Abstract:
An apparatus for transmitting charging signals on a data transmission path having a line impedance includes a driver device for setting a line voltage level that corresponds to the line impedance. The driver device is configured to provide, in response to a driver input voltage level, a driver output current and a driver output voltage level. A current detection unit provides a current signal indicative of the driver output current and a current signal matching unit receives the current signal from the current detection unit and adjusts a level of the current signal for further processing by a filter device and by a regulation device. A matching filter unit then matches the driver device to a data transmission path unit.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of the Sep. 24, 2001 priority date of German application 101 46 891.1. 
     1. Field of Invention 
     The present invention relates to a method for transmitting charging signals via a data transmission path, and particular to a method in which a line voltage level can be predetermined. 
     2. Background 
     Many different methods have been used for transmitting data via conventional telephone lines, for example via conventional double-cored copper wires. These methods are referred to as DSL (digital subscriber line) methods. 
     An asymmetric DSL method (ADSL=asymmetric digital subscriber line) is particularly widely used, in which matching filter units, also referred to as splitter filters, are used at the exchange end and subscriber end, in order to separate conventional telephony (POT=plain old telephony) and ADSL applications for data transmission. 
     In the asymmetric DSL method, data is transmitted asymmetrically, that is to say at different speeds depending on the direction. Data is typically transmitted at a data transmission rate of 8 Mbit/s from a switching center to a subscriber point, while data is transmitted from a subscriber end to an exchange end (upstream) at a maximum of 1 Mbit/s. 
     It should be mentioned that the data transmission rate of both downstream and upstream is dependent on the line length. One major advantage of using ADSL methods is that existing cable networks, for example double-cored copper lines, conventional telephone lines, etc., can still be used for data transmission. 
     In order to charge or bill for telephone calls when using data transmission lines, so-called charging signals are transmitted at specific frequencies, typically at frequencies of 16 kHz or 12 kHz from the switching center to the subscriber point. 
     In this case, voltage levels within permissible tolerances are specified for the charging signals, and these may vary between different countries. In the process, it should be noted that the required voltage levels for the charging signals (also referred to as teletax signals) must be designed to be independent of the respective line impedance of one or more data transmission path units. 
     An estimate of a driver input voltage level at the input of a driver circuit is made in a conventional manner, and is compared with a desired nominal value of a driver input voltage level. An amplitude and a voltage level for the charging signal are varied in a control unit of a regulation device until the nominal value is reached, or until the control error is 0. 
     For conventional POTs applications, it is sufficient to regulate the driver input voltage level at the input of the driver device, since any voltage drop across a downstream series resistance element or downstream circuitry, in particular downstream protection circuitry, is negligible. 
     One disadvantageous feature is that conventional methods for voltage level production for charging signals cannot be used for ADSL methods that are operated via POTs applications since voltage levels that do not conform with the standards occur on the line and at the at least one data transmission path unit. This is a result of the fact that it is no longer possible to ignore any influence of external circuitry resulting, for example, from a matching filter unit (also referred to as a splitter filter unit), a transformer for ADSL data transmission, impedance synthesis of the subscriber line interface circuit (SLIC). 
     A further disadvantage is that the overall circuitry is highly sensitive to load changes on the line due to the interaction of the matching filter unit with the ADSL data transmission path and/or with the at least one data transmission path unit. 
     It is thus disadvantageously not sufficient just to keep a driver input voltage level at the input of a driver device constant. 
     SUMMARY 
     One object of the present invention is thus to regulate a voltage level directly on the transmission line, that is to say a line voltage level, with a defined driver output voltage level being produced by a driver device. 
     One major idea of the invention is to determine a line voltage level on the basis of a detected output current from the driver device as well as known series resistance elements, series reactive elements, parallel resistance elements and parallel reactive elements of the at least one matching filter unit, and on the basis of known data transmission path capacitances, data transmission path inductances and data transmission path resistances of the at least one data transmission path unit. 
