Abstract:
A line driver coupled to a transmission path having line characteristics associated therewith and a method of operating the same. In one embodiment, the line driver includes a driver stage configured to send a signal along the transmission path. The line driver also includes a switching network, coupled to the driver stage, configured to adaptively select a power level to send the signal as a function of the line characteristics of the transmission path.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention is directed, in general, to communication systems and, more specifically, to a line driver, method of operating the same and a transceiver employing the driver and method.  
         BACKGROUND OF THE INVENTION  
         [0002]    In any electrical or electronic system, power considerations are generally a factor in circuit and system design, and line drivers, including Digital subscriber Line (DSL) drivers, are no exception. A driver typically includes an amplifier stage, having a gain (which may have a value greater than, equal to or less than one), preceding an output stage in a transceiver. Line drivers are used, among other things, to drive or compel a signal (e.g., an analog signal) through a transmission medium. DSL drivers are used to drive signals down a transmission medium such as a twisted pair telephone wire. For a general discussion of DSL, please see “DSL: Simulation Techniques and Standards Development for Digital Subscriber Line Systems,” by Dr. Walter Y. Chen, MacMillian Technical Publishing, 1998, which is hereby incorporated by reference in its entirety.  
           [0003]    The power necessary to drive a signal down a transmission medium may vary depending on the line characteristics thereof. For example, given a plurality of telephone wires connected to a central office providing DSL service, each of the plurality of the twisted pair telephone wires generally exhibits different line characteristics thereby necessitating varying power prerequisites to transmit a signal. These needs may be a function of various related or disparate factors, such as a length of the transmission medium, electromagnetic shielding of the transmission medium, and so on.  
           [0004]    Another design consideration that should be accommodated for is that the line driver should be designed with a sufficient amount of headroom. Headroom may be generally defined as a design parameter that allows a wider dynamic range associated with a driver&#39;s output than is normally associated with the typical root-mean-square average value of the driver&#39;s typical output signal.  
           [0005]    This allocation of the dynamic range for the line driver may, therefore, accommodate for the transmission of certain bursts of signals with a significantly higher voltage amplitude when compared to the typical root-mean-square average of the signal. In other words, designing for the headroom of a signal may be necessary to accommodate a bursty output signal so that the driver is not forced into distortion.  
           [0006]    However, designing for the desired headroom of a driver system, as well as compensating for various non-ideal characteristics of the transmission medium or path, may place even higher power considerations on the driver system. The inefficiencies associated with these considerations may necessitate that the voltage rails in connection with the amplifier stage of the line driver have a wider range, which in turn creates more extreme thermal characteristics, which may in turn lead to a lower density for the drivers than would otherwise be possible.  
           [0007]    To combat these above and other considerations and inefficiencies, there have been attempts in the prior art to achieve more efficient amplifier or power driver systems. For instance, driver systems such as the AD 8016 , by Analog Devices, Incorporated of Norwood, Mass. and the LT 1795  by Linear Technologies Corporation of Milpitas, Calif. allow for an adjustment of the bias current in the line driver to control quiescent consumption.  
           [0008]    Another attempt to achieve a more efficient amplifier or driver system is disclosed in the U.S. Pat. No. 3,961,280, by Sampei, entitled “Amplifier Circuit Having Power Supply Voltage Responsive to Amplitude of Input Signal,” issued on Jun. 1, 1996, which is hereby incorporated by reference in its entirety. In Sampei, a class of amplifiers, designated as class ‘G’ amplifiers, are disclosed. The amplifier disclosed by Sampei changes the power supply voltages in accordance with the magnitude of an input signal.  
           [0009]    Problems persist, however, in association with these various systems and approaches. A limitation of the AD0816 and the LT1795 driver systems from Analog Devices, Incorporated and Linear Technologies Corporation, respectively, is that the devices only control the bias current. It does not control the dominant, dynamic power consumption of the driver. In the case of the amplifier disclosed by Sampei, which is associated with the class ‘G’ amplifiers, there are the problems of high circuit complexity, poor linearity and the need for multiple power supplies, each power supply correlating to given applied power level.  
