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40m Log |
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Wed Jan 25 15:41:29 2017, Lydia, Update, IMC, 29.5 MHz modulation depth measurement plan
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Wed Jan 25 17:30:03 2017, Koji, Update, IMC, 29.5 MHz modulation depth measurement plan
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Wed Jan 25 19:39:07 2017, gautam , Update, IMC, 29.5 MHz modulation depth measurement plan
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Fri Jan 27 21:25:11 2017, Lydia, Update, IMC, 29.5 MHz modulation depth
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Sat Jan 28 01:25:51 2017, gautam, Update, IMC, 29.5 MHz modulation depth 
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Mon Jan 30 19:07:48 2017, gautam, Update, IMC, RF AM stabilization box pulled out
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Tue Jan 31 01:07:20 2017, Lydia, Update, IMC, RF AM stabilization box pulled out
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Tue Jan 31 13:46:34 2017, rana, Update, IMC, RF AM stabilization box pulled out
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Tue Jan 31 14:17:48 2017, gautam, Update, IMC, RF AM stabilization box pulled out
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Tue Jan 31 22:07:13 2017, gautam, Update, IMC, RF AM stabilization box revamp
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Tue Jan 31 22:28:39 2017, Lydia, Update, IMC, RF AM stabilization box pulled out
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Wed Feb 1 11:51:19 2017, Koji, Update, IMC, RF AM stabilization box pulled out
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Wed Feb 1 16:45:56 2017, Lydia, Update, IMC, RF AM stabilizer box Modification Plan
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Wed Feb 1 20:49:34 2017, rana, Update, IMC, RF AM stabilizer box Modification Plan
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Wed Feb 1 23:13:30 2017, Lydia, Update, IMC, RF AM stabilizer box Modification Plan
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Sun Feb 5 21:56:50 2017, Lydia, Update, IMC, 29.5 MHz stabilizer box replacement
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Tue Feb 7 12:01:10 2017, Lydia, Update, IMC, 29.5 MHz stabilizer box replacement
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Tue Feb 7 17:00:55 2017, Lydia, Update, IMC, 29.5 MHz stabilizer box replacement
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Fri Feb 10 02:14:10 2017, gautam, Update, IMC, 29.5 MHz stabilizer box replacement
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Fri Feb 10 11:41:43 2017, Lydia, Update, IMC, 29.5 MHz stabilizer box replacement
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Fri Feb 10 13:24:28 2017, rana, Update, IMC, 29.5 MHz stabilizer box replacement
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Fri Feb 10 18:21:21 2017, gautam, Update, IMC, IMC Demod board
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Fri Feb 10 19:32:15 2017, Koji, Update, IMC, IMC Demod board
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Sun Feb 12 01:16:57 2017, gautam, Update, IMC, IMC length loop - summary of changes    
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Mon Feb 13 11:55:14 2017, rana, Update, IMC, IMC length loop - summary of changes
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Mon Feb 13 13:34:44 2017, gautam, Update, IMC, IMC length loop - bad SMA cable replaced
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Mon Feb 13 19:44:55 2017, Lydia, Update, IMC, Front panel for 29.5 MHz amplifier box
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Wed Mar 1 21:15:40 2017, Lydia, Update, IMC, Front panel for 29.5 MHz amplifier box
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Wed Mar 1 23:56:09 2017, gautam, Update, IMC, Front panel for 29.5 MHz amplifier box
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Thu Mar 2 20:32:18 2017, Lydia, Update, IMC, Front panel for 29.5 MHz amplifier box
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Wed Feb 15 23:54:13 2017, gautam, Update, IMC, IMC saga continues... 
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Fri Feb 17 20:10:18 2017, gautam, Update, IMC, WFS servos turned back on
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Sat Feb 18 14:09:06 2017, rana, Update, IMC, WFS servos turned back on
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Sat Feb 18 21:50:48 2017, gautam, Update, IMC, WFS servos turned back on
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Sun Mar 5 12:41:23 2017, gautam, Update, IMC, WFS servo-steppin  
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Message ID: 12840
Entry time: Sat Feb 18 21:50:48 2017
In reply to: 12839
Reply to this: 12867
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Author: |
gautam |
Type: |
Update |
Category: |
IMC |
Subject: |
WFS servos turned back on |
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Here is a comparison of the error signal spectra after increasing the IMC modulation depth, to the contribution with RF inputs / whitening inputs terminated (which I borrowed from Koji's characterization of the same in Dec 2016, these shouldn't have changed).

Some general observations:
- This data was taken with the WFS servos disabled, but with the IMC hand-aligned to a good state (MC_TRANS ~15,000). The error signal spectra are from the new DQ channels (but still sampled at 2048Hz, I had not implemented the change to 512Hz).
- The error signals seem to have increased by ~25x
, which is consistent with how much we expect the modulation depth to have increased
- The bump around 1 Hz is now cleaerly visible in all 16 channels, as is the bounce peak at 16Hz (relative to Dec 2016). In general, between 0.1Hz and 5Hz, there is now a fair bit of daylight between the error signals and the electronics noise contribution.
I will update with the in-loop error signal spectra, which should give us some idea of the loop bandwidth.
I will look into lowering the sampling rate, and how much out-of-band power is aliasing into the 0-256 Hz band and update with my findings.
Quote: |
Yikes. Please change the all teh WFS DQ channels sample rates from 2048 down to 512 Hz. I doubt we ever need anything about 180 Hz.
There is sometimes an issue with this: if our digital AA filters are not strong enough, the noise about above 256 Hz can alias into the 0-256 Hz band. We ought to check this quantitatively and make some elog statement about our AA filters. This issue is also seen in DTT when requesting a low frequency spectrum: DTT uses FIR filters which are sometimes not sharp enough to prevent this issue.
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