     Furthermore, it is necessary to eliminate any influence of a complex (frequency-dependent) gain factor of an amplifier unit which is provided in the driver device. In addition, any influence of filter parallel currents and line parallel currents must be compensated for by the matching filter unit and/or the data transmission path unit, so as to ensure insensitivity to load changes on the transmission line. 
     The method according to the invention for transmitting charging signals via a data transmission path essentially has the following steps: 
     a) determination of an output current, which is produced by the driver device, by means of a current detection unit; 
     b) determination of a transfer function of the data transmission path for at least one frequency, at which the charging signals are to be transmitted from a switching center to a subscriber point, with any influence of the frequency-dependent gain factor of an amplifier unit in the driver device having to be eliminated; 
     c) entry of a nominal value in a nominal value comparison unit of a regulation device in order to provide a control signal; 
     d) modification of the output current such that the entered nominal value matches the output signal of a determination unit, which output signal corresponds to the line voltage level at that time; and 
     e) emission of a driver output voltage level by the driver device, which voltage level predetermines the line voltage level. 
     The dependent claims contain advantageous developments and improvements of the respective subject matter of the invention. 
     According to one preferred development of the present invention, the charging signals are produced as sinusoidal signals, which can be transmitted via the data transmission path unit. 
     According to yet another preferred development of the present invention, the charging signals are transmitted at frequencies, which can be predetermined, from the switching center to the subscriber point. The frequencies of 16 kHz and 12 kHz are advantageously used via conventional telephone lines. 
     According to yet another preferred development of the present invention, the required driver output voltage level is produced independently of the line impedance of the data transmission path. 
     An overall voltage drop across the matching filter unit and the at least one data transmission path unit is advantageously determined, so that a corresponding driver output voltage level can be predetermined. 
     According to yet another preferred development of the present invention, a feedback resistance element and a feedback reactive element in a feedback branch of an amplifier unit in the driver device define a frequency-dependent gain factor, such that a constant line voltage level, which can be predetermined, is maintained for at least one frequency, at which the charging signals are to be transmitted. 
     According to yet another preferred development of the present invention, the overall voltage drop, which can be predetermined, is produced in order to set the line voltage level as a function of the driver output voltage level. 
     According to yet another preferred development of the present invention, a filter parallel current, which can be predetermined, is produced, and can be set as a function of series resistance elements, series reactive elements, parallel resistance elements and parallel reactive elements in the matching filter unit. 
     According to yet another preferred development of the present invention, a filter series current, which can be predetermined, is produced, which, together with the filter parallel current, represents the output current from a modified driver device. 
     The apparatus according to the invention for transmitting charging signals furthermore has: 
     a) a driver device for producing an output current and a driver output voltage level as a function of a driver input voltage level, in which a line voltage level can be set on a line impedance; 
     b) a current detection unit for determining the output current which is emitted from the driver device to the matching filter unit and to the at least one data transmission path unit; 
     c) a current signal matching unit for matching a current signal, which is emitted from the current detection unit, in order to produce a matched current signal, so that further processing can be carried out in an advantageous manner in a filter device and in a regulation device; and 
     d) a matching filter unit for matching the driver device to at least one data transmission path unit. 
     Exemplary embodiments of the invention will be explained in more detail in the following description and are illustrated in the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of a circuit arrangement for transmitting charging signals for conventional transmission of charging signals via conventional telephone lines; and 
         FIG. 2  shows a block diagram of a circuit arrangement for transmitting charging signals for ADSL methods, according to one exemplary embodiment of the present invention. 
     
    
    
     In the figures, identical reference symbols denote identical or functionally identical components or steps. 
