def calc_SFDR(P, IM3):
    """
        Calculate SFDR value
        from P and IM3

        Parameters
        ----------
        P : float
            Signal power in [dBm]
        IM3 : float
            Intermodulation 3 power in [dBm]

        Returns
        -------
        SFDR : float or None
            Spurious free dynamic range in [dB]
        """
    return P - IM3 if IM3 is not None else None
