We present a comparison of digital real-time spectrometers used in ground-based radiometry for the profiling of trace gases in the middle-atmosphere. From January to June 2019, we performed parallel observations of the atmospheric ozone transition line at 110.836 GHz with three different spectrometers connected to the same front-end. It allows to compare and characterize the ozone spectra over an extended period of time and covering a wide range of meteorological conditions. We show that the spectra derived from the Acqiris AC240 is systematically biased compared to other state-of-the-art spectrometers. It has a different spectral slope and a negative bias at the line center on the order of 8 percent. The bias evolves with the atmospheric opacity and originates from various sources. Using some simple corrections, we show that the bias from the AC240 can be corrected during data processing which is of high interest for the numerous time series derived from this back-end.
