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takafumi.ushiba - 21:46 Sunday 26 March 2023 (24539) Print this report
Test of BPC at PR suspensions with PRCL signals


I tried to close BPC of PRCL but it didn't work well.
Probably, it is better to use IMMT1 but it is necessary to modify the real-time model to do so.


I tested BPC at PR suspensions, especially PRM and PR3 by using PR3 and PR2, respectively.
Frequencies of excitations are temporary decided as 70, 75, 100, and 105 Hz.
Figure 1 shows the setting of BPC.

Phase of the demodulation was decided as following procedure.
1. Set bubd-pass filters at demod sig filter banks. frequency band is +/- 1 Hz from each excitation frequency.
2. Send excitation to the susoension.
3. If signals after band-pass filter is too low, intentionally misalign the mirrors to obtain good S/N ratio.
4. Measure the relative phase (phi0) between LO signals and signals after band-pass fiter like fig2.
5. Set demod phase as -phi0.

After setting demodulation phase, I tried to close the BPC loops but it didn't work well (drifting too large and lock was lost).
It probably due to the BPC and ADS are fighting each other because if we turned off ADS, BPC itself seems wirking fine.
Currently, beam spot on IMMT2 was fixed, so it is difficult to move the beam spot on PRM while keeping good alignment of PRMI.
So, feedback to IMMT1 seems necessary but real-time model modification is necessary to do so...

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