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Entry  Thu Jan 4 12:46:27 2018, gautam, Update, ALS, Fiber ALS assay FiberALS.pdfFiberALS.svg.zip
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                Reply  Fri Jan 19 11:34:21 2018, gautam, Update, ALS, Fiber ALS assay IR_ALS_20180119.pdf
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                                  Reply  Wed Jan 31 16:29:42 2018, gautam, Update, ALS, Modulation depths PMCmodDepth.pdfXPDH.pdf
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                                        Reply  Thu Feb 1 01:24:56 2018, gautam, Update, ALS, D0902745 revamp underway PSLinterlock.png
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                                              Reply  Fri Feb 2 13:16:55 2018, gautam, Update, ALS, ALS signals whitening switching ALS_whitening_switching.pdfALS_whitening_switching_works.pdf
                                                 Reply  Mon Feb 5 14:11:01 2018, gautam, Update, ALS, Huge harmonics in ALS channels ALSpowerlineHarmonics.pdf2A1B3AC4-4059-416A-AD88-8AB0431D7A55.jpeg
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                                                       Reply  Thu Feb 8 00:33:20 2018, gautam, Update, ALS, D990694 characterization / THD measurement plan D990694THD_trial.pdf
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                                                          Reply  Fri Feb 9 13:37:44 2018, gautam, Update, ALS, THD measurement trial THD_trial.pdf
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                      Reply  Thu Jan 25 13:18:41 2018, gautam, Update, ALS, Fiber ALS assay preampProposed.pdf
Message ID: 13628     Entry time: Fri Feb 9 13:37:44 2018     In reply to: 13621
Author: gautam 
Type: Update 
Category: ALS 
Subject: THD measurement trial 

I quickly put together some code that calculates the THD from CDS data and generates a plot (see e.g. Attachment #1).

Algorithm is:

  1. Get data (for now, offile file, but can be readily adapted to download data live or from NDS).
  2. Compute power spectrum using scipy.signal.periodogram. I use a Kaiser window with beta=38 based on some cursory googling, and do 10 averages (i.e. nfft is total length / 10), and set the scaling to "spectrum" so as to directly get a power spectrum as opposed to a spectral density.
  3. Find fundamental (assumed highest peak) and N harmonics using scipy.signal.find_peaks_cwt. I downsample 16k data to 2k for speed. A 120second time series takes ~5 seconds.
  4. Compute THD as \mathrm{THD} = \frac{\sqrt{\sum_{i=2}^{N}\mathrm{V}_i^2}}{V_1}where V_i denotes an rms voltage, or in the case of a power spectrum, just the y-axis value.

I conducted a trial on the Y arm ALS channel whitening board (while the X arm counterpart is still undergoing surgery). With the whitening gain set to 0dB, and a 1Vpp input signal (so nothing should be saturated), I measure a THD of ~0.08% according to the above formula. Seems rather high - the LT1125 datasheet tells us to expect <0.001% THD+N at ~100Hz for a closed loop gain of ~10. I can only assume that the digitization process somehow introduces more THD? Of course the FoM we care about is what happens to this number as we increase the gain.

Quote:
 

I'm going to work on putting together some code that gives me a quick readback on the measured THD, and then do the test for real with different amplitude input signal and whitening gain settings.

 

Attachment 1: THD_trial.pdf  220 kB  Uploaded Fri Feb 9 17:20:30 2018  | Hide | Hide all
THD_trial.pdf
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