All plots are shown as input referred noise of whitening filter chassis.
- MIXER_DAQ: ADC noise is dominant in the DGS frequency range with out any gain. ~20dB gain is required.
- FAST_DAQ: There is large noise excess (1e-3 V/rHz) below 30Hz. It seems to be mixed in after whitening filter chassis.
- SLOW_DAQ: Not measured today, because (maybe) we can't change whitening filter configuration without lockloss.
After the measurement, whitening filter configuration was restored same as the original situation.
(21dB for MIXER, 0dB for FAST, and 0dB for SLOW)
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MIXER_DAQ (Fig1)
By setting whitening gain as larger than 0dB, noise level was improved below 1kHz compared with 0dB gain.
This means noise is mixed in after a whitening filter is dominant below 1kHz.
Since ideal ADC noise level is roughly 3e-6V/rHz, MIXER_DAQ is probably limited by ADC noise without whitening filter gain.
FAST_DAQ (Fig2)
Noise level on FAST DAQ is ~1e-3V/rHz (1000 times larger than ADC noise level).
But I was able to see the improvement of noise level by increasing whitening filter gain.
Is mV level noise mixed in after whitening filter chassis?
With 6dB gain, signal is saturated by the ADC range. So I can measure the noise only 0 and 3dB gain as proper situations.
For the calibration and noise budget on the DGS frequency range (<8kHz), we can take care of only SLOW_DAQ signal.
But something doesn't sit right with me.
If I'll have enough time, I'll try to measure these signals without input signal (unplugged DB9, directly connected CMS output to AA, etc.) in addition to SLOW_DAQ.
and confirmed that ADC and AA themselves have no large excess from expected ADC noise level.
So mV level noise in FAST_DAQ probably comes from large input to whitening filter chassis.
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[ADC+AA (blue curves in attachment)]
In the case that only ADC and AA are connected (unplugging DB9 cable between AA chassis and whitening filter chassis)
Noise level is consistent with expected ADC noise for all three channels.
[ADC+AA+WF (green curves in attachment)]
By connecting whitening filters (unplugging DB9 cable between WF chassis and CMS chassis)
some excess appeared in MIXER channel.
Because I forgot turned off 21dB gain for MIXER channel, it may comes from the difference of whitening filter gain.
Anyway, there is some small excess below 30Hz on MIXER channel with the same whitening settings as IMC lock state.
[noise in unlocked state (brown curves in attachment)]
By connecting CMS large signal can be seen even unlocked state.
Input switch of CMS is turned OFF in unlocked state. So it doesn't come from the refl light.
I'm not sure input switch of CMS really works well, I plan to measure again with turning OFF laser shutter.
We need a proper laser shutter driver ASAP. I can't do that from control room...
