S. Dwyer, J. Kissel We found that the Beckhoff error handling system was reporting an intermittent error on ALS Y WFS A Demod Local Oscillator (LO) Power (H1:ALS-Y_WFS_A_DEMOD_LOMON) having surpassed its threshold (H1:ALS-Y_WFS_A_DEMOD_LONOM). Trending reveals that (a) The error bit has been green since the Beckhoff ADC was replace on Sept 22nd 2016 (see LHO aLOG 29848) (b) the LO's power has been slowly increasing over time, and was now on the hairy edge of the threshold. After consulting Sheila, she suggests "it's probably fine," so I increased the threshold from 21 [dBm?] to 22.5 [dBm], and accepted the new threshold in both safe and OBSERVE.snaps in the SDF system. (I'm not confident that this monitor is actually calibrated into physical units, but 21 [dBm] doesn't sound crazy so I put confidence in the designer of the system to have done so.) Hopefully this doesn't turn out to be a canary for the demodulator like the diode laser power was to the now replaced ALSX laser...
Opened FRS Ticket 8248 regarding missing 6 dB attenuator on the ALS COMM VCO path.
Other thresholds updated / cleaned up: H1:ALS-C_COMM_A_DEMOD_LONOM increased from 12 to 19 [dBm?] The monitor channel H1:ALS-C_COMM_A_DEMOD_LOMON increase from about 12.4 to 18.3 [dBm?] on March 01 2017 ~23:00 UTC (mid-afternoon local time). I could find an associated aLOG about this. H1:ISC-RF_C_AMP137M_OUTPUTNOM increased from 20 to 21.5 [dBm?] The monitor channel H1:ISC-RF_C_AMP137M_OUTPUTMON has been as high as 22 [dBm?] between Feb 7 and Mar 23rd, but then was brought back to ~21 with some small steps between then and now. No aLOGs about this one either. All of these monitor channel changes were through the course of the observing run, so we presume that this is the new normal. New thresholds have been accepted into the SDF system.
A nominal LO for a length sensor is around ~10-13 dBm. For a WFS the signal level is divided between the 4 segments. In software, the readbacks are added back together, so the LO sum should be similar in value. For the demodulators the RF power is measured after an internal 10-11 dB amplifier. So, a normal readback will be around 21-24 dBm for the LSC and the ASC sum. There is no amplifier for the phase-frequency discriminators, so their readbacks will be between 10-13 dBm.
For a distribution amplifier the power is measured before the 8-way splitter and is nominally around 22 dBm.
The ALS-C_COMM demod is driven by the COMM VCO. Alog 34512 describes a measurement involving the COMM VCO. It looks like a 6dB attenuator was left out when the changes were reverted. This should be fixed.
Not sure what happened on Feb 7, but on March 23 the harmonics oscillator was swapped which required a readjustment of some of the RF attenuators. Looks like H1:ISC-RF_C_AMP137M_OUTPUTNOM was effectively reduced by 1dB, see alog 35051. This is fine.
Checking the ALS WFS RF readbacks:
- X channels seem to be fine
- In Y only ALS_Y_WFS_A shows a drift
- Among the 4 segments, only chn 3 and 4 show the drift
This indicates that the EL3104 module L7 in EtherCAT end 3 (D1400175-v2) of EY is broken too.
