Abstract:
Threshold setting apparatus controls a relative relation in DC level between a baseband signal produced by processing a received signal and a threshold for use in identifying serial data from the baseband signal. The apparatus includes a waveform distortion estimating circuit and a setting circuit. The waveform distortion estimating circuit estimates, based on the baseband signal, a relation between the center of the dynamic range of the baseband signal and a crossing point where a positive-going edge curve crosses a negative-going edge curve of an eye pattern formed by the baseband signal. The setting circuit adjustably sets either the threshold or the DC level of the baseband signal in dependence upon the relation estimated by the waveform distortion estimating circuit.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to apparatus for setting an identification threshold for use in identifying and reproducing serial data from a baseband signal received or automatically setting the DC level of a received baseband signal, and advantageously applicable to an optical signal receiver configured to receive, e.g. an intensity-modulated optical signal.  
           [0003]    2. Description of the Background Art  
           [0004]    An optical signal receiver of the type is conventional which receives from a transmitter, e.g. an optical pulse signal modulated in intensity with the logical level of data to be sent. In this type of optical receiver, the received optical signal is converted to an electric signal corresponding thereto, and then compared in level with an identification threshold in order to determine the logical level of data received.  
           [0005]    It is a common practice with the optical signal receiver of the type referred to above to set the identification threshold at the center of a dynamic range between the peak and bottom levels of the received electric signal. This scheme is extensively used because of its simplicity and versatility. However, when the transmitter uses, e.g. an EA (Electro-Absorption) modulator, the crossing point of the received signal where the curves of a positive-going and a negative-going edge cross each other when occurring at the same timing is apt to fail to coincide with the center of the dynamic range. In such a case, the optimum threshold produced in the receiver is also shifted from the center. Further, it is likely that the optimum threshold is shifted from the center due to the dispersion of wavelength or polarization mode.  
           [0006]    In light of the above, systems for automatically adjusting the threshold in accordance with the received signal have been proposed in the past. Japanese patent laid-open publication No. 265273/1996, for example, discloses a system including a number-of-errors detecting circuit configured to determine the degree of errors having occurred in serial data identified and reproduced. The degree of errors determined is reflected back to a new identification threshold. This kind of threshold setting system, however, needs a high-speed, sophisticated logic integrated circuit for detecting errors in serial data. It was therefore difficult to implement simple, low-cost apparatus for setting an identification threshold.  
         SUMMARY OF THE INVENTION  
         [0007]    It is an object of the present invention to provide threshold setting apparatus capable of automatically, relatively setting an identification threshold with low-speed, simple circuit arrangement.  
           [0008]    Threshold setting apparatus in accordance with the present invention controls a relative relation in DC level between a baseband signal produced by processing a received signal and a threshold for use in identifying data from the baseband signal. The threshold setting apparatus includes a waveform distortion estimating circuit and a setting circuit. The waveform distortion estimating circuit estimates, based on the baseband signal, a relation between the center of the dynamic range of the baseband signal and a crossing point where a positive-going edge curve crosses a negative-going edge curve of an eye pattern formed by the baseband signal to produce estimated information. The setting circuit varies either one of the threshold and the DC level of the baseband signal in dependance upon the information estimated by the waveform distortion estimating circuit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The objects and features of the present invention will become more apparent from consideration of the following detailed description taken in conjunction with the accompanying drawings in which:  
         [0010]    [0010]FIG. 1 is a schematic block diagram showing an optical signal receiver embodying the present invention;  
         [0011]    [0011]FIGS. 2A, 2B and  2 C are charts for useful for understanding a relation between a signal waveform and a threshold with the illustrative embodiment shown in FIG. 1;  
         [0012]    [0012]FIG. 3 is a schematic block diagram, like FIG. 1, showing an alternative embodiment of the present invention;  
         [0013]    [0013]FIGS. 4A, 4B and  4 C are charts useful for understanding a relation between a signal waveform and a threshold with the alternative embodiment; and  
         [0014]    [0014]FIG. 5 is a schematic block diagram, like FIG. 1, showing another alternative embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    Referring to FIG. 1 of the drawings, an optical signal receiver, generally  10 A, to which applied is threshold setting apparatus embodying the present invention is made up of an O/E (Optical-to-Electrical) converter or transducer  11 , an amplifier  12 , a clock separator  13 , an identification and reproduction unit  14 , a peak detector  15 , a DC level detector  16 , a bottom detector  17 , and a threshold calculator  18  interconnected as illustrated. The peak detector  15 , DC detector  16 , bottom detector  17  and threshold calculator  18  constitute in combination a threshold setting circuit  116  for setting an identification threshold.  
