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Kenta Tanaka - 3:06 Saturday 19 September 2020 (15059) Print this report
ALS DARM OLTF measurements

Nakano, Kenta

The follow figures are at /users/Measurements/IFO/ALS/archives. And the scripts  i used for the plot is  /users/Commissiong/scripts.ALS/ALSDARM_TF.py

What to do

  •  Design filter for ALS DARM Loop

              First, in order to check the structures, measured TFs of MN and TM(fig 1). judging from this figure, design some filters to weak some peaks so that they would not be unity gain. Fig 2 is the result that measured OLTFs with created filters. These UGF are about 0.8 Hz

  • Check Cross over

            At the frequency of cross-over of these TFs, we checked if the phse was not 180 degree relative to each other. Fig 3 is the ration between MN and TM OLTF. Blue line means designed one, Orange means measured one.

           You can see how the phase is when this line intersects with 0 dB. At cross over frequency  = ~1.4 Hz, the phase is ~ 135 degrees. Also, Above the frequency, the gain seems to have enough margin. So this loop seems to be fine.

  • Measure ALS DARM OLTF

           It was confirmed that the loop was stabe, we closed the loop and measured the overall open loop TF. Fig 4 is the results. Bkue lines is designed OLTF, Orange is the TF of MN and TM , added together in python, Green is measured one.

           There is not much difference between the model,and the actual measurements. The UGF is about 0.8 Hz. concern that  1.5Hz Length peak is close to 0dB.

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