def populate_freqs(self):
        """
         Populate frequency axis
        """

        if self.header[b'foff'] < 0:
            f0 = self.f_end
        else:
            f0 = self.f_begin

        self._setup_chans()

        #create freq array
        i_vals = np.arange(self.chan_start_idx, self.chan_stop_idx)
        freqs = self.header[b'foff'] * i_vals + f0

        return freqs
Populate frequency axis