Abstract:
A modem operating in a narrow voltage range while maintaining a high signal to noise ratio during reception. The modem may contain a coder-decoder (CODEC) and a transformer. The CODEC receives data using more windings of a primary coil than the number of windings used for transmitting. As a result, the turns ratio is higher during transmission, leading to a correspondingly high amplification during transmission. The high amplification in the transmit direction enables the modem to operate in a narrow voltage range. As more windings of the primary coil are used for receiving, a signal of interest received from the telephone line is attenuated to a corresponding lesser degree, which leads to a high signal to noise ratio.

Description:
RELATED APPLICATION(S)  
       [0001]    The present application is related to and claims priority from the co-pending U.S. Provisional Patent Application Serial No. 60/251,809, entitled, “Transformer”, filed on Dec. 7, 2000, and is incorporated in its entirety herewith. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to modems used in digital subscriber&#39;s loop (DSL) environments, and more specifically to a method and apparatus for improving the reception of data in a modem.  
           [0004]    2. Related Art  
           [0005]    Digital subscriber loop (DSL) technology is generally used to transmit and receive (“tranceives”) data at high bit rates as is well known in the relevant arts. In a typical DSL environment, a modem at one end sends data on a telephone line and another modem at another end receives the data. The data transfer happens in the reverse direction also. The same telephone line is used for both transmission and reception of the data.  
           [0006]    Modems typically contain a transformer which is used to tranceive data. Transformers generally contain a primary coil (“primary”) and a secondary coil (“secondary”) and each coil contains multiple windings. The ratio of the number of windings in the secondary to the number of windings in the primary is known as turns ratio.  
           [0007]    The strength of the signal generated by a transformer is directly proportional to the turns ratio. For example, assuming that the number of windings on the primary is ‘m’ and the number of windings in the secondary is ‘n’, the turns ratio is n/m. When the transformer transmits a signal is stepped up/down by ‘n/m’ and when receiving the signal is stepped up/down by ‘m/n’.  
           [0008]    In one prior approach commonly referred to as a single winding transformer based approach, a secondary contains more windings than a primary and the same windings of the primary are used for both transmitting and receiving. In such a situation, the signal transmitted to a telephone line is stepped up by turns ratio and a signal received from the telephone line is stepped down by the same ratio.  
           [0009]    One advantage of the single winding transformer approach is that the components of the a modem can operate in a narrow voltage range (swing). Narrow voltage ranges typically imply that the modems can operate using power supplies of correspondingly lower voltages, which generally leads to lower total implementation costs. However, the step down of the signal received from the telephone line may lead to several disadvantages. For example, the signal to noise ratio may be reduced in proportionate to the step-down factor, and low signal to noise ratio is often undesirable generally because the signal of interest is a correspondingly small fraction of the received signal.  
           [0010]    A low signal to noise ratio is particularly problematic in environments in which data is encoded at high rates in a signal received on a telephone line. For example, in remote locations (e.g., homes) implementing ADSL (Asymmetric DSL) technology, the receive bit rate (downstream direction) is much higher than the transmit bit rate (upstream direction), and it may be particularly necessary to maintain a high signal to noise ratio while recovering the data bits from the analog signal received on the telephone line.  
           [0011]    Therefore, what is needed is a method and apparatus which enables the signal to noise ratio to be maintained high while enabling modem to operate within a narrow voltage range.  
         SUMMARY OF THE INVENTION  
         [0012]    A modem provided in accordance with the present invention receives and transmits data on a telephone line while maintaining a high signal to noise ratio and operating within a narrow voltage range. Such a result may be obtained by using more turns (of a primary coil of a transformer) for receiving than for transmitting as described below.  
           [0013]    A coder-decoder (CODEC) contained in the modem may convert digital transmit data to an analog transmit signal and transmit the analog transmit signals using a first set of windings. The CODEC may receive an analog receive signal on a second set of windings and convert the analog receive signal to a digital receive data. The first set of windings and the second set of windings may be contained in a primary coil of transformer, and the first set of windings contain a fewer number of windings than the second set of windings. In an embodiment, a subset of the second set of windings are used as the first set of windings.  
