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Entry  Wed Apr 29 04:03:51 2009, Yoichi, Update, Locking, CARM RF changed to REFL_2I 
    Reply  Thu Apr 30 15:10:54 2009, rob, Update, Locking, CARM RF changed to REFL_2I 
Message ID: 1535     Entry time: Thu Apr 30 15:10:54 2009     In reply to: 1531
Author: rob 
Type: Update 
Category: Locking 
Subject: CARM RF changed to REFL_2I 

Yoichi, Peter

As Rob suggested, the optimal demodulation phase is easier to find for REFL_2I than POX_1I.
Moreover, for 166MHz LO, we have a phase shifter (delay line) already installed. So we can easily change the demodulation phase of REFL_2I.
Tonight, we switched the RF CARM signal to REFL_2I.
To do so, I changed the signal going to the REFL1 input of the common mode board from POX_1I to REFL_2I.
I moved a BNC-T installed at the output of POX_1I to the REFL_2I output to split the REFL_2I signal and send it to the CM board.
Since the gain of the REFL_2I was about 20dB lower than that of POX_1I, I increased the gain of the SR560, which is installed between the REFL_2 demodulation board and the CM board, from 1 to 10.

With some gain tweaks, we were able to hand off the CARM from REFL_DC to REFL_2I. We also succeeded in switching the REFL_2I ADC channel from PD11 to PD2_DC (the output of the length path from the CM board). This switching is necessary in order to engage the boost on the CM board.

There remains some offset in the CARM when the arm power is maximized. This is expected because the REFL_2I demodulation phase is probably not exactly right.
I will optimize the demodulation phase tomorrow.

From Optickle simulations, it looks like the SRCL/CARM gain ratio at REFL I2 is about 8e-4. So a 1 nanometer offset in SRCL yields 0.8 picometers of offset in CARM.
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