Koji, Evan
Summary
MICH feedforward seems to be doing its job, although there is room for improvement by implementing a frequency-dependent subtraction.
SRCL coupling into DARM seems to be very nonstationary. Consequently, the feedforward is not working.
Details
We injected band-limited white noise (elliptic bandpass, 10 Hz to 1 kHz, 6 ct amplitude) first into MICH, then into SRCL, to test the feedforward that was implemented a few weeks ago.
For MICH, frequency-independent subtraction is fair to middling (red) compared to no subtraction (blue); at best we get 20 dB of subtraction around 150 Hz. Note that the TFs in this plot use the whitened DARM channel. The whitening is undone for the spectra in the fourth pad.
For SRCL, the 1/f2 feedforward via ITMY L2 gives no subtraction at all. The attachment shows the TF of SRCL control → DARM with the feedforward off and with broadband noise injected into the SRCL error point. Unlike MICH, appearance of this excitation in DARM is highly nonstationary, fluctuating by a factor of 2 or so in a frequency-dependent way. Additionally, the coherence is poor above 20 Hz, despite the excitation elevating the DARM noise by more than an order of magnitude from 20 to 100 Hz.
The shape of the excess noise is more or less the shape of the 100 Hz elliptic cutoff that we put into SRCL a few weeks ago. Is it possible that the SRCL control noise explains the nonstationary, 100 Hz "scattering" shelf that we've seen in the DARM spectrum this past week?
Using the measurements described above, here is a projection of MICH and SRCL control into DARM. It seems that these two noise sources, along with DAC→ESD noise, can explain most of the DARM noise from 10 to 70 Hz. There is still some excess from 80 to 200 Hz, and an overall excess in the high-frequency noise floor.
For MICH, I used the coherent transfer function we measured earlier. For SRCL, I estimated the TF magnitude by dividing the ASDs of DARM and SRCL (after subtracting off their quiescent values). The dtt files are in evan.hall/Public/2015/04/FullIFO/Noise as MichNoise.xml and SrclNoise.xml.
Some times (all UTC):
- Quiet data: 07:55:00 to 08:02:00
- MICH excitation: 08:12:22 to 08:14:22 with MICH FF off, 08:08:58 to 08:10:58 with MICH FF on
- SRCL excitation: 09:22:10 to 09:25:10 with MICH FF and SRCL FF on, 09:26:05 to 09:28:05 with MICH FF on and SRCL FF off, and 09:28:15 to 09:30:15, no FF.
After these measurements, I also tuned the PRCL→SRCL subtraction in the LSC input matrix from 0.005 to -0.04 (using in-vac POP). This reduced the appearance of a 122 Hz PRCL excitation in the SRCL error signal by 20 dB.
For completeness, here is the same budget as above, with intensity and frequency noises included.
We suspect that the sharp shelf at 100 Hz in the frequency noise projection might be coupling via SRCL, rather than directly to DARM. So between the frequency and SRCL projections, there may be some double-counting of noise in DARM.
Frequency, intensity, and DCPD dark noise are not enough to explain the excess noise between 200 Hz and 4 kHz. It seems they can somewhat explain the uptick in noise above 4 kHz.
Slightly updated/corrected version attached.