Abstract:
Precision AC and DC voltage, current, phase, power and energy measurements and calibrations with current ranges from 1 uA to 20 kA and voltage ranges from 1V to 1000 kV are now performed with accuracies of better than one part per million. Continued demand for improved accuracy has led the inventors to address remnant magetization within the current comparators that form the basis of the measuring process within many of the measurement instruments providing the precision AC and DC measurements and calibrations. Accordingly, the inventors present current comparator and measurement system architectures together with control protocols to provide for correction of this remnant magnetization.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of priority from U.S. Provisional Patent Application US 61/842,184 filed Jul. 2, 2013 entitled “Methods and Systems for Accuracy Improvement in Current Comparators” the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to electrical measurement and calibration systems and more particularly to current comparator based measurement and calibration systems with better than parts per million accuracy. 
       BACKGROUND OF THE INVENTION 
       [0003]    Alternating Current (AC) and Direct Current (DC) electrical measurements are used in a wide variety of applications and may be performed for a variety of electrical quantities including voltage, current, capacitance, impedance, resistance etc. These tests and measurements include those relating to designing, evaluating, maintaining and servicing electrical circuits and equipment from high voltage electrical transmission lines operating at hundreds of kiloVolts (kV) and kiloAmps (kA) to industrial/medical/residential electrical and lighting, typically 400V/240V/100V and 30/15 A, to a wide variety of industrial/scientific/medical/consumer electrical and electronic devices. 
         [0004]    Within a variety of applications and test equipment comparator bridges, e.g. AC comparator bridges and DC comparator bridges, are employed to provide the required dynamic range, accuracy, and flexibility. Such bridge configurations remove many of the issues associated with achieving making measurements at accuracies of a part, or few parts, per million such as insensitivity to lead resistances, excellent ratio linearity, excellent ratio stability, and a high level of resolution. As such DCC bridges, for example, have replaced resistance ratio instruments such as the Wenner and Kelvin bridges for resistance measurements. Typically, comparator implementations provide accuracies within the range of 0.1 ppm to 1.0 ppm. 
         [0005]    However, with the continued drive for improved accuracy in calibration, standards, and measurements on circuits and components operating at hundreds of kiloVolts, thousands of Amps, with resistances into Gigaohms accuracies of parts per million is being replaced by parts per billion. For example, Guildline Instruments offers a range of DCC bridges starting at 100 parts per billion over the range 0.001Ω to 100 kΩ to extended performance DCC bridges at fifty (50) parts per billion for measurements up to 100 MΩ and currents to 3000 A or forty (40) parts per billion for measurements up to 1 GΩ and 1000V . Other DCC bridges operate at even lower errors of fifteen (15) parts per billion. 
         [0006]    At these levels the inventors have identified that a variety of factors, such as interruptions to measurement cycles, e.g. the bridge is unplugged mid-measurement, and manufacturing variations in the current comparator toroids, can generate residual flux within the toroids sufficient to generate offsets within the range of ten (10) to twenty (20) parts per billion. Accordingly, the inventors have established a current comparator design and control approach to address these offsets. 
         [0007]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an object of the present invention to mitigate limitations within the prior art relating to electrical measurement and calibration systems and more particularly to current comparator based measurement and calibration systems with parts per billion accuracy. 
         [0009]    In accordance with an embodiment of the invention there is provided a method comprising:
       providing current comparator comprising a predetermined portion of a bridge generating a bi-polar current coupled to a first end of a first winding of the current comparator;   providing a first load resistor;   selectively coupling the first winding of the current comparator to ground via the load resistor; and   generating a first predetermined current profile to initially saturate a magnetic core of the current comparator and subsequently iteratively cycle the current through negative and positive cycles to null the magnetic field within the magnetic core.       
 
         [0014]    In accordance with an embodiment of the invention there is provided a resistance measurement system for generating a bi-polar current comprising:
       a bridge comprising;
           a digital to analog converter for generating a first signal having a defined bi-polar current,   a first current tracking amplifier for generating a second signal having a defined bi-polar current, and for transmitting the generated second signal to a reference resistor,   a first comparator for receiving the output of the first current tracking amplifier en route to the reference resistor and for controlling a switch to place the resistance measurement system into the first state and the second state, the first state connecting a secondary input of the bridge to ground and the second state connecting the output of the digital to analog converter to a test resistance; and   a first current source generating a first predetermined current profile under control of a controller to initially saturate a magnetic core of the first comparator and subsequently iteratively cycle the current through negative and positive cycles to null the magnetic field within the magnetic core; and   
           an extender comprising;
           a bi-polar amplifier for receiving the output of the digital to analog converter in the bridge, for amplifying the received converter output, and for transmitting the amplified current signal as an output; and   a second comparator for receiving the output of the bipolar amplifier, for controlling a second current tracking amplifier to generate and transmit a signal having a defined bi-polar current to the secondary input of the bridge, and for receiving the output of the second current tracking amplifier en route to the secondary input of the bridge.   
               
 
         [0023]    In accordance with an embodiment of the invention there is provided a current comparator comprising:
       a first winding for receiving a first bi-polar current;   a second winding for receiving a second bi-polar current;   a third winding for generating an output current in dependence upon at least the first and second bi-polar currents;   a fourth winding coupled to ground on one end and coupled to a current source at another end.       
 
         [0028]    In accordance with an embodiment of the invention there is provided a current comparator comprising:
       a first winding for receiving a first bi-polar current;   a second winding for receiving a second bi-polar current;   a third winding for generating an output current in dependence upon at least the first and second bi-polar currents;   a magnetic shield surrounding a magnetic core and the first to third windings;   a fourth winding surrounding a predetermined portion of the magnetic shield coupled to ground on one end and coupled to a current source at another end.       
