**Summary:**
### Yesterday at the meeting, we talked about how the analog de-whitening filters in the coil driver path may be more aggressive than necessary. *I think ***Attachment #1** shows that this is indeed the case.
**Details:**
I had done some modeling and measurement of some of these noises while I was putting together the initial DRMI noise budget, but I had never put things together in one plot. In Attachment #1, I've plotted the following:
**Quadrature sum of seismic noise (from GWINC calculations) for 3 suspended optics** (I'm sticking to the case of 3 optics since I've been doing all the noise-budgeting for MICH - for DARM, it will be 4 suspended optics).
**The unfiltered DAC noise estimate**. The voltage noise was measured in this elog. To convert this to displacement noise for 3 suspended optics, I've used the value of 1.55e-9/f^2 m/ct as the actuator coefficient. This number should be accurate under the **assumption that the series resistance on the coil driver board output is **__400 ohms __(we could increase this - by how much depends on how much actuation range is needed).
**Coil driver board and de-whitening board electronics noises (added in quadrature)**. I've used the LISO model noises, which line up well with the measured noises in elogs 13010 and 13015.
- The
**DAC noise filtered** by the de-whitening transfer function, separately for the cases of **using one** or **both** of the available biquad stages. *This cannot be lower than the preceeding trace (electronics noise of de-whitening and coil driver boards), so should be disregarded where it dips below it.*
### It would seem that the coil driver + de-whitening board electronic noises dominate above ~150Hz. The electronics noise is ~10nV/rtHz at the output of the coil driver board, which is only a factor of 100 below the DAC noise - so the stopband attenuation of ~70dB on the de-whitening boards seems excessive.
### We can lower this noise by a factor of 2.5 if we up the series resistance on the coil driver boards from 400ohm to 1kohm, but even so, the displacement noise is ~1e-18 m/rtHz. I need to investigate the electronics noises a little more carefully - I only measured it for the case when both biquad stages were engaged,* I will need to do the model for all permutations - to be updated. *
__Attachment #2__ has an iPython notebook used to generate this plot along with all the data.
Edit 28 Jul 2.30pm: I've added __Attachment #3__ with traces for different assumed values of the series resistance on the coil driver board - although I have not re-computed the Johnson noise contribution for the various resistances. If we can afford to reduce the actuation range by a factor of 25, then it looks like we get to within a factor of ~5 of the seismic noise at ~150Hz. |