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Sun Apr 29 22:53:06 2018, gautam, Update, General, DARM actuation estimate
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Tue May 1 19:37:50 2018, gautam, Update, General, DARM actuation estimate 
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Mon May 7 16:23:06 2018, gautam, Update, General, DARM actuation estimate  
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Mon May 7 20:01:14 2018, Rorpheus, Update, General, Use anti-dewhitening + show CARMA/DARMA
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Thu May 10 14:13:22 2018, gautam, Update, General, More refinement of DARM control signal projection  
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Fri May 11 13:58:42 2018, rana, Update, General, More refinement of DARM control signal projection
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Fri May 11 19:02:52 2018, gautam, Update, General, More refinement of DARM control signal projection    
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Sat May 12 10:02:03 2018, rana, Update, General, More refinement of DARM control signal projection
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Sun May 20 17:43:01 2018, rana, Update, Electronics, How to choose resistors
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Mon May 14 18:58:32 2018, Kevin, Update, PonderSqueeze, Squeezing with no SRM  
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Tue Oct 2 23:57:16 2018, gautam, Update, PonderSqueeze, Squeezing scenarios
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Tue May 15 21:56:57 2018, gautam, Update, General, Stack measurement setup decommissioned
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Thu May 17 09:14:38 2018, Steve, Update, General, Stack measurement setup decommissioned 
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Message ID: 13831
Entry time: Thu May 10 14:13:22 2018
In reply to: 13823
Reply to this: 13833
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Author: |
gautam |
Type: |
Update |
Category: |
General |
Subject: |
More refinement of DARM control signal projection |
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Summary:
- It seems that after a x10 increase in the coil driver resistance, we will have enough actuation range to control (anti de-whitened) DARM without saturating the DAC.
- The Barry puck doesn't seem to help us much in reducing the required RMS for DARM control. If this calculation is to be believed, it actually makes the RMS actuation a little bit higher.
See Attachment #1 for the projected control signal ASDs. The main assumption in the above is that all other control loops can be low-passed sufficiently such that even with anti-dewhitening, we won't run into saturation issues.
DARM control loop:
- I'm now calculating the DARM control signal in counts after factoring into account a digital DARM control loop.
- The loop shape is what we used when the DRFPMI was locked in Oct 2015.
- I scaled the overall OLTF gain to have a UGF around 200Hz.
- The breakdown of how the DARM loop is constructed is shown in Attachment #2.
De-whitening and Anti-De-whitening:
- The existing DW shape in the ITM and ETM signal chains has ~80dB attenuation around 100 Hz.
- Assuming ~5uV/rtHz noise from the DAC, 60dB of low-passing gets us to 5nV/rtHz. With 4.3kohm series resistance, this amounts to ~1pA/rtHz current noise (compared to ~3pA/rtHz from the Johnson noise of the series resistance). Actually, I measured the DAC noise to be more like ~700nV/rtHz at 100 Hz, so the current noise contribution is only 0.16pA/rtHz.
- This amounts to getting rid of the passive filter at the end of the chain in the de-whitening board.
- Attachment #3 shows the existing and proposed filter shapes.
It remains to add the control signals for Oplev, local damping, and ASC to make sure we have sufficient headroom, but given that current projections are predicting using up only ~1000cts of the ~23000cts (RMS) available from the DAC, I think it is likely we won't run into saturations. Need to also figure out what the implication of the reduced actuation range will be on handling the locking transient. |
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