Abstract:
The present invention provides a method and circuit for equalizing and compensating IQ imbalance at the same time. The method includes: down-converting an RF signal to generate a baseband signal, and driving an adaptive equalizer to process the baseband signal for achieving equalization and IQ imbalance compensation simultaneously.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a signal processing method and the circuit thereof, especially to a method and a circuit for equalizing and compensating IQ imbalance simultaneously. 
   2. Description of the Prior Art 
   In communication systems, a carrier is frequently utilized to carry baseband signals that contain data. Generally, a carrier is a high frequency signal. After receiving a radio frequency signal, a receiver initially down converts the radio frequency for the convenience of further process. Recently, due to manufacturing progress and consideration of cost reduction issues, more and more RF circuits in the receiving end have adopted direct down conversion to directly down convert the radio frequency signals into baseband signals. In general, a transmitter adopts a modulation scheme with high bandwidth efficiency because of the bandwidth limitation. A quadrature amplitude modulation (QAM) is a frequently utilized modulation scheme, especially in the cases where a high-resolution digital television signal is transmitted. In such a case, a 256 QAM modulation scheme is often adopted. 
   When the front end of a receiving terminal adopts an RF front end that utilizes direct down conversion, an IQ imbalance issue is frequently introduced. The IQ imbalance results in the interfering of the function of a QAM receiver. As a result, the receiving end can be equipped with an IQ imbalance compensation circuit for compensating the received radio frequency signal that possesses an IQ imbalance problem caused by direct down conversion. In addition, a channel is generally accompanied by a multi-path issue in the transmission process, so the receiving end requires an equalizer to solve the problem caused by multi-path effects during signal transmission. 
   Please refer to  FIG. 1 .  FIG. 1  shows a block diagram of a prior art receiver  100 . The receiver  100  contains a direct down converter  110 , an IQ imbalance compensating circuit  120 , and an equalizer  130 . After the incoming signal S 1  is received by the direct down converter  110 , the incoming signal S 1  is transmitted in two different paths. Ideally, in the two paths, the incoming signal S 1  is multiplied by g sin wt and g cos wt respectively by a mixer  116  and a mixer  118  (where g is the gain, w is the angular frequency). However, the phases of these two signals, g sin wt and g cos wt, are probably not orthogonal, and the gains of these two signals may also be different. In other words, the oscillating signals utilized by the direct down converter  110  are probably (g+α) sin wt and g cos(wt+θ), where α is the gain imbalance and θ is the phase imbalance. As a result, the two signals S 1 _I and S 1 _Q, which are respectively filtered by the LPF&#39;s  112  and  114 , have an IQ imbalance issue. Typically, the IQ imbalance compensating circuit  120  generates a compensation coefficient through a calibration method for compensating the direct down converter  110 . Once the calibration process of the IQ imbalance compensating circuit  120  has been achieved, the IQ imbalance compensating circuit  120  utilizes the same compensation coefficient to compensate all signals under all kinds of operating environments. 
   The compensated signals S 1 _I′ and S 1 _Q′ enter the equalizer  130  and are therefore equalized. In general, assuming that the IQ imbalance does not exist, a signal (T) transmitted by a transmitter, passing through a channel (H), and being received by a receiver (R), the transmitting signal T and the receiving signal R are therefore expressed by the following equation:
 
 R ( n )=   H   ( n )×   T   ( n )=(   H     i ( n )+ j  H     q ( n ))×(   T     i ( n )+ j  T     q ( n ))  nε 1, 2, 3, . . .  Eq. (1),
 
   where H represents a channel model. The above equation can also be expressed in the matrix form: 
   
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           R 
                           i 
                         
                         ⁡ 
                         
                           ( 
                           n 
                           ) 
                         
                       
                       
                         
                           R 
                           q 
                         
                         ⁡ 
                         
                           ( 
                           n 
                           ) 
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               H 
                               _ 
                             
                             i 
                           
                           
                             