     DETAILED DESCRIPTION 
     In the block diagram, as shown in  FIG. 1 , of a circuit arrangement for transmitting charging signals, three major blocks are shown, that is to say a driver device  201 , a filter device  109  and a regulation device  115 . An output current  100  which is emitted from the driver device  201  is supplied to a data transmission path, in this case represented by a series resistance element  124   a , with a driver output voltage level  108  with respect to a ground connection  133  being produced at the output of the driver device  201 . The output current  100  causes an overall voltage drop  136   a  across the series resistance element  124   a , so that a line voltage level  101  across a line impedance  102  which is connected in series with the series resistance element  124   a  is reduced in accordance with a voltage divider, that is to say the driver output voltage level  108  corresponds to the sum of the overall voltage drop  136   a  and of the line voltage level  101 . The line series current  135  flowing through the line impedance  102  corresponds, in the block diagram shown in  FIG. 1 , to the output current  100  from the driver device  201 . 
     The driver device  201  will be explained in more detail in the following text. The major component of the driver device  201  is an amplifier unit  104 , which may, for example, be in the form of an operational amplifier. A current detection unit  103  is connected to the output of the amplifier unit  104  and supplies a current signal  204  which corresponds to the output current  100 , so that the output current  100  is detected precisely. 
     The current detection unit  103  may, for example, be in the form of a Hall sensor. Furthermore, the current detection unit  103  may be produced by means of a shunt resistance element, with a tap at the connections of the shunt resistance element producing a voltage drop which is proportional to the output current and can be used as a current signal  204 . The current signal  204  is supplied to a current signal matching unit  203 , in which a level of the current signal can be adapted in order to obtain a matched current signal  205 , which is supplied to the filter device  109 . 
     As is illustrated in the block diagram of the circuit arrangement for transmitting charging signals, the amplifier unit  104  may have a feedback branch comprising a feedback resistance element  105  and a feedback reactive element  106 , thus resulting in a modified driver device  202 . The units of the filter device  109  and of the regulation device  115 , which are shown in  FIG. 2 , correspond to the units which are shown in  FIG. 1 . 
     It should be mentioned that the feedback resistance element  105  and the feedback reactive element  106  in the feedback branch of the amplifier unit  104  in the modified driver device  202  may be formed not only by passive elements (such as resistance elements R, inductive reactive elements L and capacitive reactive elements C), but also by active elements. According to the invention, the two elements which are arranged in the feedback branch, that is to say the feedback resistance element  105  and the feedback reactive element  106 , are used to eliminate any influence of a complex, frequency-dependent gain factor of the amplifier unit  104 , in order to eliminate any influence of parallel currents which can occur in a matching filter unit  123 . 
     The data transmission path unit  122  and the matching filter unit  123  will be explained in more detail in the following text with reference to  FIG. 2 . In the illustrated exemplary embodiment of the present invention, the matching filter unit  123  comprises a series resistance, which is formed by a series resistance element  124  and a series reactive element  125 , in the illustrated case a series reactive inductance, while, in contrast, a parallel resistance is formed from a parallel resistance element  126  and a parallel reactive element  127 , in this case a parallel capacitance. 
     The series resistance is arranged between an input connection and an output connection of the matching filter unit, while the parallel resistance is arranged between the input connection and a ground connection  133 . The currents which occur in the matching filter unit, that is to say a filter series current  131  and a filter parallel current  132 , in total form the output current  100  of the modified driver device  201 , while the magnitudes of the filter series current  131  and of the filter parallel current  132  depend on the circuit elements  124 ,  125 ,  126  and  127  which are used in the matching filter unit  123 . The output connection of the matching filter unit  123  is connected to an input connection of the data transmission path unit  122 . 
     A parallel current, referred to as a line parallel current  134 , once again occurs in the data transmission path unit, so that the filter series current  131  flowing through the matching filter unit  123  is modified to a line series current  135  such that the filter series current  131  forms the total of the line parallel current  134  and the line series current  135 . 
     The line parallel current  134  flows from the input connection of the data transmission path unit  122 , via a data transmission path capacitance  128  and via a parallel circuit comprising a data transmission path inductance  129  and a data transmission path resistance  130 , to the ground connection  133 . 
