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Entry  Fri Jun 26 16:53:38 2020, gautam, Update, Electronics, RFPD characterization WFS1.pdfWFS2.pdfbuildupMons.pdf
    Reply  Mon Jun 29 15:56:02 2020, gautam, Update, Electronics, RFPD characterization 
    Reply  Sun Jul 5 18:25:42 2020, rana, Update, Electronics, WFS characterization WFS_attenOff.pdf
       Reply  Sun Jul 12 22:40:35 2020, gautam, Update, Electronics, WFS characterization - old SURF report 
       Reply  Sat Aug 29 20:07:48 2020, rana, Update, Electronics, WFS characterization 
          Reply  Sat Aug 29 22:10:09 2020, gautam, Update, Electronics, WFS characterization 
          Reply  Tue Sep 15 17:04:43 2020, gautam, Update, Electronics, DC adaptors delivered 
Message ID: 15433     Entry time: Fri Jun 26 16:53:38 2020     Reply to this: 15439   15450
Author: gautam 
Type: Update 
Category: Electronics 
Subject: RFPD characterization 


While the vacuum system was knocked out, I measured the RF transimpedance (using the AM laser setup, didn't do the shot noise intercept current measurement for now) of all the RFPDs (except PMC REFL). At the very least, the following photodiodes are suspect:

  1. WFS heads - expected transimpedance is 50 kohm unattenuated, and 5 kohm attenuated. I measure values that are x10 lower than this, and the segments are significantly imbalanced. Morover, the attenuators for some quadrants appear to do nothing. This could be a problem with the Acromag system I guess, but the measured transimpedance is nowhere close to the "expected" value. See Attachments #1 and #2. You can also see that the response at 55 MHz is significantly attenuated, so I'm guessing trying to measure the AS port ASC sensing response is going to be difficult.

    Note that I assumed a 1kohm DC transimpedance, which is what I expect from the schematic and also is consistent with the DC voltage I measured, knowing the approximate optical power incident on the photodiode.
  2. POP 22/ POP 110 - this is a Thorlabs PDA10CF diode. It should have a flat gain profile out to ~100 MHz, but I measure some weird features. The other PDA10CF we use, at AS110, shows a more reasonable response. See Attachment #3. I don't know what kind of failure mode this is? Anyway I'll try testing another PDA10CF and if it looks more reasonable, I'll switch out this diode. FWIW, the measured AS110 gain is ~3kohms, whereas the datasheet tells us to expect 5 kohms.

For the remaining photodiodes, I measure a transimpedance that is within ~20% of what is on the wiki page. The notches may benefit from some retuning. While I have the data, I will fit this and post a more complete report on the wiki.

Update July 6 1145am: WFS response plots now have legends mapping quadrants, and I've also added the response of a spare PDA10CF (which is now the new POP22/POP110 photodiode).

Attachment 1: WFS1.pdf  436 kB  Uploaded Mon Jul 6 12:44:45 2020  | Hide | Hide all
Attachment 2: WFS2.pdf  418 kB  Uploaded Mon Jul 6 12:44:54 2020  | Hide | Hide all
Attachment 3: buildupMons.pdf  124 kB  Uploaded Mon Jul 6 12:45:04 2020  | Hide | Hide all
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