Virgo Logbook

Similar to the search for aliasing of lines in B1, I had a quick look at the aliasing of lines in B5_d2 (acquired at 20kHz). As a reference, we can use SSFS_Visu, which is acquired at 80 kHz. These signals should be equal except for a constant gain and the addition of the 444 and 1111 Hz lines. Any lines that are aliased should thus show up in the ratio between these two, see fig 1. Only 2 lines are cleary aliased: a big line from 15625 to 4375 Hz (also visible in B1) and a smaller line from 10673 to 9327 Hz. Other than that, it seems that SSFS_Visu is more sensitive to 100 Hz harmonics. Anyhow, it is not a real problem if there is some aliasing in B5_ACp, since this is a descrete signal that is not in loop, the SSFS loop is purely analog.

Looking at the spectrum of SSFS_Visu, it appears that the line at 15625 Hz is dominating the RMS, see fig 2. According to Federico, the origin of this line is the well-known horizontal scan frequency of CRT monitors (25 Hz refresh rate x 625 lines). Since the line is huge in SSFS_Visu and pretty small in B1, it is likely that the line is entering via the SSFS electronics. A possible culprit is thus the analog monitor or some video equipment in the detection lab.