 
         [0034]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein: 
           [0036]      FIG. 1  depicts a direct current comparator bridge according to the prior art as employed by National Institute of Standards and Technology; 
           [0037]      FIG. 2  depicts a block diagram of an automatic micro-processor controlled measurement system comprising a current comparator bridge with a directly slaved extender; 
           [0038]      FIG. 3  depicts a block diagram of an automatic micro-processor controlled measurement system comprising a current comparator bridge with a directly slaved extender and high current extenders; 
           [0039]      FIG. 4  depicts a block diagram of an automatic micro-processor controlled measurement system comprising a current comparator bridge with a directly slaved extender and high current extenders; 
           [0040]      FIG. 5  depicts a block diagram of an automatic micro-processor controlled measurement system current comparator bridge according to an embodiment of the invention; 
           [0041]      FIG. 6A  depicts a block diagram of an automatic micro-processor controlled measurement system current comparator bridge according to an embodiment of the invention; 
           [0042]      FIGS. 6B  depicts alternate circuit sections for the current comparator bridge of  FIG. 6A  according to embodiments of the invention; 
           [0043]      FIG. 7  depicts a current comparator bridge according to an embodiment of the invention with dedicated degaussing current sources; 
           [0044]      FIG. 8A  depicts a block diagram of an automatic micro-processor controlled measurement system according to an embodiment of the invention comprising a current comparator bridge with a directly slaved extender and high current extenders exploiting dedicated degaussing current sources; 
           [0045]      FIG. 8B  depicts a block diagram of an automatic micro-processor controlled measurement system according to an embodiment of the invention comprising a current comparator bridge with a directly slaved extender and high current extenders exploiting dedicated degaussing current sources; 
           [0046]      FIG. 9  depicts a block diagram of an automatic micro-processor controlled measurement system according to an embodiment of the invention comprising a current comparator bridge with a directly slaved extender and high current extenders exploiting dedicated degaussing current sources within the current comparator bridge and high current extender; 
           [0047]      FIG. 10  depicts a block diagram of an automatic micro-processor controlled measurement system according to an embodiment of the invention comprising a current comparator bridge with a directly slaved extender exploiting a dedicated degaussing current source with fourth winding for the current comparator bridge; 
           [0048]      FIG. 11  depicts an exemplary flowchart for an automatic micro-processor controlled measurement system according to an embodiment of the invention comprising degaussing current sources; 
           [0049]      FIG. 12  depicts an exemplary construction of a current comparator according to an embodiment of the invention; 
           [0050]      FIGS. 13A and 13B  depict an exemplary construction of a current comparator with magnetic shield and shield degaussing winding according to an embodiment of the invention; 
           [0051]      FIG. 14  depicts a block diagram of an automatic micro-processor controlled measurement system according to an embodiment of the invention comprising a current comparator bridge with magnetic shield and shield degaussing winding such as depicted in  FIGS. 13A and 13B . 
       
    
    
     DETAILED DESCRIPTION 
       [0052]    The present invention is directed to electrical measurement and calibration systems and more particularly to current comparator based measurement and calibration systems with parts per billion accuracy. 
         [0053]    The ensuing description provides exemplary embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the disclosure. Rather, the ensuing description of the exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing an exemplary embodiment. It being understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope as set forth in the appended claims. 
         [0054]    Within embodiments of the invention described below in respect of  FIGS. 1 through 11  the instruments are described as being resistance measurement systems. However, the embodiments of the invention may be applied to other measurement systems exploiting current comparators as part of the measurement system and/or as the basis of the actual measurement. 
         [0055]    In general, the operation of a current comparator is based on Ampere&#39;s fundamental law given by Equation (1) where the line integral of the magnetic field H around a closed path dl is equal to the total current I crossing any surface bounded by this path. Referring to  FIG. 1  then for an ideal comparator at balance, the total current is carried by two ratio windings, Primary Winding  110  and Secondary Winding  120 , and the total number of ampere-turns of one winding is equal and opposite to that of the other winding such that Equation (2) is satisfied where the subscripts P and S refer to the Primary and Secondary Windings  110  and  120 , respectively. 
         [0000]                  H ·dl=Σ I   (1)
 
         [0000]      Σ I=N   P   I   P   −N   S   I   S =0  (2)
 
         [0056]    Accordingly, the current ratio of the comparator, I P /I S , is therefore equal to the inverse of the turns ratio, N S /N P . In order to obtain high ratio accuracies, the ampere-turn balance or zero flux condition is determined by some type of flux detector system that is only sensitive to the mutual fluxes generated by the ratio windings. In practice, a DCC achieves good ratio accuracy and sensitivity by utilizing high-permeability toroidal cores, magnetic and eddy-current shields, and careful winding procedures. The main component of a DCC consists of a pair of high-permeability cores, surrounded by a magnetic shield, over which are the ratio windings that carry the direct currents to be compared. Cryogenic current comparators are similar in concept, but make use of the ideal magnetic shielding properties of a self-enclosing, but non-continuous, surface made of superconducting material. 
         [0057]    The currents for the ratio windings are supplied by two isolated direct current sources, such as Constant Current Source  170  and Slave Current Source  180 . In some embodiments the cores and within the magnetic shield of the DCC is wound a Modulation-Detection Winding  130  which is used to sense the flux condition of the cores. This is achieved by modulating the core permeability through the Modulation-Detection Winding  130  with a Modulator Oscillator  190  and using a Second Harmonic Detector (SHD) Circuit  195 . The presence of dc flux in the cores due to primary and secondary ampere-turn imbalance is indicated by this detector output both in magnitude and polarity. The SHD Circuit  195  output is used in a feedback circuit to adjust the current in one of the windings, automatically maintaining ampere-turn balance. 
         [0058]    This basic self-balancing DCC resistance bridge in  FIG. 1  requires two simultaneous balances, an ampere-turn balance and a voltage balance. The Slave Current Source  180  is continuously adjusted so that ampere-turn balance is maintained. Under this condition, the ampere-turn product of the primary circuit equals that of the secondary circuit, as given by Equation (3). 
         [0000]        N   P   I   P   =N   S   I   S   (3)
 
         [0059]    The voltage balance can be achieved by the adjustment of the number of turns in the primary circuit, N P , until there is a null condition on Detector D  140 . Then the voltage drop across the unknown resistor R X ,  160  in the primary circuit is equal to the voltage drop across the dummy resistor R D    150  in the secondary circuit. Accordingly Equation (4) applies and using Equation (3) we obtain Equation (5) for the unknown resistor R X ,  160 . 
