[Tanaka, Ushiba, Komori]
Abstract:
We continued the investigation of the GR–IR coupling issue.
We successfully identified a configuration without the coupling, in which the DC power for the whitening board is supplied by an independent power supply.
A possible cause of the issue might be instantaneous saturation of the power supply during IFO flashing.
Detail
This work is a continuation of yesterday’s investigation (klog:36533).
First, we inserted an RF transformer (T1CA) between PDA3 and the demodulator after measuring the transfer function of the transformer (gain ≈ −1 dB, phase delay ≈ 1 deg).
However, the noisy behavior did not change after inserting the transformer.
The related measurement results are shown in Fig.1.
Next, we checked whether the noisy spectrum depends on which board the cable is connected to.
The spectrum did not become noisy when the cable was connected to the demodulator, but it became noisy when the cable was connected to the whitening board.
Therefore, we tried using an independent whitening board outside the rack with an independent power supply.
With this configuration, we confirmed that the spectrum remained quiet.
In contrast, the spectrum became noisy when we used the power supply inside the rack to drive even the independent whitening board (Fig.2).
Finally, we measured the spectrum using the original whitening board inside the rack but powered by the independent power supply.
The spectrum remained quiet even without the RF transformer (Fig.3).
This result indicates that the coupling issue can likely be resolved by using an independent power supply for the whitening board.
We suspect that a possible cause of this issue is instantaneous saturation of the power supply during IFO flashing.
We found that the whitening board draws a relatively large current, approximately 0.7 A at +18 V and 0.4 A at −18 V.
In addition, we checked the rack power supply located in the computer room, which currently outputs approximately 17 A, while the protection threshold is set to 21 A.
Therefore, such saturation could plausibly occur.
Tomorrow, we will measure the spectrum with the PDA1 and PDA2 cables connected to their corresponding whitening boards, which will be powered by the independent power supply.
