Eleonora, Irene, Yuhang, Federico

Yesterday we took a high resolution spectrum of the homodyne signal (no squeezing): this is shown in Figure 1.

Above 1kHz the spectrum is quite flat at around -134dBV/sqrt(Hz). Instead below 1kHz some clear structures appear. Below 10Hz the spectrum is rising with no clear structure. In Figure 2 we marked some of these peaks.

Some of these peaks we recognize from May 7th investigation (logbook 1336): 13.6Hz looks the table horizontal resonance, 24.6Hz and 49.25Hz are the scroll pump, 603Hz (turbo pump). Peaks at 33.75 and 67.6 (looks its harmonics) are not known.

We then put the accelerometer on the homodyne box and measured coherence with the homodyne signal. This is in figure 3 and figure 4. In Figure 4: we see coherence with some narrow peaks (possibly originated by something like a fan?) plus the turbo pump (603Hz) and some large coherence around 640Hz and some reasonable coherence around 577Hz, however the accelerometer has no associated peak at these same frequencies, so we would conclude these are not resonant mode of the homodyne box (see also figure 10 of  1336).

The closest match is with the "no-piezo mount" we measured yesterdy (resonant mode 576Hz, see figure 8 of 1336).

NOTE: this frequency (around 576Hz) should be typical first resonant mode of all NO-PIEZO mounts - there are several. While, PIEZO mounts have first resonance mode around 470-500Hz. Then mounts seem to have higher resonant modes up to 2kHz or so.