GB2034557A - Transmission bridge for telephone subscriber's line circuit - Google Patents

Abstract

A transmission bridge comprising, arranged between two line terminals (1, 2) a series arrangement of two primary windings (3, 4) of a transformer and an intervening capacitor (20), two supply resistors (6, 7) each connected to a respective one of the terminals and to power supply (10, 11), secondary windings (13,14) of the transformer being used to provide voltages equal to the a.c. voltages appearing at the terminal and these voltages are fed via respective alternating current circuits (8, 9) having a high input impedance, a low output impedance and a gain factor equal to unity, the outputs (17, 19) of these circuits being connected to the respective supply resistors in such a manner that the same a.c. voltage appears at both ends of each resistor. As a result, wanted anti-phase voltage signals between the terminals are not attenuated by the supply resistors whereas the supply resistors do attenuate unwanted in-phase signals. <IMAGE>

Description

SPECIFICATION Transmission bridge for a telephone subscriber's line circuit The invention relates to a transmission bridge for a telephone subscriber's line circuit, comprising two line terminals for connection to a subscriber's line, a transformer whose primary winding is arranged in series with a capacitor between the line terminals, two supply registors each connected at one end to a respective line terminal, and a supply source connected to the other ends of the supply resistors.

Such a transmission bridge is generally known from USP 3.993.880 or USP 3.300.588 wherein the supply current is applied, by-passing the series arrangement of the primary winding and the capacitor, to a subscriber's line connected to the terminals to prevent the transformer from being saturated.

In this known transmission bridge a.c. voltage difference signals, so-called differential mode signals, such as speech signals occurring between the terminals, are applied by means of the transformer to a telephone exchange and/or reversed, and in-phase a.c. voltage signals, so-called common mode signals, such as unwanted signals induced by a noise source, in the two wires of a subscriber's line, occurring on the terminals, are blocked by the transformer and attenuated by the supply resistors.

Owing to the fact that not only the common mode signals but also the differential mode signals are present across the supply resistors the latter signals are also attenuated. To mitigate this drawback it is known to substitute coils for the supply resistors.

However, coils have a frequency-dependent impedance so that they must be rather bulky to form a sufficiently high impedance for the low frequencies in the speech band. It is further known to implement these coils electronically.

It is an object of the invention to provide a new concept of the transmission bridge described in the opening paragraph, wherein attenuation of the differential mode signals by the supply resistors is avoided. According to the invention there is provided a transmission bridge for a telephone subscriber's line circuit, comprising two line terminals for connection to a subscriber's line, a transformer whose primary winding is arranged in series with a capacitor between the line terminals, two supply resistors each connected at one end to a respective line terminal, and a supply source connected to the other ends of the supply resistors, characterised in that the other end of each supply resistor is connected to the output of a respective alternating current circuit the input of which is connected to one end of a respective secondary winding at the transformer arranged to provide, at the input of the associated alternating current circuit, an a.c. voltage signal equal to phase and magnitude to an a.c.

voltage signal occurring, in operation, at the respective line terminal, and in that each alternating current circuit has a high input impedance and a low output impedance and is so arranged that its output signal is substantially equal to its input signal in magnitude and phase. This transmission bridge has the advantage that no differential mode signals are present across the supply resistors so that these signals are not attenuated.

In accordance with an embodiment of the bridge according to the invention, each of the alternating current circuits comprises a high gain differential amplifier having a non-inverting signal input, an inverting signal-input, and a signal output, the signal input being connected to the said one end of the associated secondary winding, the signal output being coupled to said other end of the associated supply resistor, a feedback path being provided between the said other end of the associated supply resistor and the inverting signal input. This furnishes an economically very attractive implementation.

In accordance with a further feature, each alternating current circuit further comprises two complementary transistors whose bases are interconnected and connected to the signal output of the differential amplifier, whose emitters are, on the one hand, interconnected and connected to said other end of the associated supply resistor and, on the other hand, to the bases, via a high-ohmic resistor, and each collector is connected to an associated respective pole of the supply source.

This circuit has the advantage that ringing with voltages considerably higher (e.g. 125 V) than the supply voltage (50 V) is possible whilst using standard differential amplifiers.

Embodiments of the invention will now be described with reference to the accompanying drawings, in the Figures of which corresponding components having been given the same reference numerals.

