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AdV-COM (AdV commissioning (1st part) )

casanueva - 16:34 Thursday 31 May 2018 (41630)

Tests with SSFS gain in LN3

I have made the test of decreasing the SSFS gain during LN3 to see which contributions to the sensitivity come from the frequency noise. The results are shown in FIgure 1, the blue curve corresponds to standard conditions (10kHz) and the violet one to reduced gain by a factor 4 (2.6kHz).

The GPS are: 1211808164 (blue, 2min), 1211808437 (violet 2min).

The worrying thing is that above 2kHz the sensitivity is coherent with frequency noise, and above 4.5 we improve the sensitivity when the SSFS loop is lower, which points to a problem in the SSFS loop.

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Comments to this report:

mwas - 11:35 Friday 01 June 2018 (41640)

This is a normal side effect of the sensitivity being limited by frequency noise and the SSFS loop having a low phase margin.

Figure 1, shows the measured SSFS loop OLTF from 2 weeks ago, with a phase margin of 25 degrees.
In general, a loop injects its sensing noise with a transfer function of L/(1-L) where L is the open loop transfer function.
Figure 2, shows L/(1-L) for the nominal gain and the for the reduced by factor 4. This explains very well the bump appearing at ~2.5kHz with lower loop gain (due to clear gain peaking). But it also highlights that above 4kHz the SSFS loop injects double its sensing noise due to the low phase margin of the SSFS loop around 10kHz. Increasing the SSFS loop gain would shift the problem to higher frequencies, as the gain peaking happens only when the gain is close to 1 (so 1/(1-L) is large).

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