Abstract:
A system and method for echo cancellation in digital subscriber line (DSL) service using passive devices. The outgoing transmitted signal is attenuated and fed back into the receiver circuit in such a way as to maximize the canceling of the transmitted signal and thus allow the receiver circuit to amplify and process the received signal without interference from the outgoing transmitted signal. The feedback circuit uses only passive elements. The feedback circuit has complex impedance branches. These complex impedance branches parallel the complex impedances of the transformer and transmission line such that any change in the transformer or transmission line impedance is similarly experienced in the feedback circuit. This allows for near total cancellation of the echo signal without the need for costly active circuits.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to transmission/receiver circuits, and more particularly to an echo cancellation circuit used in DSL communications.  
         BACKGROUND  
         [0002]    In many communications systems a single data path transmits and receives data signals. As an example, in digital subscriber line (DSL) service, the home user transmits and receives signals over a twisted pair of wires. At any given moment, the twisted pair of wires can be carrying both outgoing and incoming signals.  
           [0003]    Echo cancellation circuits aid in the reception of the incoming signals. More specifically, echo cancellation circuits compensate for the reflection, or echo, of outgoing signals into the receiver circuit. This results in the receiver circuit receiving a cleaner incoming signal for amplification and processing.  
           [0004]    In general, there are two types of echo cancellation circuits. The first type includes active circuits and memories. These echo cancellation circuits are trained to compensate for a particular transmission line and terminator impedance, and to adapt to changes in this impedance as the temperature and frequency of outgoing signals change. The second type of echo cancellation circuit includes resistors to reduce the reflection of the outgoing signals into the receiver circuit. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0005]    [0005]FIG. 1 is a circuit diagram of a resistive, passive echo cancellation circuit.  
         [0006]    [0006]FIGS. 2 a ,  2   b  and  2   c  are simplified circuit diagrams of the circuit shown in FIG. 1.  
         [0007]    [0007]FIGS. 3 and 4 are graphs of the relationship between impedance and frequency.  
         [0008]    [0008]FIG. 5 is a circuit diagram of an improved echo cancellation circuit.  
         [0009]    FIGS.  6 - 8  are simplified circuit diagrams of the circuit of FIG. 5. 
     
    
       [0010]    Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0011]    An improved echo cancellation circuit employs both reactive elements, (e.g., capacitors and inductors) and resistive elements (e.g., resistors) such that the impedance of the circuit has both real and imaginary components. This arrangement permits the circuit to more closely track variations in the impedance of a transmission line and associated transformer due to variations in a frequency of a signal being transmitted. For ease of discussion, an echo cancellation circuit including resistive elements is discussed with reference to FIGS.  1 - 4  before the improved circuit is discussed with reference to FIGS.  5 - 8 . Referring to FIG. 1, circuit  100  includes transmitter  105  and receiver  110 . Transmitter  105  issues differential output signals onto nodes T and −T, while receiver  110  receives differential input signals on nodes R and −R. A terminating resistor  115   a  is coupled between nodes T and R, and a terminating resistor  115   b  is coupled between nodes −T and −R. A transformer  120  couples nodes R and −R to a twisted pair of transmission lines  125   a  and  125   b . A resistor  130  represents the impedance of the transmitter and/or receiver circuit(s) coupled to transmission lines  125   a  and  125   b.    
         [0012]    There are two cancellation circuits  135   a  and  135   b  for cancelling the outgoing transmitted signal. Cancellation circuit  135   a  is coupled to nodes T and −R and includes three resistors  135   a   1 ,  135   a   2  and  135   a   3 . Similarly, cancellation circuit  135   b  is coupled to nodes −T and R and includes three resistors  135   b   1 ,  135   b   2  and  135   b   3 .  
         [0013]    [0013]FIG. 2 a  shows only the cancellation circuit  135   a  for ease of discussion. It should be understood that a similar diagram and analysis can be done for cancellation circuit  135   b . To simplify the analysis, the impedances of transformer  120  and transmission lines  125   a  and  125   b  are represented by impedance Z line . Voltage source  245  represents an incoming signal to be detected, amplified and processed by receiver  110 . The outgoing signal, which is transmitted as a differential signal from nodes T and −T, is reduced so as not to be amplified and processed as an incoming, received signal by receiver  110 .  
         [0014]    Cancellation circuit  135   a , in conjunction with receiver circuit  110 , operates as a voltage summer. Thus, the voltage at node A, V A , is given by the following equation where V T  is the voltage at node T, V −R  is the voltage at node −R, R ref  is the resistance value of resistor  135   a   1 , R 1  is the resistance value of resistor  135   a   2  and R 2  is the resistance value of resistor  135   a   3 :  
         V   A     =         V   T          (       R   ref       R   1       )       +         V     -   R            (       R   ref       R   2       )       .                             
 