         [0000]        R   X   I   P   =R   D   I   S   (4)
 
         [0000]        R   X =( N   P   /N   S ) R   D   (5)
 
         [0060]    Similarly, another measurement with the direct substitution of a known standard resistor R S  of the same nominal value in the primary circuit and re-balancing the detector by adjusting N P  to a new value N′ P  results in Equation (6) which when combined with Equation (5) yields Equation (7) where ΔN P =(N P −N′ P ), i.e. the number of primary turns between the two voltage balance settings. According, the value of an unknown resistor R X , is determined in terms of a standard resistor and the small relative difference in turns ratio.  FIG. 1  depict thereby depicts a direct current comparator bridge according to the prior art such as employed by the National Institute of Standards and Technology (NIST) (NIST Measurement Service for DC Standard Resistors, http://www.nist.gov/calibrations/upload/tn1458.pdf). 
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         [0061]    Referring to  FIG. 2 , a Resistance Measurement System (RMS)  200  makes use of a low Current Comparator (CC) Bridge  202  and an Intermediate Current Range (ICR) Extender  204 . The CC Bridge  202  generates a bi-polar current as directed by microcontroller  206 . Microcontroller  206  controls Amplifier  208 , which is used to amplify a signal. The Amplifier  208  is preferably a servo current tracking amplifier which can produce an output current in the range of 0 mA to ±150 mA. The microcontroller  206  also connects to digital to analog converter (DAC)  210  which also provides an output in the range of 0 mA to ±150 mA. The second input to amplifier  208  is connected to the winding of Comparator  212  of the bridge. The 0 mA to ±150 mA output of the amplifier  208  is also connected to the winding of Comparator  212  before it is connected to the reference resistance (P REF )  214 . A winding of Comparator  212  is also connected to a switch  218 . In one position, switch  218  will provide a connection to ground that feeds to the ICR Extender  204 . In the second position, it will connect a test resistance (P TEST )  216  to allow the bridge to test the resistance with a low current of 0-±150 mA. 
         [0062]    The ICR Extender  204  extends the current test range of CC Bridge  202  in the RMS  200 . It connects to the CC Bridge  202  to receive both control information and an input current. Its output is connected to P TEST    216  to provide an intermediate level current. In the illustrated embodiment, the intermediate current level is a current range of 0 mA to ±3 A due to the use of a 20×amplifier. One skilled in the art will appreciate that a different output current can be obtained by using a different amplifier stage. The ICR Extender  204  includes a bipolar DC current amplifier  120  with a range of 0 to ±3 Amperes (or higher in other embodiments) directly coupled to the high current primary winding of the integrated ICR Current Comparator  222  and a servo amplifier  224  that provides sufficient lower level current through the secondary winding of the ICR Current Comparator  222  to balance the ICR Current Comparator  222  under all operating conditions. The output of the primary winding is connected to P TEST    216  and the output of servo  224  through the extender secondary winding is connected back to the automated resistance CC Bridge  202  and as such the CC Bridge  202  can maintain balance within the bridge current Comparator  212 . 
         [0063]    The output of the DAC  210  in CC Bridge  202  is the primary input to the ICR Extender  204 . This output is a signal between 0 mA to ±150 mA which is provided to a directly coupled bi-polar current amplifier  220 . The use of the bipolar amplifier  220  allows the obviation of mechanical switches that are required in the prior art. The manner in which the ICR Extender  204  connects to the CC Bridge  202  allows the overall system to remain balanced, which is a factor in why the prior art maintained its reliance on the mechanical switches. The output of bi-polar amplifier  220  is passed through the primary winding of ICR Current Comparator  222 , and is then provided as an output to the Extender  204  through P Test  216 . Another winding on ICR Current Comparator  222  is used as the input to servo tracking amplifier  224 , whose output is passed through a third winding on ICR Current Comparator  222 . This signal (the output of amplifier  224 ) is transmitted back to the CC Bridge  202  which connects the signal to the switch  218 . When in the first position, switch  218  connects this output to ground to balance the measurement system. In the second position, switch  218  connects the output of the DAC  210  to P TEST    216 , thus providing a current of 0 mA to ±150 mA to P TEST    216 , as opposed to the output of 0 mA to ±3 A provided by the output of Extender  204 . 
         [0064]    This configuration allows the resistance measurement RMS  200  to perform resistance measurements at much higher currents than the CC Bridge  202  would be able to as a result of its limitation of 0 mA to ±150 mA. This increase in the output makes use of the cascaded configuration. The accuracy of the measurement is dependent mainly on the accuracy of the current comparison within the comparators  212  and  222 . In the illustrated implementation, the base configuration can be integrated directly within the main assembly of the low level current comparator resistance measurement CC Bridge  202  such that no manual connections are required with the exception of connections to the test resistance P TEST  to be measured. The utilization of the bipolar directly coupled DC current amplifier  220  eliminates the requirement for reversing relays, external current supplies and the resulting complexity of interconnections. The use of a voltmeter to measure the potential drop across the test resistance  216  and reference resistance  214  can be performed as used in the prior art. However, as discussed above, for an accurate reading, the difference in the voltage drops across the test resistance  216  and the reference resistance  214  is driven towards zero, and the comparators  212  and  222  are employed to determine the current ratio directed to the two resistances. Knowing this ratio and the winding ratio of the comparators  212  and  222  allows the RMS  200  to determine the unknown resistance of P TEST    216  to a high degree of accuracy. It would be evident to one skilled in the art that the CC Bridge  202  may be operated discretely without the ICR Extender  204 . 
         [0065]    An Augmented Resistance Measurement System (ARMS)  300  is illustrated in  FIG. 3 . The ARMS  300  builds upon the configuration illustrated in  FIG. 2  with the addition of a high Current Comparator (CC) Extender  232 . The CC Extender  232  includes a High Current Comparator  236  that is connected to a modular bipolar directly coupled high current amplifier  234 , a servo amplifier  238 , and the output of amplifier  220 . The bipolar high current amplifier  234  is directly coupled through the primary winding side of the High Current Comparator  236  to the resistance device P TEXT    216  and can include multiple modules such that full scale currents up to ±3000 Amperes or higher may be attained, preferably in multiples of ±150 A in the present embodiment. Servo amplifier  238  receives its input from High Current Comparator  236 , and its output is connected back into the input of the intermediate level Extender  204  and is fed to that stage&#39;s bipolar current amplifier  220 . The output of intermediate current amplifier  220  is coupled through the primary winding of the intermediate level ICR Current Comparator  222  and also through the secondary winding of the High Current Comparator  236  such that both the intermediate ICR Current Comparator  222  and the High Current Comparator  236  can maintain current balance under all operating conditions. The configuration is completed with the direct coupling of the output of the servo amplifier  224  of the intermediate level current Extender  204  through the secondary winding of the ICR Current Comparator  222  and the current Comparator  212  of the CC Bridge  202  as described in relation to the embodiment of  FIG. 2 . The utilization of a modular build-up of directly coupled high current amplifiers allows for a multiplicity of current ranges to be implemented and upgraded from ±150 A to ±3000 A or higher and also eliminates the requirement for specially designed very high current pneumatically or otherwise mechanically actuated reversing relay contacts, external commercial power supplies and the associated more complex interconnections. 
