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Entry  Wed Jan 30 15:07:32 2019, awade, anchal, HowTo, FSS, Shot noise in FSS loops? 
    Reply  Fri Feb 1 14:59:01 2019, awade, anchal, HowTo, FSS, Noise tallies CTN_FSS_south_blockdiagram.pdf
       Reply  Fri Feb 1 20:20:11 2019, awade, HowTo, FSS, Modifying North FSS EOM path 
          Reply  Tue Feb 5 11:40:02 2019, awade, HowTo, FSS, Modifying South FSS boost 
             Reply  Tue Feb 5 20:39:52 2019, anchal, Summary, FSS, Complete model of TTFSS box with TF and crossover analysis FSS_Modified_Analysis.pdfFSS_Mod_Analysis.zip
                Reply  Wed Feb 6 19:13:06 2019, anchal, Summary, FSS, Complete model of TTFSS box with TF and crossover analysis FSS_Modified_Analysis.pdf
                   Reply  Wed Feb 6 21:30:24 2019, awade, Summary, FSS, Complete model of TTFSS box with TF and crossover analysis EOM_Vrms_FunctionOfLowerFrequencyBoundOfTF.pdfTTFSS_lisomodel_FSS_Mod_Analysis.ipynb.zip
                      Reply  Thu Feb 7 10:33:27 2019, anchal, Summary, FSS, Complete model of TTFSS box with TF and crossover analysis 
Message ID: 2302     Entry time: Fri Feb 1 20:20:11 2019     In reply to: 2301     Reply to this: 2303
Author: awade 
Type: HowTo 
Category: FSS 
Subject: Modifying North FSS EOM path 

I'm implementing the suggested cap and resistor updates in the last post (PSL:2301).  

Changes to FSS board (North 36 MHz field box)

I couldn't find any 100 nF or 33 nF caps in polycarbonate in WB EEshop.  These haven't been manufactured since 2000 and I couldn't find stock at the 40 m either.  Instead I used polypropylene as they have comparable specs (as far as I can tell) and should be good up to ~1 MHz (I think).  If not, its easy enough to fix.

Replacements on D040105-C FSS servo board:

  • C24 was changed from 1 µF polycarbonate cap to a 100 nF polypropylene cap (Digikey 1928-1302-ND an FKP3 series).  This was an exact fit to the PCB board pins.
  • C23 was changed from 1 µF polycarbonate cap to 0.033 µF polypropylene cap (Digikey 1928-1231-ND and FKP2 series).  This wasn't the same size.  I made some slight pin extentions with some resistor lead cutoffs.
  • R19 was changed from 24.9 kΩ to 2 kΩ.  I used a 0805 thin film here.
  • C36 was changed from 3.3 nF to 560 pF to push the pole of U8 from 9.9 kHz out to 58.3 kHz.  I did this after I realized that there was not enough phase margin in the fast (PZT) path at the crossover frequency, it just wasn't stable for fast path gains high enough for a smooth handoff to the EOM.

I also checked the boost that hacked onto U7 of the FSS servo board.  This is a capacitor in series with R29 (5.6 kΩ).  I checked the actual value it is 6.8 nF in the North FSS box.  This combination (5.6 kΩ + 6.8 nF) in the feed back path of U7 gives a pole at 4.18 kHz.  This should be enough to give an increase of gain of 10 dB by 418 Hz and 16.2 dB by 100 Hz.  This is probably more than we need.  It could be shifted down to 1 kHz (28 nF) if we do eventually reach 1 MHz UGF.

New Loop Bandwidth Limits

The loss of gain from changing R19 means that we are getting a factor 12.45 less gain in the EOM path.  I turned the common gain  slider up to max of 10V (+32 dB) and see a UGF 224 kHz.  The loop is stable with boost on and fast gain set to -1.07 V and also with the boost off; the cross over for this configuration of gains is 11.3 kHz. The RMS noise on north PDH error monitor is 19.4 Vrms (just looking on the oscilloscope). It seems pretty stable for now, I'll take a TF later after a few more tweaks of values. Also its late and I want to go home.

Now we have a problem with gain, we need more of it. It looks like that if we could find gain of +10 dB somewhere we could push the UGF out to 593 kHz and have a healthy phase margin of 42 deg. If we can find more gain we can probably go further.  The phase margin gets to 30 deg at 806 kHz so that might be a logical point to stop.     However, there appears to be a little peaking at 750 kHz that we need to watch out for.  Not sure what it is, don't think I've seen it before. We'll have a closer look at it when it becomes a problem, but we might need to reexamine tuning of notches or putting another notch in if the combination of OLG UGF and and boost mean we need to push out this far.

We could collect error signals and actuator signal PSDs and model the optimal ballance and crossover between PZT and EOM, but this will take weeks to build an actuate model. For now its probably faster and easier to see empirically if the EOM can handle the higher gains without running out of actuation range. 

In terms of finding more gain, we can get a bit by boosting do any of the following:

  • the RF signal from the detectors (up to whatever still gives us linear range of mixer without compression)
  • turn up the laser power a little
  • increase the modulation depth (its already at 0.3 rad)
  • change R21 (in D040105-C ) from 10 kΩ to something a little higher (20 kΩ would give us +3 dB)
  • change the C23 + R22 combination to something with a larger capacitance and smaller resistance (i.e. 330 nF + 110 Ω)
  • do same/similar as above with C24+ R24
  • raise the value of R4 in the common path by factor of 10?, here we need to careful that its not going to saturate the subsequent stages


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