GB2098028A - Line circuit - Google Patents

Abstract

To be suitable for implementation in integrated circuit form, it is desirable for a telephone line circuit which is insensitive to interfering voltages to avoid the use of isolating transformers or capacitors. In addition it should be able to supply a minimum line current for even the longest lines contemplated, and be able to supply the relatively high-level ringing current. To achieve the above desiderata, the feed circuit includes two direct voltage controlled alternating voltage sources (Q2, Q2'), each having its output connected to one of the line wires (a, b) via a resistor (Z2, Z2'). These devices are AC sources as "seen" by the line wire and provide amplification for speech outgoing to the line. Each such source is direct voltage controlled in such a way that the direct voltage level of its speech wire equals that of the output of the alternating voltage source (Q2, Q2'). The direct voltage supply for the line circuit includes two controllable direct voltage sources (Q1, Q1') one per line wire, the inputs of which are each connected to the appropriate linewire via a high pass filter whose pole lies at the lower end of the speech frequency band. <IMAGE>

Description

SPECIFICATION Line circuit This invention relates to an arrangement for a telephone subscriber's line circuit.

Such a circuit has controllable voltage sources for supplying the speech wires, with an associated reference voltage source for line monitoring. It also often includes a hybrid or its equivalentfortwo-wire four-wire conversion.

In one such known circuit, the controllable voltage sources each have two inputs, one connected to the associated speech wire and, via a resistor, to the voltage-reference source, the other being connected to the other speech wire. If anti-phase signals, such as speech and ringing signals, appear at the two inputs of a voltage source, they are so copied by the voltage source that the same direct voltage levels are present at both ends of the feed resistor. As a result, the feed resistors cause no speech-path attenuation. However, in-phase signals, such as interfering longitudinal voltages induced from power lines, do not influence the respective voltage source, so that these interfering voltages are attenuated by the feed resistors. The reference voltage may have a ringing signal superimposed on it, so that the direct current supply varies in the rythm of the ringing signal.

The controllable source may include a differential amplifier whose first input is connected to the voltage reference via the resistor and to one of the inputs of the supply-voltage source via an additional resistor, the second input being connected to the other input of the voltage source via a transistor. The output of the differential amplifier is coupled to the feed resistor through a complementary transistor push-pull amplifier stage. The collectors of the transistors in this stage are coupled to terminals of the central-office battery. The reference voltage enables the differential amplifier and the push-pull amplifier stage to be adjusted so that a fixed supply voltage is delivered to the feed resistorthrough one of the transistors.Using negative feedback, the alternating voltage gain of the differential amplifier is adjusted so that the supply voltage source has approximately unity gain at high input resistances.

Speech-signal-feeding circuits in the incoming four-wire path which use amplifers are also known.

Such circuits are usually isolated from the controllable supply sources with isolation transformers or isolating capacitors. These isolating means need much space and are unsuitable for solutions using integrated circuit techniques. To achieve a symmetry which excludes crosstalk, isolating capacitors must have close capacitance tolerances, which are difficult to maintain over the intended life of a telephone system.

An object of the invention is to provide an arrangement of the above kind which eliminates the need for such transformers or capacitors and makes it possible, on the one hand, to provide a minimum supply current for long lines and, on the other hand, to deliver relatively high-level metering signals via the feed circuit.

According to the invention there is provided a circuit arrangement for a telephone subscriber's line circuit includes two controllable supply-voltage sources each associated with one speech wire and each having a single input connected to its said speech wire, in which each said supply voltage source also has its said input connected via a resistor to a voltage-reference source and a single output connected to its said speech wire via an associated feed resistor, in which a feed circuit including amplifier elements is inserted in the incoming four-wire path, in which the feed circuit consists of two direct voltage-controlled alternating voltage sources individually associated with the speech wires, in which each said alternating voltage source has its output coupled to its associated speech wire through a resistive impedance, in which each alternating voltage source is direct voltage-controlled in such a way that the direct voltage level of the associated speech wire equals that of the output of the associated alternating voltage source, and in which the input of each said controllable supply-voltage source is connected to its associated speech wire via a high-pass filter whose pole lies at the lower end of the speechfrequency band but above the interfering frequencies to be expected.

An advantage of such an arrangement is that no high-value capacitors are needed, so that the circuit can be implemented using integrated-circuittechniques.

An embodiment of the invention will now be explained with reference to the accompanying drawings, in which: Fig. 1 shows a circuit arrangement of that portion of a line circuit to which the invention relates; Fig. 2 shows a circuit arrangement of the direct voltage-controlled AC source shown as a block in Fig. 1, and Fig. 3 shows a circuit arrangement for the controllable voltage source indicated as a block in Fig. 1, the voltage-reference source, and the high-pass filter.

In Fig. 1 a andb are the subscriber lineterminals to which the telephone set T is connected via the line.

When the handset of the telephone T is off-hook, the loop has the impedance Z1.