         [0016]    The O/E converter  11  is adapted to convert an incoming optical signal  100  to a corresponding baseband electric signal  102 . In the following, signals are designated with reference numerals directed to connections on which the signals appear. With the illustrative embodiment, the optical signal  100  is subjectedto, e.g. intensity modulation by a remote transmitter, not shown, which is adapted to modulate an optical beam in intensity with serial data to be sent, which may be a pulse signal, and transmit an optical signal thus modulated whose intensity goes high or low. The serial data is implemented as, e.g. an NRZ (Non-Return-to-Zero) signal or a modified NRZ signal with the embodiment. A wavelength filter, not shown, may be located at a stage preceding, or at the input stage of, the O/E converter  11 , if desired.  
         [0017]    The amplifier  12  amplifies the electric signal or received baseband signal  102  output from the O/E converter  11 , and may include, e.g. an AGC (Automatic Gain Control) function. The amplified electric signal  104  output from the amplifier  12  is branched away into two signals, i.e. a signal for receipt processing  106  and a signal for threshold control  108 . The amplifier  12  may be omitted if the output level  102  of the O/E converter  11  is sufficiently high.  
         [0018]    The clock separator  13  is adapted to separate from the signal for receipt processing  106  output from the amplifier  12  a timing clock particular to the serial data sent from the transmitting station. The separated timing clock  110  is fed from the clock separator  13  to the identification and reproduction unit  14 .  
         [0019]    The identification and reproduction unit  14  is adapted for comparing the level of the signal for receipt processing  106  with a threshold or threshold signal  112 , which will be described later, at a timing determined by the timing clock  110  separated by the clock separator  13 . The identification and reproduction unit  14  determines, based on the result of the comparison, a code carried by, or the logical level of, the signal for receipt processing  106  to reproduce or restore serial data transmitted from the transmitter on its output port  114 .  
         [0020]    The threshold setting circuit  116 , made up of the peak detector  15 , DC detector  16 , bottom detector  17  and threshold calculator  18 , is adapted to automatically set the identification threshold to provide the identification and reproduction unit  14  with the threshold signal  112 . Specifically, the peak detector  15  detects and holds the peak level, i.e. logical ONE, of the signal for threshold control  108  output from the amplifier  12  and feeds the peak level signal  118  to the threshold calculator  18 . For the peak detector  15 , use may be made of a conventional peak hold circuit.  
         [0021]    The DC level detector  16  detects and holds the DC level of the signal for threshold control  108  output from the amplifier  12  and delivers the DC level signal  120  to the threshold calculator  18 . The DC detector  16  may be implemented as an LC filter or similar low-pass filter (LPF) by way of example. When use is made of a low-pass filter, the DC detector  16  can detect a DC level if the low-pass filter is provided with a cutoff frequency sufficiently lower than the minimum frequency of the signal for receipt processing  108 .  
         [0022]    The bottom detector  17  detects and holds the bottom level or logical ZERO of the signal for threshold control  108  output from the amplifier  12  and delivers the bottom level signal  122  to the threshold calculator  18 . The bottom detector  17  may be implemented as a conventional bottom level hold circuit.  
         [0023]    In an application in which the peak and bottom level hold circuits implementing the peak detector  15  and bottom detector  17 , respectively, are of the type needing a signal indicative of a detection timing, the timing clock  110  output from the clock separator  13  may be supplied to the peak and bottom level hold circuits.  
         [0024]    The threshold calculator  18  establishes the threshold to feed the identification and reproduction unit  14  with the threshold signal  112 , in response to the peak level, DC level and bottom level signals  118 ,  120  and  122  received from the peak detector  15 , DC detector  16  and bottom detector  17 , respectively. How the threshold calculator  18  produces the threshold will be described in detail later in conjunction with the operation of the optical signal receiver  10 A.  
