The first attachment shows the RF45_AM_CTRL_OUT (and RF9) to DARM coherence when RFWhen RF45 AM was not glitching. Left column is after we pulled RFAM measurement unit and coupler out of the PSL room, right is before we installed RF AM measurement unit and coupler, but the distinction between left and right is not that important.

There are two things worth noting:

1. When it's not glitching, there's some coherence between RFAM and DARM at around 170-190Hz, 540-580Hz, and 810-860Hz.

The coherence is almost common to both RF45 and RF9 (which is monitoring RF45 in CER), so this is probably the harmonic generator or some common environmental noise coupling into two different distribution amplifier outputs.

Seems like the shape of the noise bumps in RF45_AM_CTRL_OUT_DQ is not stable over time. Also, this might have become somewhat worse over time.

2. Wandering peak between 600 and 700 Hz (and its second harmonic) that comes and goes.

It's apparent in the right column, but there's also one trace in the left (black trace, Nov/22).

In my plot the measurement time was arbitrary and very very sparsely chosen, so it's not clear if the peak is gone after Nov/22. The peak was not present in Nov/07 data point either (right, brown traces).

Coherence between CARM and both RF45 and RF9 control outputs are very high for this peak.

I can see the second harmonic but not 3rd and higher (second attachment).

The peak also seems to be visible in FOM reference trace from 2015/Oct/14 (third attachment).

I wonder if detchar identified the cause of this.