The phase and gain margin in the DARM loop in the final configuration is quite small. In the interest of improving it, I started by comparing what is used on DARM for H1 and L1. The attached plots show the comparison; the differences are:
- The H1 DARM high frequency roll-off starts much lower than on L1 (roughly 80 Hz vs 600 Hz), and this explains the lack of phase margin on H1 compared to L1. At 30 Hz, e.g., there is about 25 deg more phase lag in H1 than L1.
- L1 DARM has a bandstop filter for the violin mode fundamental (500 Hz), but H1 does not. H1 has a notch at 60 Hz, which is not needed in the final configuration.
- The low-frequency boosts are a little different. The L1 boost starts earlier (higher freq), and has significantly more gain than H1 in the 2-5 Hz band. L1 also has more gain below 0.2 Hz. H1 has a resonant gain at 1 Hz that L1 does not have.
- The overall gain for H1 is about 60 dB higher than for L1 (which must be compensated somewhere else than in the filters)
Attached plots:
- L1H1DARMcompare: All filter modules used in the final DARM configuration. The L1 magnitude has been increased by 60 dB for easier comparison.
- L1H1DARMsuscompare: The subset of filters that are used to compensate the suspension response.
- L1H1DARMboostcompare: Comparison of boost and resonant gain fitlers.
I have modified the H1 DARM suspension compensation filter so that it is now just like the L1 version. Previously this filter (FM8) was called 'LLO', but I changed the name to 'SUScomp'. I also changed the 60 Hz notch (in FM7) to be a bandstop for the violin modes. This is not exactly the same filter as in L1 since the H1 violin mode frequencies are a bit lower (though more narrowly grouped) than L1. This filter is now called 'Violin', and is defined to be: ellip("BandStop",4,1,80,480,530)
Of course the new FM8 filter has more gain than the previous one above ~50 Hz. This could be a problem in terms of ESD range while using ALS-DIFF, so there may need to be some tailoring of other low-pass filters that are used in that state of the acquisition sequence.
I haven't made any changes to the boost/resonant gain filters yet. The spectrum of the residual (in-lock) DARM should be looked at to see if these should be modified.