         [0015]    Assuming R 1 =R ref  and R 2 =½ R ref , this simplifies to:  
           V   A   =V   T +(2) V   −R .  
         [0016]    The voltage at node −R, V −R  is a combination of the voltage output from transmitter  105 , V −T , and the input voltage, V in , received through transformer  120 . Thus V −R  may be expressed as the sum of a component, V −RT , provided by V −T  and a component, V −Rin , provided by V in :  
           V   −R   =V   −RT   +V   −Rin .  
         [0017]    Referring to FIG. 2 b , using superposition to calculate the influence that V −T  has on V −R , and setting the resistance of terminating resistor  115   b  to R T  yields the following equation:  
         V     -   RT       =         V     -   T            [       Z   line         R   T     +     Z   line         ]       .                           
 
         [0018]    Referring to FIG. 2 c  and using superposition to calculate the influence that V in  has on V −R  when V −T  is grounded produces the following equation:  
         V     -   Rin       =         V     i                 n            [       R   T         R   T     +     Z   line         ]       .                           
 
         [0019]    Substituting the equations for V −RT  and V −Rin  into the equation for V −R  yields:  
         V     -   R       =         V     -   T            [       Z   line         R   T     +     Z   line         ]       +         V     i                 n            [       R   T         R   T     +     Z   line         ]       .                             
 
         [0020]    Assuming that the terminating resistor  115   b  matches the combined impedance of transformer  120  and transmission lines  125   a  and  125   b , that is Z line =R T , the equation for V −R  reduces to:  
         V     -   R       =         V     -   T            (     1   2     )       +         V     i                 n            (     1   2     )       .                             
 
         [0021]    Since the outgoing transmitted signal is differential, it follows that V −T =−(V T ) Substituting this value into the equation for V −R , and then substituting the equation for V −R  into the equation for V A  yields:  
           V   A     =         V   T          (   1   )       +     [       -       V   T          (     1   2     )              (   2   )       ]     +         V     i                 n            (     1   2     )            (   2   )           ,                         
 
