Schematics of an LFI radiometer taken from Cal13. The two linearized polarization components are separated by an orthomode transducer (OMT), and each of them enters a twin radiometer, only one of which is shown in the figure. A first amplification stage is provided in the cold (20 K) focal plane, where the signal is combined with a reference signal originating in a thermally stable 4.5-K thermal load. The radio frequency signal is then propagated through a set of composite waveguides to the warm (300 K) backend, where it is further amplified and filtered, and finally converted into a sequence of digitized numbers by an analogue-to-digital converter. The numbers are then compressed into packets and sent to Earth.