code: def test_frequency_double_side_band_channel_str():
      """Test the frequency double side band object: test the band description."""
      frq_dsb1 = FrequencyDoubleSideBand(183, 7, 2)
      frq_dsb2 = FrequencyDoubleSideBand(183000, 7000, 2000, "MHz")
  
      assert str(frq_dsb1) == "central=183 GHz ±7 width=2 GHz"
      assert str(frq_dsb2) == "central=183000 MHz ±7000 width=2000 MHz"
  
imports: import unittest
  from datetime import datetime
  import numpy
  import pytest
  from satpy.dataset.dataid import DataID, DataQuery, ModifierTuple, WavelengthRange, minimal_default_keys_config
  from satpy.readers.pmw_channels_definitions import FrequencyDoubleSideBand, FrequencyQuadrupleSideBand, FrequencyRange
  from satpy.tests.utils import make_cid, make_dataid, make_dsq
label_desc: Use image node_id 14 for calling a global function with example usage: test_frequency_double_side_band_channel_str() without return types