  The LFI (Low Frequency Instrument) on board the ESA Planck satellite is
constituted by an array of radiometric detectors actively cooled at 20 K in the
30-70 GHz frequency range in the focal plane of the Planck telescope. In this
paper we present an overview of the LFI instrument, with a particular focus on
the radiometer design. The adopted pseudo-correlation scheme uses a software
balancing technique (with a tunable parameter called gain modulation factor)
which is effective in reducing the radiometer susceptibility to amplifier
instabilities also in presence of small non-idealities in the radiometric chain
components, provided that the gain modulation factor is estimated with an
accuracy of the order of 0.2%. These results have been recently confirmed by
experimental laboratory measurements conducted on the LFI prototype radiometers
at 30, 70 and 100 GHz.