           [0010]    Accordingly, what is needed in the art is a line driver that may adaptively modify the power selection capability associated with transmitting a signal that overcomes the deficiencies in the prior art.  
         SUMMARY OF THE INVENTION  
         [0011]    To address the above-discussed deficiencies of the prior art, the present invention provides a line driver coupled to a transmission path having line characteristics associated therewith and a method of operating the same. In one embodiment, the line driver includes a driver stage configured to send a signal along the transmission path. The line driver also includes a switching network, coupled to the driver stage, configured to adaptively select a power level to send the signal as a function of the line characteristics of the transmission path.  
           [0012]    The present invention introduces, in one aspect, a line driver that adaptively selects a power level to send a signal as a function of the line characteristics of the transmission path. As a result, a power consumption level associated with transmitting signals can be specifically tailored to the environment in which the line driver is employed. In one embodiment, the driver stage includes a plurality of amplifiers configured to amplify the signal and a reference circuit configured to provide a reference level associated with the plurality of amplifiers. In an alternative embodiment, the switching network includes a plurality of switches configured to adaptively select the power level and a plurality of switches configured to couple an output of the line driver to ground. Additionally, the line driver may form a portion of a front end of a transceiver associated with a Digital Subscriber Line (DSL) system.  
           [0013]    The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0015]    [0015]FIG. 1 illustrates a block diagram of an embodiment of a transceiver constructed according to the principles of the present invention;  
         [0016]    [0016]FIG. 2 illustrates a schematic diagram of an embodiment of a line driver constructed according to the principles of the present invention; and  
         [0017]    [0017]FIG. 3 illustrates a schematic diagram of another embodiment of a line driver constructed according to the principles of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0018]    Referring initially to FIG. 1, illustrated is a block diagram of an embodiment of a transceiver  100  constructed according to the principles of the present invention. The functionality and the various relationships between these various elements of the transceiver  100  will be detailed more fully below. In the illustrated embodiment, the transceiver  100  operates as a front end transmission system employing Digital Subscriber Line (DSL) service. Of course, the transceiver  100  may be employed in other communications networks as well.  
         [0019]    The transceiver  100  includes a digital-to-analog converter  110 , which converts an incoming digital signal to a corresponding analog signal. The digital signal is input into the digital-to-analog converter  110  through a bidirectional digital path  105 . The transceiver  100  further includes a transmitter filtering circuit  120  coupled to the digital-to-analog converter  110 . The analog signal from the digital-to-analog converter  110  undergoes appropriate signal processing functions including filtering processes.  
         [0020]    The transceiver  100  further includes a line driver  130 , which is coupled to the transmitter filtering circuit  120  and receives a signal therefrom. The line driver  130  is configured to, among other things, adaptively select a power level for driving an analog signal as a function of line characteristics or characteristics of a transmission path  160 . The line driver  130  shall be described in greater detail below.  
         [0021]    The transceiver  100  may further include a termination and hybrid circuit  140 , which is coupled to the line driver  130 . The termination and hybrid circuit  140  may perform such functions as maximizing the power transfer to a line transformer  150  and the transmission path  160 . The termination and hybrid circuit  140  may also subtract the transmitter energy of the line driver  130  from a signal received by the transceiver  100 , as the termination and hybrid circuit  140  may receive signals from either direction, transmitting or receiving, as shown in FIG. 1.  
         [0022]    The transceiver  100  may further include a line transformer  150 , which may be coupled to the termination and hybrid circuit  140 . The line transformer  150  may be one of a variety of transformers known to those skilled in the art, such as an isolation transformer, a power transformer, etc. The line transformer  150  is further coupled to the transmission path  160 . The line transformer  150  provides such functions as electrical isolation between the transmission path  160 , such as a twisted pair telephone line, and transfers information from the termination and hybrid circuit  140  to the transmission path  160  or visa verses. The transmission path  160  may be able to carry signals in either direction, transmitting or receiving, as is evidenced in FIG. 1.  
