Chris, Alexa
Loop Gains
The first two attachments show the OLTFs of the various length DOFs. In these plots, the model is the blue trace and the red trace is measured data. The DARM TF agrees pretty well. The PRCL and SRCL loops appear to have a gain missing. The CARM and MICH model loops are not correct.
Noise Budget Overview (***Rough Estimates***)
The fourth attachment depicts the noise budget produced by the model for the 8W lock on March 04, 2015 15:30 UTC. This roughly agrees with the conclusions Evan has made with his NB (LHO#17101). At high frequencies we are dominated by shot noise. At around 300 Hz we suspect intensity noise (or maybe beam jitter), as Gabriele noticed. From 30-100 Hz we are limited by DAC noise. And below 10 Hz we see some angular fluctuations.
Noise Budget Details
- Sensing Noise: The OMC dark noise was measured previously (LHO 16656) in mA/rtHz. We use the measurement with two whitening stages engaged and propagate this to darm. We find the conversion 1.56e12mA/m below the cavity pole.
- Intensity Noise: We use the second ISS loop out-of-loop PDs (currently PDs 5-8) to measure the RIN. We then measured the OMCPDSUM/ISS_PDSUM58 TF to determine the calibration from RIN to W. The third attachment shows the model (blue) optickle TF and the measured (green) TF. During this TF we only had coherence between 200-300 Hz. At this point we see that the model is -20dB relative to the measured data. This explains why the intensity noise looks too low in the NB plot. When the IFO is locked we should take this TF (/ligo/svncommon/NbSVN/aligonoisebudget/trunk/Dev/DRFPMI/Validation/H1Data/RIN2OMCDCPD.xml). Then the optickle model TF can be replaced with the actual measurement.
- Angular Noise: The model uses live parts to measure the spectra in cts at the PUM stage of the quads. We have used an undamped model to convert these counts into displacement in P and Y of the test mass. We assume no coupling of P2Y and Y2P. Our cailibration from m to radians also needs to be checked. This produces a very rough estimate of angular noise.
- We are using an ESD strength of 4.2e-10 [N/V^2], which is different than what Kiwamu uses for the calibration (2.8e-10 [N/V^2] LHO# 17206). The spectra agree within about 20%, which seems better than what one would expect with the differences in ESD strength.
The modeled MICH loop gain is looking better now. There were some aggressive BSFM plant inversion filters that weren't being linearized accurately in Simulink. This was fixed by using numerical TFs for them. The shape is still off above 60 Hz, but that's probably due to the filter change from alog 17148.
I put in sign flips as needed to ensure an even number of them are present in each OLTF. There remains some 10 dB of mystery gain to be hunted for in the DRMI loops.