         [0025]    In operation, an optical signal  100  input to the optical signal receiver  10 A is converted to an electric baseband signal  102  by the O/E converter  11  and then amplified by the amplifier  12  to a desired amplitude. The amplified electric signal  104  is input to the clock separator  13  and identification and reproduction unit  14  as a signal for receipt processing  106  on one hand and to the peak detector  15 , DC detector  16  and bottom detector  17  as a signal for threshold control  108  on the other hand.  
         [0026]    The clock separator  13  separates a timing clock  110  from the signal for receipt processing  106  and feeds the timing clock  110  to the identification and reproduction unit  14 . The identification and reproduction unit  14  identifies and reproduces a code carried by, or the logical level of, the received signal for receipt processing  106  in synchronism with the separated timing clock  110  to output the restored signal  114  having the code or logical level thus determined. For this identification and reproduction, a threshold for decision  112  is fed from the threshold calculator  18  to the identification and reproduction unit  14 , as stated earlier.  
         [0027]    How the threshold setting circuit  116  sets a threshold for decision  112  will be described more specifically hereinafter. The peak detector  15  detects and holds the peak level or absolute maximum level of the signal for threshold control  108 . The DC level detector  16  detects and holds the absolute DC level of the signal for threshold control  108 . The bottom detector  17  detects and holds the bottom level or absolute minimum level of the signal for threshold control  108 . The resulting information on peak level  118 , DC level  120  and bottom level  122  is input to the threshold calculator  18 . The threshold calculator  18  in turn determines an optimum threshold in response to the information  118 ,  120  and  122  and feeds the identification and reproduction unit  14  with the optimum threshold as a threshold signal  112 .  
         [0028]    To better understand the operation of the threshold setting circuit  116 , reference will be made to FIGS. 2A, 2B and  2 C, each of which shows a particular condition in which a peak level  124 , a DC level  126  and a bottom level  128  are detected. FIG. 2A shows an eye pattern appearing when the incoming optical signal  100  is an ideal NRZ signal. In this case, the DC level  126  is positioned just at the center of the dynamic range between the peak level  124  and the bottom level  128 . In an application where an optical amplifier is absent at a stage preceding the O/E converter  11 , the optimum threshold is on the center of the dynamic range between the peak level  124  and the bottom level  128 , i.e. corresponds to the DC level  126 .  
         [0029]    [0029]FIG. 2B shows an eye pattern appearing when the crossing point  130  is shifted downward. This kind of eye pattern, i.e. waveforms, is apt to appear when the transmitter uses, e.g. an EA modulator. That is also the case with the dispersion of wavelength or polarization mode, or the nonlinearity involved in the receiver system. As shown, the DC level  126  is positioned below the center of the dynamic range between the peak level  124  and the bottom level  128 , and so is the optimum threshold.  
         [0030]    [0030]FIG. 2C shows another eye pattern, which appears when a crossing point  130  is shifted upward. This kind of eye pattern is also apt to appear due to the dispersion of wavelength or polarization mode, or the nonlinearity of the receiver system. As shown, the DC level  126  is positioned above the center of the dynamic range between the peak level  124  and the bottom level  128 , and so is the optimum threshold.  
         [0031]    As stated above, although the optimum threshold depends on the received waveform, it can be determined by using the peak, DC and bottom levels. The threshold calculator  18  determines an optimum threshold, relying upon those three parameters, and delivers the optimum threshold thus determined to the identification and reproduction unit  14  as a threshold signal  112 .  
         [0032]    More specifically, the threshold calculator  18  produces an optimum threshold by using a ternary equation including the three parameters mentioned above as variables. For example, assuming that the peak, DC and bottom levels are represented by x, y and z, respectively, an optimum threshold TH op  is expressed as:  
           TH   op   =a ( y −( x+z )/2)+( x+z )/2,   (1)  
         [0033]    where the term (x+z)/2 corresponds to the center of the dynamic range, and the letter a denotes an adjustment coefficient that is around unity.  
         [0034]    The Equation (1) therefore indicates that the threshold calculator  18  produces an adjustment amount that reflects a difference between the center of the dynamic range and the DC level by a ratio represented by the adjustment coefficient a, and then selects a level shifted from the center of the dynamic range by the adjustment amount.  
