def load_seit_data(directory, frequency_file='frequencies.dat',
                   data_prefix='volt_', **kwargs):
    frequencies = np.loadtxt(directory + os.sep + frequency_file)
    data_files = sorted(glob(directory + os.sep + data_prefix + '*'))
    if frequencies.size != len(data_files):
        raise Exception(
            'number of frequencies does not match number of data files')
    data_list = []
    for frequency, filename in zip(frequencies, data_files):
        subdata = load_mod_file(filename)
        subdata['frequency'] = frequency
        data_list.append(subdata)
    df = pd.concat(data_list)
    return df, None, None