         [0066]    As illustrated in  FIG. 3 , the CC Bridge  202  and Extender  204  of the ARMS  300  are configured largely as they were in RMS  200  of  FIG. 2 . One notable difference is that the input to Extender  204  is routed through High Current CC Extender  232  which in turn is connected to CC Bridge  202 . Thus, though the input to Extender  204  is still the output of CC Bridge  202 , it is not a direct connection. The output of DAC  210 , which in this exemplary embodiment is a signal of 0-±150 mA, is provided to high current bipolar amplifiers  234 . These amplifiers are preferably parallel amplifiers that allow, in the illustrated embodiment, an amplification of up to 20,000×, allowing for an output signal ranging from 0 mA to ±3000 A. One skilled in the art will appreciate that higher or lower amplification ratios can be employed without departing from the scope of the present invention. This output signal is passed through the primary windings of High Current Comparator  236  and then provided as an external output of the stage to P TEST    216 . A Servo Current Tracking Amp  238  receives input from the High Current Comparator  236  and provides its output signal, of 0 mA to ±150 mA to the input of the Extender  204 , which provides the input signal to amplifier  220 . The resulting 0 mA to ±3 A signal is passed through the primary winding of ICR Current Comparator  222 , and then through the secondary winding of High Current Comparator  236 . The ICR Current Comparator  222  of the Extender  204  is connected back to the CC Bridge  202  through servo tracking amplifier  224  as was described in  FIG. 2 . In an embodiment the amplifier  234  is a modular amplifier that can be built as ±150 A modules connected in parallel to allow for an output of ±3000 A. This allows the test resistance to be supplied a reliable bipolar current of 0 mA to ±3000 A which is typically sufficient in most instances to create a potential drop across the resistances that can be measured accurately by the CC Bridge  202 . The known current ratios can then be used to determine the unknown resistance. One skilled in the art will appreciate that other factors can be determined by knowing the current ratios. 
         [0067]    Referring to  FIG. 4  there is depicted an electronically re-configurable implementation of the ARMS  300  as described and depicted in  FIG. 2 . Accordingly Electronically Reconfigurable ARMS (ERARMS)  400  comprises the CC Bridge  202  and ICR Extender  204  together with the CC Extender  232  as discussed supra in respect of  FIG. 3 . As such the ERARMS  300  may provide three output current ranges to the Test Resistance  406  based upon the configuration of the first to fourth configuration switches  401  through  404  respectively. In a first configuration the output of the DAC  210  is routed by switch  218  to the bridge output C 1 A of the Bridge  202 . In a second configuration the output of the DAC  110  is routed to the amplifier  220  within the ICR Extender  204  and thereby to the extender output C 1 B of the ICR Extender  204 . In a third configuration the output of the DAC  110  is routed to the high current bipolar amplifiers  234  within the CC Extender  232  and therein to the high current extender output C 1 C. Coupling these three outputs to the fourth configuration switch  404  allows the selected current range to be coupled to the Test Resistance  406 . The other side of the Test Resistance  406  being coupled to bridge input port C 2 A of Bridge  202  and extender input port C 1 B of CC Extender  232 . First and second configuration switches  401  and  402  manage the routing of the output of the DAC  210  to the ICR Extender  204  and High Current Extender in the second and third configurations and the routing of the Servo Current Tracking Amp  138  to the input of amplifier  220  in the third configuration. Third configuration switch  403  manages routing of the output port of the amplifier  220  between the fourth configuration switch  404  and High Current Comparator  236  in the second and third configurations respectively. 
         [0068]    Accordingly, where the first to fourth configuration switches  401  through  404  are controlled through a controller, not shown for clarity, together with switch  218  then the re-configurable system  300  provides multiple programmable output ranges such as the ±150 mA, ±3 A, or ±3000 A discussed above provided by exemplary embodiments of the Bridge  102 , ICR Extender  204 , and CC Extender  232 . It would be evident to one skilled in the art that the re-configurable system  300  may be implemented in a modular manner such as for example by providing Bridge  202 , ICR Extender  204 , and CC Extender  232  as discrete units together with first to fourth configuration switches  401  through  404 . Alternatively as first to third configuration switches  401  and  404  relate to interconnections and input/outputs of ICR Extender  204  and CC Extender  232  these may be provided within a single module with the CC Extender  232 . It would also be evident that the CC Extender  232  which within the embodiments above provides multiplication of the output of the DAC  210  may be similarly implemented in modular format either through multiple amplifier stages with single comparator stage or multiple High Current Extenders  232  with appropriate switching elements. It would be further evident that the fourth configuration switch  404  may alternatively be a 1:4, 1:5, 1:6, of 1:N switch rather than the 1:3 switch depicted. 
         [0069]    CC Extender  232  in  FIGS. 2 and 3  is depicted as being implemented with multiple high current bipolar amplifiers  234 . Accordingly CC Extender  232  may provide multiple output current ranges with the provision of additional primary windings with appropriate switching elements to provide more output current options. Alternatively it would be evident to one skilled in the art that the multiple high current bipolar amplifiers  234  may be similarly switchably engaged thereby providing additional output current ranges for the resistance measurement systems described above in respect of  FIGS. 2 through 4 . It would also be evident that alternatively multiple High Current Extenders  232  may be employed with different gain factors and maximum output current range to provide a modular approach to currents of ±3000 A or higher. 