     The line series current  135  flows through the line impedance  102 , which has already been described with reference to  FIG. 1 , as a result of which a voltage drop occurs across the line impedance, that is to say a line voltage level  101  is produced, which can be tapped off between an output connection of the data transmission path unit  122  and the ground connection  133 . 
     As illustrated in  FIG. 2 , the driver output voltage level  108  which is produced by the modified driver device  202  is reduced by an overall voltage drop  136 , which is dropped across the series circuit comprising the data transmission path unit  122  and the matching filter unit  123 . 
     A driver output voltage level  108 , which is reduced by the overall voltage drop  136 , is produced as the line voltage level  101 , as a function of the circuit components of the data transmission path unit  122  and of the matching filter unit  123 . 
     The blocks  109  and  115 , which are arranged identically in  FIGS. 1 and 2 , will be described in more detail in the following text, that is to say the filter device  109  and the regulation device  115 . 
     It should be mentioned that the devices  109  and  115 , respectively, which are shown in  FIGS. 1 and 2  have identical structures, but use different determination methods for defining the output current of the respectively corresponding driver devices  201  and  202 . The regulation device  115  operates as a digital regulation device, while all the other circuit components, including the filter units  110 ,  112 , operate in the analog domain. It can clearly be seen that analog/digital conversion is thus required, in an analog/digital converter  112 , for signals which are passed from the filter device  109  to the regulation device  115 . 
     Conversely, it is necessary for signals which are passed from the regulation device  115  to the filter device  109  to be converted from the digital domain to the analog domain in a digital/analog converter  113 . 
     The matched current signal  205  is supplied via an input connection of the filter device  109  to an input filter unit  110 , which is used as an anti-aliasing filter, with the output signal from the input filter unit  110  being supplied to the analog/digital converter  111 . The digitized output signal from the analog/digital converter  111  is supplied to a digital filter unit  114  and to a determination unit  116 . 
     Since charging signals are at a fixed frequency which can be predetermined, for example 16 kHz or 12 kHz, and, furthermore, are sinusoidal, the transfer function of the digital filter unit  114  consists of a single complex number, which is multiplied in a multiplication unit  121  by an output signal from a control unit  120 . 
     The transfer function of the at least one data transmission path is determined in the determination unit  116  for the at least one frequency at which the charging signals are to be transmitted from a switching center to a subscriber point. 
     The output signal from the determination unit  116  is supplied to a nominal value comparison unit  118 , in which a nominal value  117  may be entered, so that a control signal  119  can be produced as the output signal from the nominal value comparison unit  118 , corresponding to a difference that is to be regulated out between the nominal value  117 , which can be predetermined, and the actual signal determined by the determination unit  116 . The control signal  119  is supplied to the control unit  120 , so that, after multiplication by the output signal from the digital filter unit  114 , an output signal is produced from the regulation device  115 . The digital output signal from the regulation device  115  is supplied to the digital/analog converter  113  of the filter device  109 , in order to obtain an analog signal which is proportional to the digital output signal from the regulation device  115 , and which is supplied to an output filter unit  112  of the filter device  109 . 
     Filtering in the output filter unit  112  of the filter device  109  is used to filter out oversampled components which are outside a transmission band of a transmission frequency range. The filtered signal is emitted from the filter device  109  as a driver input voltage level  107 , which can be tapped off between an output connection of the filter device  109  and the ground connection  133 , and is supplied to the driver device  201  ( FIG. 1 ) or to the modified driver device ( FIG. 2 ). 
     Since this driver input voltage level  107  is no longer based on an estimate, as in the case of methods for transmitting charging signals, but on an analysis of a network which comprises the data transmission path unit  122 , the matching filter unit  123  and the modified driver device  202 , it is possible to eliminate any influence of parallel currents in the matching filter unit  123  and in the at least one data transmission path unit  122 , so that it is possible to obtain a constant line voltage level  101 , which can be predetermined, across the line impedance  102 . 
     Although the present invention has been described above on the basis of preferred exemplary embodiments, it is not restricted to these exemplary embodiments, but can be modified in a wide range of ways.