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   Function called by fzero to compute the carrier to noise ratio. 
   See also CNRComp.
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   Form:
   x = FPD( cNR, pD, yB, n )
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   ------
   Inputs
   ------
   CNR           (1,1)  Carrier to noise ratio
   pD            (1,1)  Probability of detection
   yB            (1,1)  Upper range of integrand
   n             (1,:)  Number of pulses

   -------
   Outputs
   -------
   x             (1,1)   Value

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