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AdV-DET (Commissioning)
swinkels - 22:53 Monday 10 July 2017 (38533) Print this report
Mixing of high-frequency lines
One of the peculiarities of the moving lines is that some of them have a negative dependency on temperature. One way to produce mirrored lines are non-linear processes such as aliasing and the mixing of lines. To better understand if we have such issues, I asked Loic to acquire the main photodiode signals at high frequency. Looking at the spectrum of the new signals shows some interesting structure, see fig 1. Some of the biggest lines were seen before and are caused by the two dither lines used for the lock of the OMCs and for the LVDTs of SDB1. One of the things that jumps out is the comb of lines of the LVDTs around 8500 (fig 2) is also seen at 21 kHz (fig 3), displaced by 12831.3 Hz, which is the dither frequency of the first OMC. This modulation line itself shows up till its 4th harmonic (4 * 12831.3 = 51325.2 Hz, aliased down to 48674.8 Hz), see fig 4. The forest around 4 kHz is caused by the difference between the LVDT lines and the OMC1 line, see fig 5.

One possible explanation for the moving lines might be some high frequency mirror modes (which are known to depend on temperature), which are beating with respect to some of the SDB1 lines. These mirror modes have not been identified yet, so for now this is just a hypothesis. Comparing the low-frequency spectrum with the regions around the big OMC lines does not show any obvious candidates either.

Proposed actions:
-change the frequency and amplitude of the biggest lines, especially the ones that modulate the OMCs, to see if the moving lines move as well
-check for aliasing.
-identify the most important mirror modes, which are also needed to monitor the etalon effect.
-try to understand what is the exact non-linear process: is it something in the electronics, the inherently non-linear method of DC detection, something inside the OMCs, something in the actuators (but this seems unlikely given the high frequency), ...
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swinkels, mantovani, was - 19:18 Tuesday 11 July 2017 (38555) Print this report
To test if some of the moving lines are caused by high frequency lines that are beating with the OMC lines, we moved these by a few Hz. A few lines at 231.7, 328.6, 633.7, 4084.3, 4117 and 4316 Hz can be seen moving in parallel, see attached figures for some examples. All of these are lines that were not drifting in time, and which were on the list of known mixed frequencies that was posted before. The forest of moving lines does however not move at all. Also the amplitude of the OMC lines and the frequency of some LVDT lines were moved, all with predictable results. The search goes on ...

Log messages of performed changes:
# move OMC lines:
2017-07-11-15h20m10-UTC>INFO...-AcSineLineChCmFreqSet> line_f1 - freq 12832.3Hz - loaded and request to apply received - sender "OMC_script"
2017-07-11-15h20m45-UTC>INFO...-AcSineLineChCmFreqSet> line_f1 - freq 12832.3Hz - loaded and request to apply received - sender "OMC_script"
2017-07-11-15h21m38-UTC>INFO...-AcSineLineChCmFreqSet> line_f2 - freq 0Hz - loaded and request to apply received - sender "OMC_script"
2017-07-11-15h22m13-UTC>INFO...-AcSineLineChCmFreqSet> line_f2 - freq 8746Hz - loaded and request to apply received - sender "OMC_script"

# lower line amplitudes:
2017-07-11-15h30m04-UTC>INFO...-AcConstChCmSet> OMC1_PZT_amp, val 0.3 rampTime 3 - loaded and request to apply received - sender OMC_script
2017-07-11-15h30m54-UTC>INFO...-AcConstChCmSet> OMC2_PZT_amp, val 0.03 rampTime 3 - loaded and request to apply received - sender OMC_script

# move LVDT lines:
2017-07-11-15h47m26-UTC>INFO...-AcSineLineChCmFreqSet> LC_LVDT_FR_H_line - freq 8717.3Hz - loaded and request to apply received - sender "SDB1_LC_script"
2017-07-11-15h53m14-UTC>INFO...-AcSineLineChCmFreqSet> LC_LVDT_FL_H_line - freq 8513.3Hz - loaded and request to apply received - sender "SDB1_LC_script"

# restore everything:
2017-07-11-15h57m41-UTC>INFO...-AcSineLineChCmFreqSet> LC_LVDT_FL_H_line - freq 8515.3Hz - loaded and request to apply received - sender "SDB1_LC_script"
2017-07-11-15h57m56-UTC>INFO...-AcSineLineChCmFreqSet> LC_LVDT_FR_H_line - freq 8714.3Hz - loaded and request to apply received - sender "SDB1_LC_script"
2017-07-11-15h58m16-UTC>INFO...-AcConstChCmSet> OMC2_PZT_amp, val 0.1 rampTime 3 - loaded and request to apply received - sender SDB1_LC_script
2017-07-11-15h58m25-UTC>INFO...-AcConstChCmSet> OMC1_PZT_amp, val 1 rampTime 3 - loaded and request to apply received - sender SDB1_LC_script
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