The invention relates to a method for increasing onhook voltage in a subscriber loop, said method being employed in a subscriber interface unit, said subscriber loop comprising lines, and at least one terminal, and said subscriber interface unit comprising adapter means for adapting said lines for the subscriber interface unit, a battery voltage being generated by a battery voltage source.

The solution of the invention can be used in telecommunication equipments, particularly in subscriber signalling equipments, which in some cases require that onhook voltage in a subscriber loop be increased. Onhook voltage refers to the voltage that occurs between subscriber lines when the handset of a terminal, e.g. a telephone, is on its mounting. Offhook voltage refers to the voltage that occurs between subscriber lines when the handset of a terminal, e.g. a telephone, is lifted from its mounting. An offhook situation occurs in the system when offhook voltage prevails. Subscriber signalling equipments are normally connected to an operating voltage, or battery voltage, of e.g. -48 V. However, the battery voltage used is not always high enough for generating a sufficiently high onhook voltage to a subscriber loop. In addition, there are telephone exchanges which require a higher voltage for a subscriber loop than is generated by means of the normal battery voltage. The object of the invention is thus to increase onhook voltage.

A previously known solution for increasing onhook voltage is based on the use of two separate battery voltages. For this reason, the prior art solution is complicated and expensive, and takes up a lot of space. Moreover, the components used in the arrangement have high power consumption.

The object of the present invention is to provide a new type of solution for increasing onhook voltage in an onhook situation, which avoids the problems associated with the prior art. The object is achieved with a method of the type described in the introductory portion, the method being characterized in that the onhook voltage supplied from the adapter means to the terminal is increased by an additional voltage generated by said battery voltage source.

A further object of the invention is an arrangement for increasing onhook voltage in a subscriber loop, said arrangement being employed in a subscriber interface unit, said subscriber loop comprising lines, and at least

one terminal, and said subscriber interface unit comprising adapter means for adapting said lines for the subscriber interface unit, said arrangement comprising a battery voltage source for generating a battery voltage.

The arrangement of the invention is characterized in that the arrangement comprises an additional voltage source connected to the battery voltage source for adding an additional voltage to the battery voltage in an onhook situation.

The method and arrangement of the invention have many advantages. The invention allows the onhook voltage of a subscriber loop to be increased using only one battery voltage. In addition, the solution of the invention allows a constant voltage of the right magnitude to be provided in a subscriber loop in an onhook situation. The solution of the invention is simple and has low power consumption. On account of the small number of components, the solution is also inexpensive. In addition, the solution of the invention takes up less space than the solution of the prior art.

In the following, the invention will be described in greater detail with reference to the examples illustrated in the accompanying drawings, in which

Figure 1 shows a preferred switching diagram of the arrangement of the invention, and Figure 2 shows another preferred switching diagram of the arrangement of the invention.

Figure 1 shows an arrangement of the invention for increasing onhook voltage Von in an onhook situation. The arrangement comprises adapter means 1 , a battery voltage source 4, and a subscriber loop B. The subscriber loop B comprises lines a and b, and a terminal 3 connected to the lines a and b. In the example illustrated, the terminal is a telephone. The adapter means 1 adapt the lines a and b for the subscriber interface unit. The adapter means 1 are preferably implemented with a subscriber line interface circuit SLIC. The SLIC can be, for example, the commercial circuit Am79M574. The arrangement further comprises an additional voltage source 2. In the example of Figure 1 , the additional voltage source 2 is implemented with a commercial DC/DC converter or with a switching mode power supply. The additional voltage source 2 comprises a primary side 2a and a secondary side 2b. The primary side 2a of the additional voltage source 2 is connected to the battery voltage source 4, whereas its secondary side 2b is connected to the adapter means 1.

In the arrangement of Figure 1 , battery voltage Vbat, which is typically -44 V to -54 V, occurs in the battery voltage source 4. The magnitude of the battery voltage Vbat has an effect on the magnitude of the onhook voltage Von occurring in the subscriber loop B. The additional voltage source 2, which is connected to the battery voltage source 4, generates an additional voltage Vad from the battery voltage Vbat. The generated additional voltage Vad is added to the battery voltage Vbat generated by the battery voltage source 4, whereby the onhook voltage Von supplied to the subscriber loop B through the adapter means 1 increases in proportion. In the arrangement of Figure 1 , a subscriber terminal 3, which in practice is a telephone machine, is connected between the lines a and b.

