I found that none of the filter banks in the LSC input signals have the precise anti-whitening filters.
I installed the precise filters on REFL11, REFL33, REFL55 and AS55 based on Jenne's measurement (#4955)
After installing them I briefly checked the REFL11 sensing matrix with the PRMI locked, but it didn't change so much from what I got (#6283).
But I felt that the PRMI became more robust after that ... I just felt so ...
(Background)
The lock of the PRMI doesn't look healthy, especially the sensing matrix doesn't make sense at all ( #6283). A very staring thing in the sensing matrix is that the REFL11 and REFL55 didn't show the 90 degree separation between MICH and PRCL.
So I suspected some electronics, particularly the demodulation boards.
(What I did)
I checked the anti-whitening filters shape to see if they are ok or not.
I found that they all had the default filters of two zeros at 150 Hz and two poles at 15 Hz. So they weren't quite tuned.
I thought this could be a problem when I measure the sensing matrix because I usually excite the length DOFs at a high frequency of 283.1 Hz
and the mismatches between the anti-whitening and whitening filters may lead to something funny at such a high frequency.
So I installed the precise filters on REFL11, REFL33, REFL55 and AS55.
After that I did a orthogonality test on each I-Q pair of the demod signals to correct the D-phases and the relative gain between I and Q.
(Next ?)
Rana and I discussed the plan and decided to go back to a simple Michelson which should be easy enough to understand what is going on and should allow us a complete set of measurements.
Our big concern behind it is that we maybe locking the PRMI at a funny operation point.
In order to assess the issue I will do the following actions on the Michelson at first and then apply the same things on the PRMI later :
- Check the amount of of the sidebands using the OSA
- Check the amount of the DC light
- Check the sensing matrix to see if the absolute values in watt / meter make sense or not
- This work needs calibrations on all the demodulated board (this is equivalent to measuring the conversion losses of the mixers in the demod boards).
- Measure the contribution from the RAMs (it must be measurable by some means)
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