         [0022]    which reduces to:  
         V A =V in .  
         [0023]    This analysis shows that the echo cancellation circuits  135   a  and  135   b  of FIG. 1 are effective at reducing the echo of V T  and V −T  onto nodes A and B as long as the impedance of terminating resistors  115   a  and  115   b  matches the combined impedance of transformer  120  and transmission lines  125   a  and  125   b.    
         [0024]    As noted above, the impedance Z line  represents both the impedance of the transmission lines  125   a  and  125   b  (e.g., the twisted pair of telephone lines outside of the user&#39;s home) and the transformer  120  of FIG. 1. The individual impedances of these components vary with the frequency of the signals they carry and the ambient temperature. In other words, Z line  is not constant and does not always equal the resistances of terminating resistors  115   a  and  115   b  (R T ).  
         [0025]    For DC signals, the impedance of transformer  120  is approximately 0 Ω. Thus, the dominant factor in impedance Z line  is the impedance of transmission lines  125   a  and  125   b . As the frequency of the signals increases from 0 Hz to about 5 kHz, the impedance of transformer  120  increases, which in turn causes the impedance Z line  to increase as shown in FIG. 3.  
         [0026]    Above 5 kHz, the impedance of transmission lines  125   a  and  125   b  decreases substantially to dominate the impedance Z line . Thus, for signals above 5 kHz (e.g., from 5 kHz to 10 kHz), the impedance Z line  decreases. FIGS. 3 and 4 show the variations in the complex impedance Z line  as the frequency increases. As shown, compensating for these variations in impedance using only resistive elements is virtually impossible.  
         [0027]    [0027]FIG. 5 illustrates a circuit having many elements that are the same as elements of the circuit of FIG. 1 and are referred to with the same reference numbers. Cancellation circuit  550  is coupled between nodes T, −T, R and −R and the receiver  110  input nodes A and B. Cancellation circuit  550  includes four separate impedance branches  554   a ,  554   b ,  558   a  and  558   b  that are coupled, respectively, between nodes T and A, and nodes −R and A, nodes −T and B, and nodes −R and B.  
         [0028]    Impedance branch  554   a  includes resistor R 554   a   1  and capacitor C 554   a   1  coupled in series. Impedance branch  554   b  includes resistor R 554   b   1  coupled in parallel with a series combination of resistor R 554   b   2  and capacitor C 554   b   1 . Impedance branch  558   a  includes a series combination of resistor R 558   a   1  and capacitor C 558   a   1 . Impedance branch  558   b  includes resistor R 558   b   1  coupled in parallel with a series combination of resistor R 558   b   2  and capacitor C 558   b   1 . In one implementation, each of R 554   a   1  and R 558   a   1  has a value of 4.6 kΩ; each of C 554   a   1  and C 558   a   1  has a value of 16 nanoFarads; each of R 554   b   1  and R 558   b   1  has a value of 1.7 kΩ; each of R 554   b   2  and R 558   b   2  has a value of 7.1 kΩ; and each of C 554   b   1  and C 558   b   1  has a value of 1 nanoFarad.  
         [0029]    Each of the four impedance branches  554   a ,  554   b ,  558   a  and  558   b  includes resistive elements (i.e., the resistors) and reactive elements (i.e., the capacitors). The use of both resistors and capacitors produces complex impedances. In other words, each branch has real impedance components based substantially on the values of the resistors and imaginary impedance components based substantially on the values of the capacitors.  
         [0030]    The circuits shown in FIGS.  6 - 8  are analyzed to describe the behavior of the circuit shown in FIG. 5. For brevity and clarity, only half of cancellation circuit  550  that includes impedance branches  554   a  and  554   b  is described. It should be understood that the following analysis also applies to impedance branch  558   a  and  558   b  of the cancellation circuit. Using superposition, several of the nodes, T, R and −R are grounded and the resulting characteristic equations are calculated. Also for the sake of brevity, the impedance of branch  554   a  is defined as Z 1  and the impedance of branch  554   b  is defined as Z 2 .  
         [0031]    The voltage at node A, V A , includes a component, V AT , attributable to the voltage at node T, and a component, V A−R , attributable to the voltage at node −R:  
           V   A   =V   AT   +V   A−R .  
         [0032]    In FIG. 6, node −R is grounded so that V A−R  equals zero and V AT  is calculated to determine the effect of echoing the transmitted voltage onto nodes A and B. By voltage division, V AT  is:  
         V   AT     =         V   T          [       Z   2         Z   1     +     Z   2         ]       .                           
 
         [0033]    In FIG. 7, node T is grounded so that VAT equals zero and V A−R  is calculated to determine the effect of the voltage at node −R on node A. By voltage division, V A−R  is:  
         V     A   -   R       =         V     -   R            [       Z   1         Z   1     +     Z   2         ]       .                           
 
         [0034]    As described earlier, V −R  is itself a combination of the signals received through transformer  120  from outside circuits as well as the signals output by transmitter  105  that are propagated to node −R through terminating resistor  115   b . The voltage applied to node −R from transformer  120  due to received input signals is ignored.  
         [0035]    Grounding node R in FIG. 5 produces the equivalent circuit shown in FIG. 8. The relationship between V −T  and V −R  is derived through voltage division to be:  
           V     -   R       =       V     -   T            [       Z   line         Z   line     +     R   T         ]         ,                         
 
         [0036]    and substituting V T  for V −T  produces:  
         V     -   R       =     -         V   T          [       Z   line         Z   line     +     R   T         ]       .                             
 
         [0037]    Substituting for V −R , V AT , and V A−R  in the equation for V A  using the equations above yields:  
           V   A     =         V   T          [       Z   2         Z   1     +     Z   2         ]       +         (     -     V   T       )          [       Z   line         Z   line     +     R   T         ]            [       Z   1         Z   1     +     Z   2         ]           ,                         
 
         [0038]    which may be rewritten as:  
         V   A     =         [       V   T         Z   1     +     Z   2         ]          [       Z   2     -       Z   1          (       Z   line         Z   line     +     R   T         )         ]       .                           
 
         [0039]    From the preceding equation, it is clear that the transmitted output voltage V T  can be eliminated from nodes A and B if  
           Z   2     =         Z   1     ×     Z   line           Z   line     +     R   T           ,                         
 
         [0040]    which may be rewritten as:  
           Z   2       Z   1       =         Z   line         Z   line     +     R   T         .                           
 
