2 Source of 1/f noise

The 1/f noise of the PLANCK-LFI receivers is generated by High Electron Mobility Transistor (HEMT) amplifier gain instabilities. If not properly corrected, it may lead to stripes in the final maps due to the satellite scanning strategy. Therefore it is of great importance to reduce the impact of such effect both by hardware and software techniques. The LFI receiver concept is driven by the need to reduce instability effects. Bersanelli et al.([1995]) described the design of LFI radiometers which are modified Blum correlation receivers (Blum [1959]; Colvin [1961]). We remind the interested reader to the Bersanelli et al.([1995]) and Seiffert et al. ([1997]) works for an analytical analysis of the source of 1/f noise in the final output of this receiver design and of the dependency of $f_{\rm k}$ upon the radiometer properties. In general the noise power spectrum writes:

$$S_{{\rm noise}}(f) = a\left[1+\left(\frac{f_{\rm k}}{f}\right)^\beta\right]\, ,$$ (1)

where a is a normalization factor (related to the ideal white noise level of the receiver) and $\beta$ takes typical theoretical values from 1 to 2.5 depending on the source of noise (gain drifts or thermal effects): $\beta = 1$ is the 1/f noise case. It is also a good approximation to take $S_{{\rm noise}}(f)$ vanishing for $f<f_{{\rm min}}$ and for $f>f_{{\rm max}}$. Delabrouille ([1998]) proposed $f_{{\rm min}}\sim 1/T_{{\rm mission}}$ and $f_{{\rm max}}\sim 1/2T_{{\rm sampling}}$.

With the current instrument specifications (Mandolesi et al. [1998]) typical values of the knee-frequency are $f_{\rm k} = 0.046$ Hz and $f_{\rm k} = 0.11$ Hz at 30 and 100 GHz repsectively with a 20 K load. Lower values of $f_{\rm k}$ can be reached by lowering the load temperature possibly up to values ($\simeq 4~$K) close to the full (sky plus environment) signal entering the horn (Seiffert et al.[1997]).