FI79431C - KOPPLINGSANORDNING MED TVAOTRAODS-FYRTRAODSOMVANDLING. - Google Patents

Description

! 79431

Interface circuit with two-wire to four-wire conversion

The invention relates to a connection circuit for telephone exchanges with a two-wire-to-four-wire variant, which connection circuit comprises a galvanic isolation with an optoelectronic switch comprising an optocoupler.

Such a circuit is used as an interface circuit between main and side telephone exchanges, e.g. between private branch exchanges and main lines, or as an interface circuit for a remote-input space-10 device. Usually the interface circuit is arranged in a side system.

At present, two-wire centers symmetrical with respect to the ground are used for analog voice transmission in the above-mentioned area, in which case in most cases a direct current component for either 15 remote inputs or telecommunication communication transmission is supplied simultaneously with voice communication.

In general, symmetry with respect to the ground is necessary to prevent crosstalk and, respectively, connection failure.

The two-wire transmission allows maximum use of the available 20 wire system and is normally used over a short distance.

Modern telephone exchange systems, especially private branch exchanges, make internal four-wire transmission necessary. Therefore, the adaptation of modern systems 25 to obsolete systems, networks, and configurations is required using two-wire-to-four-wire conversion.

Circuits are known which use a capacitive or inductive circuit or a combination of both to transmit analog signals. However, such circuits require large (heavy) and therefore expensive components, such as large single-pole capacitors, supply coils for the conductor loop, and bias current transformers.

35 There are also circuits with direct signal connection 2 79431 and electronic wire looping. Their disadvantages are the short distance, the required overvoltage strength of the switching capacitors, the high requirements for the component tolerance due to the high symmetry requirement 5 and the relatively high power loss in the transistors.

In DE-OS 2649024 and DE-OS 2844304 we know connection circuits in which galvanic isolation is performed with opto-isolators, two-wire to four-wire conversion is taken care of and a constant DC current is used for the supply.

In addition, circuits having the above-mentioned characteristics but no constant DC power supply are described in US-PS 4,039,766 and DE-AS 26,270,140.

A disadvantage of the known circuits in which the analog signal 15 is fed through the optoisolators is that the circuits are not stable to direct current and are inaccurate at low frequencies.

In addition, there is a large loss in the non-operating state of the semiconductor circuit.

The object of the invention is to avoid the above-mentioned drawbacks.

According to the invention, this object is solved by modulating speech signals and transmission signals in a pulse duration modulator, these modulated signals 25 are fed to the two-wire line for the amplitude of the frequency voltage.

The communication frequency range of this arrangement is from 0 Hz (-DC) to about half the pulse number. Therefore, it is possible to transmit low frequency signals 35 as well as DC wire signals on one and 3,79431 same circuit. An additional advantage is the considerable reduction in power loss due to the switching function.

The invention can be implemented particularly advantageously in that the pulse duration modulator consists, in a manner known per se, of a comparator and a pulse generator, the comparator comparing speech signals and transmission signals, respectively, with the amplitude of the pulses is controlled by the control variables supplied by the amplitude controller, and these rectangular pulses are converted into amplitude-controlled triangular pulses in the integrator.

Compared to an external centralized clock pulse supply, this arrangement has the advantage that it operates without the need for any external connection.

Amplitude-controlled pulse duration modulation is already known from EP 0 011 093. The amplitude control then affects the signal to be modulated in the modulator stage, which is a disadvantage if it is desired to control several modulators with central amplitude control. This known solution is also very expensive in terms of the number of components required. However, the biggest disadvantage is that the useful signal must not be proportional to the ground potential. If such a useful signal relative to the ground potential is to be processed, a transducer is required. The invention avoids this problem.

According to one aspect of the present invention, the optoelectronic switch includes a V-MOS transistor which is controlled by a displaceable useful signal via an optocoupler, and a current as a constant current source for shortening the optocoupler switching times.

Further details of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which the circuit arrangement of an active pre-current line transformer 4,79431 is shown.

This is, for example, the connection of a private branch exchange via a two-wire main line to a main telephone exchange.

The incoming signal is connected directly via the differential amplifier DIF to a two-wire to four-wire conversion.

The output DC signal forming a loop to the center is provided by a switching mode transistor T (a-conductor, rectifier bridge BG, low-pass filter TP, impe-10 dance R ', transistor T, BG, b-conductor) which is driven by an op-insulator OK.

