GB1572595A

GB1572595A - Two-wire to four-wire converters

Info

Publication number: GB1572595A
Application number: GB49303/75A
Authority: GB
Original assignee: Post Office
Current assignee: Post Office
Priority date: 1975-12-01
Filing date: 1975-12-01
Publication date: 1980-07-30
Also published as: SE7613470L; NL7613299A; DE2654495A1; JPS5284907A

Description

(54) IMPROVEMENTS IN OR RELATING TO TWO-WIRE TO FOUR-WIRE CONVERTERS (71) We, THE POST OFFICE, a British Corporation established by Statute, of 23 Howland Street, London WIP 6HQ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a two-wire to fourwire ( > w/4w) converter and line unit particularly for use in a telecommunication network.

Conversion from two-wire to four-wire working is likely to become increasingly import ant with the growth of digital networks and with pulse code modulation (PCM) encoders moving closer to the subscriber. The existing 2w/4w unit uses hybrid transformers which are rather bulky and expensive. This memorandum describes a circuit designed to eliminate the transformer from the 2w/4w unit. The complete circuit includes line current feed and d.c. signalling detection so that it can be used as a line unit. on a one-per-subscriber basis for a completely electronic exchange or as a supervisory unit in a concentrator feeding into a digital transmission system.

According to the present invention there is provided a two-wire to four-wire converter comprising a balanced resistive network having a pair of input terminals for connection to a two-wire line, a first arm comprising a first pair of resistors connected to one of said input terminals. a second arm comprising a second pair of resistors connected to said other input terminal. and a pair of series connected balance resistors connected between said arms, a first resistive path connecting said one input terminal to the second arm and a second resistive path connecting the other input terminal to the first arm, a send amplifier having differential inputs connected one to a point intermediate the ends of said first resistive path and the other to a point intermediate the ends of said second resistive path so that said amplifier can receive a differential signal from the input terminals, and a receive amplifier which is connected to the junction of said pairs of resistors such that the amplifier output is divided by the network between the input terminals and the balance resistors.

The send amplifier and the receive amplifier may each be a differential amplifier. A d.c. reference signal for each differential amplifier may be derived from said network.

The two-wire to four-wire converter may be used in combination with a current feed circuit for a telephone exchange transmission bridge which comprises a pair of electronic circuit elements each of which has one terminal for connection to a respective one of a pair of telephone lines and another terminal for connection to one terminal of an electrical power supply, each circuit element being so arranged that when connected in a transmission bridge it forms a simulated inductance and can provide a resistive path for d.c. current from the power supply to a subscriber's telephone.

Each electronic circuit element may comprise a transistor whose collector-emitter circuit is connected in series with resistive components between said terminals, an operational amplifier whose output is connected to the base of said transistor, and a resistor connected between said non-inverting input and the collector of the transistor.

The resistive network may include an input load resistor which is connected between the operational amplifiers.

The converter may be isolated from the current feed circuit by isolating capacitors.

The invention will be described now by way of example only with particular reference to the accompanying drawings. In the drawings: Figure 1 shows one form of known transmission bridge used in telephone exchanges; Figure 2 shows another known form of transmission bridge which is used in telephone exchanges; Figure 3 illustrates schematically a trans ;mission bridge employing a current feed circuit described in U.K. Patent Application No.

46375/75;(Serial No. 1563802) Figure 4 shows another type of transmission bridge employing a current feed circuit de- scribed in U.K. Patent Application No.

46375/75;(Serial No. 1563802) Figure 5 is a circuit diagram of a simulated inductance circuit used in a current feed circuit described in UK. Patent Application No.

46375/75; (Serial No. 1563802) Figure 6 is a circuit diagram of a current feed circuit described in U.K. Patent Application No. 46375/75; (Serial No. 1563802) Figure 7 is a plot which compares the insertion loss of a transformer bridge provided with the current feed circuit of Figure 6 with tile insertion loss of a conventional transformer bridge; Figure 8 is a plot which compares the insertion loss of a capacitor bridge provided with the current feed circuit of Figure 6 with the insertion loss of a conventional capacitor bridge; Figure 9 is a circuit diagram of a circuit for feeding tone signals to the circuit of Figure 6, and Figure 10 is a circuit diagram of a two-wire to four-wire converter in accordance with the present invention.

