def calc_IM3(P, IP3):
    """
        Calculate Intermodulation Value IM3 (absolute)
        from P and IP3

        Parameters
        ----------
        P : float
            Signal ower in [dBm]
        IP3 : float
            IP3 in [dBm]

        Returns
        -------
        IM3 : float or None
            intermodulation 3 power in [dBm]
        """
    return P - (2 * (IP3 - P) + 6) if IP3 is not None else None
