Virgo Logbook

Since it has been noticed some coupling between the piezo corrections of the BPC and the dark fringe signal when the ITF is locked (last plot), we tried to investigate the possible sources. We found out that the correction signal which is sent to the piezo is very noisy in the peak at 50 Hz and its harmonics.

Yesterday morning we made some in parallel measurements of the correction signal sent to the piezo. Even though a resistance has been put in series with the piezo to filter out the higher frequencies (first plot), the amplitude of the harmonics which is directly sent to the piezo is still high (second plot). It will probably excite some resonances of the piezo, and could then be injected in ITF signal as beam jitter.

We conclude that the dac introduces noise in the way it is connected now (last plot) and we are currently working on a solution to improve this signal.