Schematics of a LFI radiometer. The radiation entering the feed horns in the 20 K front end is split by an OrthoMode transducer (OMT) into its two linearly polarized components. The two signals feed two twin radiometers. (Only one of them is shown in this figure.) The 2.7 K signal is mixed with the signal emitted by a reference blackbody at 4.5 K by an hybrid coupler before being amplified by a Low Noise Amplifier (LNA). A 4096 Hz phase switch induces an alternating 0°/180° phase shift, so that the outputs of the second hybrid are two sequences of sky/reference signals, which both propagate to the warm back-end. Here they are further amplified and measured, before being compressed into packets and sent to Earth. Refer to Mennella et al. (2010) for more details.