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Entry  Wed Dec 10 15:05:32 2014, Jenne, Update, Electronics, Xend QPD whitening board modified already IMG_1776.JPG
    Reply  Thu Dec 11 16:42:12 2014, Jenne, Update, Electronics, Xend QPD whitening board plan QPDwhiteningModification_11Dec2014.pdf
       Reply  Fri Dec 12 19:54:21 2014, Jenne, Update, Electronics, Xend QPD whitening board modified QPDwhiteningModification_XtransCompleted_12Dec2014.pdfIMG_1779.JPGQPD_Xtrans_Ytrans_12Dec2014.pdfQPD_Xtrans_Ytrans_DarkVsLock_12Dec2014.pdf
          Reply  Sat Dec 13 00:35:11 2014, rana, Update, Electronics, Xend QPD whitening board modified 
          Reply  Mon Dec 15 22:30:50 2014, Jenne, Update, Electronics, Yend QPD modified 
             Reply  Mon Dec 15 22:45:59 2014, Jenne, Update, Electronics, Yend QPD modified QPD_Ytrans_oscillating_15Dec2014.pdfQPD_IOPchannels_Ytrans_oscillating_15Dec2014.pdf
                Reply  Fri Feb 13 14:27:30 2015, ericq, Update, Electronics, Second QPD Whitening Switch enabled 
                   Reply  Fri Feb 13 14:59:13 2015, ericq, Update, Electronics, Second QPD Whitening Switch enabled 
                      Reply  Fri Feb 13 18:56:44 2015, rana, Update, Electronics, Second QPD Whitening Switch enabled 
                         Reply  Thu Feb 19 04:09:21 2015, ericq, Update, Electronics, Second QPD Whitening Switch enabled 
                   Reply  Fri Feb 13 17:07:51 2015, Jenne, Update, Electronics, Second QPD Whitening Switch enabled 
Message ID: 10794     Entry time: Fri Dec 12 19:54:21 2014     In reply to: 10782     Reply to this: 10795   10799
Author: Jenne 
Type: Update 
Category: Electronics 
Subject: Xend QPD whitening board modified 

Okay, I have finished modifying the Xend QPD whitening board, although I will likely need to change the gain on Monday.

Rather than following my plan in elog 10782, I removed the AD602's entirely, and just use the AD620's as the amplifiers.  We don't need remotely adjustable gains, and the AD620s are a less noisy part.

I set the gain to be 30dB using a 1.65k resistor for R_G, which turns out to be too high.  After I installed the board and realized that my counts were much higher than they used to be, I realized that what we had been calling +30dB was in fact +13.2dB. ( I am assuming that the ADC for the gain sliders were putting out a maximum of +10V.  The AD620 used to have a 1/10 voltage divider at the input, and an overall gain of 1, so the output of the AD620 was 100mV.  This goes into pin 16 of the AD602, which has gain of 32*V_set + 10.  Which gives 32*0.1+10=13.2dB.  Ooops.  We've been lying to ourselves. )

Anyhow, before I made the gain realization, I was happily going along, setting the AD620s' gains all to 30dB. I also copied Koji's modification from April of this year, and permanently enabled the whitening filters.

Here is the schematic of what ended up happening.  The red modifications were already in place, and the greens are what I did today.

QPDwhiteningModification_XtransCompleted_12Dec2014.pdf

You can see the "before" picture in my elog Wednesday, elog 10774.  Here is an "after" photo:

IMG_1779.JPG

Here is a spectrum comparing the dark noise of the Xend QPD after modification to the current Yend QPD (which is still using the AD602 as the main instrumentation amplifier).  I have given the Yend data an extra 16.8dB to make things match.

QPD_Xtrans_Ytrans_12Dec2014.pdf

And, here is a set of spectra comparing both ends, dark noise versus single arm lock.  While I'll have to sacrifice a lot of it, there's oodles more SNR in the Xend now.  The Yend data still has the "gain fixing" extra 16.8dB.

QPD_Xtrans_Ytrans_DarkVsLock_12Dec2014.pdf

The Xend quadrant input counts (before the de-whitening filters) now go up to peak values of about 1,000 at single arm lock.  If (optimistically) the we got full power recycling and the arms got to powers of 300, that would mean we would have 300,000 counts, which is obviously way more than we actually have ADC range for.  Currently, the Yend quadrant input counts go as high as 50, which with arm powers of 300 would give 15,000 counts.  I think I need to bring the Xend gain down to about the level of the Yend, so that we don't saturate at full arm powers.  I can't remember right now - are the ends 14-bit or 16-bit ADCs?  If they're 16-bit, then I can set the gain somewhere between the current X and Y values.

 Finally, I added a section of the 40m's DCC document tree for the QPD whitening:  E1400473, with a page for each end.  Xend = D1400414, Yend = D1400415.

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