plot the pulse spectral density.

        Parameters
        ----------
        xlim : array_like, optional
            The x limits of the plotted PSD [LowerLimit, UpperLimit]
            Default value is [0, SampleFreq/2]
        units : string, optional
            Units of frequency to plot on the x axis - defaults to kHz
        show_fig : bool, optional
            If True runs plt.show() before returning figure
            if False it just returns the figure object.
            (the default is True, it shows the figure)

        Returns
        -------
        fig : matplotlib.figure.Figure object
            The figure object created
        ax : matplotlib.axes.Axes object
            The subplot object created

def plot_PSD(self, xlim=None, units="kHz", show_fig=True, timeStart=None, timeEnd=None, *args, **kwargs):
        """
        plot the pulse spectral density.

        Parameters
        ----------
        xlim : array_like, optional
            The x limits of the plotted PSD [LowerLimit, UpperLimit]
            Default value is [0, SampleFreq/2]
        units : string, optional
            Units of frequency to plot on the x axis - defaults to kHz
        show_fig : bool, optional
            If True runs plt.show() before returning figure
            if False it just returns the figure object.
            (the default is True, it shows the figure)

        Returns
        -------
        fig : matplotlib.figure.Figure object
            The figure object created
        ax : matplotlib.axes.Axes object
            The subplot object created
        """
        #        self.get_PSD()
        if timeStart == None and timeEnd == None:
            freqs = self.freqs
            PSD = self.PSD
        else:
            freqs, PSD = self.get_PSD(timeStart=timeStart, timeEnd=timeEnd)
            
        unit_prefix = units[:-2]
        if xlim == None:
            xlim = [0, unit_conversion(self.SampleFreq/2, unit_prefix)]
        fig = _plt.figure(figsize=properties['default_fig_size'])
        ax = fig.add_subplot(111)
        ax.semilogy(unit_conversion(freqs, unit_prefix), PSD, *args, **kwargs)
        ax.set_xlabel("Frequency ({})".format(units))
        ax.set_xlim(xlim)
        ax.grid(which="major")
        ax.set_ylabel("$S_{xx}$ ($V^2/Hz$)")
        if show_fig == True:
            _plt.show()
        return fig, ax