Virgo Logbook

During the afternoon shift of Tuesday July the 29th, we were looking for a possible reason that could explain the series of unlocks observed last week despite the ML T° adjustment (see logbook #67348). We checked the behavior of the Pstab servo box (Pstab rampauto) by injecting a sine pertubration at 700Hz. We then noticed that:
- intermittently, the signals of the Pstab photodiode saturates. This is less visible on the correction signal (see Fig1)
- to check the origin of this effect, we unlocked Pstab and checked that no saturation was observed even at higher perturbation amplitude. This shows that the saturation comes from rampauto itself and not from the phtodiodes (fig2)
- this effect might explain Pstab triggered unlocks.
Since, this effect was intermittent, we decided to swap the rampauto with a spare. We adjusted the DC correction level at 3V and adjusted the gain. The maximum correction is then 400mVpp which corresponds to 900mVpp on the photodiodes.
We'll be monitoring the Pstab behavior for the next few days.