Abstract

Though past DAC noise measurement was done with zero output, a possibility that noise level is different between situations with zero output and with non-zero output was pointed out.
So we measured DAC noise again with non-zero output.
Because we cannot see any difference between zero output spectrum and non-zero output one at the output of Anti-Imaging chassis, past measurements seems to be reasonable as DAC nosise.
On the other hand, because engaging de-whitening filter didn't make a 10 times reduction of DAC(?) noise in the past measurement, there may be a noise larger than DAC noise coming from downstream of de-whitening filters.
 

Details

Quick check of DARM contamination by DAC noise was done in klog#31706 (PRs), klog#31707 (BS+SRs), klog#31708 (IX), klog#31710 (IY), klog#31712 (EY), and klog#31716 (EX). In these measurements, we can know that de-whitening filter for some of actuators makes sensitivity improvement. In usual, we assume that a DAC noise with de-whitening filters in the unit of DARM displacement can be suppressed by factors of 1e-1, 1e-2, or 1e-3 above 10Hz for the case of engaging 1, 2, or 3 stages of de-whitening filters, respectively. On the other hand, past DAC noise measurement (see also link below) shows more small improvement of DAC noise by engaging the de-whitening filters.
- klog#24725 (IY), klog#24726 (IX), klog#24919 (EY), klog#25011 (EX),
- klog#24916 (BS), klog#28069 (SR2), klog#26129 (SR3), klog#26230 (SRM),
- klog#25009 (PR2), klog#25886 (PR3), klog#26205 (PRM).

If de-whitening filter doesn't make a improvement by a proper factor for the number of engaged filter stages, current noise limit may come from the downstream of the de-whitening filter stages instead of DAC noise itself. As another scenario, a possibility that noise level was different between situations with zero output and with non-zero output was pointed out. In former case, we may need to replace high power coil driver to low power one. In latter case, noise projection should be done with the spectra measured in the case of non-zero output. To know which case we are in now, we made a DAC noise measurement with the non-zero output.

Measurement was done for TM actuators of ITMX with the same manner as past measurement. At first, we measured the noise level with connecting only whitening filters and Anti-aliasing filter to ADC as a background noise of our measurement. These result are shown as blue curves in Fig.1. Upper left, lower left, upper right, and lower right panels show the spectrum of DAC channels for H1, H2, H3 and H4 coils, respectively. Next, we connected DAC and Anti-imaging filters to the input of whitening filters. We already know DAC noise is smaller than ADC noise, so we applied whitening filter gain as 36dB to see DAC noise on ADC channels. DAC noise with zero output and 1000ct output are represented as green curve and brown curve, respectively. There is no significant difference each other. So noise level in the case of zero-output did't seem to be a underestimate.

We considered a possibility that though DC output doesn't make larger noise than a case of zero output, AC output may make it. So we made a 0-3Hz Gaussian noise for the DAC output shown as magenta curve. Seeing this noise on ADC channels, we can obtained red curve. Because DAC output (magenta curve) is larger than the DAC noise (green or brown curves) below 100Hz, we can see the injected output on ADC (red curve) below 100Hz. On the other hand, red curve shows same level as green and brown curves above 100Hz. This fact means that AC output also doesn't make lager noise than a case of zero output.

According to these results, past measurements seems to be fine for the estimation of DAC noise and noise coming from the downstream of de-whitening filter is probably larger than DAC noise. Several questions still remains, but it can be investigated by using Anti-Imaging output without coil drivers. So the rest of works can be done on Mozumi standalone system.