         [0070]    Accordingly, it would be evident that systems such as RMS  200 , ARMS  300  and ERARMS  400  provide measurement capabilities over a wide range of resistances, currents, and voltages. For example, such systems can measure resistances as low as 1 μΩ at 300 A all the way through to 1 GΩ at 1 kV and others operating at 10,000 A. In parallel to DC current comparator based measurement systems there are a corresponding parallel set of AC current comparator based measurement systems. As noted supra evolving measurement requirements have shifted the error in such measurements from a few parts per million to tens or hundreds of part per billion and as a result error sources that were previously minor but known factors or unknown factors become important and require correction in order to reduce or remove these error sources. Accordingly, the behaviour of the toroidal current transformer at parts per billion becomes important. 
         [0071]    During normal operation of such test systems as RMS  200 , ARMS  300  and ERARMS  400  the test resistor  216  or test resistance  406  is cycled through a range of operational voltages and currents generated by DAC  210  under control of the Microcontroller  206 . However, during this operation interruption of the testing, e.g. turning off the system, disconnecting power supply, disconnecting device under test, etc may leave remanent flux within the core of one or more of the current comparators within the test and measurement system (TMS). Such remanence magnetization being more likely when such an interruption occurs at high current and/or voltage but it may also arise as a result of anhysteretic remanence or anhysteretic remanent magnetization (ARM) which arises when the magnetic core of the current comparator toroidal transformers is exposed to a large alternating field with a small DC bias field. It would be evident that where a large alternating field of ±1 kV is applied then a DC bias field of 1 mV is equivalent to 100 ppb and 10 μV equivalent to 10 ppb. 
         [0072]    The remanent magnetization within the transformer core of the current comparator may depend upon many factors including magnitude of the primary current, impedance of the secondary circuit, and the amplitude and time constant of an offset transients. Accordingly, when the current transformer is next energized the flux changes will start from the remanent value and not zero as expected. Accordingly, in order to remove such remanent magnetization it is necessary to degauss the magnetic core of the toroidal transformers within the AC and DC comparators. However, in contrast to prior art transformers wherein degaussing is undertaken in conjunction with primary and secondary windings or current sensors with a sole primary winding the degaussing of AC and DC current comparators requires consideration of the primary, secondary, and tertiary windings. 
         [0073]    Accordingly, the inventors have established control and excitation circuits for use in conjunction with the current comparators. The basis of the embodiments of the invention are three elements:
       simple, non-accurate, current source providing controlled excitation of current comparator to a maximum predetermined current and subsequent reduction to zero current through a predetermined cycle;   loading of primary and secondary windings with a predetermined load; and   trigger and control circuit to determine when to perform the degaussing, for example before any measurement exceeding a predetermined voltage, every time for a current comparator having an intrinsic accuracy exceeding a predetermined level, or after every interrupted measurement.       
 
         [0077]    As will become evident in addition to addressing the transformer core of a current comparator the inventors have also addressed that high accuracy alternating and direct current (AC/DC) comparators are magnetically shielded to reduce the susceptibility of the current comparator to external magnetic fields but that these magnetic shields may themselves become partially magnetized through operation of the AC/DC transformer especially at high currents. 
         [0078]    Referring to  FIG. 5  there is depicted in schematic  500 A a De-Gaussing Resistance Measurement System (DRMS)  510  according to an embodiment of the invention based upon the architecture of RMS  200  described supra in respect of  FIG. 2  based upon CC Bridge  202 . Accordingly, DRMS  510  comprises microcontroller  206  in communication with the primary and secondary current sources, namely Amplifier  208  and DAC  210  respectively, which are coupled permanently and switchably to Comparator  212 . Switch  218  in RMS  200  is replaced by first and second switches  520 A and  520 B respectively. First switch  520 A provides the same functionality of 1×2 to connect to ports  540 A and  540 B, corresponding to the output/input feeds of Extender  204  respectively. Second switch  520 B is now a 1×3 switch rather than a 1×2 switch such that in addition to providing the original connections to test resistance (P TEST )  216  and  500 F to ground it also provides a resistive connection to ground via second Load Resistor  530 B. Similarly rather than being directly connected to reference resistance (P REF )  214  after Comparator  212  the Amplifier  208  is routed via third switch  520 C that connects to reference resistance (P REF )  214  or via first Load Resistor  530 A to ground. Accordingly, through appropriate control of first to third switches  520 A to  520 C respectively Amplifier  208  and DAC  210  are coupled to ground via the primary and secondary windings of Comparator  212  and first and second Load Resistors  530 A and  530 B respectively. In this mode Amplifier  208  and DAC  210  as depicted in magnetic field graph  500 B and current profile  500 C the maximum current delivered is ±I SATURATE  rather than the operational limits ±I OP   MAX . 
         [0079]    As depicted in magnetic field graph  500 B at ±I SATURATE  the magnetic field within the comparator transformer core reaches saturation whereas in normal operation at ±I OP   MAX  the comparator transformer core is unsaturated. Accordingly, Test Cycle  540  as depicted in current profile  500 C cycles the current according to a predetermined profile, e.g. between ±I OP   MAX  or lower values as established by the Microcontroller  206  according to the Test Resistor (P TEST )  216  being tested. In Degauss Cycle  550  the Microcontroller now drives the Amplifier  208  and DAC  210  to a current past the saturation current, ±I SATURATE , and then cycles down to no current through a number, N, of cycles, C X , for a total period of time, T. Whilst all N cycles may be of equal duration it would be evident that each cycle C X  may be of different duration. Similarly, the amplitude of each cycle may follow a predetermined mathematical relationship, e.g. exponential, linear or may be arbitrarily defined. For example, the Degauss Cycle  550  may start with a number of cycles that meet or exceed the saturation current ±I SATURATE  before reducing over a number of cycles that decrease linearly. Accordingly. Degauss Cycle  550  cycles the magnetic materials within the Comparator  212  to magnetic field saturation, reverses the saturation, and then cycles the magnetic field through a series of field reversals with reducing magnitude until the field is null. 
         [0080]    Accordingly, each of the Amplifier  208  and DAC  210  are implemented to provide at least ±I SATURATE  which is in excess of the maximum operating current ±I OP   MAX . In Degauss Cycle  550  the primary and secondary of Comparator  212  are coupled to the Amplifier  208  and DAC  210  respectively at one end of each and to ground via first and second Load Resistors P LOAD    530 A and  530 B respectively, for example 1Ω resistors although other values may employed as determined by characteristics of one or more of the Comparator  212 , DAC  210 , and Amplifier  208  for example. It would be apparent that first and second Load Resistors P LOAD    530 A and  530 B respectively do not need to be accurate but rather may be nominally the target resistance. 