         [0035]    As far as the Equation (1) is concerned, the adjustment coefficient a is dealt with as a constant. However, the optimum value of the adjustment coefficient a may vary, depending on the system configuration of the optical signal transmitter, transmission path and optical signal receiver  10 A. In light of this, the threshold calculator  18  should preferably be adapted for varying the adjustment coefficient a. As the Equation (1) indicates, if the adjustment coefficient a is unity by way of example, then the optimum threshold TH op  is y, i.e. the threshold coincides with the DC level. Stated another way, when it is desired to make the threshold coincide with the DC level, the adjustment coefficient a of unity suffices.  
         [0036]    Some different configurations are available for the threshold calculator  18  to solve the Equation (1). In one specific configuration, use is made of an analog circuit including, e.g. an operational amplifier. In another specific configuration, each of the input stages of the threshold calculator  18  connected to the peak detector  15 , DC detector  16  and bottom detector  17 , respectively, may include an analog-to-digital (AD) converter, not shown. With such a configuration, the threshold calculator  18  is adapted to solve the Equation (1) by a digital circuit or software processing. In a further specific configuration, each of the input stages of the threshold calculator  18  is adapted to include an AD converter while the peak detector  15 , DC detector  16 , bottom detector  17  and threshold calculator  18  themselves are all implemented as digital circuits or by software.  
         [0037]    As stated above, the illustrative embodiment can estimate the distortion of the waveform of a received signal  102  on the basis of a peak, a DC and a bottom level detected to automatically, relatively set an adequate threshold for identification and reproduction. In addition, the peak, DC and bottom levels can be detected by low-speed or analog circuitry, so that the threshold setting circuit  116  is simple in structure and low in cost. Feedback control used to set a threshold would make the threshold setting circuit sophisticated in structure and high in cost although it might be desirable in the aspect of threshold optimization.  
         [0038]    Reference will now be made to FIG. 3 for describing an alternative embodiment of the present invention, which is also applied to an optical signal receiver. In FIG. 3, blocks like those shown in FIG. 1 are designated by identical reference numerals and will not be described specifically in order to avoid redundancy. As shown, the optical signal receiver, generally  10 B, includes a band-pass filter (BPF)  21 , a phase shifter  22 , a phase comparator  23  and a threshold calculator  24  as well as the O/E converter  11 , amplifier  12 , clock separator  13 , and identification and reproduction unit  14  as connected as illustrated.  
         [0039]    The band-pass filter  21  is adapted to separate from the signal for threshold control  108  output from the amplifier  12  a high-frequency signal component  132  whose frequency is coincident with the bit rate of the received signal  104 . The high-frequency signal component  132  thus separated is input to the phase shifter  22 . For example, if the bit rate of the signal  104  output from the amplifier  12  has a bit rate of 10 Gbit/s, then the band-pass filter  21  is designed to separate a high-frequency signal component of 10 GHz. In the following description, let the bit rate be assumed to be 10 Gbit/s by way of example. The band-pass filter  21  may be configured to separate high-frequency components of both in-phase and opposite-phase and superpose both of them on each other.  
         [0040]    The phase shifter  22  is adapted to shift the phase of the high-frequency signal  132  output from the band-pass filter  21  by a preselected amount. For this purpose, the phase shifter  22  may delay the high-frequency signal  132  by a preselected period of time. The amount of phase shift or delay time will be described later in detail.  
         [0041]    The phase comparator  23  is adapted to determine the amplitude of the high-frequency signal  134  and a phase difference of the high-frequency signal  134  from the timing clock  110  input thereto. The phase comparator  23  feeds signals  136  representative of the phase difference and amplitude thus determined to the threshold calculator  24 . The threshold calculator  24  is adapted for determining an optimum threshold on the basis of the signals  136  input from the phase comparator  23  and feeds the optimum threshold to the identification and reproduction unit  14  as a threshold signal  112 .  
         [0042]    The amount of phase shift or delay time is selected in such a fashion that, assuming that the electric signal  104  output from the amplifier  12  has an ideal waveform, the high-frequency signal  134  output from the phase shifter  22  differs in phase from the timing clock  110  output from the clock separator  13  by a preselected amount, which may be e.g. zero.  
