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Wed Mar 11 16:53:48 2009, Yoichi, Update, Locking, Junks in around kHz
Wed Mar 11 22:57:48 2009, Yoichi, Update, Locking, Calibrated XARM error signal spectrum
Fri Mar 13 22:07:14 2009, Yoichi, Update, Locking, Calibrated XARM error signal spectrum
Fri Mar 13 22:07:14 2009
In reply to:
Calibrated XARM error signal spectrum
Of course I made a mistake.
I put a pole at 1525Hz whereas it should have been a zero.
The correct calibration factor is:
I attached a revised spectrum.
I did a rough calibration of the XARM error spectrum.
See the attached calibrated spectrum.
I started from this Rana's elog entry.
I first injected a 20Hz sin signal into C1:SUS-ETMX_LSC_EXC and measured the response to the ETMX SUSPOS.
Using the calibration of the SUSPOS given in the above entry, I calibrated the ETMX coil actuation efficiency.
It was 3.4e-12 m/cnt @20Hz for C1:SUS-ETMX_LSC_EXC.
Then I locked the X-arm and injected a calibration peak at 20Hz.
From the ratio of the peaks in C1:SUS-ETMX_LSC_IN2 and C1:LSC-XARM_IN1, I calibrated the X-arm error signal to be 4.2e-13 m/cnt.
We have to also take into account the cavity pole of the arm, 1525Hz (the design value, may not be actual).
So I used the following calibration in the DTT:
Note that the attached spectrum shows the actual motion of the X-arm (or equivalent frequency noise) after suppressed by the feedback servo,
unlike conventional noise spectra showing "virtual" displacement which would have been induced in the absence of servos.