Upper panel: raw spectra measured during a hot-cold experiment with the 6.5-mm receiver in spectroscopy mode. Each spectrum is the average of a 10 min observation. The sky position was observed afterwards by pointing the telescope to the zenith. Lower panel: using the raw spectra measured for hot and cold load, one can infer the (calibrated) gain-curve, i.e. the transfer function. Residual noise is present but the overall shape and even several small features are clearly visible.