         [0043]    The phase shifter  22  is used to match the phases of the two signals  134  and  110  input to the phase comparator  23 . Such phase matching function may alternatively be implemented by disposing a phase shifter in the connection  110  between the clock separator  13  and the phase comparator  23 , if desired. The phase shifter  22  may even be omitted if the length of the connections assigned to the timing clock  110  output from the clock separator  13  and to the high-frequency signal  134  output from the band-pass filter  21  are adequately selected.  
         [0044]    The operation of the optical signal receiver  10 B will be described hereinafter. It is to be noted that the O/E converter  11 , amplifier  12 , clock separator  13  and identification and reproduction unit  14  operate to reproduce a received signal in exactly the same manner as described with reference to FIG. 1. One of the two signals branched away after the amplification of the amplifier  12  is input to the band-pass filter  21  as a signal for threshold control  108 .  
         [0045]    The band-pass filter  21  separates a high-frequency signal  132  whose frequency is coincident with the bit rate of the input signal  102 . The high-frequency signal  132  output from the band-pass filter  21  is shifted in phase by the phase shifter  22  by the preselected amount and then input to the phase comparator  23 . The timing clock  110  output from the clock separator  13  is also input to the phase comparator  23 . The phase comparator  23  determines a phase difference between the high-frequency signal  134  and the timing clock  110  and the amplitude of the high-frequency signal  134 , and delivers them to the threshold calculator  24 .  
         [0046]    The threshold calculator  24  determines an optimum threshold on the basis of the phase difference and amplitude input thereto, and feeds the resulting threshold signal  136  to the identification and reproduction unit  14 . To better understand the illustrative embodiment, FIGS. 4A, 4B and  4 C each show a particular relation between the waveforms or eye pattern of the signal for threshold control  108  and the separated high-frequency signal  134 .  
         [0047]    Specifically, FIG. 4A shows the eye pattern of an ideal signal for threshold control  108  (NRZ signal). As shown, when the signal for threshold control  108  is ideal, the upper and lower portions of the eye pattern are symmetrical to each other, i.e. the crossing point  130  is coincident with the center of the dynamic range between the peak level  124  and the bottom level  128 . In this condition, a 10 GHz high-frequency signal does not appear.  
         [0048]    [0048]FIG. 4B shows a condition wherein the crossing point  130  is lower than the center of the dynamic range. In this case, a 10 GHz high-frequency signal appears in a phase shown in FIG. 4B. As the shift of the crossing point  130  from the center of the dynamic range increases, the amplitude of the high-frequency signal  134  increases, too.  
         [0049]    [0049]FIG. 4C shows another condition, where the crossing point  130  is higher than the center of the dynamic range. In this case, a 10 GHz high-frequency signal appears in a phase shown in FIG. 4C. As shown, the phase of the high-frequency signal  132  differs from the phase of the high-frequency signal shown in FIG. 4B by 180° (         ). Again, as the shift of the crossing point  130  from the center of the dynamic range increases, the amplitude of the high-frequency signal  134  increases, too.  
         [0050]    As FIGS. 4A, 4B and  4 C indicate, by detecting the phase and amplitude of the high-frequency signal  132 , it is possible to quantitatively determine the direction in which the crossing point  130  is shifted as well as the amount of shift. It is to be noted that the cases shown in FIGS. 4B and 4C are distinguished from each other in consideration of their relation with the timing clock  110  output from the clock separator  13 .  
         [0051]    The threshold calculator  24  produces a threshold on the basis of the signal  136  representing the phase and amplitude of the high-frequency signal  134  detected. To produce a threshold, the threshold calculator  24  may be implemented as either one of an analog and a digital circuit. In the case of a digital circuit, there should be included a device for digitizing the phase and amplitude of the high-frequency signal  134 . Alternatively, the threshold calculator  24  may be implemented by software so long as it includes a device for digitizing the phase and amplitude of the high-frequency signal  134 .  
         [0052]    As stated above, the illustrative embodiment can separate a high-frequency signal  134  whose frequency is coincident with the bit rate of the signal for threshold control  108  and estimate the distortion of a received signal  102  on the basis of the high-frequency signal  108  to automatically set an adequate threshold for identification and reproduction. Further, the circuitry for the detection of a high-frequency signal can be implemented as analog circuitry, making the threshold setting circuit  116  simple in structure and low in cost.  
