AU653659B2 - A branching circuit - Google Patents

Description

3659 S F Ref: 217539

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICA710N FOR A STANDARD PATENT

ORIGINAL

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V S I* S Name and Address of Applicant: Actual Inventor(s): Address for Service: Invention Title: ke Kommunikations-Elektronik GmbH Co Kabelkamp 3000 Hannover 1

GERMANY

Werner Klamt,Klaus Pollak, Andreas Tinimermann Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia A Branching Circuit *4 ID ID ID 5$

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The following statement is a full description of this invention, Including the best method of performing it known to me/us:- 5845/4 TECHNICAL FIELD The invention relates to a branching circuit for connecting a two-wire circuit to a four-wire circuit of a telephone system wherein the four-wire circuit is composed of a transmitting branch and a receiving branch, wherein between the two-wire circuit and the four-wire circuit there is inserted a transformer having a primary winding and a secondary winding, and wherein with the two-wire circuit there is associated a dummy circuit connected with one of the windings of the transformer, with the impedance of the dummy circuit dimensioned with proper regard for the impedance of a peripheral unit connected to the primary winding of the transformer and for the transformation ratio of the transformer.

BACKGRUND OF THE INVENTION Various embodiments of branching circuits have been known for many years in communications engineering and are used particularly in telephone traffic. They serve to connect a four-wire system comprising a transmitting branch and a receiving branch to a two-wire system and vice versa. Reflections are to be minimized both within the two-wire system and from the transmitting branch of the four-wire system in the receiving branch to minimize transmission losses, on the one hand, and to avoid 20 feedback within the four-wire system, on the other hand, Known branching circuits, which are actually in use, comprise a transformer having a primary winding and a secondary winding, with the two lines connected to this transforrner. Ideally, the transformer's side to which the two-wire circuit is connected, should be matched to be free of reflections. This would be achieved if the dummy circuit transformed to' I through the transformer had the sime wave impedance as the two-wire circuit. On the other hand, the attenuation on the side of the four-wire circuit should be high so that there is no interference from the S" transmitting branch with the receiving branch. This ideal state cannot be obtained in practice with the aid of the known branching circuit because transformer-inherent losses resulting from the resistance of the windings and stray Inductances cannot be fully accounted for neither on the secondary side nor on the primary side but can be brought into account only within the scope of optimization. Therefore, in the known branching circuit there are reflections on the side of the two-wire circuit and feedback from the emitting branch to the receiving branch on the side of the four-wire circuit, which causes transmission losses and deterioration of echo stability, 745z/jrb 74Sz/rb SUMMARY OF THE INVENTION The problem underlying the invention is to configure the above-described branching circuit so that refleLtions on the one side of the transformer and feedback from the transmitting branch to the receiving branch on the other side of the same are precluded.

According to the invention, there is provided a branching circuit for connecting a two-wire circuit to a four-wire circuit of a telephone system wherein the four-wire circuit comprises a transmitting branch and a receiving branch, wherein between the two-wire circuit and the four-wire circuit there is inserted a transformer having a primary winding for connection to a peripheral device and a secondary winding, and wherein the four-wire circuit comprises a balance network connected in series with the secondary winding of the transformer, with the impedance of the balance network being dimensioned with proper regard for the impedance of the peripheral unit and for the transformation ratio of the transformer, wherein: for the purpose of properly dimensioning the impedance of the balance network, the resistance of the windings of the transformer and the stray inductances of the transformer are also taken into account; to a point between the transformer's secondary winding and the balance network there is connected, via an ohmic resistance, an inverting amplifier which has an inverting input, and which is connected to the receiving branch of the four-wire circuit; and a compensating network is connected between the inverting input of the inverting amplifier and the secondary winding on the opposite side of the transformer to the balance network.

BRIEF DESCRIPTION OF THE DRAWINGS There show: Figure 1, the branching circuit according to the invention; Figures 2 to 5, various embodiments of useable networks.

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DETAILED DESCRIPTION OF PREFRRED EMBODIMENTS The branching circuit illustrated in Figure 1 comprises a transformer U which is shown bordered by the dashed line. The transformer U has a primary winding 1 and a secondary winding 2. To the primary winding 1 there is connected a two-wire circuit to the far end of which a peripheral unit, a telephone, Is connected. The termination of the two-wire circuit corresponds to its wave impedance.

A four-wire circuit comprising a transmitting branch and a receiving branch is connected to the secondary winding 2. A dummy circuit 3, the Impedance or dimensioning of which are described below, is connected in series with the secondary winding 2. The secondary winding 2 of the transformer U is symmetrically arranged in the two-wire circuit of the transmitting branch in which amplifiers 4 and 5 are inserted. The transmitter signal is inputted Into input E of the transmitting branch.

