Evan, Alexa
Even after the ETMY L2P work performed yesterday (alog 14832), last night we found the L 2 angle for ETMY was still bad. So today we did some more investigation, again. I don't think we made any big head-way, but I will post what we did and the results anyways...
Setup: We excited ETMY_L1_LOCK_L at various frequencies with an amplitude of 5e5 cts and 0 deg phase. The LOCK filters that are nominally installed were left on. Using the pitch lock-in, we demdoluated the pitch oplev signal at the respective frequencies and noted the I and Q signals. We then turned off this excitation and excited ETMY_L1_DRIVEALIGN_Y2P_EXC. This Y2P filter bank was empty, and we changed the gain from 0 to 1. The phase of this excitation was also set to 0 deg. We adjusted the gain such that the magnitude of the I,Q signals was close to that induced by the LOCK_L excitation. Then, we adjusted the phase of the Y2P excitation until we reduced both I, Q signals to zero. In other words, we drove in length with the LOCK_L, and examined the motion in the pitch oplev. We reduced this motion with an excitation in the Y2P drive align matrix. We had hoped this information would help us create a patch and improve our L2P in L1.
Results:
0.8 Hz
| LOCK_L EXC = 5e5cts only | Y2P Gain = 16, 0 deg only | |
| I | -0.412 | 0.174 |
| Q | -0.082 | 0.387 |
| |z| | 0.42 | 0.43 |
| ang(z) | 11 deg | 66 deg |
With both excitations, and Y2P phase set to 55 deg, then I = -0.026, Q = -0.006
0.6 Hz
| LOCK_L EXC = 5e5cts only | Y2P Gain = 16.5, 0 deg only | |
| I | 0.54 | -0.585 |
| Q | 0.44 | 0.46 |
| |z| | 0.7 | 0.74 |
| ang(z) | 39 deg | 38 deg |
With both excitations, and Y2P phase set to 1 deg, then I = -0.01, Q = -0.01
0.5 Hz
| LOCK_L EXC = 5e5cts only | Y2P Gain = 10, 0 deg only | |
| I | 1.02 | -0.85 |
| Q | -0.33 | 0.73 |
| |z| | 0.42 | 1.12 |
| ang(z) | -18 deg | -40 deg |
With both excitations, and Y2P phase set to -22 deg, then I = -0.06, Q = -0.1
0.4 Hz
| LOCK_L EXC = 5e5cts only | Y2P Gain = 3.8, 0 deg only | |
| I | 0.045 | -0.092 |
| Q | 0.083 | -0.019 |
| |z| | 0.095 | 0.094 |
| ang(z) | 3 deg | -2 deg |
With both excitations, and Y2P phase set to -5 deg, then I = -0.003, Q = -0.095
We then decided to take a transfer function from L1_LOCK_L_EXC to the pitch oplev, from 0.1 to 1 Hz, which was our region of interest. We forgot to save the DTT template and closed it, but the results were consistent with the measurement above within 10 dB. So we created a notch at 0.5 Hz since it seemed like we were over compenstating this resonance. However, our oplev spectrum did not improve at all. So we moved onto the damping which seemed to help a good amount at these frequencies. One thing we still need to check is confirm which of the stages has the worst L 2 angle.
Another thing... I talked to Arnuad at LLO and all their L2P, L2Y filters in the ETMs are simple low pass filters with the DC gain set to minimize the coupling. The LOCK filters then have nothces at 9, 10 Hz to handle those resonances, but no other resonance are compenstated for. Their M0 damping filters are "aggressive" and help reduce the angular motion.