The present two-wire to four-wire converter has been designed for use with the current feed circuit described in U.K. Patent Application No.46375/75 (Serial No. 1563802). Before describing the converter a description of the current feed circuit will be given.

Figures 1 and 2 each show a known form of transmission bridge used in telephone ex changes. The form shown in Figure 1 is a capacitor transmission bridge. The bridge is connected to a subscriber's telephone by lines 10 and 11. Current feed to a subscriber's telephone is from a battery 12 which is connected to the lines 10 and 11 by way of relay coils 14, 15. Capacitors 17, 18 connected in the lines 10, 11 isolate direct current signals flowing on one side of the exchange from those on the other side. The impedance of the relay coils are sufficient to minimise the shunting effect of the battery on the line. Typically the resistance of the current feed is 20011 in each leg to maintain the line current within suitable limits for different lengths of line.

The form of bridge shown in Figure 2 is a transformer transmission bridge. In this case d.c. isolation is performed by a transformer 20. Current feed is again from a battery 12 by way of relay coils 14, 15 and the coils 21, 22 of the transformer primary. In this bridge the high impedance current feed is provided by the coils 21,22. The transformer bridge has a major advantage over the capacitor bridge in that it rejects longitudinal or common mode interference thereby reducing the possibility of the build-up over a number of links of very large longitudinal voltages which could affect the transmission and signalling performance.

This advantage makes the use of a transformer bridge preferable in many cases even though it is larger and more expensive than a capacitor bridge.

Both of the above known transmission bridge circuits are rather bulky particularly when compared with the electronic circuits.

Figures 3 and 4 show respectively a capacitor bridge and a transformer bridge provided with a current feed circuit 25 which employs electronic components. The transformer version of such a transmission bridge can be made relatively small if a parallel line current feed circuit is provided so that no d.c. current flows in the transformer windings which are designed purely for their a.c. characteristics.

A parallel line current feed circuit needs to be of low resistance and high a.c. impedance which means employing inductors which provide inductances as large as those provided in known transformer bridges. Large value inductances can be simulated using electronic components and a current feed circuit which uses simulated inductances is shown in Figure 6. This circuit supplies a direct current through a low resistance but can be connected across the transmission line without any appreciable transmission loss. It can also be used in a capacitor type transmission bridge.

Before describing the circuit of Figure 6 reference will be made to Figure 5 which shows the basic simulated inductance used in the circuit of Figure 6. The simulated inductance comprises an operational amplifier 50, the noninverting input of which is connected by a capacitor 51 to earth line E and by a resistor 52 to the collector of a transistor 54. The inverting input of the amplifier 50 is connected to the junction of two resistors 55 and 56 which are connected in series with a further resistor 57 between earth line E and supply line -V. The output of the operational amplifier 50 is connected to the base of the transsitor 54 and the emitter of the transistor 54 is connected to the junction of the resistors 55 and 57. The collector of the transistor 54 is connected by a resistor 59 to a terminal 60.

Under steady state conditions the inverting input to the operational amplifier 50 is maintained at approximately two volts below the voltage at the emitter of the transistor 54 by the resistor chain 56, 55, 57. The non-inverting input of the amplifier is at substantially the same voltage as the collector of the transistor 54, the current in the resistor 52 being negligible. Since the amplifier 50 operates to equalise the voltages on its two inputs the collector voltage of the transistor 54 will always be maintained at substantially two volts below its emitter voltage. When the resistor 57 has a resistance of 3011 and the resistor 59 a resistance of 120Q the d.c. characteristics of Figure 5 are defined as a resistance of 15011 in a series with a constant two volts.