This DC loop signal is directly proportional (via comparator K2) to the transfer signal (S), which is affected by pulse duration modulation. The basic arrangement for pulse duration modulation is an oscillator circuit comprising a comparator K1 and an integrator 0P4, which at the output of 0P4 generates a triangular voltage symmetrical with respect to ground. This triangular voltage is the input to comparator K2, which provides a rectangular wave signal with a duty cycle of 50% of the loop value prevailing on the main line. Therefore, the wire is terminated with a resistor R, the value of which is twice as large as the resistance R 'of the circuit.

tx + Ty 25 R = R '· - tx = switching time tx Ty = non-switching time

---------- R

30 when tx = ty R1 = - 2 In the main line to modulate the direct current correctly with respect to the transmission signal S, the reference voltage 35 of the comparator K2 is modulated. In this way, the output voltage of the comparator K2 generates a pulse duration modulated rectangular wave signal which is directly used to control the optoisolator OK.

5,79431

The clock cycle number is determined by R9, R10, R11 and C4 together with comparators K1 and K4.

The pulse duration modulated signal is applied to the main line through the optoelectronic switch OS and the low pass filter 5 through the TP.

The optoelectronic switch OS comprises a V-MOS transistor T, which is controlled by the optoisolator OK. The current limiting circuit acts as a constant current source KS to reduce the switching time of the optoisolator.

The pulse duration modulated signal, which is switched via T, is filtered by a low pass filter TP to prevent line interference. As an additional feature, the filter TP protects the transistor T from overvoltage.

Since the "coupled DC signal" also determines the amplitude (directly proportional) of the transmission signal, which, however, is not constant due to the different wire lengths, a gain adjustment that depends on the resistance of the wire is necessary for the transmission signal.

These are achieved by reducing the triangular amplitude 20 (output of 0P4) when operating at lower currents and thus increasing the gain in the transmission direction.

Since the current due to the constant current supply IK (which affects the increase of the switching frequency OK) is very small, the voltage on the main line can be used for automatic gain control if the signal-to-state ratio is known and R 'is real. This voltage is converted by the operational amplifier to a logic level of control similar to the incoming signal (a relatively large reduction is necessary due to the common mode training ratio) and integrated via R3 / C3. This signal is inverted by 0P2 and amplified by non-inverting 0P3, which is used as the reference voltage. These two output signals control a comparator K1, the output of which varies between the output voltage 0P2 (positive) and the output voltage 0P3.

35 By modulating the amplitude of the square wave voltage 6,79431, the amplitude of the triangular voltage is also modulated.

By using an FET operational amplifier with leakage current values (via R1) it is possible to get slightly above the insulation resistance range.

5 transmission frequency region of the direct current to the pulse generator pulse half-cycle apart. If the selected clock frequency is high (about 100 kHz), the low-pass filter TP may be correspondingly small.

Claims (3)

7 79431 A connection circuit for telephone exchanges with a two-wire-to-four-wire conversion, the connection circuit comprising a galvanic isolation by an optoelectronic switch comprising an optocoupler, characterized in that the speech signals (S) and the transmission signals (GKZ) are modulated in a via a switch (OS) 10 and a demodulating low-pass filter (TP) and a bridge rectifier (BG) at its output to the two-wire line (a, b) and that the direct current component derived from the signals coming to the two-wire line (a, b) is fed to control-15 for the amplitude of the switching frequency voltage of the large pulse duration modulator (PDM). Interface circuit according to Claim 1, characterized in that the pulse duration modulator consists in a manner known per se of a comparator (K2) and a pulse generator (PG), the comparator (K2) comparing the speech signals (S) and the transmission signals (GKZ) and that the pulse generator (PG) comprises a self-oscillating rectangular generator (K1, R9, R10 and R11) and an integrator (0P4 and C4) in a manner known per se, the amplitude of the rectangular pulses being adjusted by the control variables supplied by the amplitude controller (AR) ) into amplitude-controlled triangular pulses. Interface circuit according to Claim 1 or 2, characterized in that the optoelectronic switch (OS) comprises a V-MOS transistor (T) which is controlled by a displaceable useful signal via an optical switch (OK) and a current limiter circuit as a constant current source for shortening the switching times of the optical switch 35.