[John / Kiwamu]
We tried to figure out what is causing spikes in the REFL33 signal, which is used to lock PRCL.
No useful information was obtained tonight and it is still under investigation.
One thing preventing us from doing smooth measurements of the noise budget and the sensing matrix is some sharp spikes in the LSC error signals.
For example when we lock PRMI with REFL33 and AS55 fedback to PRCL and MICH respectively, both the REFL33 and AS55 signals show some spikes in time series.
Those spikes then bring the noise spectra higher than how they should be.
So for the reason, taking the noise budget doesn't give us much information about the interferometer rather than there are spikes.
Also the sensing matrix measurement has been suffered from those spikes, which excite the impulse responses of the low pass filters in the LOCKIN detection systems a lot.
(What we did)
We looked into the actual analog signals to see if there are indeed spikes or not before they are acquired to the ADCs.
But we didn't find any corresponding spikes in the signals that are after the mixers.
It maybe because the signals we looked into didn't have high enough SNR because they were coming out from the monitor lemo outputs on the demod boards.
Then we thought the spikes are from the whitening circuits, due to some kind of saturation.
We decreased the gain of the whitening filters by a factor of 10, but it didn't help and the spikes were still there.