Aritomi, Yaochin, and Yuhang

For CC2 phase shifter, we could always hear the sound around it and we wanted to check what is the frequency of this sound. So we used a sound spectrum analyzer and it shows only one frequency component which is 6.9kHz. Note that we couldn't see this oscillation from the oscilloscope. 

We checked the squeezing spectrum and found that this frequency appears in the spectrum of squeezing. Also in the squeezing spectrum we could find the peak at ~23kHz. These peaks come from the resonance of CC2 phase shifter.

However, we found there was also a clear peak at 16kHz. And we found this peak when we measure OLTF of CC2. So it seems this peak comes from servo? Maybe we can consult with Pierre.