Abstract:
At the output of a line driver ( 1 ) connected to a long transmission line, signals received on the transmission line are only a fraction of signals transmitted by the line driver on the transmission line. To minimize the transmitted signal with its distortion and noise in the receive path of the line driver, an additional driver ( 4 ) having the same voltages and currents in the signal path, is added. The additional driver ( 4 ) drives an additional load (Z 2 ). The signal transmitted to the transmission line together with its superimposed received signal, is subtracted from the signal from the additional driver ( 4 ), which is not superimposed on any received signal. The resulting signal is reduced from transmitted signal, transmitted distortion and transmitted noise. To minimize die size and power consumption, the additional driver ( 4 ) is scaled, but the distortion remains unchanged compared to the “normal” line driver.

Description:
TECHNICAL FIELD 
     The invention relates generally to line drivers and more specifically to an echo and distortion cancellation arrangement in line drivers. 
     BACKGROUND OF THE INVENTION 
     In a line driver connected to a telephone line for transmitting signals generated by a signal source and receiving signals incoming on the transmission line, echoes and distortions caused by the line driver itself as well as by the signal source appear together with the signals to be transmitted and the signals to be received across the output terminals of the line driver. 
     With long transmission lines, the received signals are only fractions of the transmitted signals at the line driver output. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to reduce such echoes and distortions in the receive path of the line driver. 
     This is attained in accordance with the invention by providing an additional driver for driving a separate load with the same voltages and currents in its signal path as the “normal” line driver. 
     The additional driver is, thus, not affected by signals received on the telephone line since it, instead, is connected to that separate load. 
     Thus, the additional driver mirrors the signal to be transmitted by the “normal” line driver with the same distortion and signal levels to the separate load. 
     By subtracting the signal on the telephone line, which includes both the large transmitted signal with its related distortion and the often small received signal, from the signal “transmitted” to the separate load by the additional driver, which only includes the large transmitted signal and its related distortion, the difference will ideally be the signal received on the telephone line, reduced from the transmitted signal as well as the transmitted distortion and noise. 
     Since the transmitted signal has the same currents and voltages in the signal path through both the “normal” line driver and the additional driver, and both the line driver and the additional driver are preferably fabricated on the same die, the difference in distortion and voltage levels of the transmitted signals will be minimized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be described more in detail below with reference to the appended drawing on which 
     FIG. 1 illustrates a known line driver with a first embodiment of an arrangement according to the invention, 
     FIGS. 2 a ,  2   b  and  2   c  are signal diagrams illustrating different signals in the embodiment in FIG. 1, 
     FIG. 3 illustrates another known line driver with a second embodiment of an arrangement according to the invention, and 
     FIG. 4 illustrates a further known line driver with a third embodiment of an arrangement according to the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a known line driver  1  with a first embodiment of an echo and distortion cancellation arrangement in accordance with the invention. 
     In a manner known per se, the known line driver  1  has two input terminals and two output terminals. 
     In the embodiment in FIG. 1, the output terminals of the line driver  1  are connected via terminating impedances ZT to respective wire of a two-wire transmission line as well as to input terminals of receiving means  2  for receiving signals incoming on the transmission line. A line impedance of the transmission line is denoted ZL in FIG.  1 . 
     The input terminals of the line driver  1  are connected to output terminals of a signal source  3  for generating output signals to be transmitted on the transmission line. 
     The output signals of the line driver  1  on the transmission line, emanating from the signal source  3 , are superimposed on signals coming in to the line driver  1  on the transmission line to be received by the receiving means  2  for further processing. The receiving means  2  is connected with its output terminals to signal processing means, e.g. an analog-to-digital converter (not shown). 
     The output signals of the line driver  1  on the transmission line, i.e. the transmitted signals, are accompanied by distortion and noise generated by the line driver  1  itself as well as by the signal source  3 . 
     At the output terminals of the line driver  1 , the output signals are large. 
     At the same time, signals coming in to the line driver  1  on the transmission line, i.e. the received signals, are small in comparison with the transmitted signals. 
     FIG. 2 a  schematically illustrates the power spectral density (PSD) in dBm versus frequency f of the signals at the output of the line driver  1  in FIG. 1 towards the transmission line. 
     The transmitted signal is represented by solid lines, the distortion associated with the transmitted signal is represented by dotted lines, and the received signal is represented by dashed lines. 