         [0081]    Now referring to  FIG. 6A  there is depicted a De-Gaussing Resistance Measurement System (DRMS)  600 A according to an embodiment of the invention similarly based upon the architecture of RMS  200  described supra in respect of  FIG. 2  based upon CC Bridge  202  as was DRMS  510  in  FIG. 5 . In contrast to DRMS  500  the first and second Load Resistors  630  and  640  are disposed within the circuit between the Comparator  212  and the Amplifier  208  and DAC  210  respectively. Accordingly, first switch  620 A still selects between DAC  210  which is also coupled to port  540 A or port  540 B to the secondary of Comparator  212 . In normal measurement mode second and third switches  620 B and  630 C connect the primary and secondary of Comparator  212  to reference resistor P REF    214  and test resistor P TEST    216  respectively. In degauss mode these switches connect the primary and secondary of Comparator  212  to ground on those ends and fourth and fifth switches  620 D and  620 E switch in the first and second Load Resistors  630  and  640  respectively. 
         [0082]    Referring to  FIG. 6B  a first Circuit Section  600 B is shown depicting part of the secondary circuit path from DAC  210  through Comparator  212  to test resistor P TEST    216 . However, from second switch  620 B rather than switching to ground directly in degauss mode the DRMS switches to ground via additional Load Resistor  650 . As such a similar additional Load Resistor may be applied to the primary side of the Comparator  212  such that each of the primary and secondary sides of Comparator  212  have Load Resistors in series when the DRMS of which first Circuit Section  600 B forms part. Also depicted in  FIG. 6B  is second Circuit Section  600 C depicts an alternative embodiment according to the invention wherein the DAC  610  is employed to drive both the primary and secondary sides of the Comparator  212 . Accordingly first switch  620  now couples to sixth switch  620 F to select either the primary or secondary side of the Comparator  212  and fourth switch  620 D is replaced with seventh switch  620 G to either couple DAC  210  via first load resistor  630  to the primary side in degauss mode or couple Amplifier  208  directly to the primary side. Accordingly, in degauss mode the DAC  210  may be executed once on the primary side and once on the secondary or one only in one degauss step and then the other in a later degauss step. Optionally, when the degauss mode is executed on the primary side the secondary may be grounded via second load resistor  640  rather than floating as depicted in second Circuit Section  600 C through the addition of additional elements or modification of fifth switch  620 E. 
         [0083]    Now referring to  FIG. 7  there is depicted a schematic of a De-Gaussing Resistance Measurement System (DRMS)  700  according to an embodiment of the invention similarly based upon the architecture of RMS  200  described supra in respect of  FIG. 2  based upon CC Bridge  202  as were DRMS  510  and  600 A in  FIGS. 5 and 6  respectively. As depicted Microcontroller  206  is now connected to first and second Degauss Current Sources  730 A and  730 B as well as Amplifier  208  and DAC  210 . First Degauss Current Source  730 A is switchably connected to the primary of Comparator  212  by first switch  720 A whilst the other end of the primary of Comparator  212  is connected to either the Reference Resistor  214  or ground by third Switch  720 C. Second Degauss Current Source  730 B is switchably connected to the secondary of Comparator  212  by second switch  720 B whilst the other end of the secondary of Comparator  212  is connected to either the Test Resistor  216  or ground by fourth Switch  720 D. Accordingly, first and second Degauss Current Sources  730 A and  730 B, which include Load Resistors, are driven during the Degauss Cycle whilst Amplifier  208  and DAC  210  are driven during the measurement cycle. 
         [0084]    Accordingly first and second Degauss Current Sources  730 A and  730 B may be lower accuracy, but higher current, sources than Amplifier  208  and DAC  210 . First and second Degauss Current Sources  730 A and  730 B may be programmable via the Microcontroller  206  providing flexibility in current versus time, such as described supra with multiple cycles alternating to ±I SATURATE  followed by a series of cycles with reducing peak current until the cycles collapse to zero. However, first and second Degauss Current Sources  730 A and  730 B may also be resonant circuits providing decaying oscillatory behaviour established in dependence upon, for example, capacitance, inductance, and resistance parameters of elements which may be fixed or variable under Microcontroller  206  control. 
         [0085]    Now referring to  FIG. 8A  there is depicted an Electronically Reconfigurable ARMS (ERARMS)  800  according to an embodiment of the invention comprising De-Gaussing Resistance Measurement System (DRMS)  700  together with ICR Extender  204  and CC Extender  232  such as discussed supra in respect of  FIGS. 3 and 4  respectively. Accordingly, DRMS  700  may execute a degaussing cycle for Comparator  212  within DRMS  700  as discussed supra whilst providing, through the configuration switches  401  through  404  which are controlled through the Microcontroller  206  or another controller, not shown for clarity, together with the switches internally to the DRMS  700 , multiple programmable output ranges such as ±150 mA, ±3 A, and ±3000 A for example. 
         [0086]    Referring to  FIG. 8B  there is depicted an ERARMS  8000  employing a Modified DRMS  7000  together with first Switch  810  which replaces fourth switch  404  in ERARMS  800 . Modified DRMS  7000  replaces second switch  720 B with second and third Switches  820  and  830  respectively. Second Switch  830  provides for selective coupling of the DAC  210  or second Degauss Current Source  730 B to first port  540 A. Similarly, first Switch  820  selectively couples either the output of second Switch  830  or second port  540 B to the secondary of Comparator  212 . First Switch  810  provides the same 3×1 switching connectivity as fourth switch  404  but now also provides 1×2 switching to either Test Resistance  406  or ground. Accordingly, second Degauss Current Source  730 B can be coupled to first port  540 A and thereafter to either ICR Extender  204  or CC Extender  232  with the other end of the ICR Current Comparator  222  or High Current Comparator  236  respectively coupled to ground through First Switch  810 . Accordingly, each of the magnetic cores with the ICR Current Comparator  222  and High Current Comparator  236  may be degaussed through second Degauss Current Source  730 B being executed through a current profile, e.g. Degauss Cycle  550  as discussed in respect of  FIG. 5 , under control of Microcontroller  206 , or another controller not shown for clarity. 
