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Fri Mar 6 05:44:14 2009, Yoichi, Update, Locking, Locking distracted by the QPD whitenning problem again
Fri Mar 6 15:23:39 2009, Yoichi, Update, Locking, Locking distracted by the QPD whitenning problem again
Fri Mar 6 15:23:39 2009
In reply to:
Locking distracted by the QPD whitenning problem again
By looking at the time series of DARM signal at the time of a lock loss, the oscillation frequency was about 3.5kHz (see the attm1 and its zoomed version attm2).
I will measure the DARM loop gain around this frequency next.
Tonight, I was able to ramp up the arm power to around 20. Then the DARM loop started to oscillate and the IFO lost lock in a few seconds.
I repeated this several times, then realized that the transmission QPDs were not working properly again due to the well known sticky slider problem.
I should have run slider_twiddle script. Since the DARM RF signal is normalized by the sqrt(TRX+TRY), it is reasonable that the DARM loop got unstable.
The fact that I was able to go up to arm power = 20 means there is nothing saturating below this power level.