     As apparent from FIG. 2 a , the distortion may be of the same level as the received signal. 
     In accordance with the invention, to cancel echo and distortion that appear across the output terminals of the line driver  1 , an echo and distortion cancellation arrangement in the form of a modified version of the known line driver described above, is added to the known line driver  1  to cooperate therewith. 
     The echo and distortion cancellation arrangement according to the invention, i.e. the additional driver  4  also has two input terminals and two output terminals. 
     The output terminals of the additional driver  4  are connected via impedances Z 1  across a load Z 2  as well as to additional input terminals of the receiving means  2 . 
     The input terminals of the additional driver  4  are connected to the output terminals of the signal source  3  for generating the same signals across the load Z 2  as generated by the line driver  1  across the telephone line. 
     FIG. 2 b  schematically illustrates the signals at the output of the additional driver  4  towards the load Z 2 . 
     The signal transmitted to the load Z 2  is represented by solid lines, and the distortion associated with that signal is represented by dotted lines. 
     As apparent, the signals transmitted on the telephone line and to the load Z 2  as illustrated in FIGS. 2 a  and  2   b , respectively, as well as the distortion associated with these signals, are the same. 
     In accordance with the invention, the receiving means  2  is adapted to subtract the signal generated across the load Z 2  as illustrated in FIG. 2 b  from the signals on the transmission line as illustrated in FIG. 2 a  in order to cancel the transmitted signal with its distortion and noise from the incoming signal on the transmission line. 
     This is schematically illustrated in FIG. 2 c  in which only the received signal remains after the subtraction by the receiving means  2 . In practice, it is however to be understood that a portion of the transmitted signal as well as a portion of its associated distortion also can remain after the subtraction. 
     In accordance with the invention, to reduce the power consumption of the additional driver  4 , the impedances Z 1  are of an impedance value that is k times the impedance value of the terminating impedances ZT, k being an integer. Also, the impedance value of the load Z 2  is k times the impedance between the output terminals of the line driver  1 . 
     Also, in accordance with the invention, the size of the output transistors (not shown) of the additional driver  4  is 1/k times the size of the output transistors (not shown) of the line driver  1 . 
     The reason for scaling the output transistors in the additional driver  4  is to achieve the same current density in both the “normal” line driver  1  and the additional driver  4  in order to get the same distortion. 
     Hereby, echoes and distortions will be efficiently canceled from the input signals on the transmission line to be received by signal processing means (not shown) connected to the output terminals of the receiving means  2 . 
     FIG. 3 shows another embodiment of a known line driver  5  with a second embodiment of an echo and distortion cancellation arrangement in accordance with the invention. 
     In a manner known per se, the known line driver  5  in FIG. 3 has two input terminals and two output terminals. 
     In the same manner as described above in connection with FIG. 1, the output terminals of the line driver  5  are connected to respective wire of a two-wire transmission line as well as two input terminals of a receiving means  6  for receiving signals incoming on the transmission line. In FIG. 3, the transmission line has a line impedance Z 3 . 
     The input terminals of the line driver  5  are connected to output terminals of a signal source  7  for generating output signals to be transmitted on the transmission line. 
     The input terminals of the line driver  5  are also connected in a manner known per se to output terminals of a current-to-voltage converter  8  which is connected with its input terminals to the wires of the transmission line. 
     The line driver output signals on the transmission line, emanating from the signal source  7 , are superimposed on signals coming in to the line driver  5  on the transmission line to be received by the receiving means  6  for further processing. The receiving means  6  is connected with its output terminals to signal processing means, e.g. an analog-to-digital converter (not shown). 
     The line driver output signals on the transmission line, i.e. the transmitted signals, are accompanied by distortion and noise generated i.a. by the signal source  7 . 
     As described above in connection with FIG. 1, the output signals at the output terminals of the line driver  5  are large, while incoming signals to the line driver  5  on the transmission line, i,e. the received signals, are small in comparison with the transmitted signals. 
     Also in the line driver  5  in FIG. 3, the power spectral density, PSD, versus frequency, f, of the signals at the output of the line driver  5  will be as illustrated in FIG. 2 a.    
     Also in the embodiment in accordance with FIG. 3, to cancel echo and distortion that appear across the output terminals of the line driver  5 , an echo and distortion cancellation arrangement in the form of a modified version of the known line driver  5  described above, is added to the known line driver  5  to cooperate therewith. 