           [0014]    In one implementation, the CODEC contains a network transmitting the analog transmit signal using the subset of windings. The network further generates a subtraction component representing an echo voltage generated by transmitting the analog transmit signal. An echo cancellation unit subtracts the subtraction component from the analog receive signal to generate a signal of interest representing data received on the telephone line.  
           [0015]    An analog to digital converter (ADC) generates the digital receive data from the signal of interest. A digital signal processor (DSP) performs signal processing operation on the digital receive data to recover the data encoded on a signal received on the telephone line. The modem may also include a digital to analog converter (DAC) to convert the digital transmit data to the analog transmit signal.  
           [0016]    An embodiment of the network includes multiple impedances, with the subtraction component being measured across one of the impedances. The echo cancellation unit may contain a differential amplifier and multiple resistors. The differential amplifier subtracts the subtraction component from the analog receive signal to generate the signal representing the data received (or transmitted at the other end) on the telephone line.  
           [0017]    Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The present invention will be described with reference to the accompanying drawings, wherein:  
         [0019]    [0019]FIG. 1 is a block diagram illustrating the details of a DSL modem in accordance with the present invention;  
         [0020]    [0020]FIG. 2 is a block diagram illustrating the details of an embodiment of a CODEC in accordance with the present invention;  
         [0021]    [0021]FIG. 3 is a block diagram illustrating the details n embodiment of a hybrid network and an echo cancellation unit;  
         [0022]    [0022]FIG. 4 is a block diagram illustrating the details of an embodiment of hybrid network; and  
         [0023]    [0023]FIG. 5 is a block diagram illustrating an example environment in which the present invention can be implemented. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    1. Overview and Discussion of the Invention  
         [0025]    The present invention enables a modem to maintain a high signal to noise ratio while operating within a narrow voltage range. The features are achieved by using a multi-winding transformer approach, in which the data is received on several windings of a primary, but transmission is performed using only a subset of the same windings. By using more windings in the receive direction, the attenuation of a signal received from a telephone line is minimized, thereby potentially increasing the signal to noise ratio. By using less windings (of a primary) in the transmit direction, the modem operates in a low voltage range.  
         [0026]    Several aspects of the invention are described below with reference to example environments for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One skilled in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details, or with other methods, etc. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the invention.  
         [0027]    2. Modem  
         [0028]    [0028]FIG. 1 is a block diagram illustrating a DSL modem in which the present invention can be implemented. There is shown DSL modem  100  containing digital signal processor (DSP)  110 , coder-decoder (CODEC)  120  and multi-winding transformer  150 . Each component is described below in further detail.  
         [0029]    DSP  110  performs operations such as frequency transformation on the stream of data bits received on input paths  101  and  121 , and generates a corresponding stream of data bits on output paths  112  and  102  respectively. DSP  110  may be implemented in a known way.  
         [0030]    Transformer  150  is shown containing primary coil  130  and secondary coil  140 , each in turn containing multiple windings. Secondary coil  140  is connected to telephone line pair ( 198  and  199 ) over which transmission and reception of data occurs. Primary coil  130  is shown containing transmit tap pair  137  and  138  and receive tap pair  135  and  136 . As may be readily observed, receive tap pair ( 135  and  136 ) contains all the windings of primary  130 , and transmit tap pair ( 137  and  138 ) contains only a subset of the windings. The manner in which the tap pairs are used is described below in further detail.  
         [0031]    According to an aspect of the present invention, CODEC  120  transmits analog transmit signal using transmit tap pair  137 / 138 , and receives analog receive signal using receive tap pair  135 / 136 . By using less windings in the transmit direction, CODEC  120  may be implemented to operate at a lower voltage range for a desired strength of the signal on secondary  140  during transmission. By using more windings for reception, a signal of interest received on telephone wire pair  198 / 199  is amplified to a desired high degree.  
         [0032]    The analog transmit signals transmitted on path  133 / 134  are generated based on the digital transmit data present on path  112 . In addition, CODEC  120  converts the analog receive signal received from transformer  150  (on path  131  and  132 ) into digital receive data for transmission on path  121 .  
