Abstract:
A technique for converting asymmetric waveforms into symmetric ones. The technique is used in disk drive read channels which receive asymmetric waveforms from magnetoresistive heads. Conversion of these waveforms into symmetric ones results in improved bit error rate of the read channel. The correction technique can be used for any general asymmetry transfer function, and in any general application where the correction of asymmetric waveforms is needed. The technique involves splitting the input signal into two rectified paths and applying correction independently on each of the paths.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to circuitry used to correct asymmetric waveforms, such as those in disk drive signals, into symmetric waveforms. 
     BACKGROUND OF THE INVENTION 
     The resistance of the sensing layer in a magnetoresistive (MR) head varies with the field in the vicinity of the heads. This feature is used to read data from magnetic disk drives. Ideally, the resistance of the sensing layer is a linear function of its magnetic orientation. In practice, however, the output of the MR head is asymmetric, owing to an offset in the biasing of the head. The percentage of asymmetry is defined as:          percentage                 asymmetry     =                max                 peak          -          min                 peak                   max                 peak            ×   100                            
     Asymmetry could also be defined as:          percentage                 asymmetry     =                max                 peak          -          min                 peak                (            max                 peak          +          min                 peak            )     /   2       ×   100                            
     Both the above definitions of asymmetry provide a quantitative way of expressing the degree of asymmetry in the waveform. The waveforms could be asymmetric with either polarity. The waveform is considered to be positively asymmetric if the positive side of the waveform has a higher peak than the negative side of the waveform. A waveform distorted in the opposite direction is considered negatively asymmetric. The asymmetric waveforms result in a high bit error rate (BER) being generated in the read channel electronics that process the signal from the MR head and extract data from this signal. 
     U.S. Pat. No. 6,043,943, entitled “Asymmetry correction for a read head,” (F. Rezzi, G. Patti, ST Microelectronics Inc., issued on Mar. 28, 2000), discloses an arrangement in which, in the context of correcting asymmetric waveforms, essentially one correction function is applied to both sides of a waveform. This often results in sub-optimal correction of the asymmetry at hand. 
     In view of the foregoing, a need has been recognized in connection with optimally converting asymmetric waveforms into symmetric waveforms in signal processing circuits or other environments in which such conversion would be appropriate or desirable. 
     SUMMARY OF THE INVENTION 
     In accordance with at least one presently preferred embodiment of the present invention, asymmetric waveforms are converted into symmetric waveforms via splitting the input signal into two rectified paths and applying correction independently on each of the paths. 
     In summary, one aspect of the invention provides an apparatus for correcting asymmetric waveforms, the apparatus comprising: an input arrangement which accepts an input signal, the input signal having an associated waveform; a splitting arrangement which splits the input signal into at least two paths; a first altering arrangement, associated with a first of the two signal paths, which alters a first portion of the split signal; a second altering arrangement, associated with a second of the two signal paths, which alters a second portion of the split signal; and an output arrangement which recombines the altered split signals into an output signal; whereby the output signal exhibits a substantially symmetric waveform. 
     Another aspect of the invention provides a method of correcting asymmetric waveforms, the method comprising the steps of: accepting an input signal, the input signal having an associated waveform; splitting the input signal into at least two portions; altering a first portion of the split signal; altering a second portion of the split signal; and recombining the altered split signals into an output signal; whereby the output signal exhibits a substantially symmetric waveform. 
     For a better understanding of the present invention, together with other and further features and advantages thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, and the scope of the invention will be pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a read channel circuit in a hard disk drive, showing the location of an asymmetry correction circuit. 
     FIG. 2 is a block diagram showing an asymmetry correction technique. 
     FIG. 3 is a block diagram of the signal path with linear gain. 
     FIG. 4 is a block diagram of the signal path with a rectifier. 
     FIG. 5 illustrates a DC transfer function showing the asymmetric and symmetric waveforms. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Generally, the signal at the output of an MR head is asymmetric, due to commonly exhibited encountered characteristics as observed in practice. This signal is amplified in the arm electronics of the disk drive system, and then goes to the read channel electronics. 
     A simplified block diagram of a read channel circuit  100  is shown in FIG.  1 . The first stage in the read channel is a variable gain amplifier (VGA)  102 , which is then typically followed by a continuous time filter (CTF)  104 . In accordance with a preferred embodiment of the present invention, in order to correct the asymmetric waveform into a symmetric one, an asymmetry correction circuit  106  is inserted in the signal path between the VGA  102  and CTF  104 . The remainder of the circuit, which could include an analog-to-digital converter (ADC)  108  and some digital signal processing  110 , need not necessarily be changed. 
     A block diagram of an asymmetry correction circuit  106 , in accordance with an embodiment of the present invention, is shown in FIG.  2 . There can be a unity gain buffer  202  at the input, in order to present minimal loading to the output of the VGA ( 102  in FIG.  1 ). At the output of the buffer  202 , the signal is preferably split into three paths, PATH 1 , PATH 2  and PATH 3 . PATH 1  goes through a linear gain stage  204 . The purpose of this path is to provide the same delay to the signal as in the other two paths. The gain associated with PATH 1  is preferably selected based on the gain requirements of the entire asymmetry correction circuit. 