The receiving branch of the four-wire circuit comprises basically an inverting amplifier 6 the Inverting input El of which is connected through a resistor 7 to point the T between the secondary winding 2 and the dummy circuit 3. The second Input E2 of the amplifier 6 is connected to grounbd as the reference potential. The received signal of the receiving branch is available at output A.

Between the Inverting input El of the amplifier 6 and the transmitting branch of the four-wire circuit there Is connected a compensating net Nl which Is in the same loop as the secondary winding 2 of transformer U.

25 The Impedance of the dummy circuit 3 is chosen with proper regard for the Impedance of the periphe 1 unit connected on the primary side, the transformation ratio of the transformer U, and the resistance of the S" secondary winding 2. In this way optimal matching of the transformer U is obtained so that there are no reflections on Its primary side.

In this way, the four-wire circuit is necessarily detuned so that a current flows In the direction of arrow P from the transmitting branch into the r( ring branch. In order to overcome Its effects, there is provided the c. jnsating net NI which is configured so that a current of equal amplitude and phase can be discharged. Therefore, a current resulting from the output signal of the transmitting branch does not appear at the point of current summation at the input El of amplifier 6.

The branching circuit can be supplemented by a second compensating net N2 indicated by dashed lines In Figure 1, which is connected between S4- 745z/jrb the inverting input El of the amplifier 6 and the transmitting branch's output directly connected to the dummy circuit 3. The second compensating net N2 facilitates compensation in a larger frequency range. With it, currents having an additional 1800 phase shift can be discharged.

In preferred embodiment, RC networks are used as compensating nets N1 and N2. Figures 2 to 5 show examples of such nets.

According to Figure 2, a series circuit and, according to Figure 3, a parallel circuit of an ohmic resistor 8 and a capacitor 9 can be used as the compensatfng net N1 or N2. According to Figure 4, the compensating net according to Figure 2 can be supplemented by a capacitor 10 connected in parallel, Similarly, according to Figure 5, the network according to Figure 2 can be supplemented by a series resistor 11.

The invention is not limited to the use of RC networks. Other appropriate compensating networks can be employed too.

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Claims (4)

1. A branching circuit for connecting a two-wire circuit to a four-wire circuit of a telephone system wherein the four-wire circuit comprises a transmitting branch and a receiving branch, wherein between the two-wire circuit and the four-wire circuit there is inserted a transformer having a primary winding for connection to a peripheral device and a secondary winding, and wherein the four-wire circuit comprises a balance network connected in series with the secondary winding of the transformer, with the impedance of the balance network being dimensioned with proper regard for the impedance of the peripheral unit and for the transformation ratio of the transformer, wherein: for the purpose of properly dimensioning the impedance of the balance network, the resistance of the windings of the transformer and the stray inductances of the transformer are also taken into account; to a point between the transformer's secondary winding and the balance network there is connected, via an ohmic resistance, an inverting amplifier which has an inverting input, and which is connected to the receiving branch of the four-wire circuit; and e'.r t a compensating network is connected between the inverting input of the ib einverting amplifier and the secondary winding on the opposite side of the transformer to the balance network.

S2. A branching circuit according to claim 1, wherein a second compensating network is inserted between the inverting input of the inverting amplifier and the transmitting branch's input, which input is directly connected to the balance network, o c.

3. A branching according to claim 1 or 2, wherein the compensating networks S: are configured as RC networks,

4. A branching circuit substantially as hereinbefore described with reference to the accompanying drawings. *see DATED this Twenty-seventh Day of July 1994 ke Kommunikations-Elcktronik GmbH Co Patent Attorneys for the Applicant SPRUSON FERGUSON AT tN:\libcl00tb:rhk ABSTRACT A Branching Circuit There is disclosed a branching circuit for connecting a two-wire circuit to a four-wire circuit of a telephone system, wherein between the s two-wire circuit and the four-wire circuit there is inserted a transformer having a primary winding and a secondary winding The two-wire circuit is associated with a dummy circuit which is connected with one of the windings of the transformer and the impedance of which is dimensioned with proper regard for a peripheral unit connected to the to primary winding of the transformer, the transformation ratio of the transformer and the resistance of the windings and the stray inductances of the transformer An inverting amplifier with its inverting input (E1) is connected between the secondary winding of the transformer and the dummy circuit in the receiving branch of the r s four-wire circuit, with an ohmic resistance interposed. In the transmitting branch, a compensating network (Ni) is connected between the inverting input (El) of the amplifier and the secondary winding's (2) terminal far from the dummy circuit o S Figure 1 *e: *5*4 *0 *e 0e 00 0