         [0087]    Optionally, the current profile executed by second Degauss Current Source  730 B may be different for each of the Comparator  212 , ICR Current Comparator  222 , and High Current Comparator  236  according to one of more factors including, but not limited to, the characteristics of their magnetic core, the history of the comparator since last degaussing, and the current required to saturate relative to the gain provided or normal operating current. Optionally, the timing of degaussing for each of Comparator  212 , ICR Current Comparator  222 , and High Current Comparator  236  may be varied such as discussed below in respect of  FIG. 11 . 
         [0088]    Now referring to  FIG. 9  there is depicted an ERARMS  900  according to an embodiment of the invention wherein ERARMS  900  comprises Modified DRMS  7000 , such as described supra in respect of  FIG. 8B , ICR Extender  204 , and High Current (HC) Extender  900 A. As depicted HC Extender  900 A provides the same functionality as CC Extender  232 , such as described supra in respect of  FIG. 2 , through Bipolar High Current Amplifier  234 , High Current (HC) Comparator  236 , and Servo Amplifier  238  but is augmented with an internal degaussing function through Integral Degauss High Current Source (IDHCS)  905 . As depicted IDHCS  905  may be switchably connected to either the primary and secondary windings of the HC Comparator  236  through appropriate control of first to fifth switches  915  to  935  respectively which HC  900 A may be disconnected from ERARM  900  through sixth switch  910 . Accordingly, degaussing of the comparators within Modified DRMS  7000  and ICR Extender  204  may be undertaken using the integral degauss current sources within the DRMS  7000  whilst degaussing of HC Comparator  236  is undertaken with IDHCS  905  within HC Extender  900 A. As depicted IDHCS  905  includes the current source and load resistance although optionally multiple load resistance may be disposed after the primary and second windings and be switched into the circuit when the third and fifth switches  925  and  935  respectively switch the appropriate winding to ground. 
         [0089]    Optionally, HC Extender  900 A may include a pair of IDHCS  905  to simultaneously drive the primary and secondary windings of HC Comparator  236 . Alternatively IDHC  905  may within a degaussing cycle only be driven through one of the primary and secondary windings of HC Comparator  236  and the functionality reduced with respect to the other winding or within one degaussing cycle the primary is employed followed by the secondary within a subsequent degaussing cycle. It would also be apparent that a similar functionality may be integrated within ICR Extender  204  such that each stage of the ERARM  900  may be degaussed simultaneously as opposed to sequentially or in different sequences. 
         [0090]    Referring to  FIG. 10  there is depicted a Resistance Measurement System (RMS)  1000  which makes use of a low Current Comparator Bridge  1010  and an Intermediate Current Range (ICR) Extender  204 . As depicted low Current Comparator Bridge  1010  has a similar construction to CC Bridge  202  described supra in respect of  FIG. 5  in that the primary and secondary windings are connected to Amplifier  208  and DAC  210 /second Port  540 B. However, in addition to primary and second windings  1012 A and  1012 B and comparator winding  1012 C which feeds back to Amplifier  208  there is a fourth winding  1012 D which is coupled to Degauss Current Source  1060  via Load Resistor  1040 . When Degauss Current Source  1060  is operated under action of Microcontroller  206  or another controller, not shown for clarity, first to fourth switches  1020 ,  1030 ,  1050 , and  1070  couple the primary and secondary windings  1012 A and  1012 B to ground as well as comparator winding  1012 C. Accordingly, Degauss Current Source  1060  may as described supra in respect of other embodiments of the invention be a low accuracy current source rather than an accurate current source such as Amplifier  208  and DAC  210 . Optionally, ICR Extender  204  may be similarly configured with an additional winding and dedicated Degauss Current Source. 
         [0091]    Now referring to  FIG. 11  there is depicted an exemplary flowchart  1100  for a controller within a measurement system exploiting one or more degaussing circuits such as described supra in respect of  FIGS. 5 through 10  for example. The process begins with Instrument Power Up  1105  wherein the measuring instrument is either connected to mains power and turned on or if handheld or remote and being operated from a battery simply turned on. Next the process checks in step  1110  whether Auto-Degauss has been enabled wherein a positive determination results in the process proceeding to step  1115  wherein a Power-Up Degauss Sequence is executed such that, for example all compensators within the instrument are cycled to reduce residual magnetization and the process proceeds to step  1120  wherein the instrument performs the first measurement. If no Auto-Degauss was enabled then in step  1110  the process would have proceeded directly to step  1120 . In step  1120  a determination is made as to whether the Instrument is to be Powered Down or whether other measurements may be made. If to be Powered Down the process proceeds to step  1135  and stops. Optionally, another Auto-Degauss or other Degauss Process may be performed as part of a controlled Power Down of the instrument. 
         [0092]    If more measurements are to be performed then the process proceeds to step  1130  wherein a Measurement Protocol is selected. This selection may be automatically determined by the controller in dependence upon the test to be performed or alternatively may be established as a factory default or selected by a user of the Instrument. Based upon the selection the process flow proceeds to one of three Sub-Flows  1100 A to  1100 C respectively. Optionally, only one, two, or all may be implemented within the Instrument as well as others not described within respect of  FIG. 11 . Additional Sub-Flows may be added to the Instrument through one or more interfaces such as USB, memory card, wireless, and Wi-Fi for example. For simplicity, the measurement steps in each of first to third Sub-Flows  1100 A to  1100 C respectively have been omitted but it would be evident to one skilled in the art where these would be inserted into the Sub-Flows. 