     The echo and distortion cancellation arrangement according to the embodiment in FIG. 3, i.e. the additional driver  9 , has two input terminals and two output terminals, 
     The output terminals of the additional driver  9  are connected to a load Z 4  as well as to additional input terminals of the receiving means  6 . 
     The input terminals of the additional driver  9  are connected to the output terminals of the signal source  7  for generating the same signals across the load Z 4  as generated by the line driver  5  across the telephone line. 
     Also in this case, FIG. 2 b  schematically illustrates the signal on the output of the additional driver  9  towards the load Z 4 . 
     The input terminals of the additional driver  9  are also connected to output terminals of a current-to-voltage converter  10  which is connected with its input terminals to the output terminals of the additional driver  9 . 
     As in the embodiment in FIG. 1, the receiving means  6  in FIG. 3 is adapted to subtract the signal generated across the load Z 4  from the signals on the transmission line in order to cancel the transmitted signal with its distortion and noise from the incoming signals on the transmission line. 
     In accordance with the invention, to reduce the power consumption of the additional driver  9 , the impedance value of the load Z 4  is k times the impedance between the output terminals of the line driver  5 , k being an integer. 
     Moreover, in accordance with the invention, the current-to-voltage converter  10  has a current-to-voltage conversion factor that is k times the current-to-voltage conversion factor of the current-to-voltage converter  8 . 
     Also in this case, the size of the output transistors (not shown) of the additional driver  9  are 1/k times the size of the output transistors (not shown) of the line driver  5  to achieve the same current density in both the “normal” line driver  5  and the additional driver  9  in order to get the same distortion. 
     FIG. 4 shows an embodiment of a further known line driver with an echo and distortion cancellation arrangement in accordance with the invention. 
     The known line driver comprises two current amplifiers  11 ,  12  connected with their outputs to the respective wire of a two-wire transmission line  20 . The inputs of the current amplifiers  11 ,  12  are connected, on the one band, to outputs from a signal source  14  for generating output signals on the transmission line  20 , and, on the other hand, to outputs from a voltage-to-current converter  13 , the inputs of which are connected to the transmission line  20 . The voltage-to-current converter  13  is part of a loop that sets the termination impedance of the line driver. The voltage-to-current converter  13  feeds back a current which is a function of the output voltage. This current is then amplified and fed back by the line driver to the transmission line  20 . The result will then be an active impedance. 
     The current amplifiers  11 ,  12  are identical. In view hereof, in FIG. 4, only an embodiment of the current amplifier  11  is shown in more detail. 
     The current amplifier  11  comprises an operational amplifier OA 1  and three resistors R 11 , R 12  and R 13 . 
     The resistor R 11  is connected between the output terminal of the operational amplifier OA 1  and the output terminal of the current amplifier  11 , connected to one of the wires of the transmission line  20 . 
     The resistor R 12  is connected between the output terminal of the current amplifier  11  and the (+)-input terminal of the operational amplifier OA 1 . 
     The resistor R 13  is connected between the output terminal of the operational amplifier OA 1  and the (−)-input terminal of the operational amplifier OA 1 . 
     The line driver output signals on the transmission line  20 , emanating from the signal source  14 , are superimposed on line driver input signals on the transmission line  20  to be received by receiving means  18  to be processed in the line driver. The receiving means  18  are connected to signal processing means, e.g. an analog-to-digital is converter (not shown). 
     The line driver output signals on the transmission line  20 , i.e. the transmitted signals, are accompanied by distortion and noise generated within the line driver, i.a, by the signal source  14 . 
     At the same time, input signals on the transmission line  20  to the line driver, i.e. the received signals, are small in comparison with the transmitted signals. 
     Also in this case, FIG. 2 a  schematically illustrates the power spectral density (PSD) in dBm versus frequency f of the signals on the output of the line driver towards the transmission line  20 . 
     The transmitted signal is represented by solid lines, the distortion associated with the transmitted signal is represented by dotted lines, and the received signal is represented by dashed lines. 
     As indicated above, the distortion may be of the same signal level as the received signal as is also apparent from FIG. 2 a.    
     In accordance with the invention, to cancel the distortion that appears across the output terminals of the line driver connected to the telephone line  20 , an echo and distortion cancellation arrangement in the form of a modified version of the known line driver described above, is added to the known line driver to cooperate therewith. 