A controllable supply-voltage source Q1 has its output B coupled to the wire a through a feed resistorW1. It provides at its output B a fixed supply potential derived from the central office battery U under control of a signal applied to an input S. The control input S of the source Q1 is connected to a voltage-reference source UR via a decoupling resis torW2. This control input is also coupled to the wire a through a high-pass filter F, which has a pole (cutoff frequency) at the lower end of the frequency band but above the interfering frequency to be expected. The main interferences are due to power lines acting on the line circuit, i.e. by the public supply mains, having a frequency of 50 Hz, and in some countries by the power supply of an electric traction system of 16 2/3 Hz.The filter pole is advantageously at a frequency of 300 Hz.

Toward the central office, th-e two-wire line a, t is divided into two branches, an outgoing branch (neitherthis branch nor the electronic hybrid circuit is shown here), and an incoming branch, whose input E is connected to the wire a via a direct voltage-controlled AC source Q2 and a balancing network Z2. The DC control of this source is accomp lished with an RC section W3, W4, C.

Elements corresponding to those just described are provided for the wireb; their reference characters are marked with a prime. The input E from the four-wire side is also connected to an AC source Q2' via an inverter I, which phase shifts the incoming signal by 1809 Each of the balancing network2 and Z2' preferably has half the value of the impedance Z1. Each of the feed resistors W1 and W1' preferably has a value of 150 ohms, while the decoupling resistors W2, W2' may each have a value of about 100 kohms. The resistors of the RC sections have high values.

The circuit works as follows. First, DC operation will be described. By meansofthevoltage-reference source UR, the supply potential at the output B of the source Q1 is so adjusted that for the longest line to be expected, a minimum permissible supply current flows. For a maximum loop resistance of 2,240 ohms, for example, a minimum supply current of 17.5 mA must flow. Thus the loop must be supplied with 40 V, in which case about 2.5 V is dropped across the 150-ohm feed resistor Thus a supply potential of-52.5 V must be set at the output B which value is controlled by the voltage-reference source UR and a supply potential òf -8.5V must be set at the output B' under control of the voltage-reference source UR'.Slow changes in the line supply potential of -50 V do not affect the control input S through the high-pass filter F, because the latter does not pass such low-frequency changes.

The potential of the wire a is applied to the input of the AC source Q2 through resistors W3 and W4, speech frequencies being filtered out by the capacitor C, so that only slow changes such as variations in supply potential, reach the input E. Thus the alternating voltage source Q2 is so adjusted that the same direct potential as that of the wire a appears at its output A. Thus no direct current can flow through the balancing network Z2. The supply potential on the wire b is adjusted in the same manner. The output B' of the source Q1' provides a supply potential of -8.5 V, while the wire b has the line supply poten tial of-1 1 V connected thereto, which also appears at the output A' of the AC source Q2'. Thus the balancing network2' is not traversed by direct current either.The supply circuit and the incoming four-wire AC circuit are thus DC-isolated from each other Since slow changes in line supply potential have no effect on the control input of the respective sup ply source, the internal output resistance of the AC source forms a short circuit for such changes. The interfering voltages, which may also be unsymmet rical, are thus greatly reduced by the feed resistors W1,W1'. The interpretability of the dial pulses (loop-disconnect signals) is not adversely affected by the circuit arrangement. There are no pulse distortions. The dial-pulse-interpreting circuits (not shown) are connected to the wiresa, b.

AC operation is as follows. Speech signals from the subscriber pass through the filters F, F' to control inputs S, S' and are copied via the supply-voltage sources Q1, Q1'to the output B, B', so that equal alternating potentials appear at the points a, B and the pointsb, B', respectively, and no alternating current can flow through the feed resistors W1, W1'.

The supply circuits thus offer high AC resistances, so that the speech signals are not attenuated by these circuits.

A speech signal arriving at the incoming four-wire input E is amplified by the AC source Q2 and applied to the wire a; it is also shifted in phase, and amplified by the AC source Q2', and applied to the wireb. In this connection it is particularly important that the relatively high-level metering signal is also transmitted over this path. For transmiting the metering signal, which usually has a frequency of 16 kHz, a large control range of the AC sources Q2, Q2' is needed. This large range is insured particularly since the same direct potential as that on the wire a appears at the output A, while the same direct poteitial as that on the wire b appears at the output A', so that an extension of the control range by the amount of the voltage drops across the feed resistors W1, W1' is obtained. The metering signal is not attenuated by the supply circuits (Q1, Q1'), either.

