craig.cahillane@LIGO.ORG - posted 17:36, Monday 08 August 2022 - last comment - 10:20, Tuesday 09 August 2022(64371)
IM4 Trans stage one whitening engaged
We engaged the IM4 trans stage one whitening, by Evan H's request.
Noise is further from the 0.01 cts/rtHz floor. I don't actually know the citation of this ADC noise floor, I assume we are limited by some thermal noise in the ADC.
The segments are not close to the rails either. Even if we doubled the power we'd be fine.
SEG1_OUT16 = 5.2 mW incident
SEG2_OUT16 = 1.6 mW
SEG3_OUT16 = 0.8 mW
SEG4_OUT16 = 2.7 mW
I accepted these changes into the SDF so the guardian will not strike them down.
Code I used to make the plot also attached, it lives in /ligo/home/craig.cahillane/Git/IFO/IMC/scripts/im4_trans_quadrant_adc_noise_check.py
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I took the IMC offline for a dark noise measurement at around gpstime 1344100121 to gpstime 1344100924. Looks like we are dark noise limited as Evan suggests.
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Thanks. This probably made an improvement at 10 Hz, but it is hard to tell because IM4 trans seems to have wandering lines in it on the lock stretches that I've checked. Attachment shows the sum channel before (no whitening filter) and after (1 stage of whitening), with +18 dB of whitening gain in both cases. I am not sure why the sum does not achieve its shot noise limit (the total light on the photodiode is about 10 mW these days, according to the installed calibration filters). Might be worth a dark noise measurement.
For ADC noise, I think the right reference is T070213. The usual number is 4 µV/rtHz or 0.007 ct/rtHz.
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