Returns an array of frequencies in Hertz ranging from -sw/2 to
        sw/2.

        :return: an array of frequencies in Hertz ranging from -sw/2 to sw/2.

def frequency_axis(self):
        """
        Returns an array of frequencies in Hertz ranging from -sw/2 to
        sw/2.

        :return: an array of frequencies in Hertz ranging from -sw/2 to sw/2.
        """
        return numpy.linspace(-self.sw / 2, self.sw / 2, self.np, endpoint=False)