     PATH 2  and PATH 3  are similar, but not identical. Each of them preferably has a half-wave rectifier ( 206 ,  208 ), followed by a wave-shaping circuit ( 210 ,  212 ), followed by a programmable gain stage ( 214 ,  216 ). PATH 2  operates on the positive side of the input waveform, while PATH 3  operates on the negative side of the input waveform. The rectifiers in PATH 1  and PATH 2  therefore produce a rectified version of the two asymmetric halves of the input waveform. 
     Each of these signals (in PATH 2  and PATH 3 ) goes through a different wave- shaping block. The transfer function of the wave-shaping blocks  210 / 212  will depend on the transfer function of the asymmetry. For a piecewise linear asymmetry transfer function, the wave-shaping circuits  210 / 212  have linear gain. For square-law or sinusoidal asymmetry transfer functions, the wave-shaping circuit  210 / 212  will have the appropriate transfer function to generate the correction term for each half of the input waveform. The two independently programmable gain stages in the two paths are also intended to give greater flexibility for the correction of high percentages of asymmetry. 
     The three paths, PATH 1 , PATH 2  and PATH 3  are preferably configured in such a way that the signal has identical delay going through any one of the paths. The signals at the outputs of the three paths are then added (at  218 ) to produce the output waveform of the asymmetry correction circuit. This output waveform is a symmetric one, which then goes to the next stage in the read channel which is the CTF circuit ( 104  in FIG.  1 ). 
     A block diagram of PATH 1 , the path with linear gain stage  204 , is shown in FIG.  3 . The gain associated with this path is preferably configured so that the total gain through the asymmetry correction circuit after adding the outputs of PATH 1 , PATH 2  and PATH 3  meets the gain requirements of the system. PATH 1 , as such, preferably includes gain stages, buffers between the stages, and delay stages. In a preferred embodiment, the gain stages include source-coupled field effect transistor (FET) differential pairs with source degeneration (indicated at  302   a  and  302   b ). These could also be designed with emitter-coupled bipolar junction transistor (BJT) devices. Also, in a preferred embodiment, the buffers between the stages are implemented as FET&#39;s connected as source followers ( 304   a  and  304   b ). 
     The delay of PATH 1  is preferably made very similar to PATH 2  and PATH 3  by using a similar number of circuit stages in PATH 1  as are used in PATH 2  and PATH 3 . The circuit stages are also preferably made similar between the different paths, to the extent possible by the functional requirements of the circuits. In a preferred embodiment, a differential pair ( 306 ) with a topology matching the rectifier circuits in PATHS  2 / 3  is added to PATH 1 . The purpose of this matched differential pair is to match the delay of PATH 1  with those in PATHS  2 / 3 . Any further mismatch in delay can be compensated for by adding a delay stage. The delay stage could be implemented with a simple circuit, such as capacitive loading on the nodes in the signal path, as is done in the “delay matching cap” block  308  of FIG.  3 . In a preferred embodiment, the capacitive loading is provided by an FET gate capacitor. An additional source follower  310  may preferably be added subsequent to the delay stage at  308 . The delay stage could also be added to PATH 2  and PATH 3 , if the delay in these paths is less than that of PATH 1 . As shown, dummy path switches  312  may be provided subsequent to source follower  304   b.    
     A block diagram illustrating the composition of PATH 2  (also applicable to PATH 3 ) is shown in FIG.  4 . The paired reference numerals provided in FIG. 4 relate, respectively, to PATH 2  and PATH 3 . Each path preferably includes a half-wave rectifier stage  402  in order to extract one or other half of the asymmetric waveform. In a preferred embodiment of the present invention, the rectifier used is that described in the copending and commonly assigned U.S. patent application Ser. No. 09/753,311 entitled “MOSFET Rectifier Circuit with Operational Amplifier Feedback”. 
     As shown in FIG. 4, in a preferred embodiment, the wave shaping function (indicated at  210 / 212  in FIG. 2) is implemented within the transfer characteristics of differential pairs with source degeneration  404   a / 454   a  and  404   b / 454   b,  as well as the half-wave rectifier  402 / 452 . This wave-shaping function is optimized based on the asymmetry transfer function that is exhibited by the MR head. In addition, the two differential pairs with source degeneration ( 402 /. 452 ) shown in FIG. 4 provide additional flexibility to adjust the gain depending on design requirements. The two paths also preferably have a programmable gain stage, which is implemented as part of the summing stage (indicated at  218  in FIG. 2) where PATH 1 , PATH 2  and PATH 3  end. Note that each of these two paths, PATH 2  and PATH 3 , has its own wave-shaping function, and that the gains in the two paths are independently programmable. PATH 2  and PATH 3  also preferably contain switches  406 / 456  which are used to direct the signals in order to correct for opposite polarities of asymmetry. As shown, source followers  408   a / 458   a,    408   b / 458   b  and  408   c / 458   c  may also preferably be provided in PATH 2  and PATH 3  to serve similar purposes as described heretofore in connection with PATH 1  (FIG.  3 ). 
     A DC transfer curve is shown in FIG.  5 . The dashed lines  502   a  and  502   b  indicate the input to the asymmetric correction circuit, which is an asymmetric waveform with different linear gains on either side of the origin. After the input waveform goes through the asymmetry correction circuit and correction has been applied independently to the two halves of the signals coming out of the half-wave rectifiers, the symmetric output signal (indicated by line  504 ) is produced. 
     A more detailed explanation of the working of an asymmetry correction circuit in accordance with a preferred embodiment of the present invention follows. 
     Consider a symmetric signal with a period T, written as a function of time (t): 
     