In the drawings: Figure 1 shows, for explanatory purposes, a circuit diagram of an embodiment of half of a transmission bridge according to the invention; Figure 2 shows an embodiment of a transmission bridge according to the invention using the circuit shown in Figure 1; Figure 3 shows an embodiment of an alternating current circuit for use in the embodiment of Figure 2; and Figure 4 shows an embodiment of a supply source for use in the transmission bridge shown in Figure 2.

The circuit diagram shown in Figure 1 shows one of the lineterminals 1 of a transmission bridge coupled to a balance point for a.c. signals drawn as a virtual earth point 12 via a first portion 3 of the primary winding of a transformer 5. In addition, a voltage supply source, not shown in this Figure, having poles 10 and 11 is connected to the terminal 1 via a supply resistor 6.

The circuit described so far forms one half of a generally known transmission bridge wherein the problem is encountered that an a.c. voltage signal present at terminal 1 is attenuated in the resistor 6, which forms a shunt path for the signal.

To prevent this attenuation from occurring there is provided, in a transmission bridge in accordance with the invention, a series arrangement of a first secondary winding 13 of the transformer 5 and an alternating current circuit 8 between a balance, point, for a.c. signals drawn as a virtual earth point 15 and that end of the supply resistor 6 not connected to the terminal 1. The alternating current circuit 8 has a high impedance at input 16, a low impedance at output 17 and a gain factor substantially equal to unity.

The series arrangement operates as follows: An a.c. voltage, present at input terminal 1, is transferred through the primary winding 3 to the secondary winding 13 and applied to input terminal 16 as a signal having the same magnitude and phase as the voltage at terminal 1. The alternating current circuit 8, having a gain factor equal to unity, applies this signal to that end of the supply resistor 6 not connected to the terminal 1, via output 17. This signal again has the same magnitude and phase as that on terminal 1 and results in the same a.c.

voltage signal at both sides of the supply resistor 6, so that the signal present at terminal 1 is not attenuated by the supply resistor 6. Furthermore, owing to the high impedance of input 16, the signal at terminal 1 is not attenuated (via the transformer 5) by the alternating current circuit 8.

Figure 2 is a block schematic circuit of the complete transmission bridge, with the exception of a supply source, operating in accordance with the principle described above. This bridge is arranged mirror-symmetrically around the balance points 12 and 15 of Figure 1 and comprises, in addition to the circuit shown in Figure 1, a second lineterminal 2, a second partial winding 4 of the primary winding of the transformer 5 which is arranged in series with the first partial winding 3 via a capacitor 20, a second supply resistor 7, and a series arrangement of a second secondary winding 14 and an alternating current circuit 9.

The second secondary winding 14 is wound in the opposite sense to the first secondary winding and the alternating current circuit 9 is identical to the alternating current circuit 8.

An a.c. voltage, a so-called differential mode signal such as a speech or ringing signal, appearing with opposite phases at the terminals 1 and 2, produces a current through the windings 3 and 4 (wound in the same direction) and capacitor 20 and is consequently transferred to the secondary windings 13 and 14 as well as to another secondary winding (not shown) fortranmission to the exchange side of the bridge. As described with referpence to Figure 1,these signals of the same magnitude and phase as at terminals 1 and 2 are applied to the supply resistors 6 and 7 via the outputs 17 and 19 of the alternating current circuits 8 and 9 respectively. Consequently, these differential mode signals are present at both sides of the supply resistors 6 and 7 and are therefore not attenuated by these supply resistors.

An In-phase a.c. voltages at the terminals 1 and 2, a so-called "common mode signal" such as, for example, a noise voltage induced by a noise source in the two wires a and b of a subscriber's line connected to the terminals 1 and 2, do not produce a current through the primary windings 3 and 4 and are therefore not transferred to the secondary windings 13 and 14. Owing to the low impedance of the outputs 17 and 19 of the alternating current circuits 8 and 9 substantially the full in-phase signal voltages are present across the supply resistors 6 and 7 and are attenuated therein.

Because the differential mode voltages at those ends of the secondary windings 13 and 14 not connected to the inputs 16 and 18 are of the opposite phase, a balance or virtual earth point for a.c. signals is created for these ends with respect to those voltages in a simple manner by means of the capacitor 21 arranged between these ends.

The symmetrical implementation of the transmission bridge prevents the unwanted in-phase signals from being converted into audible anti-phase signals.

The transmission bridge shown in Figure 2 has therefore the advantage that anti-phase signals on terminals 1 and 2 are not attenuated, whereas in-phase signals at these terminals are greatly attenuated and are not converted into audible antiphase signals.