Fig. 2 shows the direct voltage-controlled AC source Q1 of Fig. 1. In addition,theelementsZ2,W3, W4, and C are shown in the same configuration as in Fig. 1. The incoming four-wire input E is connected to the resistor W4 and to the positive input of an operational amplifierVl whose negative input is connected to the point A. The output of this amplifier is connected to the point A via a resistor W7 and to the bases of an npn transistorT1 and a pnp transis torT2. These transistors form a push-pull stage connected in the emitter-follower configuration. The emitter of the transistor T1 is connected via a resistor W5 to the point A, which is also connected to the emitter of the transistor T2 via a resistor W6.The collector of T1 is at the direct potential -U/2, while the collector of T2 is at a direct potential -U. The amplifier V1 is adjusted from the wire a via the highly resistive RC section W3, W4, C such that the point A is at the same direct potential as the wire a.

The alternating voltage dropping across the resistor W7 is amplified by T7, T2 and applied via the point A and the balancing network Z2 to the wire a. The internal output resistance of the push-pull stage is very low, so that only Z2 + Z2' is seen as the line terminating impedence.

The AC source Q2' is similar. The collector of the corresponding transistorTl is at zero potential instead of the potential -U/2, while the collector of the corresponding transistorT2 is at the potential -U/2 instead of the potential -U.

Fig. 3 shows the supply-voltage source Q1 of Fig.

1, the voltage-reference source UR, and the highpass filter F. The source Q1 includes an operational amplifier V2 and a push-pull stage with a pnp transistor T3 and an npn transistor T4 and connected in the configuration of Fig. 2. This includes the emitter resistors W8, W9, and the base resistor W1 0. The voltagereference source UR is formed by a voltage divider consisting of resistors W1 3 and W14, to which the potentials-U and-U/2 are connected and a resistor W12, which connects the tap of the voltage divider to the control input S of the supply-voltage source, i.e. to the positive input of the operational amplifierV2. The high-pass filter F is a second-order filter using positive feedback and contains an operational amplifier V3, two capacitors C1, C2, and a resistor W11. The wire a is connected to the positive input of the amplifier V3, whose output is coupled to its negative input and, via the series combination of the capacitors C1, C2, to the control input S. The junction of these capacitors is coupled to the point B through the resistor W1 1.

At the control input S a reference voltage is applied to the operational amplifier V2 whose value is such that the push-pull stage T3, T4 provides a fixed direct potential (-52.5V) to the point B. The AC signals above 300 Hz pass through the filter V3, C1, C2, W1 1 to the control input S and so control the operational amplifier V2 and the push-pull stage T3, T4 that the alternating voltage level at the point B is the same as that of the wire a.

The elements Q1', UR' and F' forthe wireb are analogous to those of Fig. 3, with the potentials-U/2 and -U being again replaced by zero potential and the potential -U/2, respectively.

As can be seen from the circuit arrangements, no high-value capacitors are used; because of the heavy degeneration of the operational amplifiers, no close-tolerance components are required.

Therefore, the arrangements can be readily implemented using integrated-circuit techniques.

Claims (6)

1. A circuit arrangement for a telephone subscriber's line circuit includes two controllable supply-voltage sources each associated with one speech wire and each having a single input connected to its said speech wire, in which each said supply voltage source also has its said input connected via a resistor to a voltage-reference source and a single output connected to its said speech wire via an associated feed resistor, in which a feed circuit including amplifier elements is inserted in the incoming four-wire path, in which the feed circuit consists of two direct voltage-controlled alternating voltage sources individually associated with the speech wires, in which each said alternating voltage source has its output coupled to its associated speech wire through a resistive impedence, in which each alternating voltage source is direct voltage controlled in such a way that the direct voltage level of the associated speech wire equals that of the output of the associated alternating voltage source, and in which the input of each said controllable supplyvoltage source is connected to its associated speech wire via a high-pass filter whose pole lies at the lower end of the speech-frequency band but above the interfering frequencies to be expected.

2. A circuit arrangement as claimed in claim 1, in which each said alternating voltage source includes an operational amplifierfollowed bya push-pull stage connected in the emitter-follower configuration, and in which the associated speech wire is connected via a speech-frequency-rejecting RC section to the positive input of the operational amplifier, which is also fed with the signal of the incoming four-wire branch.

3. A circuit arrangement as claimed in claim 1 or 2, and in which each said controllable supply-voltage source consists of an operational amplifier followed by a push-pull stage connected in the emitterfollower configuration.

4. A circuit arrangement as claimed in claim 2 or 3, in which each said high-pass filter has an operational amplifier, two capacitors and a resistor, in which the positive input of that operational amplifier is connected to the associated speech wire, in which the output of that operational amplifier is connected to its negative input and, via the series combination of the capacitors to the control input of the controllable supply-voltage source, and in which the juno tion of these capacitors is connected to the output of the supply-voltage source via the resistor.

5. A circuit arrangement as claimed in claim 4, and in which the voltage-reference source is formed by a voltage divider consisting of two resistors connected to the operating-voltage source and a resistor connected between the tap of the voltage divider and the control input of the supply-voltage source.

6. A circuit arrangement for a telephone subscriber's line circuit, substantially as described with reference to the accompanying drawings.