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Entry  Tue Jul 14 16:52:47 2020, gautam, Update, Electronics, Coil drivers for the test masses coilDriverSchem.pdf
    Reply  Wed Jul 15 21:08:43 2020, gautam, Update, Electronics, ETM coil outputs DQed CDS.pngcoilOutDQed.png
    Reply  Mon Jul 20 17:23:46 2020, gautam, Update, Electronics, Coil drivers for the test masses ALSlock_timeDomain.pdfALSlock.pdfRFlock.pdf
Message ID: 15494     Entry time: Mon Jul 20 17:23:46 2020     In reply to: 15480
Author: gautam 
Type: Update 
Category: Electronics 
Subject: Coil drivers for the test masses 

Summary:

Looking at the signals to the test mass coils, it seems borderline to me that we will be able to acquire lock and run in a low noise configuration with the same series resistor in the coil driver circuit. The way I see it, options are:

  1. Use a moderately high series resistance (e.g. 5 kohms) for the time being, and go ahead with the HAM-A coil driver.
    • This will mean a current noise of ~3pA/rtHz, which translates to ~3e-18 m/rtHz @ 100 Hz in DARM displacement noise (assuming the ITMs have much higher series resistance than the ETMs).
    • If the lock acquisiton looks smooth, double the resistance to 10 kohms.
    • With 5 kohm series resistance, there is negligible possibility of measuring ponderomotive squeezing for any of the input powers we consider feasible, but this is under the assumption that we will expose coil driver noise, which is very optimistic imho.
  2. Re-design a new coil driver that allows switchable impedance, so we can have a higher noise acquisition mode for acquiring and holding the ALS lock, then transition to a lower noise, lower range config once the RF / BHD lock has been acquired.
    • On paper, this solves all the problems, but the design of such a circuit is probably pretty non-trivial and time consuming.

Details:

I only looked at the ETMs for this study. The assumption is that we will have no length actuation on the ITMs, only local damping and Oplev loops (and maybe some ASC actuation?), which can be sufficiently low-pass filtered such that even with coil de-whitening, we won't have any range issues.

Attachment #1 shows the time-domain traces of the coil driver signals as we transition from POX/POY lock to the ALS lock. There are some transients, but I think we will be able to hold the lock even with a 5 kohm resistor (~twice what is on ETMX right now). From just these numbers, it would seem we can even go up to 10 kohms right away and still be able to acquire lock, especially if we re-design the digital feedback loop to have better low-pass filtering of the high-frequency ALS noise, see the next attachment.

Attachment #2 shows the f-domain picture, once the arm lengths are fully under ALS control (~25 seconds onwards in Attachment #1). The RMS is dominated by high frequency ALS length loop noise, which we can possibly improve with better design of the digital control loop.

Finally, Attachment #3 shows the situation once DARM control has been transitioned over to AS55_Q. Note that the vertex DoFs are still under 3f control, so there is the possibility that we can make this even lower noise. However, one thing that is not factored in here is that we will have to de-whiten these signals to low-pass filter the DAC noise (unless there is some demonstrated clever technique with noise-mons or something to subtract the DAC noise digitally). Nevertheless, it seems like we can run safely with 5 kohms on each ETM coil and still only use ~2000 cts RMS, which is ~1/10th the DAC range (to allow for dealing with spurious transients etc). 

Quote:

Looking at signals to the ETMs from the current lock acquisition sequence, the RMS current to a single coil is approximately _____ (to be filled in later).

Attachment 1: ALSlock_timeDomain.pdf  303 kB  Uploaded Mon Jul 20 18:43:55 2020  | Hide | Hide all
ALSlock_timeDomain.pdf
Attachment 2: ALSlock.pdf  383 kB  Uploaded Mon Jul 20 18:44:07 2020  | Hide | Hide all
ALSlock.pdf
Attachment 3: RFlock.pdf  617 kB  Uploaded Mon Jul 20 18:44:16 2020  | Hide | Hide all
RFlock.pdf
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