Akutsu-san, Uraguchi-san and Tanioka @Mitaka

Today, we measured Transfer Function of TMS VIS under both fixed and not fixed condtion.

[Background]
As previous report (5558), Geophone TFs showed flat floor above 10 Hz.
Shoda-san doubted that this is not die to mechanical response but to coupling by coil actuator or something.
So we fixed the VIS stage in order to know the such flat behavior is dervied from some mechanical response or not. 

[What we did]
We measured TFs as the same procefure as previous report.
In this measurement, we measured with various excitaion amplitude.

The result is the figure below.

The upper left graph shows gain, the lower left shows coherence and the upper right shows phase.
In the gain figure, blue, pink and brown lines are Geophone TFs, and red, cyan and green lines are LVDT's.
Here we focus on Geophone TFs.
Pink line is measured by excitation amp. 5000 and blue correspond to exc. amp. 10000.
Brown line is measured when we fixed VIS stage.

We can see difference below 20 Hz between VIS fixed or free.
However, they have same flat behavior above 20 Hz.
In addition, the flat floor level did not change when we increased exc. amp. from 5000 to 10000.

For some reasons, some couplings contaminate through some path above 20 Hz.

After that, we measured TFs with disconnecting 1 or 2 Geophone cables (see attached picture).
The result is here.

Concerning Geophone TFs, only blue line is different from previous figure.
Blue line is obtained when 2 out of 3 Geophone cables are disconnected (H2 & H3).
The gain decreased about 10dB and this value seemes 1/3 of previous one. 
But still have same flat trend.

We will continue to investigate about this issue.

Some details will be uploaded on below link.
http://gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA/subgroup/AOS/TMS-VIS0/2018-07-20