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

[Brian Mason]

Thanks Roger- there were actually 3 successive 5 minute drift scans 
plotted there, not 1, in different colors. Just a consistency check.

We also quickly checked into the effect of the HEMT LEDs on the noise 
performance. These are (apparently) included to keep the devices' behavior 
consistent at cryogenic temperatures, but Ed Wollack noted a 35% reduction 
in low-frequency noise from HEMTs with them off. On Wednesday 22nd we did 
some hot/cold measurements with and without LEDs. Attached are 2 "regular" 
cold load power spectra (white/red) and one (green) taken with the LEDs 
off, all 3 of which are consistent within the noise.

Also, I could now be fully persuaded of the water-dripping-from the 
subreflector explanation of that one days' bad ccb data we got when the 
weather was perfect. On the Wednesday we were up there getting the cold 
load data it was not actually precipitating at all during the day, but it 
was warmer after previous light snow, and even as late as 3pm it was like 
a rain forest up on top of the receiver cabin. In future such situations I 
guess we should spend an hour or two tipped over tracking the sun as if 
there were snow on the dish.

  -Brian

--------------------------------------------------------------------
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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On Mon, 27 Feb 2006, Roger Norrod wrote:

> Brian,
>
> Thanks.  Quite interesting.
>
> There seem to be three solid and three dotted lines in each frame.  What do
> the three colors represent?
>
> Roger
>
> At 11:23 AM 2/27/2006 -0500, Brian Mason wrote:
>>
>> 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.
>>
>>
>> --------------------------------------------------------------------
>> 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/       |
>> --------------------------------------------------------------------
>> Attachment Converted: "c:\program
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