Naoki, Sheila
In the AS72 sensing matrix measurement in alog74106, Daniel suggested to increase the whitening gain of AS72 since it could be limited by ADC noise. We checked the whitening filter of AS72 A and B. Both of them have 12dB whitening gain, but one stage whitening is engaged for AS72 A, while two stage whitening is engaged for AS72 B. We decided to engage the 2 stage whitening for AS72 A, which is used in SRC1. The IFO locks without any problem with this additional whitening. We accepted some SDFs as shown in the attached figures. We will try to increase the whitening gain later.
I think the attached plot shows this was a good idea. I have some old data measuring whitening and no whitening on at RF72, but I never posted it because I couldn't figure out the correct RF72 transimpedance. The attached plot shows an estimation of the ADC noise level comparison with the noise spectrum in lock. Naoki and Daniel were kind enough to help me figure out the proper transimpedance for the RF72 (see 37065).
The measurement procedure and calculation procedure is detailed in alog 66734. At the time, RF72 was using 1 stage of whitening with 12 dB whitening gain.
I think it's likely my shot noise calculation here is incorrect. Correcting that calcuation is in progress...
During Tuesday maintenance and IMC_LOCK is OFFLINE, I measured the AS72 dark noise with different whitening setting. The attached figure shows the dark noise of AS72 A Q PIT/YAW, which are used in SRC1. The 2 stage whitening and 12 dB whitening gain are the current nominal setting. In the previous Elenna's measurement, there was a bump around 25 Hz, but there is no bump in today's measurement.