The a.c. characteristics of the circuit of Figure 5 can be best understood by considering the application of a voltage step V1 between the terminal 60 and earth line E. When such a voltage step is applied the current through resistor 59 does not change instantaneously because the capacitor 51 prevents the voltage at the inputs of the operational amplifier 50 from changing. The capacitor 51 begins to charge slowly through the resistor 52 at an initial rate of CS1 x R52 second where x RS2 volts per second where C51 is the capacitance of the capacitor 51 and R52 is the resistance of the resistor 52.The voltage at the emitter of the transistor 54 changes at the same rate so that the current through resistor 59 starts to change at a rate of C51 x V1 x E < 57 amps per second where RS2 x RS7 R57 is the resistance of the resistor 57. Thus the circuit shown in Figure 5 appears as an inductance of CSl x R52 x R57 Henries connected between the terminal 60 and earth line E.

Referring now to Figure 6 the current feed circuit employs two of the circuits of Figure 5 connected in a balanced configuration. Components corresponding to those of Figure 5 are shown by like reference numerals the components of the two simulated inductances being distinguished by the references A and B. A common capacitor 51 is employed for the two simulated inductance circuits to provide an improved balance. In the case of the simulated inductance having the operational amplifier 50A the resistor 56 is constituted by two resistors 70 and 71 which form part of a resistance chain 70, 71,72 and 73 connected between supply line V and resistor 55A.In the case of the simulated inductance having the amplifier 50B the resistor 56 is constituted by two resistors 75 and 76 which form part of a resistance chain 75, 76, 77 and 78 connected between earth line E and the resistor 55B. A diode 80 is connected between the noninverting input of the amplifier 50A and the junction of resistor 77 and 78 and a diode 81 is connected between the non-inverting input of the amplifier 50B and the junction of resistors 72 and 73. A diode 87 is connected between the collector of the transistor 54A and the junction of the resistors 70 and 71 and a diode 88 is connected between the collector of the transistor 54B and the junction of the resistors 75 and 76.

Each simulated inductance circuit has a zener diode 85, 86 which is connected between earth and the collector of the transistor 54A,54B.

A line 90 is maintained at a potential of substantially 2 by connection to the emitter of a transistor 95 and is used as a voltage reference for diodes 91 and 92 whose purpose is to define the overload characteristics of the circuits. The diodes 91 and 92 are connected respectively to the junction of the resistors 75 and 77 and the junction of the resistors 71 and 72. The collector of the transistor 95 is connected to line V. The base of the transistor 95 is connected to the junction of two resistors 96, 97 which are connected in series between earth line E and supply line V.

Stabilisation for the power supply to the circuit is provided by an arrangement consisting of transistors 100 and 101, resistors 102 and 103 and capacitor 104. The junction of the resistor 102 and the base of the transistor 101 is connected by a resistor 105 to the collector of a transistor 108 the emitter of which is connected to a supply line V2. The base of the transistor 108 is connected to a supply line V2 by a resistor 109 and to a terminal 110 by a resistor 112.

The current feed circuit of Figure 6 is connected to form a transmission bridge by connecting the terminal 60A to the line 10 of Figure 3 or Figure 4 and the terminal 60B to the line 11 of Figure 3 or Figure 4. Such an arrangement acts as a current feed for a subscriber's telephone and also acts as an impedance for a.c. speech signals transmitted to the exchange. By appropriately selecting the values of resistors 57A, 59A and 57B, 59B each part of the current feed circuit can be made to behave as 15052 in series with the constant two volts. The circuit can then provide a suitable transmitter current which is within the required limits for all lengths of line normally used. The current feed characteristic of the circuit of Figure 6 is very similar to that of existing feed circuits.

When a.c. speech signals are transmitted along lines 10, 11 there is a relative voltage change between the collectors of the transistors 54A and 54B so that the capacitor 51 charges and discharges in a similar manner to a single circuit. However, under longitudinal, i.e. common mode noise, signals there is no relative voltage change between the collector of the transistors 54A and 54B and the complete circuit behaves as a 150Q resistor connected between each line and earth. This provides considerable attenuation to longitudinal hum and noise voltages allowing the circuit to operate on lines subject to such interference.

