I have subtracted SRCL coupling from DARM, and found that residual noise has the shape of 1/f4-5 at 15-30Hz. I remember when L2 was in acquire state we could see actuator noise in this frequecy band in DARM. For this reason I decided to look into L2 noise again even though actuators are in the low noise (state 3) during current locks.
All 16 actuators show similar noise levels. Attached plot shows spectra for one of them (ITMX LL) in different states. When there is no control signal, actuator noise in low noise state is a factor of 10 smaller compared to acquire state in the frequency range 10-30Hz. However, if a small (few counts) low frequency excitation is applied, then actuator noise increases by a factor of 10. The noise increases almost by the same level if a large excitation is applied. I decided to check if upconversion happens in DAC, actuator or noise monitor. Control signal was shifted by a DC offset (10000 cnts) and actuator noise has reduced by a factor of 2-3. From this measurement one might conclude that DAC is responsible for extra noise. Also, we run oplev servo on the ITMs and even though control signal is aggressively low passed, actuator noise still increases above 10Hz.
Since L2 actuator noise depends on the low frequency control signal (or DC offset), then actuator noise may or may not be coherent to DARM at 10-30Hz. However, when we looked at DARM spectrum with bandwidth of 1Hz, we saw that noise goes up and down by a factor of 1.5 at 10-30Hz.
We have measured the differential actuator noise in low noise by notching ETMX control signal at 20Hz. Spectrum is 0.1 cnts/sqHz and is consistant with the noise when small low frequency excitation is applied (and a factor of 10 higher compared to the case when no excitation is applied). Total noise from L2 is 1.4e-19 m/sqHz at 10Hz and from L1 is 2.2e-18 m/sqHz at 10Hz. This result is consistant to the previous measurement using pringles technique (Chris alog 21070).
Evan, Kiwamu, Den
We measured L2 actuator noise directly from the coils on ITMX. We did not find any irregular noises, current noise looks normal. We have also measured the noise on each coil end relative to the ground. At 20Hz the noise level is 4uV/sqHz. We drive one of the coils in common relative to the ground with 300uV at 30Hz but did not see any noise in DARM.
Kiwamu, Den
Tonight we have shut down ITMX and ITMY L2 drivers while in low noise to double check that these drivers do not cause extra noise at 15-30Hz. We did not see any change in DARM. We would like to do the same test for ETMs.