The aim of the measurement reported is to find the origin of the increase of the F0 transfer function at frequencies above 8 Hz. Three different measurements were carried out:

• Usual transfer function with the keystone free.
• Transfer function with the cable disconnected.
• Transfer function with the keystone locked.

The first screenshot shows the TF measurement in the three cases listed above. When the keystone is free, the TF changes quality about 7 Hz and begins to increases at about 8 Hz. The second screenshot shows the amplitude spectral density (ASD) of the LVDT readout and actuation signal in units of µm/√Hz and counts/√Hz respectively. The third screenshot shows the coherence.

From the ASD is possible to recognize that the LVDT readout when the keystone is locked (red) becomes as large as the displacement (black) at about 7 Hz. The coherence in that region is relatively high, suggesting there is electromagnetic pick up from the actuation coil into the LVDT coils. The ASD also shows the noise floor below measured with the cable disconnected but not terminated or shielded.

It's also worth pointing out the following:

• The transfer function slopes up at higher frequencies because the actuation signal decreases in the region where the readout becomes limited by electromagnetic pick up. This also happens with the cable disconnected when the signal is relatively constant in frequency.
• The coherence with the keystone free becomes high at about 10 Hz rather than at about 7 Hz as when the keystone is locked. The reason for this is not clear yet.