The impedance presented by the circuit of Figure 6 to transverse speech signals is equivalent to about 2.5 Henries so that the insertion loss of the circuit is very small (less than 0.1dB at 300Hz). Most of the loss in the transmission bridge is then caused by the transform or capacitors. Figures 7 and 8 respectively show the insertion loss of transformer and capacitor bridges using the present current feed circuit compared with the insertion loss in conventional transmission bridge circuits.

In Figure 7 curve A represents the insertion loss of a transformer transmission bridge having the present electronic current feed circuit and curve B represents the insertion loss of a conventional transmission bridge whilst in Figure 8 curve C represents the insertion loss of a capacitor transmission bridge having the present electronic current feed circuit and curve D represents the insertion loss of a conventional capacitor transmission bridge.

The use of an electronic current feed circuit enables other features to be provided which will now be referred to. The circuit has protection for short circuit fault conditions. The resistance of a telephone can reduce to as low as 50Q so it is difficult to design a circuit which will cut off under short circuit fault conditions but not affect the operation of a telephone on a very short line. D.C. fault protection therefore has been limited to protecting against fault conditions applying an earth to terminal 60B or -V volts to terminal 60A.

The operation of the protection elements in the present circuit can be seen by considering a situation when a variable resistance is connected between for example terminal 60B and earth. When this resistance decreases the current increases until it is limited by the diode 81 becoming conductive. As the resistance decreases still further the current remains almost constant but the voltage across the transistor 54B increases until the diode 88 becomes conductive. When the diode 88 conducts it pulls the inverting input of the operational amplifier 50B positive so that the amplifier causes the transistor 54B to become nonconductive. This renders the circuit inoperative. As soon as the fault is removed the circuit will automatically become operational again.The protection on the other part of the circuit operates in a similar manner if a -V volts battery fault occurs.

Protection against lightning strikes and induced voltage surges is provided by the zener diodes 85 and 86. Any of a number of suitable zener diodes can be used depending upon the surge rating required.

In applications where line current is fed from a supervisory unit via reed relay crosspoints, it is necessary to ensure that the reeds do not actually switch the line current. This can be done by providing heavy duty contacts to switch the line currents after the reed relay crosspoints have been operated. However with an electronic current feed circuit it is possible to switch off the current electronically while the crosspoints are being operated. The electronic switch is provided by the transistor 103 and is controlled by voltages applied to terminal 110.

In conventional transmission bridges tones are usually sent to line from a third winding on the transformer or from an extra relay coil.

However with an electronic current feed circuit it is possible to send tones directly to the line from the circuit. The tones are fed to the circuit via a transistor which acts as a switch controlled by external voltages and also as a phase splitter to provide a balanced signal. A number of tones can be provided by using one transistor for each tone and switching these in when required. Such a circuit is illustrated in Figure 9. This circuit has three transistor switches 111, 112, 114 and the tone signals appear at terminals 115, 116. These terminals are connected to the points 1 15A and 116A on the circuit of Figure 6.

A signal detector circuit can be connected to the points 119, 120 (Figure 6) to detect for example loop/disconnect pulses for dialling and coin signals from coin boxes. Such a detector circuit forms the subject of British Patent Application No. 48692/75 (Serial No.

1563803).

Referring now to Figure 10 a two-wire to four-wire converter has two input terminals 150 and 151. The terminal 150 is connected to the junction of the non-inverting input of the amplifier 50A and the diode 80 of Figure 6 and the terminal 151 is connected to the junction of the non-inverting input of the amplifier 50B and the diode 81. The capacitor 51 is removed. The input 150 is connected by a capacitor 153 to one side of the resistor 154 and the input 151 is connected by a capacitor 155 to the other side of the resistor 154. The resistor 154 forms part of a balanced resistive network 152 consisting of resistors 154, 156, 157, 158, 159, 160 and 161.The values of the resistors 156, 157, 158 and 159 are equal whilst the values of the resistor 1 54 and the resistors 160 and 161 are determined in a manner to be described.

The junction of the resistors 154 and 156 is connected by resistors 163 and 164 to the junction of resistors 161 and 159. The junction of resistors 157 and 160 is connected by resistors 1 66 and 1 67 to the junction of resistors 154 and 158.

