J. Kissel, for K. Izumi, and S. Ballmer

Now that Stefan and Kiwamu have the first reasonably successful evening with ALS DIFF (see LHO aLOG 11759), I wanted to make an assessment of how their filter design ended up compared to my original design (see LHO aLOG 11676). I attach the frequency dependence of each stage, divided into the distribution filters, plant inversion filters. Then I attach total path (with gain filters included).

There's still a little bit of confusion about the distribution of over-all gain of the system i.e 
- the ALS DIFF PLL's error signal, which serves as the DIFF sensor, had a some calibration filters turned OFF for some reason, leaving the output in [V] instead of the planned [um]
- converting the overall SUS actuator gain to be [ct_{TST DAC} / um_{ISC IN}] has not yet been installed, leaving the actuator gain still in [ct_{TST DAC} / ct_{ISC IN}]
so it would take some work to understand the over gain of the loop and/or with which UGF Stefan and Kiwamu ended up. 

Contrasting the creation of the super actuator between SB + KI vs. mine:
- They've used the blend filters I've installed identically, so no mystery there.
- For the plant inversion, they've diverted from my design in two ways:
     - A much more accurate inversion of the resonant forest for each stage, resulting in high Q filters with long impulse responses -- the price paid for phase accuracy
     - They've inverted the resonant forest for the TST stage, creating a resultant very high-Q resonant feature in the inversion filter to balance the zero in the TST to TST plant at 0.87 [Hz]. I did not invert the TST stage, and managed to scrounge up enough phase for the *model* to be stable (but this means little compared with reality.)
     - Instead, they've turned the TST (and each subsequent stage above) into a 1 [Hz], Q = 1, single pendulum.

Unfortunately, since the ALS DIFF lock isn't too terribly stable, we can't get more than a few frequency data points to confirm stability, but we'll try again tonight. 

I think the reasons why they were saturating with the overall unity gain frequency only at ~1.2 [Hz] (as opposed to my model of 15 [Hz]) is because of the mystery factor of 4 missing from the TST mass drive (see LHO aLOG 11676), and because we still don't have ETMY up and running because of ESD woes (yet to be aLOGged) -- i.e. a factor of 8 in drive strength that's missing. According to all the modeling done thus far and now a measurement, we really need it!

Next up -- find the factor of four in the ESD and fix it!

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Relevant Configuration Record (gain = 1.0 if not mentioned):
H1:ALS-C_DIFF_PLL_CTRL
FM3 "antiVCO"
FM4 "cnts2V"
G = 10.0

(LSC Input Matrix element = 1.0)

H1:LSC-DARM_IN
FM1 "ALSDIFF"
FM2 "invPlant"
FM3 "Boost"
G = 10.0

(LSC Output Matrix element = 1.0)

H1:SUS-ETMX_L3_ISCINF_L (these were the extra last minute filters added to try and manipulate the overall super-actuator from saturating)
FM6 "p5z50"
FM7 "p5z50"

H1:SUS-ETMX_M0_LOCK_L
FM1 "invL2L1" (part one of plant inversion filter)
FM2 "invL2L2" (part two of plant inversion filter)
FM3 "top/tst" (gain only filter to compensate for the gain ratio)

FM5 "blend LP"

H1:SUS-ETMX_L1_LOCK_L
FM6 "invL2LNEW" (part one of plant inversion filter)
FM7 "patch" (part two of plant inversion filter)
FM8 "LISOfit" (part three of plant inversion filter)
FM9 "uim/tst" (gain only filter to compensate for the gain ratio)

FM3 "blendHP" (UIM-TOP HP complement)
FM4 "blendLP" (UIM-TST LP complement)

H1:SUS-ETMX_L3_LOCK_L
FM6 "MatchedinvL2L" (part one of plant inversion filter)
FM7 "patch" (part two of plant inversion filter)

FM4 "blendHP-M0" (TST-TOP HP complement)
FM5 "blendHP-L1" (TST-UIM HP complement)

H1:SUS-ETMX_L3_LOCK_BIAS_INMON = 125000 [cts]
No linearization algorithm installed.