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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  Wed Jan 31 22:32:11 2018, gautam, Update, ALS, ALS signal chain + power budget FiberALS_PowerBudget.pdfpreampProposed.pdf
                                        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  Sun Feb 25 00:03:12 2018, gautam, Update, ALS, Daughter board prototyping schematic.pdfdaughterBoard_TF.pdfdaughterBoard_noise.pdfTEK00000.PNGdaughterBoard_noise.pdf
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                      Reply  Thu Jan 25 13:18:41 2018, gautam, Update, ALS, Fiber ALS assay preampProposed.pdf
Message ID: 13655     Entry time: Sun Feb 25 00:03:12 2018     In reply to: 13648     Reply to this: 13657
Author: gautam 
Type: Update 
Category: ALS 
Subject: Daughter board prototyping 

Using one of the prototype PCB boards given to me by Johannes, I put together v1 of this board and tested it. 

Attachment #1 - Schematic with stages grouped by function and labelled. 

Attachment #2 - Measured vs modelled Transfer function.

Attachment #3 - Measured vs modelled noise. Measurement shown only between positive output and ground, the other port is basically the same. I will update this attachment to reflect the expected signal level in comparison to the noise, but suffice it to say that given the measured input referred noise, we will have plenty of SNR between 0.1Hz and 10kHz. The single stage of whitening should also be sufficient to amplify the signal above ADC noise in the same frequency band

Attachment #4 - Positive output as viewed on a fast (300 MHz) scope using a Tektronix x1 voltage probe.

Attachment #5 - Daughter board noise with measured ALS noise overlaid (the gain of x10 on the existing audio pre-amp has been divided out). 

Comments:

  • I may have overlooked the GBW of the OP27 in the design - specifically, the negative feedback is wired for gain x100 at high frequencies, and so the input signal should be filtered above 8MHz/100 ~80kHz. But the LC poles are at ~500kHz. I wonder if the small deviation seen between modelled and masured TFs is reflecting this. Practically, the easier fix is to add a feedback capacitor that rolls off the gain at high frequencies. 300pF WIMA should do the trick, and we have these in stock.
  • I don't understand why the modelled response starts to roll off around 5kHz, even though the poles of the LC filter at the input stage are at 500kHz. This happens because at low frequencies, the 1.5uH inductor is basically a short - so the RC divider at the input of the Op27 has a pole at 1/2/pi/R/C ~5kHz for R=499, C = 68nF.
  • I am not sure what to make of the peaky comb seen in Attachment #3, but I'm pretty sure it's electronic pickup from something. The GPIB adapter power suppy is not to blame. The peaks are 10 Hz spaced.
  • From Attachment #4, I don't suspect any opamp oscillations given that the signal seen is tiny, but I don't know what amplitude is characteristic of an oscillating op amp, so I am not entirely confident about this conclusion. 
  • Initially while thinking about the design, I was trying to think of making the design generic enough that we could use these signals for high-bandwidth ALS control (a.k.a. Fast ALS) but in the current incarnation, no consideration was given to minimizing phase lag at high frequencies. 
  • Putting the PCB board together was more painful than I imagined as the board is configured for 4 single op amps whereas my design requires 5 - so I needed to do some trace cutting surgery. Rather than make 3 more of these, I'm just going to finish the characterization, and if the design looks good, we can get some custom PCBs printed.
  • Power decoupling caps (47nF) are added to all op amp power pins, but is not shown in the schematic.

Given the overall good agreement between model and measurement, I am going to test this with the actual RF beat. For this test, we will need a differential receiving AA board to interface the output of the daughter board with the ADC input

Quote:

Next step is to actually make a prototype of this.

Attachment 1: schematic.pdf  840 kB  | Hide | Hide all
schematic.pdf
Attachment 2: daughterBoard_TF.pdf  230 kB  | Hide | Hide all
daughterBoard_TF.pdf
Attachment 3: daughterBoard_noise.pdf  302 kB  | Hide | Hide all
daughterBoard_noise.pdf
Attachment 4: TEK00000.PNG  30 kB  | Hide | Hide all
TEK00000.PNG
Attachment 5: daughterBoard_noise.pdf  327 kB  Uploaded Tue Feb 27 22:06:19 2018  | Hide | Hide all
daughterBoard_noise.pdf
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