Abstract:
A method for deskewing a differential signal is provided. A common-mode voltage of a differential signal and an average for the common-mode voltage of the differential signal are measured. A difference between first and second portions of the differential signal is determined, and deskew information is derived from the common-mode voltage and the average. The deskew information can then be combined with the difference to deskew the differential signal.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to deskewing circuitry and, more particularly, to intra-pair deskewing circuitry. 
       BACKGROUND 
       [0002]    Turning to  FIG. 1 , an example of a conventional communication system  100  can be seen. This system  100  generally comprises a transmitter  102 , a transmission medium  104 , and a receiver  106 . Typically, the transmission medium  104  is comprised of a pair of transmission lines that are configured to carry a differential signal from the transmitter  102  to the receiver  106 . This type of system  100  is used in a wide variety of applications ranging from transmitting information over long distance (through cables) to on-chip communications. One issue with system  100  is that there is usually a length mismatch between the transmission lines in medium  104  that can lead to skew between the portions of the differential signal carried by the medium  104  (known as “intra-pair skew”), and an example of intra-pair skew can be seen in  FIG. 2 , where the transmission medium  104  for this example introduces 1 unit interval (UI) of skew. For low frequency signals, intra-pair skew can be largely ignored, but, for high frequency signals (i.e. &gt;1 Gb/s), intra-pair skew can significantly impair communications. 
         [0003]    To address intra-pair skew, several solutions have been proposed. These proposed solutions approach intra-pair skew as being a propagation delay issue, and an example of such a proposed solution can be seen in  FIG. 3 . As shown, skew compensator  200  (which is included within receiver  106 ) uses several delay elements  202 - 1  to  202 -N coupled in series with one another to delay each portion of the differential signal accordingly to compensate for the intra-pair skew. Adjustments to the delay elements  202 - 1  to  202 -N are made through adjustment of the control voltage VCNTL, but it can often be difficult to adequately adjust the relative delays to compensate for the intra-pair skew. Therefore, there is a need for an improved skew compensator. 
         [0004]    Some other conventional circuits are: U.S. patent application Ser. No. 12/948,757; U.S. Pat. No. 6,335,647; U.S. Pat. No. 6,909,980; U.S. Pat. No. 7,729,874; U.S. Patent Pre-Grant Publ. No. 2005/0099216; U.S. Patent Pre-Grant Publ. No. 2006/0244505; U.S. Patent Pre-Grant Publ. No. 2009/0174448.pdf; Zheng et al., “A 5 Gb/s Automatic Sub-Bit Between-Pair Skew Compensator for Parallel Data Communications in 0.13 μm CMOS,” 2010  Symposium on VLSI Circuits/Technical Digest of Technical Papers , Jun. 16-18, 2010; Olisar, Robert, “Unbalanced Twisted Pairs Can Give You the Jitters!,”  Maxim Engineering Journal , Vol. 64, September 2008, pp. 5-12. 
       SUMMARY 
       [0005]    An embodiment of the present invention, accordingly, provides an apparatus. The apparatus comprises a measuring circuit that is configured to receive a differential signal and that is configured to determine a common-mode voltage; an averaging circuit that is configured to receive the differential signal and that is configured to determine an average for the common-mode voltage; and an output circuit having that is coupled to the measuring circuit and the averaging circuit and that is configured to receive the differential signal, wherein the output circuit determines a difference between first and second portions of the differential signal, and wherein the output circuit determines deskew information from the common-mode voltage and the average, and wherein the output circuit generates a deskewed differential signal from the deskew information and the difference. 
         [0006]    In accordance with an embodiment of the present invention, the output circuit further comprises: a first differential amplifier that is configured to receive the differential signal; a second differential amplifier that is coupled to the measuring circuit and the averaging circuit; and an adder that is coupled to the first and second differential amplifiers. 
         [0007]    In accordance with an embodiment of the present invention, the first and second amplifiers have first and second gains, respectively, wherein the second gain is twice the first gain. 
         [0008]    In accordance with an embodiment of the present invention, the measuring circuit further comprises a voltage divider. 
         [0009]    In accordance with an embodiment of the present invention, the averaging circuit further comprises: a first resistor that is configured to receive the first portion of the differential signal and that is coupled to the output circuit; a second resistor that is coupled to the first resistor and that is configured to receive the second portion of the differential signal; and a capacitor that is coupled to the first and second resistors. 
         [0010]    In accordance with an embodiment of the present invention, a method is provided. The method comprises measuring a common-mode voltage of a differential signal; measuring an average for the common-mode voltage of the differential signal; determining a difference between first and second portions of the differential signal; determining deskew information from the common-mode voltage and the average; and combining the deskew information with the difference to deskew the differential signal. 
         [0011]    In accordance with an embodiment of the present invention, the step of determining the deskew information further comprises determining a difference between the common-node voltage and the average. 
         [0012]    In accordance with an embodiment of the present invention, the step of determining the difference between the first and second portions of the differential signal further comprises applying a gain to the difference between the first and second portions of the differential signal. 
         [0013]    In accordance with an embodiment of the present invention, the gain further comprises a first gain, and wherein the step of determining the deskew information further comprises applying a second gain to the difference between the common-node voltage and the average. 
         [0014]    In accordance with an embodiment of the present invention, the second gain is at least twice the first gain. 