     The echo and distortion cancellation arrangement according to the invention, i e. the additional driver, comprises two current amplifiers  15 ,  16  connected with their outputs to a load  19  and with their inputs, on the one hand, to outputs from the signal source  14  for generating the same signals across the load  19  as those generated on the telephone line  20 , and, on the other hand, to outputs from a voltage-to-current converter  17 , the inputs of which are connected to the load  19 . The voltage-to-current converter  17  is also part of a loop that sets the termination impedance of the added driver, i.e. the impedance between the output terminals of the current amplifiers  15 ,  16 . The voltage-to-current converter  17  feeds back a current which is a function of the output voltage, and this current which is then amplified and fed back by the added driver to the load  19 . The result will then be an active impedance. 
     The current amplifiers  15 ,  16  are also identical. In view hereof, only the current amplifier  15  is shown in more detail. 
     The current amplifier  15  comprises an operational amplifier OA 5  and three resistors R 51 , R 52  and R 53 . 
     The resistor R 51  is connected between the output terminal of the operational amplifier OA 5  and the output terminal of the current amplifier  15 , i.e. one end of the load  19 . 
     The resistor R 52  is connected between the output terminal of the current amplifier  15 , i.e. one end of the load  19 , and the (+)-input terminal of the operational amplifier OA 5 . The resistor R 53  is connected between the output terminal of the operational amplifier OA 5  and the (−)-input terminal of the operational amplifier OA 5 . 
     The load  19  is also connected between inputs of the receiving means  18 . 
     In this case, FIG. 2 b  schematically illustrates the signals on the output of the added driver towards the load  19 . 
     The signal transmitted to the load  19  is represented by the solid lines, and the distortion associated with that signal is represented by the dotted lines 
     As apparent, the transmitted signals as illustrated in FIGS. 2 a  and  2   b  as well as the distortion associated with these signals, are the same. 
     In accordance with the invention, the receiving means  18  are adapted to subtract the signals generated across the load  19  from the signals on the transmission line  20  in order to cancel the transmitted signal with its distortion and noise from the input signals on the transmission line  20 . 
     This is schematically illustrated by FIG. 2 c  in which only the received signal remains after the subtraction by the receiving means  18 . 
     In practice, it is however to be understood that a portion of the transmitted signal as well as its associated distortion also may remain after the subtraction. 
     To enable-this cancellation, in accordance with the invention, the current amplifiers  15 ,  16  have a current amplification factor that is 1/k times the current amplification factor of the current amplifiers  11 ,  12 , and the impedance of the load  19  is k times the impedance seen on the transmission line  20  from the outputs of the current amplifiers  11 ,  12 . 
     The current amplification factor is set in an identical manner in all current amplifiers  11 ,  12 ,  15  and  16 . 
     Thus, in e.g. the current amplifier  11 , the current amplification factor on the (+)-input of the operational amplifier OA 1  is set by (R 12 +R 11 )/R 11 , and on the (−)-input by−R 13 /R 11 . In practice, the current amplification factor is equal on both inputs but its phase is shifted 180° on the (−)-input. If R 12 =R 13 −R 11 , the current amplification factor is the same on both inputs (however phase shifted). 
     As indicated above, in accordance with the invention, the input active impedance between the output terminals of the current amplifiers  15 ,  16 , is k times the impedance between the output terminals of the current amplifiers  11 ,  12 , i.e. the impedance of the transmission line  20  as seen from the output terminals of the line driver. 
     Furthermore, in accordance with the invention, the resistance of the resistor R 51  in the current amplifier  15  and, consequently, also of the same resistor (not shown) in the current amplifier  16 , is chosen to be k times the resistance of the resistor R 11  in the current amplifier  11  and, consequently, of the same resistor (not shown) in the current amplifier  12 . 
     Also, in accordance with the invention, in the current amplifiers  15 ,  16 , the size of the output transistors (not shown) is chosen to be 1/k times the size of the output transistors (not shown) in the current amplifiers  11 ,  12 . 
     The reason for the scaling of the output transistors in the added driver is to achieve the same current density in both the “normal” line driver and the added driver in order to get the same distortion. 
     Hereby, echoes and distortions will be efficiently canceled from the input signals on the transmission line to be received by signal processing means connected to the receiving means  18 .