Unfortunately the excess noise came back already at the following lock acquisition, with the same features (Fig. 1, purple, red and blue are noise, no noise, and noise again respectively).

Looking at the B1_PD3_DC spectrum we can see many big lines (and sidebands) (Fig. 2); the most offending in terms of SNR is the one at ~6885 Hz (Fig. 3), but what is striking at a first glance is that all of them are separated by ~1722 Hz, although the lowest one is not the strongest one, so maybe also some aliasing is involved.

For debugging purposes, the channels Audio and DC related to the B1_PD3 and B1s_PD1 photodiodes acquired at 100KHz are now stored with the "_FS" suffix and available in the RAW_FULL stream .

• operations complete at 2024-04-17-15h25m26-UTC

Below the list of the channels, acquired by the SDB2_DBOX_RightDown - SDB2_MezzPD2  mezzanines at 100KHz and stored with the  "_FS" suffix:

 V1:SDB2_B1_PD3_Audio_raw_FS              nData= 100000 min= -6.11e+03  mean=-4.89e+03  max=-3.56e+03 rate= 100000Hz
 V1:SDB2_B1_PD3_DC_raw_FS                 nData= 100000 min= -3.55e+03  mean=-3.31e+03  max=-3.07e+03 rate= 100000Hz
 V1:SDB2_B1sP_PD1_Audio_raw_FS            nData= 100000 min= -2.69e+05  mean=-2.67e+05  max=-2.65e+05 rate= 100000Hz
 V1:SDB2_B1sP_PD1_DC_raw_FS               nData= 100000 min= -3.62e+03  mean=-3.38e+03  max=-3.14e+03 rate= 100000Hz
 V1:SDB2_B1s_PD1_Audio_raw_FS             nData= 100000 min=  6.12e+04  mean=  6.2e+04  max= 6.28e+04 rate= 100000Hz
 V1:SDB2_B1s_PD1_DC_raw_FS                nData= 100000 min=  1.54e+04  mean= 1.57e+04  max=  1.6e+04 rate= 100000Hz

Today we had another event of the noise suddenly disappearing, and this happened before the lock of the OMC, at 16:02:59 UTC (Figure 1); the lock of OMC and DC Readout went through fine (Figure 2).

The error signal SDB1_LC_TY_err also shows a similar behaviour to B1_PD3 regarding the 1722 Hz line, but a much more broadband effect as well (Figure 1 and spectra in Figures 3 and 4).

The following plots shows a 300s FFTTime of the SDB2_B1p_PD1_Audio, SDB2_B1_PD3_Audio and the  SDB2_B1s_PD1_Audio channels during the OMC lock acquisition where the purple rectangle highlights  the periods with the presence of unexpected lines

• successfull OMC lock
  • 2024-04-01-00h45m00-UTC : FFTTime plot
  • 2024-04-04-13h34m00-UTC:  FFTTime plot,
    • this FFT plot compare the ASD  of the related channels (Audio and sample) for 2 periods: when some lines are present(purple)  and a quiet period(blue}
  • 2024-04-07-19h41m00-UTC:  FFTTime plot
  • 2024-04-09-20h43m00-UTC:  FFTTime plot
    • this FFT plot compare the ASD  of the related channels (Audio and sample) for 2 periods: when some lines are present(purple)  and a quiet period(blue}
  • 2024-04-14-10h52m00-UTC:  FFTTime plot :
    • During the OMC temperature scan, there is a period (blue rectangle) where the lines amplitude increases in the SDB2_B1s_PD1_Audio channels and appears in the  SDB2_B1_PD3_Audio channel
    • this FFT plot compare the ASD  of the related channels (Audio and sample) for 2 periods: when some lines are present(purple) and a quiet period(blue}
  • 2024-04-16-00h09m00-UTC: FFTTime plot
    • same statement for the blue rectangle as the previous event
  • 2024-04-17-03h40m40-UTC: FFTTime  plot
    • same statement for the blue rectangle as the previous event
  • From all these plots it seems that
    • there is a noisy time period (purple rectangle) when the OMC shutter is opened . This time period has increased with the time  and seems to start to reduce (see last event)
    • when the noisy time period is long, during the OMC temperature scan  a more noisy period occurs (blue rectangle