         [0033]    The analog receive signal received on path  131 / 132  contains an echo voltage corresponding to the analog transmit signal transmitted on path  137 / 138 . CODEC  120  operates to eliminate the echo voltage from the analog receive signal as described below in further detail.  
         [0034]    3. CODEC  
         [0035]    [0035] 
         [0036]    [0036]FIG. 2 is a block diagram illustrating the details of CODEC  120  in one embodiment. CODEC  120  is shown containing digital to analog converter (DAC)  210 , analog to digital converter (DAC)  220 , echo cancellation unit  230  and hybrid network  240 . Each component is explained in further detail below.  
         [0037]    DAC  210  converts digital transmit data (received on path  112 ) into analog form. The analog equivalent of the digital transmit data is provided as input (on path  214 ) to hybrid network  240 . ADC  220  converts analog signals received on path  232  into digital format, and transmits the resulting data on path  121 . DAC  210  and ADC  220  may be implemented in a known way.  
         [0038]    Hybrid network  240  transmits analog transmit signal (path  214 ) on path  133 / 134 . Hybrid network  240  may also generate an echo voltage (on path  243  and  244 ) proportionate to an echo component present on the analog receive signal caused due to the strength analog transmit signal transmitted on path  134 / 134 . As described below, the echo voltage is used to cancel echo component present in the analog receive signal received on path  131 / 132 .  
         [0039]    Echo cancellation unit  230  subtracts the echo voltage (received on path  243  and  244 ) from the analog receive signal received on receive path  131 / 132  and generates a corresponding analog signal on path  232 . The analog signal contains data encoded in a signal of interest received on secondary  140 , and thus the data is recovered when sampled by ADC  220 . The description is continued with reference to an example embodiment of hybrid network  240  and echo cancellation unit  230 .  
         [0040]    4. Hybrid Network and Echo Cancellation Unit  
         [0041]    [0041]FIG. 4 is a block diagram illustrating the details of hybrid network  240  and echo cancellation unit  230  in one embodiment. Hybrid network  240  is shown containing impedances  330 A (“ZA”),  340  (“ZB”) and  330 B (“ZC”) connected in series. Resistors  320 A (“R6”) and  320 B (“R6”) are connected to impedances ZA and ZC respectively. Echo cancellation unit  230  is shown containing a differential amplifier  370  and four resistors  310 A (“R1”),  310 B (“R2”),  310 C (“R3”) and  310 D (“R4”).  
         [0042]    Vint represents the analog receive signal voltage containing the noise components (including echo component) and exists between nodes  131  and  132 . Vtx represents the analog transmit signal voltage between nodes  133  and  134  and is transmitted by transformer  150 . Vext represents the voltage that is received at secondary  140  of transformer  150 . Voltage across nodes  325  and  355  is represented by Vh and Vecho represents the voltage across ZB.  
         [0043]    In an embodiment, hybrid network  240  contains three impedances ZA, ZB, ZC connected in series and may be implemented to generate an echo voltage. Echo voltage Vecho is measured across ZB.  
         [0044]    Echo voltage Vecho and internal voltage Vint are provided as inputs to differential amplifier  370 . Differential amplifier  370  subtracts echo voltage Vecho from internal received voltage Vint to generate output Vext which may approximately equal the voltage received at secondary  140 .  
         [0045]    The manner in which the impedances  330 ,  340  and  350  are designed is described in detail below.  
         [0046]    Vtx may mathematically be represented as  
         [0047]    [0047] Vtx=Vh*ZL /( ZL+ 2 R 5)  Equation (1)  
         [0048]    wherein Vtx represents the analog transmit signal voltage transmitted, Vh represents the voltage across hybrid network  240  (i.e., between nodes  133  and  134 ), ZL represents the impedance of the portion of primary between the transmit tap pair  133 ,  134 ; R 5  represents the resistance  320 A, and “*” represents multiplication operation.  