         [0023]    The transceiver  100  is also configured to receive an analog signal from the transmission path  160 . The transceiver  100  further includes a receiver filtering circuit  170 , and still further includes an analog-to-digital converter  180 . The termination and hybrid circuit  140  is coupled to the receiver filtering circuit  170 , the receiver filtering circuit  170  is coupled to the analog-to-digital converter  180 , and the analog-and-digital converter  180  is coupled to the bidirectional digital path  105 . The filtering by the receiver filtering circuit  170  is done in preparation for the analog-to-digital conversion performed by the analog-to-digital converter  180 . The analog-to-digital converter  180  then converts the signal into digital form, and then this digital signal is output through the bidirectional digital path  105 .  
         [0024]    It should be understood that the representative transceiver  100  is submitted for illustrative purposes only and other transceiver configurations compatible with the principles of the present invention may be employed as the application dictates. Also, configurations and implementations of various elements (e.g., the transmitter filtering circuit  120 ) of the transceiver  100  are generally known in the art and as such detailed explanations were not heretofore submitted.  
         [0025]    Turning now to FIG. 2, illustrated is a schematic diagram of an embodiment of a line driver  200  constructed according to the principles of the present invention. The line driver  200  includes a driver stage  205  which includes first and second amplifiers  220 ,  225  and a mid-level voltage reference circuit  265  coupled to first and second nodes A, B. The line driver  200  further includes a switching network  215  having a positive rail switch SW 1  and a negative rail switch SW 2 .  
         [0026]    A value corresponding to an analog signal is input into the driver stage  205  through a first signal differential line pair  210 ,  212 . The first signal differential line pair  210 ,  212  is input into the first amplifier  220 . Although using a line pair is preferable to deliver a less error-prone signal to a respective amplifier, a single signal line input into the respective amplifier is within the scope of the present invention. In an analogous manner, a second value also corresponding to an analog signal is input into the driver stage  205  through a second signal differential line pair  216 ,  218 . The second signal differential line pair  216 ,  218  is input into the second amplifier  225 . Again, although using a line pair is preferable to deliver a less error-prone signal to a respective amplifier, a single signal line input into the respective amplifier is within the scope of the present invention.  
         [0027]    The mid-level voltage reference circuit  265  is configured to provide a voltage reference level  268  associated with the first and second amplifiers  220 ,  225 . The voltage reference level  268  is provided to calculate a median voltage between the positive rail switch SW 1  and the negative rail switch SW 2  by measuring a voltage at the first and second nodes A, B. This median value may also be employed in a transceiver employing the line driver  200  to achieve a more balanced feedback loop with other components of the transceiver. Although use of a mid-level voltage reference circuit  265  is preferable, a mid-level voltage reference circuit  265  need not necessarily be used with the present invention.  
         [0028]    The upper and lower voltage rails associated with the first and second amplifiers  220 ,  225  of the driver stage  205  are coupled to a switching network  215 . As is well known by those skilled in the art, the voltage rails of an amplifier determine the upper and lower dynamic range of an output voltage thereof. An adaptive selection of an upper and lower voltage range for the first and second amplifiers  220 ,  225  leads to a more efficient driver stage  205  and therefore a more efficient line driver  200 . This adaptive voltage selection, perhaps based upon such factors as detailed below, plays a role in accordance with the principles of the present invention.  
         [0029]    The adaptive voltage selections of the present invention are based upon such factors as transmission path characteristics, which may in turn vary upon such factors as the length of the path, and the electromagnetic shielding of the path, etc. The adaptive voltage selections may also be a function of certain aspects of the signal itself, such as an allowance for the necessary headroom for a given signal, and so on.  
         [0030]    The adaptive voltage selection is made in connection with the switching network  215  which in turn uses the positive rail switch SW 1  and the negative rail switch SW 2 . Although first and second ground terminals  260 ,  280  will be described, the ground terminals  260 ,  280  will typically be connected to a common ground. The positive rail switch SW 1  is couplable either to a +5 voltage source  250  or the first ground terminal  260 , although the present invention is not limited to these values. The negative rail switch SW 2  is couplable to a −16 voltage source  270  or the second ground terminal  280 , although the present invention is certainly not limited to these values.  
