I have created a DTT template that makes it easier to decide when it's okay to turn on more OMC DCPD whitening.
Evan wrote an alog some time ago about the new OMC DCPD whitening on/off guardian states (alog 20578), and Cheryl and Evan made some notes on when it's okay to go to these new states (alog 20787).
As of right now, the guardian will automatically turn on one stage of whitening, but we get better high frequency noise performance if we add a second stage. However, if some mode (eg. a violin mode) is rung up, then we can't add the second stage of whitening without being in danger of saturating the ADC. So. The new DTT template should help decide when it's okay to add the second stage.
The template is /ligo/home/ops/Templates/dtt/DCPD_saturation_check.xml (screenshot below). The template should be run after we have arrived at NOMINAL_LOW_NOISE for the main lock sequence. If the dashed RMS lines are below the green horizontal line, it's okay to add the second stage of whitening.
To engage the second stage of DCPD whitening:
Open the full list of guardian states for the OMC_LOCK guardian, and select "ADD_WHITENING". It will take a minute or two, and automatically return to the nominal "READY_FOR_HANDOFF" state.
could you explain the math & logic a little bit more?
I would have thought that an RMS of 3000 cts is as high as we want to go. Increasing the RMS by a factor of 10 would make it so that its always saturating = not OK. Or isn't this IN1 channel the real ADC input?
Yes, 3000 ct rms = 8500 ct pkpk = too many counts to add a second stage of whitening.
We run with about 10 mA dc on each DCPD, which shows up as 13000 ct or so of dc on the IN1 channels. That means we have something like 19000 ct of headroom before the ADCs saturate on the high side (+32 kct). Assuming the ac fuzz is symmetric about the mean, saturation will certainly occur if the ac is greater than 38000 ct pkpk with two stages of whtening, or 3800 ct pkpk with one stage of whitening.
That's why the criterion I've been using for turning on a second stage of whtening is to look at the IN1 channels and verify that the ac is less than 3000 ct pkpk, or 1000 ct rms when there is only one stage of whitening on. If we find the DCPDs saturating too often with two stages, we should be even more restrictive.