Washimi, Akutsu following 28985.

Summary

For the further inspection of the invac OMC geophone that had a strange lower signal boundary (see 28985), the analog output of the geophone distributor was inspected, and we confirmed that the strange lower bound was already there. Is it of specification of this geophone? or some mulfunction?? In any case, it might be nice to open OMM-OMC chamber soon.

Details

• Setup: [Distributor] - [R. Takahashi-san's Dsub-LEMO convertor (Figs. 1 and 2)] - [Washimi-san's portable oscilloscope driven by a battery]
• Measurements: tapped (rather touched) a sticking out part (mass spectrometer) as like yesterday, then immediately the oscilloscope showed the signal was strangely bounded in the lower region (Fig. 3). From this figure, you may be able to estimate how large the signal is.

Note

• We used an battery-driven oscilloscope. Please be careful not to use "usual" oscilloscope that might have its signal GND connected to power supply's GND. KAGRA's analog signals are differential types, and if you connect the negative side of the differential pair signals, the whole system might be, in the worst case, broken. Never do it.
• In any case, the coupling path from the part touched to the geophone is unknown. This has to be known for the better sensitvity of KAGRA. We need to open the OMM-OMC chambers soon to check the coupling.
• On the other hand, even when the geophone responded, OSTM's OSEMs are not so much (Fig. 4). In this figure, the lower one shows the invac geophone, and the upper four are of these OSEMs. The signal was made by my light tapping on the floor near this desk. Hmmm... is this due to difference of the frequency range?? I did not take time for this point today so much, so further investigation should be done later by someone.