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Entry  Tue Sep 7 18:00:54 2021, Tega, Summary, Calibration, System Identification via line injection raw_timeseries.pdfdemod_signals.pdfcal_noise_asd.pdf
    Reply  Tue Sep 21 11:09:34 2021, Anchal, Summary, CDS, XARM YARM UGF Servo and Oscillators added 
       Reply  Wed Sep 22 12:40:04 2021, Anchal, Summary, CDS, XARM YARM UGF Servo and Oscillators shifted to OAF 
    Reply  Tue Sep 21 11:13:01 2021, Paco, Summary, Calibration, XARM calibration noise XARM_signal_asd.pdfXARM_demod_timeseries.pdfXARM_demod_asds.pdfXARM_cal_0921_timeseries.pdf
       Reply  Wed Sep 22 11:43:04 2021, rana, Summary, Calibration, XARM calibration noise 
          Reply  Tue Sep 28 16:31:52 2021, Paco, Summary, Calibration, XARM OLTF (calibration) at 55.511 Hz OLTF_Calibration_Scheme.jpgXARM_POX_Lock_Model_TF.pdfXARM_OLTF_Total_Model.pdfXARM_OLTF_55p511_Hz_timeseries.pdfGmag_55p511_Hz_ASD.pdf
             Reply  Thu Sep 30 18:04:31 2021, Paco, Summary, Calibration, XARM OLTF (calibration) with three lines XARM_OLTF_Model_and_Meas.pdfGmag_ASD_nb_withTRX.pdf
Message ID: 16352     Entry time: Tue Sep 21 11:13:01 2021     In reply to: 16315     Reply to this: 16353
Author: Paco 
Type: Summary 
Category: Calibration 
Subject: XARM calibration noise 

Here are some plots from analyzing the C1:LSC-XARM calibration. The experiment is done with the XARM (POX) locked, a single line is injected at C1:LSC-XARM_EXC at f0 with some amplitude determined empirically using diaggui and awggui tools. For the analysis detailed in this post, f0 = 19 Hz, amp = 1 count, and gain = 300 (anything larger in amplitude would break the lock, and anything lower in frequency would not show up because of loop supression). Clearly, from Attachment #3 below, the calibration line can be detected with SNR > 1.

We read the test point right after the excitation C1:LSC-XARM_IN2 which, in a simplified loop will carry the excitation suppressed by 1 - OLTF, the open loop transfer function. The line is on for 5 minutes, and then we read for another 5 minutes but with the excitation off to have a reference. Both the calibration and reference signal time series are shown in Attachment #1 (decimated by 8). The corresponding ASDs are shown in Attachment #2. Then, we demodulate at 19 Hz and a 30 Hz, 4th-order butterworth LPF, and get an I and Q timeseries (shown in Attachment #3). Even though they look similar, the Q is centered about 0.2 counts, while the I is centered about 0.0. From this time series, we can of course show the noise ASDs in Attachment #3.

The ASD uncertainty bands in the last plot are statistical estimates and depend on the number of segments used in estimating the PSD. A thing to note is that the noise features surrounding the signal ASD around f0 are translated into the ASD in the demodulated signals, but now around dc. I guess from Attachment #3 there is no difference in the noise spectra around the calibration line with and without the excitation. This is what I would have expected from a linear system. If there was a systematic contribution, I would expect it to show at very low frequencies.

Attachment 1: XARM_signal_asd.pdf  34 kB  Uploaded Tue Sep 21 21:46:40 2021  | Hide | Hide all
Attachment 2: XARM_demod_timeseries.pdf  52 kB  Uploaded Tue Sep 21 21:54:07 2021  | Hide | Hide all
Attachment 3: XARM_demod_asds.pdf  1.140 MB  Uploaded Tue Sep 21 21:54:24 2021  | Hide | Hide all
Attachment 4: XARM_cal_0921_timeseries.pdf  57 kB  Uploaded Tue Sep 21 21:58:18 2021  | Hide | Hide all
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