The alternating current circuit 8 (9) may comprise a differential amplifier whose non-inverting signal input is the input 16 (18) of the alternating current circuit 8(9) and the signal output 17(19) is connected to the inverting signal input. Such differential amplifiers are commercially available as so-called "operational amplifiers". If ringing signals must be applied to the terminals 1 and 2 by means of this transmission bridge these operational amplifiers are less suitable in view of the 250 Volts peak-peak voltage required for the ringing signals.

Figure 3 shows an embodiment of an alternating current circuit 8 (9) which satisfies the above condition. In addition to a differential amplifier 22 this alternating current circuit comprises a balanced amplifier stage implemented by means of emitter followers. This stage comprises two complementary transistors 23 and 24 whose bases are interconnected and connected to the signal output of the differential amplifier 22, whose emitters are interconnected, and are connected to the output 17 of the alternating current circuit 8 and to the inverting (-) signal input of the differential amplifier 22 and whose collectors are respectively connected to the poles 10 and 11 of the supply source.

A signal applied to the non-inverting (+) signal input of the differential amplifier 22 via the input 16 is applied, after having been amplified, to the bases of the emitter followers 23 and 24. Transistor 23 is conductive in response to the positive output signals of the differential amplifier and transistor 24 is conductive in response to the negative output signals, the line current for a subscriber's line connected to terminal 17 via supply resistor 6 being obtained from the positive pole 10 or from the negative pole 11 of the supply source. The output signal of the circuit is taken from the emitters of the transistors, so that a low output impedance is obtained. The feedback path 25 keeps the total gain of the a.c. circuit 8, comprising the differential aplifier 22 and the emitter follower stage, accurately at unity, because the differential amplifier 22 causes the non-inverting signal input voltage and the inverting signal input voltage to equal the signal voltage at the signal input.

On the occurrence of a polarity change in the output voltage of the output of the differential amplifier 22, neither of the transistors 23 and 24 will conduct over the voltage range from a base-emitter junction voltage above the line voltage level to a base-emitter junction voltage below the line voltage level. The distortion thus produced is markedly decreased by means of a high-ohmic resistor 26, for example 1 K ohms, provided between the base and the emitters, These alterhating current circuits allow the use of a high voltage line supply. To this end the supply voltage of the differential amplifiers 22 must vary with the control signal. Means which render this possible are, for example, described in "Applications of operational amplifiers. Third generation Techniques" by Jerals G. Graeme, page 40, Figure 2.5, published by Mc. Graw-Hill Company.

Furthermore, these circuits make a polarity reversal of the line voltage possible.

As described above, the output 17 varies with the input 16. By applying a suitable reference d.c.

voltage to the input 16 any desired d.c. voltage can be taken from terminal 17, the current being supplied by the supply source via the poles 10 and 11 and the emitter followers 23 and 24. Control terminals 27 and 28 (Figure 2), which are connected to the inputs 16 and 18 via the respective secondary windings 13 and 14, are provided for the control of the output voltages at output 17 and 19.

Figure 4 shows a supply source suitable for the control and the supply of power to the circuit shown in Figures 2 and 3.

This supply source comprises two series-arranged voltage sources 29 and 30 of, for example, 125 V each, provided with taps 29-1 and 30-1 at, for example, 25 Volts. When change-over switches 31-1 and 31-2, respectively, are in the opposite position to that shown in Figure 4, each of the poles 10 and 11 of the supply source is coupled to a respective one of the two ends of the series-arranged voltage sources 29 and 30 and, when the switches 31-1 and 31-2 are in the position shown in the drawing, these poles are respectively coupled to said taps 29-1 and 30-1. In addition, the ends of the series-arranged voltage sources 29 and 30 and said taps 29-1,30-1, respectively, are connected to the junction 39 of the voltage sources 29 and 30 through change over switches 32-1 and 32-2, individual voltage dividers 33,34 and 35,36, and individual switches 37 and 38.The voltage divider taps are connected to the control terminals 27 and 28.

The change-over switches 31-1,31-2; 32-1 and 32-2, and the two switches 37 and 38, are controlled by a binary signal source 40.

The normal line supply voltage e.g. (50 V) is applied to the terminals 1 and 2 when the change over switches 31-1 and 31-2 and the switches 37 and 38 are in the positions shown in the drawing.

