One concern we had in the last CARM in-loop noise spectrum (see alog 6859) was that there might be more noise at high frequencies in which the digital system is normally unable to monitor. To check the high frequency situation I took a new CARM spectrum at the floor with an SR785 up to 100 kHz.

It looks like there is lots of noise at high frequncies mainly above 1 kHz. They are not identified yet at this moment. The plot is shown below.

The red curve is the one I newly took with the SR785 -- it matches with the previous measurement shown in cyan at around 2-5 kHz, indicating the calbiration of the analog signal is about correct. The cyan curve goes down rapidly above 5 kHz due to probabaly a down sampling filter. A huge peak in the red curve is located to be at 37 kHz corresponding to the FSR of the arm cavity. As for calibrating the analog signal I corrected the loop surpression of the PLL so that the plot should be valid even if it is above the UGF of the PLL which is at 60 kHz. The raw data is attached.