                               H 
                               _ 
                             
                             q 
                           
                         
                         ⁢ 
                         
                           
                             - 
                             
                               
                                 H 
                                 _ 
                               
                               q 
                             
                           
                           
                             
                               H 
                               _ 
                             
                             i 
                           
                         
                       
                       ] 
                     
                     ⁡ 
                     
                       [ 
                       
                         
                           
                             T 
                             _ 
                           
                           i 
                         
                         
                           
                             T 
                             _ 
                           
                           q 
                         
                       
                       ] 
                     
                   
                 
                 , 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
   
   It is obvious that the two elements on the two respective diagonals of the channel model have a certain relation: the two elements on the main diagonal are the same, and the signs of the two elements on the other diagonal are opposite. Generally, the channel model H is not known in advance, so the equalizer  130  executes an adaptive algorithm to find the adaptive form of the channel model H. One frequently utilized adaptive algorithm is the Least-Mean-Square Algorithm. (Please refer to “Least-Mean-Square Adaptive Filters”, Ch. 5 of “Adaptive Filter Theory”, by Simon Haykin, 4th Ed., 2004, ISBN: 0-1304-8434-2.) As a result, the adaptive algorithm can be expressed as: 
   
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             w 
                             _ 
                           
                           i 
                         
                         
                           
                             w 
                             _ 
                           
                           q 
                         
                       
                       ⁢ 
                       
                         
                           - 
                           
                             
                               w 
                               _ 
                             
                             q 
                           
                         
                         
                           
                             w 
                             _ 
                           
                           i 
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               H 
                               _ 
                             
                             i 
                           
                           
                             
                               H 
                               _ 
                             
                             q 
                           
                         
                         ⁢ 
                         
                           
                             - 
                             
                               
                                 H 
                                 _ 
                               
                               q 
                             
                           
                           
                             
                               H 
                               _ 
                             
                             i 
                           
                         
                       
                       ] 
                     
                     
                       - 
                       1 
                     
                   
                 
                 , 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
   
   where 
                 w   _     i     ⁡     (     n   +   1     )       =           w   _     i     ⁡     (   n   )       +     u   ·     (           e   i     ⁡     (   n   )       ·         R   _     i     ⁡     (   n   )         +         e   q     ⁡     (   n   )       ·         R   _     q     ⁡     (   n   )           )                             w   _     q     ⁡     (     n   +   1     )       =           w   _     q     ⁡     (   n   )       +     u   ·     (           e   q     ⁡     (   n   )       ·         R   _     i     ⁡     (   n   )         +         e   i     ⁡     (   n   )       ·         R   _     q     ⁡     (   n   )           )           ,         
u being the step-size, e i  and e q  being errors, and  R   i  and  R   q  being data.
 
