Andrei, Sheila, Naoki

We've tried to link the observable whistles with working VCOs. We've manually checked the correlation between channel H1:SQZ-FC_LSC_DOF2_OUT_DQ and channels that record the frequency of squeezer related VCOs: H1:SQZ-CLF_VCXO_FREQUENCY, H1:SQZ-FC_VCO_FREQUENCY, and H1:SQZ-LASER_VCO_FREQUENCY.

Upon quick inspection Figure 1, we observed visible dependance of H1:SQZ-FC_LSC_DOF2_OUT_DQ channel excess noise with frequency noise of H1:SQZ-FC_VCO_FREQUENCY. However, the H1:SQZ-FC_VCO_FREQUENCY frequency deviates on far less scale than whistles of the channel H1:SQZ-FC_LSC_DOF2_OUT_DQ, therefore we believe it is safe to conclude that this frequency noise is the consequence of these whistels rather its cause.

Figure_2 shows the step-like frequency drift of the whistle-signal.

UPD: We've also disabled this VCO and checked whether its noise is the cause of our problems. However, since in the observation was only for a few hours, it was hard to say whether that solved the problem (see Figure 3). We will continue to make observation with turned off VCO.

UPD2: We've added channel H1:TCS-ITMX_CO2_ISS_CTRL2_OUT_DQ for analysis (Figure 1 and  Figure 3). It didn't show any dependance on whistles.