Offset balancing in pseudo-correlation radiometers for CMB measurements

Radiometeric CMB measurements need to be highly stable and this stability is best obtained with differential receivers. The residual 1/ f noise in the differential output is strongly dependent on the radiometer input offset which can be cancelled using various balancing strategies. In this paper we discuss a software method implemented in the PLANCKLFI pseudo-correlation receivers which uses a tunable gain modulation factor, r, in the skyload difference. Numerical simulations and experimental data show how proper tuning of the parameter  r ensures a very stable differential output with knee frequencies of the order of few mHz. Various approaches to calculate  r using the radiometer total power data are discussed with some examples relevant to PLANCK-LFI. Although the paper focuses on pseudo-correlation receivers and the examples are relative to PLANCK-LFI, the proposed method and its analysis is general and can be applied to a large class of differential radiometric receivers.