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AdV-ISC (Commissioning up to first full interferometer lock)

mantovani, ruggi, allocca, casanueva, brooks - 16:14 Thursday 19 September 2019 (46970)

MICH and ALPHA improvement

Today we have worked to improve the alpha substraction (MICH to DARM) in the low frequency region (10-20Hz) where the substraction was not properly working. I have modified the script to compute the filter (from MICH to DARM) in order to have a proper fitting @ 20Hz.

The alpha filter is really more simple than before (essentially 1 pole and 1 zero instead of 7 poles and 7 zeros for the old alpha). The improvement was visible at low frequency but there was a spoiling of the performance above 60 Hz. Thus the MICH corrector has been modified, the new filter TF will be shown in a comment to this entry, to have a better roll-off.

In Figure 1 the various alpha performance are visible:

- Purple : no alpha

- Green : old alpha

- Yellow : new alpha

- Blue : new alpha and new MICH filter

A noise injection has been done in order to further improve the current alpha

we will observe the loop in the following days

Images attached to this report

Comments to this report:

ruggi - 18:17 Thursday 19 September 2019 (46972)

Fig 1: Model of the old and new filters (without the notches).

Fig 2: Expected change of the in loop error signal, calculated applying the model to data with the old filter running.

Fig 3: Expected change of the loop correction, calculated applying the model to data with the old filter running.

Fig 4: Actual change of the in loop error signal.

Fig 5: Actual change of the loop correction.

This is not yet an optimal design, even because it is not clear what is the required accuracy. If we realize that the current accuracy is plentiful, we can study a way to lower significantly the 'noisy' correction.

Images attached to this comment

ruggi - 10:59 Friday 20 September 2019 (46978)

Just an example of a 'low noise low gain' design: with an UGF ~ 5Hz it is possible to have about the same accuracy of the old filter, and a significantly lower correction (noise). The risk for a design like that is the lack of gain in the region 1-2Hz, which in certain environmental condition could become a problem. An adaptive strategy, at a very basic level, could allow a safe use of a low gain filter.

Images attached to this comment