40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  40m Log  Not logged in ELOG logo
Message ID: 15783     Entry time: Thu Jan 28 22:34:21 2021
Author: gautam 
Type: Update 
Category: SUS 
Subject: De-whitening 

Summary:

  1. We will need de-whitening filters for the BHD relay optics in order to meet the displacement noise requirements set out in the DRD. I think these need not be remotely switchable (depends on specifics of LO phase control scheme). SR2, PR2 and PR3 can also have the same config, and probably MC1, MC3 as well.
  2. We will need de-whitening filters for the non test mass core IFO optics (PRM, SRM, BS, and probably MC2).
  3. I am pretty sure we will not be able to have sufficient DAC range for the latter class of optics if we have to:
    1. Supply the DC bias.
    2. Do the LSC and ASC actuation in the presence of reasonable sensing noise levels.
    3. Engage de-whitening to low-pass-filter the DAC noise at ~200 Hz.

Details:

Attachment #1 shows the DAC noise models for the General Standards 16-bit and 18-bit DACs we are expecting to have.

  • The 16-bit model has been validated by me at the 40m a few years ago.
  • We have never used the 18-bit flavor at the 40m, and there are all manner of quirks apparently related to zero crossings and such. So the noise may be up to x2 higher (we won't have as much freedom necessarily as the sites to bias the DAC on one side of the zero crossing if we also need to use the same DAC channel to supply the DC bias current for alignment.

Attachment #2 shows the expected actuation range for DC optic alignment, assuming we use the entire DAC range for this purpose.

  • Clearly, we need to do other things with the same DAC channels as well, so this is very much an upper bound of what will be possible.
  • Let's assume we will not go lower than 100ohms.
  • For all new optics we are suspending, we should aim to get the pitch balancing to within 500urad. With a 2x2m=4m optical lever arm, this corresponds to a 2mm spot shift. Should be doable.
  • This could turn out to be a serious problem for PRM, BS and SRM if we hope to measure squeezing - the <AUX DOF>-->DARM coupling could be at the level of -40dB, and at 200 Hz, the DAC noise would result in PRCL/MICH/SRCL noise at the level of ~10^-15m/rtHz, which would be 10^-17m/rtHz in DARM. I don't think we can get 20dB of feedforward cancellation at these frequencies. For demonstrating locking using a BHD error signal, maybe this is not a big deal.

Attachment #3 shows the current and proposed (by me, just a rough first pass, not optimized in any way yet) de-whitening filter shapes. These shapes can be tweaked for sure.

  • The existing de-whitening filter is way too aggressive. FWIW, the DRD "models" a "4th order Chebyshev low pass filter" which doesn't exist anywhere as far as I know.
  • Since the DAC noise is below 1 uV/rtHz at all frequencies of interest, we never need to have >60dB de-whitening anywhere as the input referred noise of any circuit we build will exceed 1 nV/rtHz.
  • I propose 3 poles, 3 zeros. In the plot, these poles are located at 30Hz, 50Hz, 2kHz, and the zeros are at 300 Hz, 300 Hz, 800 Hz. 
  • The de-whitening is less agressive below 100 Hz, where we still need significant LSC actuation ability. Considering the sensing noise levels at the 40m, I don't know if we can have reasonable LSC and ASC loop shapes and still have the de-whitening.
  • Once again, PRM, SRM and BS will be the most challenging.
  • For the BHD relay optics, once we have the de-whitening, we won't have the option of turning on a high-frequency (~kHz) dither line because of insufficient DAC range. 

Attachment #4 puts everything into displacement noise units. The electronics noise of the coil driver / de-whitening circuit have not been included so at high frequencies, the projection is better than what will actually be realizable, but still well below the BHD requirement of 3e-17 m/rtHz.

Attachment 1: DACnoiseModels.pdf  130 kB  | Hide | Hide all
DACnoiseModels.pdf
Attachment 2: actuationRange.pdf  65 kB  | Hide | Hide all
actuationRange.pdf
Attachment 3: deWhiteTFs.pdf  86 kB  | Hide | Hide all
deWhiteTFs.pdf
Attachment 4: dispNoiseModels.pdf  160 kB  | Hide | Hide all
dispNoiseModels.pdf
ELOG V3.1.3-