[Gb-ccb] ka/ccb low frequency data -- microphonics?

[Brian Mason]

all- Last night we had great weather and everything went smoothly for 
ka/ccb tests. To investigate the instability in the low frequency CCB 
channels I followed a suggestion of Roger & Galen's which was to turn the 
LO1's down and reroute them away from ka + mmconverter. I verified that 
indeed LO power monitor points read ~0 V at Ka and converter. The idea 
here was that perhaps a low-level instability in the spectral line LO, 
coupling back into the continuum section, might contribute to the 
corruptions of the low-frequency ccb data that we see. This appears not to 
be the case.

On closer inspection I wonder if we might have some microphonics. Attached 
are some plots showing the problem more clearly, all collected last night 
with LO1's "off".

1) cbi14hBk-2005_12_22_13:05:54-raw.png

Shows the 16 ccb channels of uncalibrated beamswitched data and the fit of 
source flux (given a target position). We were observing a blank spot on 
the sky so the signal should be nice and stable gaussian noise, perhaps 
superposed on a modest drift. channels J1 and J5 stand out as anomalous 
here, in a way that's fairly typical of the couple of dozen clearly 
corrupted scans I've seen in a couple of hundred scans' worth of CCB data.

2) 2005_12_22_13:05:54-j0-zoom.png

Shows a zoom of the ccb differenced data over a 20 second interval. This 
looks to me like 1 to 1.2 Hz stuff-- microphonics from the fridge perhaps?

3)  2005_12_22_13:05:54-allchan-zooms.png

shows zooms on the same range for all 16 channels with completey illegible 
annotations. j1 + j5 (the low frequency pair on one radiometer) stand out 
here as having similar corruptions, but j4 + j8 (the high frequency pair 
for the same radiometer) appear to have a similar phenomenon going on.

  Brian

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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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