Calibrated and baseline-subtracted observed ¹³CH spectra at an IF setting of 1.4 GHz (top panel), 1.6 GHz (central panel), and 1.8 GHz (bottom panel) towards Sgr B2(M). The secondary y-axis in each panel presents the optical depth scale. Signal bandpass contamination, possibly from the transitions of H₂S at 1996.589 GHz, is displayed by the green shaded region. The hyperfine transitions corresponding to the 1997 GHz Λ-doublet arising from the signal band are displayed in pink, while that of the 2001 GHz Λ-doublet arising from the image band is displayed in blue. The change in IF pushes both the second Λ-doublet and the atmospheric ozone feature at 2002.347 GHz (telluric rest frame, marked in cyan) towards the signal band features. The secondary x-axis in each panel represents the image band LSR velocity scale.