An operational amplifier 170 is associated with the send part of the four-wire network.

The inverting input of the operational amplifier 170 is connected to the junction of the resistors 166 and 167 and the non-inverting input is connected to the junction of resistors 163 and 164. The inverting input of the amplifier 170 is connected to earth line E by a resistor 171 and the non-inverting input is connected to the earth line E by a resistor 172. The earth line E is connected to the junction of the resistors 160 and 161. The output of the operational amplifier 170 is connected by a feedback network consisting of a resistor 174 and a capacitor 175 to its inverting input.

The receive part of the four-wire network has an operational amplifier 180. The inverting input of the amplifier 180 is connected to the four-wire receive line by resistors 181 and 1 82.

The junction of resistors 181 and 182 is connected to earth by a resistor 184, and by a line 185 to the junction of resistors 156 and 157.

The non-inverting input of the amplifier 180 is connected to the junction of resistors 160 and 161. The amplifier has a feedback resistor l86 and the output of the amplifier is connected to the junction of resistors 158 and 159.

In use speech signals from the telephone line are fed to the resistive network 152 via terminals 150 and 151. As these signals are applied to one end of the network there is a larger signal at this end than at the other end and this results in a signal appearing at the output of the amplifier 170 for transmission along the four-wire send line. Signals passing along the four-wire receive line are fed to the resistive network and because the network is symmetrical the same voltage appears at each end of the network. The four-wire output is derived from the differential amplifier 180 which is connected across the resistive network so that its output is zero with two identical inputs.The resistive network 152 allows signals applied to the four-wire receive terminal to reach the two-wire circuit but not the four-wire send side whilst signals from the twowire section can pass freely to the four-wire send line.

The two-wire to four-wire part of the circuit should present a 600Q a.c. impedance at the two-wire termination, have its best balance return loss when the two-wire is terminated in 60011 and have a balance return loss of more than OdB for any line termination including short circuit and open circuit conditions. It can be shown that removing the capacitor 51 and connecting the resistor 154 into the current feed circuit shown in Figure 6 transforms the impedance of that circuit when seen from the two-wire to four-wire converter from a very high inductance to an almost pure resistance.

It still acts as a simulated inductance for current feed operation. By choosing the value of resistor 154 appropriately the resistance can be made to be approximately 600Q without affecting the d.c. conditions. The resistor 154 provides a useful termination for feeding signals to and receiving them from the two-wire part of the circuit.

The correct operation of the resistive network 152 depends on the two sides of the circuit being substantially identical. The resistor 154 provides a load at one end of the network but this is also shunted by the impedance of the two-wire circuit which varies with the load on the line. This means that the balance return loss varies for varying two-wire loads. An analysis of the circuit shows that its performance can be made very similar to that for a transformer two-wire to four-wire converter. That is to say for correct termination 600Q the balance return loss is very high but it approaches OdB as the termination approaches a short circuit or open circuit.

This is a fundamental property of any two-wire to four-wire converter. Analysis of the circuit shows that the following are suitable values for the resistance: Resistor 154: 100K11 Resistors 156, 157, 158 and 159: 47KQ Resistors 160 and 161: 5.6KQ For a two-wire to four-wire gain of OdB the resistor 174 should be 1MQ whilst for a gain of - 6dB this resistor should be 470KQ.

For a four-wire to two-wire gain of OdB the resistors 182 and 184 should each be 300so whilst for a gain of 6dB the resistor 182 should be on and the resistor 184 600he.

The present circuit has advantages over the conventional transformer circuit in its balance return loss with extreme terminations. Under open circuit conditions the transistors in the current feed circuit of Figure 6 turn off so that the balance return loss becomes quite small again. Also as the terminating resistance approaches a short circuit the current cut off begins to operate and loads the two-wire to four-wire network enough to increase the balance return loss. Thus the balance return loss is always greater than 3dB even under extreme termination conditions.

The capacitors 153 and 155 are provided to isolate the two-wire to four-wire converter from the current feed circuit. The d.c. level of the two-wire to four-wire converter is taken from the four-wire circuits thereby eliminating the need for any capacitors in series with the four-wire send and receive lines.