   Similarly, in the adaptive matrix w, the two elements on the main diagonal are the same, and the signs of the two elements on the other diagonal are opposite. 
   The equalizer  130  takes the adaptive matrix was its equalization coefficient to equalize the signals S 1 _I′ and S 1 _Q′. The required signals S 1 _I″ and S 1 _Q″ are then generated. Therefore, the equalizer  130  solves the multi-path problem occurring from signals passing through the channel. 
   Consequently, after the down-converted signals S 1 _I and S 1 _Q are processed by the IQ imbalance compensating circuit  120  and the equalizer  130 , the IQ imbalance caused by the direct down converter  110  is compensated and the multi-path issue caused by the channel is also solved. However, as mentioned above, the IQ imbalance compensating circuit  120  utilizes a calibration method whose characteristic is that the IQ imbalance compensating circuit  120  calibrates only one time according to a specific frequency, and afterwards the IQ imbalance compensating circuit  120  utilizes the same compensation coefficient to compensate the signals of all kinds of frequencies in all operational conditions. In practical operation, a change in temperature may directly affect the direct down converter  110 , so the IQ imbalance changes due to the change in temperature. In addition, the IQ imbalance also changes with respect to signals of different frequencies. As a result, a compensation circuit is required to compensate IQ imbalance of signals of different frequencies in all kinds of changes in the operational conditions, such as a temperature change. 
   SUMMARY OF THE INVENTION 
   One of the objectives of the present invention is to provide a method and a circuit for equalizing and compensating for IQ imbalances simultaneously to solve the above-mentioned problem. 
   One of the objectives of the present invention is to provide a method and a circuit which compensates the IQ imbalance of the signal and equalizes the signal. Hence the cost of the circuitry is lowered. 
   One of the objectives of the present invention is to provide a method and a circuit which utilizes the characteristics of the adaptive algorithm. The compensation can find more proper compensation coefficients according to different signal characteristics (e.g., different frequencies) and different operating environments (e.g., different temperatures). As a result, the IQ imbalance is more effectively compensated. 
   According to an embodiment of the claimed invention, a method for equalizing a signal is disclosed. The method comprises: down-converting an RF signal to generate a baseband signal; and utilizing an adaptive equalizer to process the baseband signal such that the baseband signal is equalized and an IQ imbalance of the baseband signal is compensated simultaneously. 
   According to another embodiment of the claimed invention, a circuit for equalizing and compensating IQ imbalance simultaneously is disclosed. The circuit comprises a down converter and an adaptive equalizer. The down converter down-converts an RF signal to generate a baseband signal. The adaptive equalizer, which is coupled to the down converter, receives the baseband signal and processes the baseband signal such that the baseband signal is equalized and an IQ imbalance of the baseband signal is compensated simultaneously. 
   According to another embodiment of the claimed invention, a method for equalizing a signal is disclosed. The method comprising: down-converting an RF signal to generate a baseband signal; generating a plurality of equalization coefficients, wherein the equalization coefficients correspond to channel response and IQ imbalance; and processing the baseband signal according to the equalization coefficients to equalize the baseband signal and compensate an IQ imbalance of the baseband signal. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a block diagram of a prior art receiver  100 . 
       FIG. 2  is a diagram illustrating a receiver  200  according to the present invention. 
       FIG. 3  shows a decision feedback equalizer according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   To generate a real time compensation coefficient for IQ imbalance with respect to signals of different frequencies in all kinds of operation conditions, the present invention performs signal equalization and IQ imbalance compensation simultaneously in one single equalizer based on the characteristic of the adaptive algorithm. 
   Please refer to  FIG. 2 .  FIG. 2  is a diagram illustrating a receiver  200  according to the present invention. The same as prior art, after the signal S 1  passes through the direct down converter  210 , two paths of signals, S 1 _I and S 1 _Q, are generated. Because of the imperfection of the direct down converter  210 , the two signals, S 1 _I and S 1 _Q, which respectively pass through the low pass filters  212  and  214 , have an IQ imbalance between each other. However, in the receiver  200 , the two signals, S 1 _I and S 1 _Q, are equalized and IQ imbalance compensated at the same time by the adaptive equalizer  220 . 
   As mentioned above, in the direct down converter  210 , the two signals (g sin wt and g cos wt) utilized by the mixer  216  and the mixer  218  may have un-matched gains and non-orthogonal phases. Assuming that the gain imbalance is α and the phase imbalance is θ, for example, one signal is (g+α) sin wt, and the other signal is g cos(wt+θ), accordingly, the characteristic matrix of the direct down converter  210  is obtained as follows: 
   
     
       
         
           [ 
           
             
               
                 
                   ( 
                   
                     1 
                     + 
                     α 
                   
                   ) 
                 
                 ⁢ 
                 
                   cos 
                   ⁡ 
                   
                     ( 
                     
                       θ 
                       / 
                       2 
                     
                     ) 
                   
                 
               
               
                 
                   - 
                   
                     ( 
                     
                       1 
                       - 
                       α 
                     
                     ) 
                   
                 
                 ⁢ 
                 
                   sin 
                   ⁡ 
                   
                     ( 
                     
                       θ 
                       / 
                       2 
                     
                     ) 
                   
                 
               