• unsuccessfull OMC lock
  • 2024-04-16-21h24m20-UTC: FFTTime plot
    • the noise does not disappear but seems to increase according to the OMC temperature analysis (blue rectangle) to remain when the OMC is locked
    • this FFT plot and its zoom compare the ASD  of the related channels (Audio and sample) for 2 periods: at the beginning when the OMC shutter is opened (purple) and when the OMC is locked with B1_PD3 (blue) : the noise has increased when the OMC is locked with B1_PD3
  • 2024-04-17-00h14m00-UTC: FFTTime plot
  • For reasons unknown today, the noise present when opening the OMC shutter does not disappear and moreover increases triggering the unlocking of the OMC

Here we report a few more observations concerning the excess noise causing problems to reach DC readout.

The excess noise is visible on the audio channels of several photodiodes (Fig.1 and Fig.2): B1s, B1sP and B1_PD3 (which are on the same service mezzanine) as well as on the B1p_PD1 audio channel (however in this later case, only lines are present, while for B1s and B1_PD3 there is a clear impact on the noise floor).

As reported already by Diego, an extra noise is also present on the SDB1_LC_TX_err and SDB1_LC_TY_err signals at the same time as the noise is present on the photodiodes. This SDB1_LC_TX/TY_err signals are constructed from the SDB1 B5 QD2 H and V signals, which also see the extra noise. This observation concerning the SDB1 local control was already true yesterday (Fig.3)

Both the spectrum of the B1_PD1 audio channel, and those of SDB1_B5_QD2_H/V signals are clean when we are in CARM_NULL_1F (Fig.4).

The extra lines on B1p_PD1 and the extra noise on the B5 QD2 signals show up as soon as the OMC shutter is open (Fig.5). The lines showing up on B1_PD1 are at the same frequency as the lines seen on B1s (Fig.6), although the B1s noise floor changes when the lock of the OMC is acquired.

The first lock at 16h00-UTC was luky as  the OMC lock noise disappeared just after the OMC lock with B1_PD3_Audio (plot) but not the others ones  (ie 21h50m-UTC one)

Figure 1. Another channel that sees the noise is SDB1_B5_QD1_H (and all the other channels of the quadrants on SDB1). It sees the noise start when the slow shutter on SDB1 starts opening 15:59:16, then the noise level is constant, and the it stops. On B1 PD3 the noise level stops when it stops on B5 QD1, and it is present only when there is light on B1 PD3, so that is the reason why the level of noise on B1 PD3 varies in time during these 5 minutes.

This is most likely due to the slow shutter of the OMC which is mounted on an Agilis translation stage with electronic end of range stops. The stops are failing and the piezo-motor of the shutter keeps trying to push it further even though the translation stage arrived at the end of its range.

Figure 2. Shows that the OMC thermistor sees a line at ~1721 Hz when the slow shutter is in motion. This is likely the electrical cross-coupling of the voltage sent to the slow shutter showing up on the voltage readout used for the temperature measurement.  In blue is when the shutter moves and in purple when the shutter doesn't move. Unfortunately dataDisplay can't seem to handle the dynamic, and making an FFTtime plot of this channel doesn't seem to work.

FIgure 3. When the shutter is closing after an unlock there is also a 1721Hz line preens in OMC1 T1, but it is 10 times smaller.

Figure 4. OMC T1 also sees the 1721Hz when opening the OMC shutter in single bounce. The line stays on for about 30 minutes, before stopping by itself. The shutter is closed 20 minutes after the line stops by itself according to the shutter log file.

Action items to debug this further and mitigate it:

• To open the OMC slow shutter use a given number of step instead of moving the shutter to the limit. The needed number of steps to open the shutter need to be tested, for example in single bounce after the next unlock.
• Keep using the move to limit command for closing the shutter, so the shutter always goes back to the same position after a round trip.
• Test the operation of the OMC slow shutter in single bounce. The problem can be seen on OMC1 T1 when opening the shuter. Turn on/off the driver of the slow shutter and see if that restores the functioning of the translation stage stops.
• Investigate in past data if the shutter has started to slow down, if it takes more seconds between when the shutter command is sent and when the light appears on B1s.
• Check if the height of 1721Hz line is constant on OMC1 T1 when the shutter is opened, or if it changes in height after a fews tens of seconds when the translation stage arrives at the stop.
• Investigate in past data if the height of the 1721Hz line when the shutter is being opened has increased over the past months.