[Gb-ccb] blank sky power spectra / r-factor correction

[Brian Mason]

Attached is the power spectrum from a 5 minute blank-sky drift scan with 
the CCB collected on Saturday. Solid line is the power spectrum of the 
differenced data, and the dashed line is the power spectrum of the total 
power data; CCB input ports are J1-J4 (top, left to right), J5-J8 (next 
row down left to right), etc.  Axes are log10(frequency) and log10(power 
spectra density). Typical 1/f corner frequencies for the better channels 
are 10^-0.5 - 10^-0.7 =0.2-0.3 Hz. The two first low frequency channels 
(J1 and J5) show similar performance in total power and differenced power.

With the working assumption that the 1/f we see is due to receiver gain 
fluctuations multiplying an imperfectly balanced input load, we tried 
applying the "r-factor" described in the Planck LFI papers by Seiffert, 
Menella et al. The idea is to let

  Diff Data = Sig - r * Ref

and adjust r so that the mean difference is zero, which reduces the effect 
of gain fluctuations. In effect this is a software gain-balancing made 
possible by the fact that we measure total power not just the difference. 
Doing this made no appreciable difference in the power spectra. We have 
more drift scan and hot/cold load data that we need to analyze, and we 
also need to look into the validity of the approximations made in our 
simple r-factor correction (such as detectero zero levels). But on the 
face of it this would disfavor offsets x imbalance as the cause of our 
1/f, and suggest an actual pre-hybrid intrinsically variable offset.

Several channels also have notable refrigerator microphonics (J10, J14; 
J4, J8, J12, J16) and most have 60 Hz. These are more prominent than in 
hot/cold load power spectra owing to lower overall noise levels.

  Brian & Larry W.


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Brian Mason                           | office: +1(304)456-2338
Associate Scientist                   | fax:    +1(304)456-2229
National Radio Astronomy Observatory  | mail:   PO Box 2 
bmason at gb.nrao.edu                    |         Green Bank, WV 24944
http://www.gb.nrao.edu/~bmason/       |
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