             
             ⁢ 
             
               
                 
                   - 
                   
                     ( 
                     
                       1 
                       + 
                       α 
                     
                     ) 
                   
                 
                 ⁢ 
                 
                   sin 
                   ⁡ 
                   
                     ( 
                     
                       θ 
                       / 
                       2 
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   
                     1 
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                   ) 
                 
                 ⁢ 
                 
                   cos 
                   ⁡ 
                   
                     ( 
                     
                       θ 
                       / 
                       2 
                     
                     ) 
                   
                 
               
             
           
           ] 
         
       
     
   
   Taking the multi-path issue of the channel and the IQ imbalance of the direct down converter  210  as a whole effect, the equation (2) can be further expressed as: 
   
     
       
         
           
             [ 
             
               
                 
                   R 
                   i 
                 
                 ⁡ 
                 
                   ( 
                   n 
                   ) 
                 
               
               
                 
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                   q 
                 
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                   ( 
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             ] 
           
           = 
           
             
               
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                         H 
                         _ 
                       
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                         H 
                         _ 
                       
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                         _ 
                       
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                         ( 
                         
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                         ) 
                       
                       ⁢ 
                       
                         cos 
                         ⁡ 
                         
                           ( 
                           
                             θ 
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                           ) 
                         
                       
                     
                     
                       
                         - 
                         
                           ( 
                           
                             1 
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                           ) 
                         
                       
                       ⁢ 
                       
                         sin 
                         ⁡ 
                         
                           ( 
                           
                             θ 
                             / 
                             2 
                           
                           ) 
                         
                       
                     
                   
                   ⁢ 
                   
                     
                       
                         - 
                         
                           ( 
                           
                             1 
                             + 
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                         sin 
                         ⁡ 
                         
                           ( 
                           
                             θ 
                             / 
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                           ) 
                         
                       
                     
                     
                       
                         ( 
                         
                           1 
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                         ) 
                       
                       ⁢ 
                       
                         cos 
                         ⁡ 
                         
                           ( 
                           
                             θ 
                             / 
                             2 
                           
                           ) 
                         
                       
                     
                   
                 
                 ] 
               
             
             ⁡ 
             
               [ 
               
                 
                   
                     T 
                     _ 
                   
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                     T 
                     _ 
                   
                   q 
                 
               
               ] 
             
           
         
       
     
   
   
     
       
         
           
             
               
                 = 
                 
                   
                     [ 
                     
                       
                         
                           
                             H 
                             _ 
                           
                           11 
                         
                         
                           
                             H 
                             _ 
                           
                           21 
                         
                       
                       ⁢ 
                       
                         
                           
                             H 
                             _ 
                           
                           12 
                         
                         
                           
                             H 
                             _ 
                           
                           22 
                         
                       
                     
                     ] 
                   
                   ⁡ 
                   
                     [ 
                     
                       
                         
                           T 
                           _ 
                         
                         i 
                       
                       
                         
                           T 
                           _ 
                         
                         q 
                       
                     
                     ] 
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 4 
               
             
           
         
       
     
   
   Similarly, because the channel model H is unknown, the adaptive algorithm is adopted to find the adaptive matrix w′ of the matrix 
   
     
       
         
           
             [ 
             
               
                 
                   
                     H 
                     _ 
                   
                   11 
                 
                 
                   
                     H 
                     _ 
                   
                   21 
                 
               
               ⁢ 
               
                 
                   
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           ⁢ 
           
               
           
         
       
     
   
   
     
       
         
           
             
               
                 
                   [ 
                   
                     
                       
                         
                           
                             w 
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                           11 
                         
                         ⁡ 
                         
                           ( 
                           n 
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                           21 
                         
                         ⁡ 
                         
                           ( 
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                           12 
                         
                         ⁡ 
                         
                           ( 
                           n 
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                             w 
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                           22 
                         
                         ⁡ 
                         
                           ( 
                           n 
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                   ] 
                 
