aLIGO LHO Logbook

H1 ISC

kiwamu.izumi@LIGO.ORG - posted 17:21, Friday 02 June 2017 - last comment - 18:15, Friday 02 June 2017(36629)

Update on ALS Y issue

Fil, Daniel, Jenne, Keita, Patrick, Richard, Sheila, Kiwamu,

The investigation continues. No solid conclusion or resolution yet.

We tried locking the laser to the Y arm by only using the PDH servo board in order to test the PDH board.
- The output of the PDH board was connected to the cable which is nominally connected to the output of the PLL board for this test operation.
- The same filter settings as usual but with a lower gain (I had to lower the fast path gain as well).
- This locked the laser somewhat but kept unlocking roughly every 10 sec. Note that this behavior is different from the glitches we have seen recently.
- It is inconclusive to say whether the PDH board is to blame for the glitches.
Regardless of whether the PDH board is the culprit, we decided to swap the board.
- See 36622.
- The first replacement unit turned out to be not working (non funzione in Italian) because it didn't responds to changes in the remote offset adjusters.
- Fil then installed another spare unit with which we were able to control the electronic offset using the MEDM sliders.
We then found that the FET demodulator gave us a big offset.
- We drove to the end station and then determined that the demodulator unit was non funzione either. The I-signal was just flat 5 V.
- We switched the system to the second half of the same demodulator unit.
Now we are waiting for the Alaskan earthquake (LLO 34072) to settle so that we can test out the new PDH board.

Comments related to this report

sheila.dwyer@LIGO.ORG - 18:15, Friday 02 June 2017 (36631)

Just to summarize what we think we know so far:

Things that were exonerated by bypassing PLL test:

PDH sensing (diode, demodulator (until it broke), LO, cables)
Laser (temperature wasn't very different between PLL bypass and normal locking configuration lock)
PLL servo board and beckhoff control cables.

Other things that we think are exonerated:

PDH servo board, because we swapped the board and still have glitches
loop instabilities, I measured both loops this morning in the normal configuration. PLL has a bandwidth of 27kHz and 43 degrees of phase, PDH has 2.5kHz ugf and 66 degrees of phase
The PSL laser shouldn't be the problem, because if it were we would see the problem at End X.
The beat note from the fiber BBPD looks like a nice clean sine wave

Things that we haven't eliminated:

PFD- we could try replacing this with a mixer as a test
VCO- we could try to replace this with an IFR as a test
BBPD
PDH servo board beckhoff cables- we could try to replace these with a CM board tester. Keita calling to try to find out where one of these is stored.