To test the influence of the different calibration schemes on the reconstructed flux values, synthetic spectra were produced. The upper panel shows the input power spectral densities. The antenna temperature on-source (T_sou) is a superposition of three Gaussians with an amplitude of 3 K on top of a continuum spectrum with a spectral index of  − 0.7, i.e. T_sou ~ ν^-2.7. For the system temperature, T_sys,off ~ ν^-2.1 is used for convenience. We also simulate a slight frequency-dependence of the calibration diode using the power law T_cal ~ ν^-0.5. Gaussian noise was added to the four measurement phases according to their total temperature values (see text). Note that each synthetic spectrum contains a different noise realisation. In addition, a more complicated case is included with a standing wave contribution, which is parametrised as a monochromatic sine wave component (lower panel).