GB2126049A - Interface circuit - Google Patents

Abstract

A circuit for the bidirectional transmission of voice and signalling, e.g. as an interface between a PABX and a public exchange, uses electrical isolation by an opto-coupler (OK), and DC line looping effected by pulse duration modulation by a comparator (K2). Band width range is from DC to about half pulse frequency. The pulse generator is a self- oscillating circuit including a comparator (K1) and an integrator (OP4). The pulse duration modulated signal is controlled in amplitude from the incoming signal via a differential amplifier (OP1). <IMAGE>

Description

SPECIFICATION Interface circuit The invention relates to a circuit for the analog transmission of voice and communication signalling, especially between telephone exchanges with 2-wire-4-wire translation and electrical isolation by opto-coupler.

Such circuitry is used as an interface between superior and subordinate telephone exchanges, e.g. between private branch exchanges and main lines, or as an interface of subscriber equipment with remote power feeding. Usually the interface is arranged in the subordinate system.

At present balanced-to-ground two-wire exchanges are used for analog transmission in the above-mentioned range, and a direct current component, either for remote feed or for the transmitting signalling (e.g. seizure, release, dialling), is sent simultaneously to the voice signalling. Balance to earth is necessary to prevent crosstalk and interference noise respectively.

Two-wire-transmission enables a maximal use of the available line system, and is normally used for short distance range.

Modern telephone exchange systems, especially PBXs, make an internal four-wiretransmission necessary. Hence the adaption of modern systems to antiquated systems, networks and configurations by using a two-wire-four-wire conversion is required.

There are circuits known in which for transmission of analog signals capacitive or inductive coupling or a combination of both are used. Such circuits, however, need big and thus expensive components, e.g. large capacitors, feeding coils for the line-loop and transformers with DC bias.

There are also circuits with direct signal coupling and electronic line looping. Their disadvantages are short range, the necessary over-voltage strength of the coupling capacitors, high requirements for component tolerance because of the high demand of symmetry, as well as reiatively high power dissipation at the transistors.

The disadvantage of known circuits with analog signal feeding via opto-couplers is that they are not DC stable, and at low frequencies they are inaccurate. Moreover, semiconductor circuitry shows high dissipation in the off state.

It is an object of the invention to avoid or minimise the above-mentioned disadvantages.

According to the invention, there is provide a circuit for the analog transmission of voice and communication signalling, especially between telephone exchanges with two-wire-four-wire transmission and electrical isolation via optocouplers, in which the low frequency signal input and the outgoing direct current signal performing the loop to the exchange are obtained by pulse duration modulation.

The communication bandwidth range is thus from 0 c's (=DC) to about the half pulse frequency.

Thus it is possible to transmit low frequency signals as well as direct current line signals with one and the same circuitry. Further advantage is a reduction of power dissipation due to switched operation.

The invention is even more advantageous if the pulse generator for pulse duration modulation is a self-oscillating circuit arrangement which, for exampie, consists of a comparator and an integrator. This arrangement has, compared with an external central clock pulse supply, the advantage that it operates without external interference.

Another aspect of the invention is that the pulse duration modulation signal can be controlled in its amplitude as follows. A direct DC component is derived from the incoming signal via a differential amplifier for amplitude control, so that a control-signal for the pulse duration modulation circuit is made available. Because the control comes from a DC component, this has the advantage that its stability is guaranteed for the whole transmission range.

Further details of an embodiment of the invention are shown in the following description of an active DC biased line transformer, as seen in the accompanying drawing. This is, for example, the connection of a PBX via a two-wire main line to a superior telephone exchange.

The incoming signal is switched through directly via differential amplifier DIF to the twowire-four-wire conversion.

The outgoing DC signal for the loop to the exchange is produced in a switched way by the transistorT (a-wire, bridge rectifier BG, low-passfilter TP, impedance R', transistor T, BG, b-wire), driven by an opto-coupler OK.

That DC loop signal is related directly (via comparator K2) to the transmission signal (S), which is influenced by pulse duration modulation.

The basic arrangement for the pulse duration modulation is an oscillator circuit consisting of the comparator K1 and the integrator OP4, which at the output of OP4 generates a delta voltage symmetrical to ground. That delta voltage is input to the comparator K2, which outputs a square wave signal with a duty cycle of 50 per cent in case of loop-on condition on the main line. Thus the line is terminated by a resistor R, which has a value twice as big as the resistor in the circuit R'.

tx+ty R=R'.

tx tx=on-time ty=off-time when tx=ty R R' 2 To modulate the DC in the main line properly to the transmission signal S, the reference voltage of the comparator K2 is to be modulated. Thus the output voltage of the comparator K2 is performing a square wave signal with a pulse duration modulation directly used to control the opto-coupler OK. The clock frequency is governed by R9, R10, R1 1 and C4 together with comparator K1 and K4.

The pulse duration modulated signal is transmitted to the main line via the optoelectronic switch OS and the low-pass filter TP.

The switch OS consists of a V-MOS transistorT, controlled via an opto-coupler OK. A currentlimiting-circuit serves as a constant current source JK to reduce the switching time of the opto-coupler.

The pulse duration modulated signal which is switched via T is filtered by the low-pass filter TP to prevent line interferences. Further, the filter TP protects the transistor T against overvoltage.

As the "switched DC signal" also determines the amplitude of the transmission signal (directly proportional), which, however-due to the different line lengths-is not constant, a gain control dependent on line resistance is necessary for the transmitted signal. This is obtained by reducing the triangle amplitude (output OP4) when operating with smaller currents and thus increasing the amplification in the direction of transmission.

Because the current, due to the constant current supply JK (which increases the switching frequency of OK), is very low, the voltage at the main line can be used for automatic gain control, if the mark-to-space ratio is known and R' is real.

The voltage is converted by an operational amplifier OP1 is control logic level similar to the incoming signal (relatively high reduction is necessary because of common mode rejection ratio) and integrated via R3/C3. This signal is inverted by OP2 and amplified by the non inverting OP3, used as reference voltage. These two output signals control the comparator K1 whose output is switching between the output voltage of OP2 (positive) and the output voltage of OP3. By modulation of the amplitude of the square-wave voltage the amplitude of the delta voltage is modulated as well.

By the use of a FET-operational amplifier leakage current values (via Ri) slightly above the insulation resistance range can be attained.

The transmission frequency range is from direct current to the half pulse frequency of the pulse generator. If the chosen clock frequency is high (about 100 kHz), the low pass filter TP can be accordingly small.

Claims (4)

Claims

1. A circuit for the analog transmission of voice and communication signalling, especially between telephone exchanges with two-wirefour-wire transmission and electrical isolation via opto-couplers, in which the low frequency signal input and the outgoing direct current signal performing the loop to the exchange are obtained by pulse duration modulation.

2. A circuit according to claim 1, in which the pulse generator (PG) for the pulse duration modulation is a self-oscillating circuit, a comparator (ski) and an integrator (OP4).

3. A circuit according to claim 1 or 2, in which the pulse duration modulated signal is amplitudecontrolled.

4. A circuit for the analog transmission of voice and signalling, substantially as described with reference to the accompanying drawing.