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   Convert decibels to power.

   Type DBSignalToPower for a demo.

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   Form:
   power = DBSignalToPower( dB )
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   ------
   Inputs
   ------
   powerDB       (1,:)  Power (dB)
                        
   -------
   Outputs
   -------
   power         (1,:)  Power (nW)
                        
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