TECHNICAL FIELD

The present invention relates to an apparatus in the current supply of a .subscriber's telephone set from a telephone exchange.

BACKGROUND ART

In telephone installations the subscriber sets are connected to the exchange by so-called subscriber lines, which are terminated in the exchange at an adaption

5 circuit, or subscriber line interface circuit (SLIC). Such a circuit is used, inter alia, to supply DC current to the associated subscriber apparatus.

*

A fixed DC voltage may be utilized for this current supply in the prior art. In such a case, the current flowing through the subscriber line will be a function of the internal resistance of the subscriber apparatus or station, and of course the ^U line length. The internal resistance in the apparatus can be kept within narrow limits, but the line length and line resistance may vary considerably. If a fixed DC voltage is used, this must be given a value such that sufficient current can be fed out on long lines as well.

In modern line circuits, the supply impedances are often simulated with the aid 5 of feed-back amplifiers. The physical supply resistances are low-ohmic and the power generation in the resistance is low. In order to reduce the power generation in the output step of the amplifier as well, it is known to utilize controllable DC/DC converters, e.g. so-called choppers, instead of batteries in the voltage supply.

Two principles are described in the litterature for the impedance simulation, c.f. PROCEEDINGS OF THE IEEE, VOL. 68, NO 8, AUGUST 1980, pages 991- 1009, for example.According to one principle, the line current is sensed to form an instant value in a control system controlling the line voltage. This method gives simple conditions for working point setting of the participating amplifiers and control of DC/DC converters. However, the method has the considerable

disadvantage that the amplifier which has floatingly to sense the line current out on the line will be difficult to achieve with sufficient precision.

- According to the other method, the line voltage is sensed and the control system controls the line current. This method gives more simple conditions for realizing the sensing amplifier, but on the other hand it gives difficulties in ' setting the working points of the amplifiers and control of DC/DC converters.

In electronic subscriber line circuits according to the above, the amplifiers require a supply voltage for their function which exceeds by a given amount the transversal line voltage, i.e. the voltage between the outputs of the amplifiers. If this condition is not met, there is the risk of the signal voltage being cut. This relationship defines the maximum length of aline which can be connected to the line circuit without risk of transmission degeneration. Consideration

* must also be taken here to the fact that the battery voltage can vary considerably, depending on the state of charge, load etc.

DISCLOSURE OF INVENTION

In the present invention, the line-feeding two-pole is implemented with the aid of two analogue amplifiers, through which the combined DC current and current representing the speech signal are fed outoπ the line. The DC line feed voltage, as with the internal voltage supply to said amplifier, is taken from a chopper or a battery. The inventive principle is, however, also applicable to systems utilizing two choppers per line. It is also applicable to the arrangement where the mean potential at one output terminal of the subscriber line circuit is kept at a constant level by a control system, not illustrated here.

The technical problem which is solved by the present invention resides in preventing the amplifiers from becoming saturated and thus cutting the signal voltage, even if a very long line were to be connected to the line circuit and the battery voltage vary.

The solution of this problem is characterized as will be apparent from the characterizing portion of claim 1.

OMH

An advantage of the invention is that the available battery voltage is optimally utilized at every instant. The telephone system thus functions in a desired manner for an optimum line length interval and for line lengths exceeding this interval the system still functions, although not as well from the quality aspect.

'BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described in detail with reference to the accompa¬ nying drawing, whereon

Figure 1 illustrates an electronic subscriber line circuit with voltage sensing and current supply, Figure 2 illustrates the current-voltage characteristic for the circuit according to figure 1,

Figure 3 illustrates an embodiment of the invention,

Figure 4 illustrates the current-voltage characteristic for the circuit according to figure 3, Figure 5 illustrates an electronic subscriber line circuit with current sensing and voltage supply,

Figure 6 illustrates the current-voltage characteristic for the circuit according to figure 5,

Figure 7 illustrates an embodiment of the invention, Figure 8 illustrates the current-voltage characteristic for the circuit according to figure 7.

BEST MODES FOR CARRYING OUT THE INVENTION

The principle of the invention will be illustrated below, as applied to a subscriber line circuit, for which the voltage supply to the line circuit is represented by the symbol for a fixed DC voltage. In this connection, however, the symbol must also stand for system solutions with one or two DC/DC converters connected in series with the source of supply.