         [0041]    Thus, by designing the impedances within each of the branches in cancellation circuit  550  to be correlate with the impedances of Z line  and the terminating resistors R T , the echo of the outgoing transmission signal, V T  and V −T , into receiver circuit  110  is reduced or eliminated. In other words, as long as Z 2  varies in the same proportion with Z line  as Z 1  varies in proportion with Z line  and R T , the reflection or echo of the transmitted output voltage into the receiver  110  is reduced or eliminated. Impedance branches  554   a ,  554   b ,  558   a  and  558   b  have complex impedances in order to correlate more closely with the complex impedance of the combination of Z line  and R T .  
         [0042]    In FIG. 5, each impedance branch  554   a ,  554   b ,  558   a  and  558   b  includes capacitors. Capacitors are reactive elements. By using resistors and capacitors in the impedance branches, the frequency response of echo cancellation circuit  550  more closely models the frequency response of the transformer  120  and transmission lines  125   a  and  125   b  combination. Thus, the amount of the outgoing transmitted signal from transmission circuit  105  that is echoed into receiver circuit  110  is attenuated or eliminated even as the impedance of the transmission line and transformer combination varies with frequency.  
         [0043]    Generally, cancellation circuit  550  operates as follows. As the frequency of the output signals from transmitter  105  increases, the impedance of transformer  120  increases. This results in more of the output transmission voltages V T  and V −T  being present on nodes R and −R, respectively. To compensate for this, relatively large capacitors C 554   a   1  and C 558   a   1  and resistors R 554   a   1  and R 558   a   1  are used to propagate more of the opposite polarity signals V T  and V −T  directly into nodes A and B, respectively. In other words, as the voltage at node −R rises due to the increase in the impedance of transformer  120 , a larger portion of the opposite polarity signal V T  is propagated into node A through impedance branch  554   a  to compensate for the increase in voltage at node A caused by the increase in voltage at node−R. Thus, V T  is attenuated less by branch  554  so as to balance the increase in the voltage at node −R. Similar behavior occurs at node B as a result of the behavior of impedance branch  558   a.    
         [0044]    As the output signal frequencies increase beyond a certain point (e.g., 5 kHz), the impedance of transmission lines  125   a  and  125   b  increases and the impedance of transformer  120  decreases. This causes an overall decrease in Z line  as described above. With a decrease in Z line , the effect of the output voltage becomes less of a factor in V −R . However, V T  is still propagated to V A . To compensate for this, V −R  is attenuated less so that a larger portion of V −R  is fed into node A. This is accomplished by having the impedance of series combination R 554   b   2  and C 554   b   1  decrease with increasing frequency. The decrease in impedance in that combination causes an overall decrease in impedance in branch  554   b  and a resulting increase in the voltage V −R  propagated onto node A.  
         [0045]    The echo cancellation circuit  550  has at least two advantages over other echo cancellation circuits. First, no active elements are used. Thus, this circuit is relatively inexpensive and simple in design while still providing a close correlation to the combined impedance of transformer  120  and transmission lines  125   a  and  125   b . In addition, it does not need to be trained or biased with a DC power supply in order to operate properly.  
         [0046]    Second, the echo cancellation circuit  550  maps more closely with changes in the combined transmission line and transformer impedance resulting from changes in the frequency of the transmitted signals. In other words, the echo cancellation circuit  550  better compensates for changes in the transmission line and transformer impedance than echo cancellation circuits that only include resistors. This is because each branch  554   a ,  554   b ,  558   a  and  558   b  has complex impedance (i.e., real and imaginary components). Thus, the outgoing transmission signal reflection into receiver  110  is substantially reduced over a wider range of frequencies.  
         [0047]    A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, inductors can be used instead of the capacitors shown in FIG. 5. When using inductors, the value and arrangement (i.e., serial vs. parallel and vice versa) with the resistors will differ from the arrangement and values of resistors described above. In addition, while one implementation has the resistors formed on an integrated circuit along with either the transmitter circuit  105 , the receiver circuit  110 , or both, and the capacitors being discrete and external to the integrated circuit, other implementations may have all elements of the cancellation circuit integrated with the transmitter  105 , the receiver  110 , or both. Additionally, all elements of the cancellation circuit may be implemented externally to the integrated circuit containing the transmitter  105 , the receiver  110 , or both.  
         [0048]    Accordingly, other implementations are within the scope of the following claims.