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Entry  Tue Feb 16 14:56:44 2021, gautam, Update, SUS, aLIGO Sat Amp installed, powered MC1_deadUL.pngIMG_9100.jpg
    Reply  Wed Feb 17 16:11:53 2021, gautam, Update, SUS, aLIGO Sat Amp installed, powered 
       Reply  Thu Feb 18 15:33:21 2021, gautam, Update, SUS, aLIGO Sat Amp installed, powered and commissioned 
          Reply  Thu Feb 18 18:05:04 2021, gautam, Update, SUS, aLIGO Sat Amp characterization OSEMnoise.pdfLEDdriveNoise.pdf
             Reply  Thu Feb 18 20:20:25 2021, Koji, Update, SUS, aLIGO Sat Amp characterization 
Message ID: 15818     Entry time: Thu Feb 18 18:05:04 2021     In reply to: 15817     Reply to this: 15819
Author: gautam 
Type: Update 
Category: SUS 
Subject: aLIGO Sat Amp characterization 

Before installation, I performed a bunch of tests on the aLIGO sat amp. All the measurements were made with the dummy suspension box substituting for an actual suspension. Here are the results.

Attachment #1: Transimpedance amplifier noises.

  • Measurement setup: J7 of the Satellite Amp goes to J9 on D1900068 front end (even though the connector is actually labelled "J3" on the box we have - maybe a versioning problem?). The outputs then go to a G=100 SR560 in AC coupled mode (the main purpose here was to block the large DC from the SR785, but I tacked on G=100 while I was at it).  
  • Top panel shows the raw measured voltages.
  • The bottom panel does a bunch of transformations:
    • Undoes the z:p = 3:30 Hz whitening on board the sat amp.
    • Undoes the G=100 gain of the SR560, and the AC coupling poles/zeros of SR560 and SR785.
    • Converts from voltage to current by dividing by the transimpedance gain, 242 kohms. 
  • Some model curves are shown for comparing to the measured spectra. It may be possible that we don't need to modify the nominal z:p = 0.4:10 Hz - I don't think the nominal seismic level will saturate the output even with the 0.4:10 Hz whitening, and it gives us even more clearance to the ADC noise (although we don't need it, we are gain limited at those frequencies, this is mostly a suggestion to reduce the workload).
  • The neon green curve is measured with the actual MC1 suspension plugged in, local damping enabled. It doesn't line up with the nosie floor of the bench tests, probably because the cts/um conversion factor could be off by some factor? Around 1 kHz, you can also see some broad peaks that are reminiscent of those seen in the MC_F spectrum after the c1psl Acromag upgrade. I hypothesize this is due to some poor grounding. Hopefully, once we get rid of the single-ended sending/receiving components in the suspension electronics chain, these will no longer be an issue.

Attachment #2: LED drive current source noises. I mainly wanted to check a claim by Rich in a meeting some time ago that the LED intensity fluctuations are dominated by inherent LED RIN, and not by RIN on the drive current. 

  • Measurement setup: a pair of pomona mini-grabbers was used to clip onto TP3. I found the voltage noise to be sufficiently high that no preamplification was required, and the DC level was <1V, so I just used the SR785 in AC coupled mode. 
  • The dummy suspension box was being driven while the measurement was being made (so the current source is loaded).
  • One channel (CH6) shows anomalously high nosie. I confirmed this was present even after the box was plugged in for ~1 day, so can't be due to any thermal / equilibriating transients.
  • I didn't check for consistency at the monitor testpoint, but that is exposed even with the MC1 suspension plugged in, so we can readily check. Anyways, from the corresponding photodiode curve in Attachment #1, it would seem that this excess RIN in the drive current has no measurable effect on the intensity fluctuations of the LED (the DC value of the paired PD is consistent with the others, ~6V DC). I must say I am surprised by this conclusion. I also checked for coherence between TP3 and the PD output using the SR785, and found none. 🤔 
  • Nevertheless, for the remaining channels, it is clear that the drive current is not shot noise limited for <1kHz. This isn't great. One possible reason is that the collector voltage to Q1 is unregulated (my modeling suggests only ~10dB rejection of collector voltage fluctuations at the output). I believe the current source designed by Luis for A+ makes some of these improvements and so maybe Rich was referring to that design, and not the aLIGO Satellite Amplifier flavor we are using. Anyways, this is just academic I think, the performance is the unit is fine for our purposes.

I will update with the MC1 suspension characterization (loop TFs, step responses etc) later.

Attachment 1: OSEMnoise.pdf  420 kB  | Hide | Hide all
OSEMnoise.pdf
Attachment 2: LEDdriveNoise.pdf  231 kB  | Hide | Hide all
LEDdriveNoise.pdf
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