In the solution of Figure 1 , the battery voltage Vbat supplied from the battery voltage source 4 to the primary side 2a of the additional voltage source 2 is negative. The battery voltage Vbat may, however, also be positive; in this case, the coupling must be adjusted to correspond to the polarity of the battery voltage Vbat. The additional voltage source 2 converts the negative battery voltage Vbat supplied from the battery voltage source 4; as a result of this conversion, a criterion voltage Vkr of -60 V occurs at point A between the secondary side 2b of the additional voltage source 2 and the adapter means 1. Figure 2 shows an arrangement comprising adapter means 1 , an additional voltage source 2, a criterion voltage source 4 and a subscriber loop B. The subscriber loop B comprises lines a and b, and a terminal 3 connected to the lines a and b. In addition, the arrangement comprises current limiting means 5, and diodes 6, 7. The current limiting means 5 are connected to the adapter means 1 by means of diode 6: the anode of diode 6 is connected to the adapter means 1, and the cathode of diode 6 is connected to the current limiting means 5. In practice, the current limiting means 5 are implemented with a constant current generator. The anodes of the diodes 6, 7 are also connected to each other. The cathode of diode 7 is connected to the battery voltage source 4 and the additional voltage source 2. The diodes 6, 7 have been selected in such a manner that they endure a current of about 50 mA, and a voltage of over 60 V. A subscriber terminal 3, in this case a telephone, is connected between the lines a and b extending from the adapter means 1.

It can be seen from Figure 2 that the battery voltage Vbat generated by the battery voltage source 4 is typically in the range of -44 V to -54 V. The additional voltage source 2 and the current limiting means 5 shown in Figure 2

allow the voltage applied to the adapter means 1 in an onhook situation to be increased. At the same time, the onhook voltage Von occurring in the subscriber loop B in an onhook situation also increases. In the arrangement of the invention, the onhook voltage Von is thus higher than onhook voltage generated merely by the battery voltage source 4. The additional voltage Vad generated by the additional voltage source 2 is switched on only in an onhook situation. The current limiting means 5 comprise a voltage regulation circuit 8 for keeping the criterion voltage Vkr constant at point A between the diodes 6, 7 and the adapter means 1 , even if the battery voltage Vbat generated by the battery voltage source 4 should oscillate. By means of the regulation circuit 8, it is ensured that the maximum supply voltage of the adapter means 1 is not exceeded at point A. If disturbance voltages, such as ripple voltage, occurs in the battery voltage generated by the battery voltage source 4, the regulation circuit 8 keeps the voltage Vkr at point A constant. In the arrangement illustrated in Figure 2, a current I is supplied through the adapter means 1. The current limiting means 5 are designed so as to take the amount I of current in an onhook situation, i.e. the same amount as is taken by the adapter means 1. In this case, the voltage occurring at the anodes of the diodes 6, 7, at point A, is the sum of the battery voltage Vbat generated by the battery voltage source 4 and the additional voltage Vad generated by the additional voltage source 2. In an offhook situation, most of the current supplied through the adapter means 4 passes through diode 7, whereby the additional voltage Vad generated by the additional voltage source 2 is not added to the battery voltage Vbat generated by the battery voltage source 4. In this case, only the battery voltage Vbat occurs at point A.

If the nominal value of the battery voltage Vbat is for instance -48 V in the arrangement of Figure 2, the voltage at point A, regulated by means of the regulation circuit 8, is -60 V. If the desired magnitude of the onhook voltage Von in an onhook situation is -44 V, for example, and the battery voltage Vbat used is -44 V, the voltage required at point A is -60 V. In this case, the additional voltage source 2 generates an additional voltage Vad, which is about -18 V. If the additional voltage Vad and the battery voltage Vbat are simply added together, the sum obtained is -62 V. However, about 2 V of this voltage goes to various voltage drops. The voltage occurring at point A in Figure 2 is thus -60 V, which allows an onhook voltage of -44 V to be generated in an onhook situation.

Although the invention has been described above with reference to the example illustrated in the accompanying drawings, it will be clear that the invention is not limited to this example, but it can be modified in many ways within the scope of the inventive concept disclosed in the appended claims.