Jim, Hugh, Krishna, Jeff, Fabrice:
We keep investigating the sensor corrction issue on HAM3. What we found yesterday is that it depends on which blends are engaged. We can't explain why yet. We did additional tests today:
- we turned off all CPSs of all HAM-ISI and BSC-ISI in the corner station. No change.
- we checked the jumpers of all HAM3 CPS boards. All good.
- we tried to apply large offset in case it would reduce some kind of cable touching/rubbing (+/-400 um in HEPI Z, and +/-400 um in ISI X,Y, Z). No change.
Finally, we tried to do the Z sensor correction to HEPI. In the plot attached:
- Red curves is HAM3 ISI isolated, no sensor correction
- Green Curve, we turn ON the sensor correction in X and Y to the HAM-ISI
- Blue Curve, we also turn on Z sensor correction to ISI. The 0.6 HZ peak shows up. For some reason it also reduces X at the microseism.
- Brown, we do the Z sensor correction to HEPI instada of ISI. The peak is still there in the CPS, but not in the GS13. It's unclear why.
The last configuration looks good from the GS13s, but it's unclear yet how good it is for the cavity. More info on that is coming.
One more thing that Fabrice forgot to mention in this recap: - they restarted the front-end processes for H1ISI and HPI HAM3 (see 15755) -- and also saw no change. Perhaps during a future maintenance day, we can hard-reboot the entire chassis. Some further speculation / questions: - That we *don't* see the feature in the GS13s when we're in low-frequency blend when we feed Z sensor correction to HPI (but we still see the feature in the CPS) rules out the GS13s as the source of the problem. - The 0.6 [Hz] feature is modifiable by changing the RX / RY blend filters -- higher blend frequency means less 0.6 [Hz] feature. RX/RY implies it's a differential vertical noise, in that one of two of the three CPS are causing the problem. - Higher blend means more CPS is being used. Wouldn't you think that if the problem is in the CPS, then using more of them would make the problem worse? - Could it be some subtle, small electronics cross-talk between the STS and the CPS that goes into oscillation? - We're grasping at straws. This stinks. @DetChar -- I know it's impossible to figure out the state of the ISIs offline, but can you track this chamber over time and see at least how long we've seen a 0.6 [Hz] feature? It might take Keith Riles type *days* worth of averaging to find it... It would be also good take Keith Riles type high-resolution ASDs to find out how sharp the feature is, and to quantify how the heck 1.12 [Hz] is related to 0.6 [Hz]...
In case detchar people are curious about the configuration of this chamber over break, when I came in this morning I found the ISI in what we thought was the good state in December. That is, X&Y sensor correction on the ISI, Z on HEPI and normal blends, isolation loops. I doubt anyone changed the configuration since the 19th of December.