To mitigate the demodulation noises on B1p_PD1_56MHz , the best signals are

• for the phase noise : the B1s_PD1_112MHz_phi  as we can use any phase signals coming for the demodulated channels inside SDB2  tank
• for the amplitude noise : the B1p_PD1_6MHz_mag   as we have use a signal related to the same photodiode. Unfortunately  there is a bump in the 2KHz-3KHz region only  on the 6MHz

Looking at all the SDB2 's demodulated signals at 6MHz (B1p_PD1, B1p_PD2, B1_PD2 and B1_PD3), we found that the bumps are present on the B1p_PD1,  B1p_PD2, B1_PD3 ones but not in the B1_PD2 one (see plot)

These plots shows the FFTime over 2 days  for the B1p_PD1, B1_PD2 and B1_PD3 6MHz channels

• from the 20250315-01h00m-UTC to 20250316-01h00m-UTC
• from the 20250416-01h00m-UTC to 20250317-01h00m-UTC
• and the trend related to this period for the ITF state and the INJ_LNFS_6MHz_raw_dFreq channels

This plot compare the B1 and B1p PD demodulated at 6MHz when the ITF is in LOW_NOISE_3  the 20240101-01h00m-UTC(red curve) and 20250321-12h00m-UTC(black curve): the bumps were already there and are moving with the time

The last plot and its zoom  show the all the sample channels related to the longitudinal photodiodes located on the SDB2 bench

• the 4 top graphs  are related to the  PD channels acquired by the  SDB2_DBOX_LETFDOWN DAQ box
• the 4 middlle graphs are related to the PD channels acquired by the SDB2_DBOX_LETFUP DAQ box
• the  4 botton graphs are related to the PD channels acquiered by the SDB2_DBOX_RIGHTUP DAQ box
• All the channels were the moving bumps is presents , B1p_PD1, B1p_PD2 and B1_PD3 show  noisy structures around the 6MHz
• As a check, it might be useful to complete the B1pP_PD1 demodulation at 6MHz to check if the bump is also present

It remains to find the source of this noise

The attached plots show the noise on the SDB2 longitudinal photodiodes demodulated at 6MHz

• the first plot  and its zoom show the FFTTime of the B1_PD3 photodiode and the ITF_LOCK state for the 20250322
• the third  plot and its zoom one show the FFT of the  B1_PD photodiodes demodulated at 6MHz according the ITF_LOCK state
• the B1_PD3 photodiode is located after the OMC, meaning that for the ITF_STATE lower than 109, there is no light reaching the B1 photodiode as the OMC shutter is closed

Remarks

• As the bump is present on the B1_PD3_6MHz even if the  OMC shutter is closed, this bump is probably due to electronic noise
• for the frequencies greater than 4KHz,  one may expect to have the shape of a lowpass 8th order Butterworth filter but it s not the case for all the ITF_LOCK state , meaning that there is aliasing

This morning, the demodulation noises mitigation has been setup on the B1_PD2_56MHz

• the demodulation phase noise mitigation is done using the B1s_PD1_112MHz phase signal
• the demodulation amplitude noise mitigation is done using the B1_PD2_6MHz magnetude signal

For the B1p_PD1_56MHz only the demodulation phase noise mitigation is done using the B1s_PD1_112MHz phase signal

The B1_PD2_56MHz  and the B1p_PD1_56MHz demodulation noises  mitigation are engaged at at LOCKED_DC_READOUT and  disengaged at DOWN