Most of the resistors in the current feed circuit of Figure 6 can be of 5% tolerance or greater. However the tolerances of the circuit of Figure 10 are more critical as they effect the gain through the circuit. Analysis of the circuit shows that with 2% tolerance resistors the gain from two-wire to four-wire and visa versa can vary by up to +0.3dB from the design value. The ratio of the resistors in the network is more critical than their absolute values so that if the absolute values are allowed to vary by +5% but the ratios are deEmed to within 0.5% the range of gains across the half bridge is +0.ldB to -0.3dB from the designed values.

In some applications it may be desirable to inject tones to line from the line unit. This can be done with a circuit similar to that shown in Figure 9. A number of transistor switches can be included so that any one of a number of tones can be fed to line under the control of external voltages. Even when all the transistors are off the injection circuit still loads the twowire to four-wire circuit so the changes have to be made to the values of the resistors in the two-wire to four-wire network to compensate for this. In the case of a circuit with tone injection the value of the resistor 154 should be 43K11 and the resistors 160 and 161 should each be 3.6KQ.

A current cut-off circuit can be provided to turn off the current feed circuit if this facility is required. Such a circuit has already been described with reference to Figure 6.

WHAT WE CLAIM IS: 1. A two-wire to four-wire converter comprising a balanced resistive network having pair of input terminals for connection to a twowire line, a first arm comprising a first pair of resistors connected to one of said input terminals, a second arm comprising a second pair of resistors connected to said other input terminal, and a pair of series connected balance resistors connected between said arms, a first resistive path connecting said one input terminal to the second arm and a second resistive path connecting the other input terminal to the first arm, a send amplifier having differential inputs connected one to a point intermediate the ends of said first resistive path and the other to a point intermediate the ends of said second resistive path so that said amplifier can receive a differential signal from the input terminals, and a receive amplifier which is connected to the junction of said pairs of resistors such that the amplifier output is divided by the network between the input terminals and the balance resistors.

2. A two-wire to four-wire converter as claimed in claim 1 wherein the send amplifier and the receive amplifier are each a differential amplifier.

3. A two-wire to four-wire converter as claimed in claim 2 wherein a d.c. reference signal for each differential amplifier is derived from said network.

4. A two-wire to four-wire converter as claimed in any preceding claim in combination with a current feed circuit for a telephone exchange transmission bridge which comprises a pair of electronic circuit elements each of which has one terminal for connection to a respective one of a pair of telephone lines and another terminal for connection to one terminal of an electrical power supply, each circuit element being so arranged that when connected in a transmission bridge it forms a simulated inductance and can provide a resistive path for d.c. current from the power supply to a subscriber's telephone.

5. A two-wire to four-wire converter as claimed in claim 4, wherein each electronic circuit element comprises a transistor whose collector-emitter circuit is connected in series with resistive components between said terminals, and operational amplifier whose output is connected to the base of said transistor, and a resistor connected between said non-inverting input and the collector of the transistor.

6. A two-wire to four-wire converter as claimed in claim 5, wherein said resistive network includes an input load resistor which is connected between the operational amplifiers.

7. A two-wire to four-wire converter as claimed in claim 6, wherein the converter is isolated from the current feed circuit by isolating capacitors.

8. A two-wire to four-wire converter substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. described with reference to Figure 6. WHAT WE CLAIM IS:

1. A two-wire to four-wire converter comprising a balanced resistive network having pair of input terminals for connection to a twowire line, a first arm comprising a first pair of resistors connected to one of said input terminals, a second arm comprising a second pair of resistors connected to said other input terminal, and a pair of series connected balance resistors connected between said arms, a first resistive path connecting said one input terminal to the second arm and a second resistive path connecting the other input terminal to the first arm, a send amplifier having differential inputs connected one to a point intermediate the ends of said first resistive path and the other to a point intermediate the ends of said second resistive path so that said amplifier can receive a differential signal from the input terminals, and a receive amplifier which is connected to the junction of said pairs of resistors such that the amplifier output is divided by the network between the input terminals and the balance resistors.