         [0093]    First Sub-Flow  1100 A, a simple sub-flow, begins with step  1140  wherein all current comparators within the Instrument are degaussed before the measurement and afterwards in step  1145 A the process determines whether another measurement will be made. If yes then the process returns to step  1130  otherwise the first Sub-Flow  1100 A proceeds to step  1150 A degausses all current comparator stages and stops in step  1155 A. Second Sub-Flow  1100 B begins with step  1160 A wherein the measurement to be performed is configured upon the instrument. Next in step  1165  those current comparators to be employed within the measurement are degaussed before the process in step  1145 B determines whether another measurement will be made. If yes then the process returns to step  1130  otherwise the second Sub-Flow  1100 B proceeds to step  1150 B degausses all current comparator stages and stops in step  1155 B 
         [0094]    In third Sub-Flow  1100 C the process starts with step  1160 B wherein the measurement to be performed is configured upon the instrument. In step  1165  a determination is made as to whether the measurement to be performed exceeds a predetermined threshold. If yes the process proceeds to step  1170  wherein all current comparator stages are degaussed and if not the process proceeds to step  1175 . After step  1170  the process proceeds to step  1145 C to determine whether another measurement is to be performed or not. If yes the process proceeds back to step  1130  otherwise it proceeds to step  1150 C, degausses all current comparator stages, and then stops in step  1155 C. If in step  1165  the measurement to be performed do not exceed the predetermined threshold then the process proceeds to step  1175  wherein a determination is made as to whether the cumulative measurements since a last degauss process was executed have exceeded a predetermined threshold. If yes the process proceeds to step  1150 C wherein all stages are degaussed and then step  1145 C and if not the process proceeds directly to step  1145 C. 
         [0095]    It would be evident that optionally within third Sub-Flow  1100 C step  1175  may be a determination against multiple predetermined thresholds with degaussing of different combinations of current comparators. For example, in  FIG. 8B  an ERARMS  8000  providing programmable resistance measurements over multiple programmable output ranges such as ±150 mA , ±3 A, and ±3000 A for example may have predetermined thresholds for step  1165  established as 50 mA, 1 A, and 30 A such that:
       if I MEAS &gt;50 mA degauss Comparator  212  in Modified DRMS  7000 ;   if I MEAS &gt;1 A degauss Comparator  212  and ICR Current Comparator  222  in Modified DRMS  7000  and ICR Extender  204  respectively; and   if I MEAS &gt;30 A degauss Comparator  212 , ICR Current Comparator  222 , and High Current Comparator  236  in Modified DRMS  7000 , ICR Extender  204 , and CC Extender  232  respectively.       
 
         [0099]    Similarly, cumulative thresholds for step  1175  may be 25 mA, 500 mA, and 20 A as well such that:
       ΣI MEAS &gt;25 mA degauss Comparator  212  in Modified DRMS 7000;   if ΣI MEAD &gt;500 mA degauss Comparator  212  and ICR Current Comparator  222  in Modified DRMS  7000  and ICR Extender  204  respectively; and   if ΣI MEAS &gt;20 A degauss Comparator  212 , ICR Current Comparator  222 , and High Current Comparator  236  in Modified DRMS  7000 , ICR Extender  204 , and CC Extender  232  respectively.       
 
         [0103]    Referring to  FIG. 12  there is depicted a Current Comparator (CC)  1200  according to an embodiment of the invention showing the CC  1200  sequentially stripped from the outermost layer towards the magnetic core  1210 . Accordingly as shown the CC  1210  comprises a magnetic core  1210  over which is wrapped first tape layer  1230 A separating the first winding  1240 A from it. Second tape layer  1230 B is then wrapped over first winding  1240 A upon which is then wrapped second winding  1240 B. These layers are then over-wrapped with third tape layer  1230 C followed by third winding  1250 , fourth tape layer  1230 D, shielding  1260 , fifth tape layer  1230 E and fourth winding  1270 . As depicted first winding  1240 A corresponds to the primary winding of the CC  1200 , second winding  1240 B corresponds to the secondary winding of the CC  1200 , and third winding  1250  corresponds to comparator output, e.g. the winding on ICR Current Comparator  222  which provides the input to servo tracking amplifier  224  as depicted in  FIG. 2 . Fourth winding  1270  provides the winding for a dedicated degauss current source connection such as depicted supra in respect of  FIG. 10  with fourth winding  1012 D. 
         [0104]    Referring to  FIG. 13A  there is depicted a Current Comparator (CC)  1300  according to an embodiment of the invention showing the CC  1300  sequentially stripped from the outermost layer towards the magnetic core  1210 . Accordingly as shown the CC  1210  comprises a magnetic core  1210  over which is wrapped first tape layer  1230 A separating the first winding  1240 A from it. Second tape layer  1230 B is then wrapped over first winding  1240 A upon which is then wrapped second winding  1240 B. These layers are then over-wrapped with third tape layer  1230 C followed by third winding  1250 , fourth tape layer  1230 D, shielding  1260 , and fifth tape layer  1230 E. As depicted first winding  1240 A corresponds to the primary winding of the CC  1200 , second winding  1240 B corresponds to the secondary winding of the CC  1200 , and third winding  1250  corresponds to comparator output, e.g. the winding on ICR Current Comparator  222  which provides the input to servo tracking amplifier  224  as depicted in  FIG. 2 . Also depicted is magnetic shield  1310  surrounding the CC  1300  upon which is wound shield degauss winding  1320 . 
         [0105]    Perspective view  1350  shows a partial cross-section three-dimensional view of the CC  1300  showing square magnetic core  1210  with the magnetic shield  1310  surrounding upon which is wound Shield Degauss Winding  1320 . Also it is evident how the windings on CC  1300  pass through a portion of magnetic shield  1310 . Referring to  FIG. 14  there is depicted a DRMS  1410  incorporating magnetic shield  1310  around Comparator  1300  together with Shield Degauss Winding  1320  which is coupled to Degauss Current Source  1420  via Load Resistor  1430 . Accordingly, under action of a controller the Degauss Current Source  1420 , which may be a coarse current source, may cycle at predetermined points within periods of use of the Instrument of which DRMS  1410  forms part in order to degauss the magnetic shield. Subsequently degaussing of the magnetic core of the Comparator  1300  may be executed through first winding  1140 A, being the primary, from Amplifier  208  and second winding  1140 B, being the secondary, from DAC  210 . Third winding  1150  being coupled to Amplifier  208  as part of the feedback from Comparator  1300 . 
         [0106]    Specific details are given in the above description to provide a thorough understanding of the embodiments. However, it is understood that the embodiments may be practiced without these specific details. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. 
         [0107]    Implementation of the techniques, blocks, steps and means described above may be done in various ways. For example, these techniques, blocks, steps and means may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the processing units may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described above and/or a combination thereof. 
         [0108]    Also, it is noted that the embodiments may be described as a process which is depicted as a flowchart, a flow diagram, a data flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be rearranged. A process is terminated when its operations are completed, but could have additional steps not included in the figure. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function. 
         [0109]    The foregoing disclosure of the exemplary embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents. 
         [0110]    Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.