         [0049]    The voltage Vint between nodes  131  and  132  equals sum of the voltage received Vext by secondary  140  and noise components that are introduced due to transmitted voltage Vtx. Thus, Vint may be represented as  
           Vint=Vext+Vtx   Equation (2)  
         [0050]    The voltage Vecho between nodes  247  and  248  may be mathematically represented as:  
           Vecho=Vh*ZB /( ZB+ZA+ZC )  Equation (3)  
         [0051]    wherein ZA represents impedance  330 A, ZB represents impedance  340  and ZC represents impedance  330 B.  
         [0052]    The output Vext generated by differential amplifier  370  is Vecho subtracted from Vint. Thus  
           Vext=Vint−Vecho   Equation (4)  
         [0053]    On substituting Equation (2) in Equation (4) VRX may be represented as  
           Vext=Vext+Vtx−Vecho   Equation (5)  
           Vext=Vext+Vh*ZL /( ZL+ 2 R 5)− Vh*ZB /( ZB+ZA+ZC )  Equation (6)  
         [0054]    Equation (6) can be balanced when the coefficients of Vh are equated with each other.  
           ZL /( ZL+ 2 R 5)= ZB /( ZB+ZA+ZC )  Equation (7)  
         [0055]    Thus, the impedances of hybrid network  240  can be designed according to equation (7). The description is continued with reference to an embodiment of hybrid network  240  designed according to Equation (7).  
         [0056]    5. Embodiment of Hybrid Network  
         [0057]    [0057]FIG. 4 illustrates the details of an embodiment of hybrid network  240 . Hybrid network  240  is shown containing resistors R20 through R29 and capacitors C13 through C15 and C18 through C20. R23 and C14 represent impedance ZA, R25 and C18 represent ZB and R26 and C19 represent ZC. The values corresponding to each component is described below.  
         [0058]    R22, R23, R24, R25 and R26 have values of 121, 169, 464, 2430 and 121 ohms respectively. C13 and C20 have a value of 0.247 micro farad, C14 and C19 have a value of 50 pico farads, C15 is of 5600 pico farads and C18 is of 0.15 micro farads. R20 and R28 are of 732 ohms each and R21 and R29 have a value of 1391 ohms.  
         [0059]    The description is continued with reference to an example environment where DSL modem  100  may be implemented.  
         [0060]    6. Example Environment  
         [0061]    [0061]FIG. 5 is a block diagram illustrating an example environment in which the present invention can be implemented. There is shown remote system  530  and DSL Access Multiplexor (DSLAM)  540 , which are examples of systems using a modem implemented in accordance with the present invention. Remote systems are typically present in locations such as homes which are connected to a central office using telephone wires. DSLAMs are present in the central offices. Each component is described below in further detail.  
         [0062]    Remote system  530  is shown containing processor  560  and DSL modem  510 . Processor  560  performs various operations on digital data which may have to be transmitted. DSL modem  510  transmits digital data by converting digital data into analog form and transmitting over a telephone line as described above.  
         [0063]    Similarly, DSL modem  510  converts analog signals received over telephone line ( 534 ,  543 ) into digital format before providing the data to processor  560 . The analog data is transmitted to and received from central exchange  540  on paths  534  and  543 .  
         [0064]    DSLAM  540  is shown containing DSL modem  520  and DSL MUX  570 . DSL modem receives analog signals over a telephone line and converts the signals into digital format. DSL MUX  570  receives and transmits digital data to various others systems (potentially using routers and/or switches, not shown) that are connected to central exchange  540 . DSL modems  510  and  520  may correspond to modem  100  of FIG. 1.  
         [0065]    In an embodiment, DSL modem  520  can be implemented in an asymmetric digital subscriber&#39;s loop (ADSL) technology. As is well known in the relevant arts, ADSL technologies enable the user to receive data at a faster rate (downstream rate). A high signal to noise ratio is desired when data is received at a fast rate. Thus, the present invention is particularly useful in implementing modem  510  in ADSL environment.  
         [0066]    Thus, the embodiments described above can be used to improve reception of data in DSL modems. The present invention enables a modem to operate within a narrow voltage range while maintaining a high signal to noise ratio.  
         [0067]    7. Conclusion  
         [0068]    While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.