Starting from a voltage of -25 V at point 39 e.g. by means of an d.c. voltage source of 25 V (not shown in the figure) connected between this point and earth and from the above-mentioned values of the voltages at the taps 29-1 and 30-1, the voltage at pole 10 and tap 27 is zero volt and at pole 11 and tap 28 fifty volts negative. To enable ringing, i.e. superimposing a larger a.c. voltage on the d.c. voltage, the changeover switches 31-1 and 31-2 are adjusted to the opposite positions to those shown in the drawing.

When the switches 37 and 38 are closed, and at the above-mentioned voltages of the voltage sources 29 and 30 and at the mutual junction 39 and with a division ratio of the resistors 33 and 34 respectively 35 and 36, of four to one, the voltage at the control terminal 27 is again zero and the voltage at the control terminal 28 is again fifty volts negative.

When the change-over switches 32-1 and 32-2 are switched over, the voltage across the control terminals 27 and 28 and, consequently, the voltage at the terminals 1 and 2, is inverted.

When the switches 37 and 38 are open, the voltage at the control terminal 27 is one hundred volts positive and the voltage at the control terminal 28 is one hundred and fifty volts negative, these high voltages being required for ringing. By switching the change-over switches 32-1 and 32-2 at ringing frequency, the polarisation of the voltage at the control terminals 27 and 28 is alternately reversed in a simple manner to provide ringing signals at terminals 1 and 2.

To enable the superimposition of a.c. voltages on the line supply voltages by means of the supply source, such as for example, an analog 50 Hz ringing signal, the signal source 40 is arranged so that it applies, for example, a delta modulated ringing signal to the switch 31 as the control signal. Owing to the integrating action of the networks formed by the resistors 33 and 35 and the capacitor 21, analog ringing voltages of opposite phases are obtained at the control terminals 27 and 28. It should be noted that other binary signal representation can be used e.g. DCDM etc.

When all the switches are implemented electronically an electrical isolation between the signal source and the switches can be obtained by means of optical couplers, now shown. Because the optical couplers transmit binary signals no non-linear signal distortion is introduced and the optical couplers can be adjusted to such a value that a long life is obtained.

Claims (8)

1. A transmission bridge for a telephone subscriber's line circuit, comprising two line terminals for connection to a subscriber's line, a transformer whose primary winding is arranged in series with a capacitor between the line terminals, two supply resistors each connected at one end to a respective line terminal, and a supply source connected to the other ends of the supply resistors, characterized in that the other end of each supply resistor is con nected to the output of a respective alternating current circuit the input of which is connected to one end of a respective secondary winding at the transformer arranged to provide, at the input of the associated alternating current circuit, an a.c. voltage signal equal in phase and magnitude to an a.c.

voltage signal occurring, in operation, at the respec tive line terminal, and in that each alternating current circuit has a high input impedance and a low output impedance and is so arranged that its output signal is substantially equal to its input signal in magnitude and phase.

Atransmission bridge as claimed in Claim 1, characterized in that each of the alternating current circuits comprises a high-gain differential amplifier having a non- inverting signal input, an inverting signal input, and a signal output, the signal input being connected to the said one end of the associated secondary winding, the signal output being coupled to said other end of the associated supply resistor, a feedback path being provided between the said other end of the associated supply resistor and the inverting signal input.

3. Atransmission bridge as claimed in Claim 2, characterized in that each alternating current circuit further comprises two complementary transistors whose bases are interconnected and connected to the signal output, whose emitters are, on the one hand, interconnected and connected to said other end of the associated supply resistor and, on the other hand, to the bases via a high-ohmic resistor, and each collector is connected to an associated respective pole of the supply source.

4. Atransmission bridge as claimed in Claim 1, characterized in that a capacitor is connected between the other ends of the secondary windings.

5. Atransmission bridge as claimed in Claim 1 or 4, characterized in that the primary winding comprises two identical partial windings which are arranged in series with the capacitor between them.

6. Atransmission bridge as claimed in Claim 4 or 5, characterized in that the supply source comprises control means for varying the voltage at its said poles and for providing a respective voltage, variable in sense and magnitude, at each said other end of the secondary windings.

7. Atransmission bridge as claimed in Claim 6, characterized in that the said means comprises switches for effecting said changes and a control signal source for operating said switches.

8. Atransmission bridge for a telephone subscriber's line circuit substantially as herein described with reference to Figures 1 and 2, to Figures 1,2 and 3, or to Figures 1 to 4, of the accompanying drawings.