         [0031]    In other words, the switching network  215  adaptively selects, based upon certain criteria, either the +5 voltage source  250  or the first ground terminal  260  through the use of the positive rail switch SW 1 . The switching network  215  also adaptively selects a −16 voltage source  270  or the second ground terminal  280  through the use of the negative rail switch SW 2 . These are the positive and negative voltage rails which are applied across the first and second amplifiers  220 ,  225 . The first and second amplifiers  220 ,  225  then output a dynamic power level through first and second driver outputs  290 ,  295 , respectively.  
         [0032]    The switching network  215  determines which of the following configurations is the advantageous, based upon such criteria as has been referenced above. For illustration, the switching network  215  may begin its adaptive selection with testing for a given response when the maximum voltage range selected, i.e. the positive rail switch SW 1  is connected to the +5 voltage source  250 , and the negative rail switch SW 2  is connected to the −16 voltage source  260 , for the greatest differential between the two applied rail voltages to the first and second amplifiers  220 ,  225 . The switching network  215  then couples the positive rail switch SW 1  with the first ground terminal  260 , for an intermediate voltage differential between the two applied rail voltages to the first and second amplifiers  220 ,  225 .  
         [0033]    The switching network  215  then switches the positive rail switch SW 1  from coupling to the first ground terminal  260  to the +5 voltage source  250 , and switches the negative rail switch SW 2  from coupling to the −16 voltage source  270  to the second ground terminal  280 . This switching network  215  configuration creates the smallest voltage differential across the positive and negative rails of the first and second amplifiers  220 ,  225 . The switching network  215  then determines, based perhaps upon the criteria disclosed above, a voltage rail differential that is to be applied across the first and second amplifiers  220 ,  225 , and implements that advantageous differential. The switching network  215  can also periodically retest the line conditions after startup to determine an advantageous voltage rail differential, and again may implement that advantageous differential.  
         [0034]    Turning now to FIG. 3, illustrated is a schematic diagram of another embodiment of a line driver  300  constructed according to the principles of the present invention. The line driver  300  includes a driver stage  305  which includes first and second amplifiers  320 ,  325 , a mid-level voltage reference circuit  365  coupled to first and second nodes A, B. The line driver  300  further includes a switching network  315  having a positive rail switch SW 1  and a negative rail switch SW 2 .  
         [0035]    In an analogous manner as disclosed in FIG. 2, a value corresponding to an analog signal is input into the line driver  300  through a first signal differential line pair  310 ,  312 . The first signal differential line pair  310 ,  312  is input into the first amplifier  320 . Although using a line pair is preferable to deliver a less error-prone signal to a respective amplifier, a single signal line input into the respective amplifier is within the scope of the present invention. In an analogous manner, a second value also corresponding to the analog signal is input into the line driver  300  through a second signal differential line pair  316 ,  318 . The second signal differential line pair  316 ,  318  is input into the second amplifier  325 . Again, although using a line pair is preferable to deliver a less error-prone signal to a respective amplifier, a single signal line input into the respective amplifier is within the scope of the present invention.  
         [0036]    The mid-level voltage reference circuit  365  is configured to provide a voltage reference level  368  associated with the first and second amplifiers  320 ,  325 . The voltage reference level  368  is provided to calculate a median voltage between the positive rail switch SW 1  and the negative rail switch SW 2  through measuring a voltage at the first and second nodes A, B. This median value may also be employed in a transceiver employing the line driver  300  to achieve a more balanced feedback loop with other components of the transceiver. Although using a mid-level voltage reference circuit  365  is preferable, a mid-level voltage reference circuit  365  need not necessarily be used with the present invention.  
         [0037]    The upper and lower voltage rails associated with the first and second amplifiers  320 ,  325  of the driver stage  305  are coupled to the switching network  315 . As is well known by those skilled in the art, the voltage rails of an amplifier determine the upper and lower dynamic range of an output voltage thereof. An adaptive selection of an upper and lower voltage range for the first and second amplifiers  320 ,  325  leads to a more efficient driver stage  305  and therefore a more efficient line driver  300 . This adaptive voltage selection, perhaps based upon such factors as detailed below, plays a role in accordance with the principles of the present invention. The adaptive voltage selections of the present invention are based upon such factors as discussed above.  