                 = 
                 
                   
                     [ 
                     
                       
                         
                           
                             H 
                             _ 
                           
                           11 
                         
                         
                           
                             H 
                             _ 
                           
                           21 
                         
                       
                       ⁢ 
                       
                         
                           
                             H 
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                           12 
                         
                         
                           
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                           22 
                         
                       
                     
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                     - 
                     1 
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
   
   where, 
   
     
       
         
           
             
               
                 w 
                 _ 
               
               11 
             
             ⁡ 
             
               ( 
               
                 n 
                 + 
                 1 
               
               ) 
             
           
           = 
           
             
               
                 
                   w 
                   _ 
                 
                 11 
               
               ⁡ 
               
                 ( 
                 n 
                 ) 
               
             
             + 
             
               u 
               · 
               
                 
                   e 
                   i 
                 
                 ⁡ 
                 
                   ( 
                   n 
                   ) 
                 
               
               · 
               
                 
                   
                     R 
                     _ 
                   
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                 ⁡ 
                 
                   ( 
                   n 
                   ) 
                 
               
             
           
         
       
     
     
       
         
           
             
               
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                 _ 
               
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             ⁡ 
             
               ( 
               
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                 + 
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               ) 
             
           
           = 
           
             
               
                 
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                 12 
               
               ⁡ 
               
                 ( 
                 n 
                 ) 
               
             
             + 
             
               u 
               · 
               
                 
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                   ( 
                   n 
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                     _ 
                   
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                   ( 
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               ( 
               
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                 ( 
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             + 
             
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               · 
               
                 
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                   ( 
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                 w 
                 _ 
               
               22 
             
             ⁡ 
             
               ( 
               
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                 + 
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           = 
           
             
               
                 
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                   _ 
                 
                 22 
               
               ⁡ 
               
                 ( 
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                 ) 
               
             
             + 
             
               u 
               · 
               
                 
                   e 
                   q 
                 
                 ⁡ 
                 
                   ( 
                   n 
                   ) 
                 
               
               · 
               
                 
                   
                     R 
                     _ 
                   
                   q 
                 
                 ⁡ 
                 
                   ( 
                   n 
                   ) 
                 
               
             
           
         
       
     
   
   u being the step-size, e i  and e q  being errors, and  R   i  and  R   q  being data. In general, one frequently utilized adaptive algorithm is Least-Mean-Square (LMS) Algorithm. The more detailed description of LMS Algorithm can refer to “Adaptive Filter Theory, Chapter 5 Least-Mean-Square Adaptive Filters”, SIMON HAYKIN, p 231-247, which is incorporated by reference. The Least-Mean-Square Algorithm serves as an example of the adaptive algorithm in the present invention, but is not meant to limit the scope of the present invention. 
   Equation (5) shows that the four elements of the adaptive matrix w′ have the factors for compensating channel response and IQ imbalance. These four elements are derived independently and therefore independent to each other. Generally, the two elements on the main diagonal of the adaptive matrix w′ are not definitely equal, and the absolute values of the two elements on the other diagonal of the adaptive matrix w′ are not definitely equal. The values of these four elements are probably different. The adaptive equalizer  220  adopts the adaptive matrix w′ as its equalization coefficient to compensate the IQ imbalance of the signals S 1 _I and S 1 _Q and solve the multi-path iswsue caused by the channel. In this embodiment, the adaptive equalizer  220  can be feed-forward linear equalizer (Ffe) or a decision feedback equalizer (DFE). 
   Please refer to  FIG. 3 .  FIG. 3  shows the decision feedback equalizer according to an embodiment of the present invention. This embodiment serves as an example of the adaptive equalizer in the present invention, but is not meant to limit the scope of the present invention. The decision feedback equalizer comprises a feed-forward equalizer (FFE)  310 , a slicer  320 , and a feedback equalizer (FBE)  330 . The feed-forward equalizer  310 , the slicer  320 , and the feedback equalizer  330  are well known to those skilled in the art, so the description is omitted for brevity. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.