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Entry  Wed Apr 22 17:29:14 2020, gautam, Update, PSL, FSS debugging attempts FSSspec.pdf
    Reply  Thu Apr 23 10:42:02 2020, rana, Update, PSL, FSS debugging attempts 
Message ID: 15310     Entry time: Wed Apr 22 17:29:14 2020     Reply to this: 15312
Author: gautam 
Type: Update 
Category: PSL 
Subject: FSS debugging attempts 

Summary:

On Monday, I hooked up an AG4395 to the PMC error point (using the active probe). The idea was to take a spectrum of the PMC error point every time the FSS PC drive RMS channel indicated an excursion from the nominal value. An initial look at the results don't suggest that this technique is particularly informative. I'll have to think more about a workaround, but please share your ideas/thoughts if you have some.

Also, the feature in the spectrum at ~110 kHz makes me suspect some kind of loop instability. I'll measure the IMC loop OLG at the next opportunity.

Details:

  • The PMC servo bandwidth is ~2 kHz, so above this, the PMC error point should be a faithful monitor of the PSL frequency noise, provided the sensing noise is low enough.
  • The PMC error point sensing noise is ~100nV/rtHz (I'm monitoring this straight after the Minicircuits mixer+bandpass filter that we are using as a demodulator). This corresponds to ~2 Hz/rtHz, using the ~10 MHz/V PDH discriminant calibration from January. Seems consistent with this elog.
  • I was hoping to see if there was a particular frequency band in which the noise gets elevated, and if the crossover frequency is a few kHz and the IMC servo BW is ~110 kHz, I would have expected this to be in the 10-100 kHz region. Possibly my frequency resolution isn't good enough? But with the Agilent, doing a finer grid would mean a longer measurement time, in which case the IMC might lose lock before the measurement is done.
  • But, as shown in Attachment #1, there isn't any clear evidence, from the ~20 excursions that were recorded last night. The color of the line is meant to be indicative of the average value of the PC drive RMS channel in the measurement time.
  • A significant bottleneck in this whole process is that it takes ~1 minute to initiate the GPIB measurement, and download the data. The pseudo-code I used is:
    • While the IMC is locked, watch PCdrive RMS EPICS channel's "ALARM" state, which becomes non-zero when the PCdrive RMS exceeds 1 V (this is how it is defined in the EPICS db record right now).
    • Make sure this isn't a transient feature - I do this by waiting 5 seconds and checking that the ALARM flag is still flagged.
    • Initiate a AG4395 measurement over GPIB - I use the measurement span of 1 kHz - 1 MHz with a BW/span ratio of 0.1%, 5 averages.
    • Check that the IMC is still locked (if it got unlocked while the measurement was made, presumably the measurement is garbage).
  • Is there a better monitor of the laser frequency noise? I can imagine using POX/POY which I think have a lower electronics noise floor but I'm not sure if that's true at 100 kHz and having the arms locked in addition to the IMC seems more complicated...
  • Since we are planning a laser upgrade, is this worth spending more time on? I may leave the measurement running on pianosa in a tmux session while I'm not in the lab...
Attachment 1: FSSspec.pdf  644 kB  Uploaded Thu Apr 23 00:07:56 2020  | Hide | Hide all
FSSspec.pdf
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