       
           A ( t )=| x ( t )| for 0 &lt;t&lt;T/ 2 and  
       
     
     
       
           A ( t )=−| x ( t )| for  T/ 2 &lt;t&lt;T,    
       
     
     
       
         where  x ( t )=0 at  t= 0 , T/ 2 , T    
       
     
     Assume that there are distorting functions, f1(x) and f2(x) for positive and negative x, respectively. f1(x) and f2(x) could be any form of distortion, including linear, square wave, sine wave, etc. The asymmetric signal entering the correction circuit described here is of the form: 
     
       
           B ( t )=| x ( t )|+ f 1( x ( t )) for 0 &lt;t&lt;T/ 2 and  
       
     
     
       
           B ( t )=−| x ( t )|− f 2( x ( t )) for  T/ 2 &lt;t&lt;T    
       
     
     Let it also be assumed that the rectifiers are configured in such a manner that PATH 2  extracts the positive half of the input waveform and PATH 3  extracts the negative half of the input waveform. In that case, the correction functions and gains built into PATH 2  and PATH 3  result in correction terms P2(t) and P3(t) respectively, such that: 
     
       
           A ( t )= G×B ( t )+ P 2( t ) for 0 2   t&lt;T/ 2 and  
       
     
     
       
           A ( t )= G×B ( t )+ P 3( t ) for  T/ 2 &lt;t&lt;T    
       
     
     where G is the gain of the linear gain stage shown in PATH 1  of FIG.  2 . 
     Hence, the output of the asymmetry correction circuit is the original symmetric signal, A(t). 
     It is to be understood that the embodiments of the present invention, as described and illustrated herein, need not necessarily be employed solely in the environment of disk drive systems. Indeed, it is contemplated that the embodiments of the present invention may be employed in essentially any environment in which asymmetric waveforms are to be corrected. 
     If not otherwise stated herein it is to be assumed that all patents, patent applications, patent publications and other publications (including web-based publications) mentioned and cited herein are hereby fully incorporated by reference herein as if set forth in their entirety herein. 
     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention.