J. Kissel, D. Sigg

Investigating the last corner station Beckhoff error, regarding the 9 MHz EOM driver offset. There are complaints that the read back (H1:LSC-MOD_RF9_AM_RFOUT and H1:LSC-MOD_RF9_AM_RFSETMON) is out of range at ~18.6 [dB], where the request (H1:LSC-MOD_RF9_AM_RFSET) is 16.8 [dB].  Unclear what the tolerance is; appears to be internally set and not broadcast over EPICs. Side note: leading up to and during O2, when IFO is down/acquiring, it's reduced to 10.8 in NLN.

The offset was set to 16.8 dB on Feb 23 2016 (see LHO aLOG 25687), to balance the RF power of the driver, and its readback has been slowly drifting *up* from there since.
Looking back further, we see similar upward drift during O1, even though the offset's request was different.
45 MHz Beckhoff readbacks are stable.

Fast ADC version of the control voltage (H1:LSC-MOD_RF9_AM_CTRL_OUT16) doesn't show the drift. Also the values don't agree. 

Tolerance increased LHO aLOG 30406 (perhaps unbenowst to the increaser) to accommodate the drift.

Chassis assembly E1400445
    Controller Circuit D0900761.

These channels are all readout by a single 4 channel ADC as the readback: ecatc6 Middle Rail 10 is the module (from D1300745). Maybe internal voltage reference drift?

Recommendations:
- either check the cabling at the ADC
- measure the input to the ADC to confirm the expected voltages
- replace the ADC.