Figure 1 illustrates an electronic subscriber line circuit with voltage sensing and current supply. A condition for the continued discussion here is that that the point a in the figure, i.e. the a-terminal of the subscriber line circuit, is

given a fixed mean potential with the aid of a control system not shown here. This system can be implemented as will be apparent from the Swedish patent application SE 82.04185-6.

Via its a and b terminals, the subscriber line circuit supplies a load, e.g. in the 5 form of a telephone set with line, symbolized here by a resistor with resistance .R. . Each terminal is connected to the output of an analogue amplifier 2,3, which is coupled as a current source with the current gain factor A.. Via its final step, each of these amplifers can drive positive or negative current out on the line to the load from an earth-connected voltage terminal on the amplifier,

10 and a voltage terminal connected to the minus pole on a plus-earthed voltage source 7 with the voltage E _R . The output current from the amplifiers 2,3 is controlled via their inputs, to which two current generators 5,6 are connected via a lowpass filter 4. The current generator 6 is a fixed reference current source, supplying the current I , while the generator 5 is a voltage-controlled

15 current source with the transconductance G . A differential amplifier 1 is arranged to sense the transversal line voltage U, , and via its output to control the current generator 5. The following is applicable for the line current I, :

= ^A i W V"

Figure 2 illustrates I, as a function of U-, . It will be seen that the subscriber

20 line circuit simulates a resistive supply with a battery voltage equal to /G. and a supply resistance equal to 1/A.G... When the transconductance G. is equal to zero there is obtained constant current supply and a horizontal characteris¬ tic. If the voltage E _β were to drop, for reasons described above, there may be problems concerned with clipping the signal voltage for long lines, i.e. for low 25 line current I, .

Figure 3 illustrates an embodiment of the invention: A non-linear comparator 8 with a current output is arranged parallel to the current sources 5,6 and drives current in the same direction as the source 5. The comparator has two inputs. One input is supplied the voltage -U-, and the other a voltage -E _B + LI,- is an ^****** internal voltage reference in the comparator, here depicted as an external voltage source. The comparator is such that for

its output current will be

^lim^B^lim)

where G,. is the transconductance for the comparator. The output current is lim equal to zero for remaining values of the voltage U-, .

Analogue with the preceding there thus obtained for

^U 1 ^>E B- ^U lim

^U r ^G t- ^(U l- ^E B ^+U lim ^)G lirr?

This function is illustrated graphically in figure 4. The characteristic has a break point which is at a distance U.. from the battery voltage E _β , thus following the variations of the battery voltage. The control system, which keeps the potential on the a-terminal constant, is dimensioned such that this potential will be equal to -U.. /2, and furthermore, if the voltage U,. is put equal to the voltage with which the actual battery voltage must exceed the actual transverse voltage supplemented by the slope of the characteristic for ensuring that the amplifiers so not become saturated,the above-mentioned problem is avoided.

The inventive principle, which has been illustrated here on an unsymmetric system, may also be realized in a system with a symmetric supply.

An electronic subscriber line circuit with current sensing and voltage supply is illustrated in figure 5. In the dual method, the output current is thus sensed from the circuit by a combination of two transconductance amplifiers 1 and an adding circuit 10. The transfer impedances for the amplifiers 1 is equal to R _fc . In this case the analogue amplifiers are two voltage amplifiers with the gain factor A . These amplifiers are supplied on the input side by two series- connected voltage sources, a fixed reference voltage source with the voltage U and a voltage source controlled by the output current from the circuit. The

output voltage from the voltage source 5 is equal to I^-R**.- The following is applicable for these conditions:

Figure 6 illustrates this relationship graphically.

An implementation of the inventive idea for the dual system according to figure 5 is illustrated in figure 7. In this case the comparator 8 has a voltage output and adds a voltage to the inputs of the amplifiers 2,3. When U-,>E _B -U,. with the same denotations as previously, the comparator output voltage U. is

U _k =k(U _r E _B+ U _Um )

otherwise U, =0 k

U-, may then be written

This relationship is illustrated graphically in figure 8. In this case also, the characteristic has a deflection point, the position of which follows the instant battery voltage E. at the distance U.. , and this relationship gives the advantages mentioned above.

By putting a lowpass filter 4 between the inputs of the amplifiers 2,3 and the outputs from the current- and voltage sources, respectively, which control the output magnitude from these amplifiers, the line circuit from the terminal side is made independent, seen from the signal aspect, of the characteristic being given a deflection point in accordance with the inventive idea.