def _setup_freqs(self, f_start=None, f_stop=None):
        """ Setup frequency axis """
        ## Setup frequency axis
        f0 = self.header[b'fch1']
        f_delt = self.header[b'foff']

        i_start, i_stop = 0, self.header[b'nchans']
        if f_start:
            i_start = int((f_start - f0) / f_delt)
        if f_stop:
            i_stop  = int((f_stop - f0)  / f_delt)

        #calculate closest true index value
        chan_start_idx = np.int(i_start)
        chan_stop_idx  = np.int(i_stop)

        #create freq array
        if i_start < i_stop:
            i_vals = np.arange(chan_start_idx, chan_stop_idx)
        else:
            i_vals = np.arange(chan_stop_idx, chan_start_idx)

        self.freqs = f_delt * i_vals + f0

#         if f_delt < 0:
#             self.freqs = self.freqs[::-1]

        if chan_stop_idx < chan_start_idx:
            chan_stop_idx, chan_start_idx = chan_start_idx,chan_stop_idx

        return i_start, i_stop, chan_start_idx, chan_stop_idx
Setup frequency axis