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Entry  Wed Nov 18 22:49:06 2020, gautam, Update, ASC, Some thoughts about AS WFS electronics 
    Reply  Sun Nov 22 21:09:37 2020, gautam, Update, ASC, Planned mods for WFS head aLIGO_wfs_v5_40m.pdfTFs.pdfnoise.pdf
       Reply  Wed Nov 25 18:18:41 2020, gautam, Update, ASC, Planned mods for WFS head anodeVsOutput.pngsensitivity.png
          Reply  Sat Nov 28 21:44:53 2020, rana, Update, ASC, Planned mods for WFS head 
          Reply  Thu Dec 10 22:46:57 2020, gautam, Update, ASC, WFS head assembled aLIGO_wfs_v5_40m.pdfTF_meas.pdf
             Reply  Thu Dec 17 15:23:56 2020, gautam, Update, ASC, WFS head characterization oNoise.pdfZ_Hi.pdfZ_Low.pdfIMG_9030.jpgiNoise.pdf
    Reply  Wed Nov 25 18:30:23 2020, gautam, Update, ASC, Some thoughts about AS WFS electronics 
       Reply  Wed Dec 2 23:07:19 2020, gautam, Update, ASC, Electrical LO signal for AS WFS RF_Frequency_Source.pdf
          Reply  Thu Dec 3 11:02:35 2020, rana, Update, Electronics, Electrical LO signal for AS WFS 
          Reply  Thu Dec 3 13:55:52 2020, Koji, Update, ASC, Electrical LO signal for AS WFS 
             Reply  Thu Dec 3 20:38:46 2020, gautam, Update, ASC, Electrical LO signal for AS WFS RF_Frequency_Source.pdfdemodPath.pdfEOMpath.pdf
                Reply  Thu Dec 3 21:14:38 2020, Koji, Update, ASC, Electrical LO signal for AS WFS 
                   Reply  Thu Dec 3 21:44:49 2020, gautam, Update, ASC, Electrical LO signal for AS WFS 
                      Reply  Fri Dec 4 03:02:40 2020, Koji, Update, ASC, Electrical LO signal for AS WFS 
                         Reply  Fri Dec 4 22:41:56 2020, gautam, Update, ASC, Freq Gen Box revamp 
                            Reply  Sat Dec 5 20:44:35 2020, gautam, Update, ASC, Freq Gen Box re-installed 6x
                               Reply  Mon Dec 7 12:38:51 2020, gautam, Update, IOO, IMC loop char OLTFcomparison.pdfdemodMons.pdfOLTFcomparison.pdf
                               Reply  Mon Dec 7 22:54:30 2020, gautam, Update, LSC, Modulation depth measurement modDepth.pdf
                                  Reply  Sat Jan 16 18:59:44 2021, gautam, Update, LSC, Modulation depth measurement PLL.pdfmodDepth.pdfmodProfile.pdfEOMpath_postMod.pdf
Message ID: 15713     Entry time: Mon Dec 7 12:38:51 2020     In reply to: 15711
Author: gautam 
Type: Update 
Category: IOO 
Subject: IMC loop char 

Summary:

There seems to be significant phase loss in the TTFSS path, which is limiting the IMC OLTF to <100 kHz. 

Details:

See Attachment #1 and #2. The former shows the phase loss, while the latter is just to confirm that the optical gain of the error point is roughly the same, since I noticed this after working on and replacing the RF frequency distribution unit. Unfortunately there have been many other changes also (e.g. the work that Rana and Koji did at the IMC rack, swapping of backplane controls etc etc - maybe they have an OLTF measurement from the time they were working?) so I don't know which is to blame. Off the top of my head, I don't see how the RF source can change the phase lag of the IMC servo at 100 kHz. The only part of the IMC RF chain that I touched was the short cable inside the unit that routes the output of the Wenzel source to the front panel SMA feedthrough. I confirmed with a power meter that the power level of the 29.5 MHz signal at that point is the same before and after my work.

The time domain demod monitor point signals appear somewhat noisier in todays measurement compared to some old data I had from 2018, but I think this isn't significant. Once the SR785 becomes available, I will measure the error point spectrum as well to confirm. One thing I noticed was that like many of our 1U/2U chassis units, the feedthrough returns are shorted to the chassis on the RF source box (and hence presumably also to the rack). The design doc for this box makes many statements about the precautions taken to avoid this, but stops short of saying if the desired behavior was realized, and I can't find anything about it in the elog. Can someone confirm that the shields of all the connectors on the box were ever properly isolated? My suspicion is that the shorting is happening where the all-metal N-feedthroughs touch the drilled surfaces on the front panel - while the front and back surfaces of the panel are insulating, the machined surfaces are not.

This is an unacceptable state but no clear ideas of how to troubleshoot quickly (without going piece by piece into the IMC servo chain) occur to me. I still don't understand how the freq source work could have resulted in this problem but I'm probably overlooking something basic. I'm also wondering why the differential receiving at the TTFSS error point did not require a gain adjustment of the IMC servo? Shouldn't the differential-receiving-single-ended-sending have resulted in an overall x0.5 gain?


Update 8 Dec 1200: To test the hypothesis, I bypassed the SR560 based differential receiving and restored the original config. I am then able to run with the original gain settings, and you see in Attachment #4 that the IMC OLTF UGF is back above 100 kHz. It is still a little lower than it was in June 2019, not sure why. There must be some saturation issues somewhere in the signal chain because I cannot preserve the differential receiving and retain 100 kHz UGF, either by raising the "VCO gain" on the MC servo board, setting the SR560 to G=2, or raising the "Common Gain Adjust" on the FSS box by 6 dB. I don't have a good explanation for why this worked for some weeks and failed now - maybe some issue with the SR560? We don't have many working units so I didn't try switching it.

So either there is a whole mess of lines or the frequency noise suppression is limited. Sigh.

Attachment 1: OLTFcomparison.pdf  88 kB  | Hide | Hide all
OLTFcomparison.pdf
Attachment 2: demodMons.pdf  164 kB  | Hide | Hide all
demodMons.pdf
Attachment 3: OLTFcomparison.pdf  105 kB  Uploaded Tue Dec 8 11:48:04 2020  | Hide | Hide all
OLTFcomparison.pdf
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