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Entry  Thu Apr 5 10:38:38 2018, gautam, Update, CDS, CDS puzzle MC1_outputs.pngEllipTFCheck_MC1.pdfMC1_ELP.tgzEllipTFCheck_MC1.pdf
    Reply  Fri Apr 6 10:00:29 2018, gautam, Update, CDS, CDS puzzle 
Message ID: 13729     Entry time: Thu Apr 5 10:38:38 2018     Reply to this: 13733
Author: gautam 
Type: Update 
Category: CDS 
Subject: CDS puzzle 

I'm probably doing something stupid - but I've not been able to figure this out. In the MC1 and MC3 coil driver filter banks, we have a digital "28HzELP" filter module in FM9. Attachment #1 shows the MC1 filterbanks. In the shown configuration, I would expect the only difference between the "IN1" and "OUT" testpoints to be the transfer function of said ELP filter, after all, it is just a bunch of multiplications by filter coefficients. But yesterday looking at some DTT traces, their shapes looked suspicious. So today, I did the analysis entirely offline (motivation being to rule out DTT weirdness) using scipy's welch. Attachment #2 shows the ASDs of the IN1 and OUT testpoint data (collected for 30s, fft length is set to 2 seconds, and hanning window from scipy is used). I've also plotted the "expected" spectral shape, by loading the sos coefficients from the filter file and using scipy to compute the transfer function.

Clearly, there is a discrepancy for f>20Hz. Why?

Code used to generate this plot (and also a datafile to facilitate offline plotting) is attached in the tarball Attachment #3. Note that I am using a function from my Noise Budget repo to read in the Foton filter file...

*ChrisW suggested ruling out spectral leakage. I re-ran the script with (i) 180 seconds of data (ii) fft length of 15 seconds and (iii) blackman-harris window instead of Hanning. Attachment #4 shows similar discrepancy between expectation and measurement...

Attachment 1: MC1_outputs.png  29 kB  Uploaded Thu Apr 5 11:45:05 2018  | Hide | Hide all
Attachment 2: EllipTFCheck_MC1.pdf  200 kB  Uploaded Thu Apr 5 11:45:24 2018  | Hide | Hide all
Attachment 3: MC1_ELP.tgz  3.492 MB  Uploaded Thu Apr 5 11:45:32 2018
Attachment 4: EllipTFCheck_MC1.pdf  222 kB  Uploaded Thu Apr 5 21:03:50 2018  | Hide | Hide all
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