         [0015]    In accordance with an embodiment of the present invention, an apparatus is provided. The apparatus comprises a first terminal that is configured to receive a first portion of a differential signal; a second terminal is configured to receive a second portion of the differential signal; a measuring circuit that is coupled to the first and second terminals and that is configured to determine a common-mode voltage; an averaging circuit that is coupled to the first and second terminals and that is configured to determine an average for the common-mode voltage; a first difference circuit that is coupled to the first and second terminals and that is configured to determine a difference between the first and second portions of the differential signal; a second difference circuit that is coupled to the measuring circuit and the averaging circuit and that is configured to determine deskew information from the common-mode voltage and the average; and a combiner that is coupled to the first and second output circuits. 
         [0016]    In accordance with an embodiment of the present invention, the first difference circuit further comprises a differential amplifier having a gain. 
         [0017]    In accordance with an embodiment of the present invention, the differential amplifier further comprises a first differential amplifier, and wherein the gain further comprises a first gain, and wherein the second difference circuit further comprises a second differential amplifier having a second gain. 
         [0018]    In accordance with an embodiment of the present invention, the combiner further comprises a node. 
         [0019]    In accordance with an embodiment of the present invention, the second gain is at least twice the first gain. 
         [0020]    In accordance with an embodiment of the present invention, the measuring circuit further comprises: a first resistor that is coupled to the first terminal and the second differential amplifier; and a second resistor that is coupled between the second terminal and the first resistor. 
         [0021]    In accordance with an embodiment of the present invention, the averaging circuit further comprises: a node that is coupled to second differential amplifier; a third resistor that is coupled between the first terminal and the node; a fourth resistor that is coupled between the second terminal and the node; and a capacitor that is coupled to the node. 
         [0022]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0024]      FIG. 1  is a diagram of an example of a conventional communication system; 
           [0025]      FIG. 2  is a diagram illustrating intra-pair skew for the system of  FIG. 1 ; 
           [0026]      FIG. 3  is a diagram of an example of a conventional skew compensator; 
           [0027]      FIG. 4  is a diagram of an example of a skew compensator in accordance with an embodiment of the present invention; and 
           [0028]      FIGS. 5A-8  are diagrams depicting the operation of the skew compensator of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
         [0030]    Turning to  FIG. 4 , an example of a skew compensator  300  in accordance with an embodiment of the present invention can be seen. This skew compensator  300  is generally included in a receiver (i.e., receiver  106 ) so as to perform intra-pair skew compensation, and it generally comprises an averaging circuit  302 , a measuring circuit  304 , and an output circuit. The output circuit generally uses two difference circuits (which are typically differential amplifiers  306  and  308 ) that generate differential signal information and deskew information on a main signal path and a deskew path, respectively. This differential signal information and deskew information can then be combined by combiner  310  to generate an output signal VOUT that should generally match the desired, deskewed differential signal. Typically, differential amplifier  306  (which is in the main signal path) is coupled to the input terminals of the skew compensator  300  (which carry the portions INM and INP of the differential signal), while the differential amplifier  308  (which is in the deskew path) is coupled to the averaging circuit  302  and measuring circuit  304  so as to receive a common-mode voltage VCM and an average of the common-mode voltage VCMA. The common-mode voltage VCM is usually generated by the measuring circuit  304  through the use of a voltage divider (i.e., resistors R 3  and R 4 ), while the average VCMA is generated by using a voltage divider (i.e., resistors R 1  and R 2 ) to continuously measure the common-mode voltage VCM and a memory device (i.e., capacitor C 1 ) to average it over a long period of time. 
         [0031]    To illustrate the function of skew compensator  300 , signals propagating through system  100  (which includes skew compensator  300 ) are shown in  FIGS. 5A and 5B . In this example, transmitter  102  outputs a differential signal, where the portions INM and INP can have a value of +1 or −1 (assuming differential amplifier  306  has unity gain). As the differential signal propagates across medium  104 , 1 UI of intra-pair skew is introduced, which significantly distorts the differential signal. It can, however, be recognized that the intra-pair skew converts the differential signal to common-mode, since there is a difference between the common-mode voltage VCM and the average VCMA (which for this example is 0). When the difference between portions INM and INP (i.e., INP-INM) is taken by differential amplifier  308 , this difference can have values of +2, 0, and −2, and the difference does not match the desired (deskewed) output from transmitter  102 . Because the difference between the common-mode voltage VCM and the average VCMA can have values of −1, 0, or +1, differential amplifier  308  (for this example) applies again of 2 so as to generate the deskew information (i.e., 2*(VCM-VCMA)). This deskew information can then be combined with the difference between the portions INM and INP by combiner  310  (which can, for example, be an adder or node) to generate the output signal VOUT, which generally matches the output of the transmitter  102 . It should also be noted that the gains of differential amplifiers  306  and  308  are typically different, and that the gain of differential amplifier  308  will typically be at least twice or double the gain of differential amplifier  306 . 
         [0032]    Turning now to  FIG. 6-8 , other examples of the operation of the skew compensator  300  can be seen. In  FIG. 6 , there is no intra-pair skew, so the skewed and deskewed signals generally match (but are shown with different scales). In  FIG. 7 , ½ UI (which is about 50 ps for this example) of skew is introduced, and the skew compensator  300  is able to fully recover the eye. In  FIG. 8 , there is 1 UI (i.e., about 100 ps for this example) of skew (which significantly distorts the differential signal), and the skew compensator  300  is able to recover the differential signal. 
         [0033]    Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.