The transfer functions of AA loop was measured with swipe sine method and reported in elog2230. But the signal at high frequency was not quite clear. Therefore, Eleonora suggested me to measure with fft method. The excitations were uniform noise and sent to each degree of freedom, their amplitude are

Note that if the excitation is not large enough, the coherence between these two signals will be exactly 1 at almost all the frequencies.

The measurement result is shown in the attached figures.