         [0038]    Adaptive voltage selection is made in connection with the switching network  315 , which in turn uses the positive rail switch SW 1  and the negative rail switch SW 2 . Although first and second ground terminals  360 ,  380  will be described, the first and second ground terminals  360 ,  380  will typically be connected to a common ground. The positive rail switch SW 1  is couplable either to a +5 voltage source  350  or the first ground terminal  360 , although the present invention is certainly not limited to these values. The negative rail switch SW 2  is couplable to a −16 voltage source  370  or a second ground terminal  380 , although the present invention is certainly not limited to these values. The positive and negative voltage rails may then be applied across the first and second amplifiers  320 ,  325 , which output a dynamic power level through first and second driver outputs  390 ,  395 , respectively.  
         [0039]    The switching network  315  is set to the configuration that is advantageous, based upon such criteria as has been referenced above. For example, the switching network  315  may begin its adaptive selection with testing for a given response when the maximum voltage range, an intermediate voltage differential between the two applied rail voltages and the smallest voltage differential across the positive and negative rails of the first and second amplifiers  320 ,  325  analogous to the procedure described with respect to FIG. 2. The switching network  315  then determines, based perhaps upon the criteria disclosed above, a voltage rail differential that is to be applied across the first and second amplifiers  320 ,  325 , and implements that advantageous differential. The switching network  315  can also periodically retest the line conditions after startup to determine an advantageous voltage rail differential, and again may implement that advantageous differential.  
         [0040]    The switching network  315  is coupled to a controller  307  and includes a first enablement switch ESW 1  and a second enablement switch ESW 2 . The controller  307  determines whether to have either of the first and second driver outputs  390 ,  395  at ground voltage, perhaps based on the analog signal (s). After making this determination, the controller  307 , through the use of the first enablement switch ESW 1 , either couples the output of the first amplifier  320  to the first line driver output  390 , or instead couples the first line driver output  390  directly to the second ground terminal  380 , as may be appropriate. In an analogous manner, the controller  307 , through the use of the second enablement switch ESW 2 , either couples the output of the second amplifier  325  to the second line driver output  395 , or instead couples the second line driver output  395  directly to the second ground terminal  380 , as may be appropriate.  
         [0041]    Coupling the second ground terminal  380  to the first and second line driver outputs  390 ,  395  of the line driver  300  (i.e. grounding the outputs) as manifested through the first and second enablement switches ESW 1 , ESW 2  may in the present invention have a correlation with grounding of the positive and negative rail switches SW 1 , SW 2 . That is, whenever the first enablement switch ESW 1  is coupled to the second ground terminal  380 , the high voltage rails of the first and second amplifiers  320 ,  325  may be coupled to the first ground terminal  360 . Likewise, whenever the second enablement switch ESW 2  is coupled to the second ground terminal  380 , the low voltage rails of the first and second amplifiers  320 ,  325  may be coupled to the second ground terminal  380 . This relationship is set forth in the table below.  
                                         TABLE 1                           OUTPUT VALUES FROM SWITCHING NETWORK                Power Setting   SW1   ESW1   SW2   ESW2                       High   C   C   C   C           Medium High   0   0   C   C           Medium Low   C   C   0   0           Off   0   0   0   0                      
 
         [0042]    It should be understood, that the embodiments of the line driver employing the driver stage and switching network constructed according to the principles of the present invention illustrated and described with respect to the preceding FIGUREs are submitted for illustrative purposes only and other configurations compatible with the principles of the present invention may be employed as the application dictates. For a better understanding of communications theory, in general, and digital subscriber line services including the standards and systems that support the technology, see also “Understanding Digital Subscriber Line Technology” by Thomas Starr, Peter Silverman, and John M. Coiffi, Prentice Hall (1998), and “Digital Communication” by Edward A. Lee and David G. Messerschmitt, Kluwer Academic Publishers (1994), which are incorporated herein by reference.  
         [0043]    Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.