         [0053]    Reference will further be made to FIG. 5 for describing another alternative embodiment of the present invention, which is also applied to an optical signal receiver. To enhance the accuracy in reproduction of a received signal  114 , the embodiment shown in FIG. 1 varies the level of the identification threshold  112  to be compared with the signal for receipt processing  106  by the identification and reproduction unit  14  for thereby compensating for the waveform distortion included in the received signal  102 . By contrast, the embodiment shown in FIG. 5 achieves the same object by varying the DC level of the signal for receipt processing  106 . In FIG. 5, blocks like those shown in FIG. 1 are designated by identical reference numerals and will not be described specifically in order to avoid redundancy.  
         [0054]    As shown in FIG. 5, the optical signal receiver, generally  10 C, includes a DC level shifter  19  interconnected between the amplifier  12  and the identification and reproduction unit  14 C. The signal for receipt processing  104  output from the amplifier  12  is input to the clock separator  13  and DC level shifter  19 . The clock separator  13  separates a timing clock from the signal for receipt processing  106  and feeds an identification and reproduction unit  14 C with the timing clock  110  as in the embodiment of FIG. 1.  
         [0055]    The DC level shifter  19  is adapted to shift the DC level of the signal for receipt processing  106  by a shift amount  142  indicated by a shift calculator  18 C, which will be described later, and feeds the identification and reproduction unit  14 C with a signal  138  representative of the resultant, shifted DC level. By so shifting the DC level, the DC shifter  19  shifts the entire waveform of the signal for receipt processing  106 .  
         [0056]    In the illustrative embodiment, the identification and reproduction unit  14 C is adapted to compare the signal for receipt processing  106  output from the DC shifter  19  with a fixed threshold value  140  at a timing determined by the timing clock  110 , which is output from the clock separator  13 . The identification and reproduction unit  14 C determines, or reproduces, the code or logical level of serial data  102  sent from a transmitter in accordance with the result of the above comparison.  
         [0057]    The shift calculator  18 C, which corresponds to the threshold calculator  18  shown in FIG. 1, is adapted to produce a shift amount  142 , which is supplied to the DC level shifter  19 . The shift amount  142  corresponds to a difference of the threshold  112  which the threshold calculator  18  of the FIG. 1 embodiment calculates in accordance with the Equation (1) from the fixed threshold  140  input to the identification and reproduction unit  14 C with its sign (positive or negative) inverted. The shift calculator  18 C then feeds the DC shifter  19  with the resultant shift amount  142 .  
         [0058]    It is to be noted that the identification and reproduction units  14  and  14 C are identical with each other as to the relative result of comparison between the signal for receipt processing  106  and the threshold although the former varies the level of the threshold  112  while the latter varies the DC level of the signal for receipt processing  106 . The illustrative embodiment shown in FIG. 5 therefore achieves the same advantages as the embodiment shown in FIG. 1.  
         [0059]    The illustrative embodiment shares the same concept with the embodiment shown in FIG. 1, but varies the DC level of the signal for receipt processing  106  instead of the level of the threshold, as stated above. Alternatively, on the basis of the concept of the embodiment shown in FIG. 3, the illustrative embodiment may be adapted to estimate the waveform distortion of a signal and vary the DC level of the signal for receipt processing  106  instead of the level of the threshold, if desired.  
         [0060]    The adjustment coefficient a included in the Equation (1) of the embodiment described with reference to FIG. 1 is dealt with as a constant. If desired, the adjustment constant a may be varied in accordance with, e.g. the bit error rate of data  114  output from the comparison and reproduction unit  14 .  
         [0061]    While the illustrative embodiments have been shown and described as being applied to an optical signal receiver, the present invention is similarly applicable to a receiver of the kind receiving an electric or an electromagnetic signal. For example, the present invention is applicable to setting a threshold for determining the code of a baseband signal, into which a bilevel FSK (Frequency Shift Keying) or similar digital signal is modulated.  
         [0062]    In summary, it will be seen that the present invention provides threshold setting apparatus capable of automatically, relatively setting an identification threshold with a low-speed and simple circuit arrangement.  
         [0063]    The entire disclosure of Japanese patent application No. 2002-168156 filed on Jun. 10, 2002, including the specification, claims, accompanying drawings and abstract of the disclosure is incorporated herein by reference in its entirety.  
         [0064]    While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.