatively high. The conventional way of achieving a high output impedance for amplifier A2 is to arrange the internal feedback circuits of the amplifier to give a current output rather than a voltage output. By arranging that amplifier A2 exhibits a negative resistance characteristic resistors R1 and R2 together can be less than the characteristic line termination allowing for example the existing telephone instrument to be driven from a 50 volts source using 200 ohm line feed resistors in the conventional manner.

The above description has been of one embodiment only and alternative arrangements will readily be seen by those skilled in the art. For example the transformerT could be a simple isolating device with the hybrid function provided by other conventional hybrid circuit connections.

Claims

1. A telecommunications two-to-four wire line circuit comprising line current feed resistors providing line current to the two-wire circuit, a line isolation transformer connected on one side to the two-wire circuit and on the other to go and return amplifiers of the four-wire circuit, in which the return amplifier is arranged to have a high output impedance to provide a current output rather than a voltage output and the go amplifier is arranged to have a high input impedance so that the line current feed resistors are also used to provide the terminating impedance for the line.

2. A telecommunications two-ta-four wire line circuit according to claim 1 in which the return amplifier is arranged to exhibit a negative resistance characteristic to compensate for the line current feed resistors having an actual value less than the characteristic terminating impedance for the line.

3. A telecommunications two-to-four wire line circuit according to claim 1 or 2 in which the isolation transformer also acts as a hybrid transformer.

4. A telecommunications two-to-four wire line circuit according to any one of the preceding claims in which line signalling detection arrangements are connected across the two wire line.

5. A telecommunications two-to-four wire line circuit as claimed in claims 1, 2 or 3 in which line signailing detection arrangements are connected in series with the line current feed resistors.

6. A telecommunictaions two-to-four wire line circuit as claimed in claim 5 in which the line signalling detection arrangements are optoisolator devices.

7. A telecommunications two-to-four wire line circuit substantially as described with reference to the accompanying drawing.