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Entry  Thu Dec 16 19:25:27 2010, Zach, Misc, GYRO, Murphy's Law verified, science rejoices hanky.png
    Reply  Fri Dec 17 03:34:55 2010, rana, Misc, GYRO, Murphy's Law verified, science rejoices 
       Reply  Tue Dec 28 14:22:03 2010, Zach, Misc, GYRO, Murphy's Law verified, science rejoices 
Message ID: 1225     Entry time: Thu Dec 16 19:25:27 2010     Reply to this: 1227
Author: Zach 
Type: Misc 
Category: GYRO 
Subject: Murphy's Law verified, science rejoices 

 Earlier this week I planned to do the following things before I went home:

  1. Finish with EOM resonant circuit and install
  2. Reduce primary PDH box gain in order to increase optical gain without instability
  3. Re-optimize primary loop in light of (1) and (2), take new OLTFs
  4. Finalize low-frequency noise-budget generation using second trend and stitching, observe improvement in noise from the above modifications
  5. Work with Alastair to finalize the RFPD board design

With the exception of (5), for which Alastair deserves the bulk of the credit, all attempts to get the above done have blown up in my face. What's worse, I have finally contracted the cold that has been lingering about. It feels irresponsible to leave for vacation without having gotten anything I wanted done, but I just physically don't have it in me. The gyro is currently in an unlockable state and so I have left everything shut off until my return. Below is a quick recap of what happened, with the numbers corresponding to the individual goals above.

  1. I followed Koji and Kiwamu's advice and put a resistor to ground at the input side of the circuit to better match the impedance. I was not 100% sure of the exact impedance value I had gotten using the second 1:16 transformer at the half-tap point, so I empirically switched resistors in until I got the greatest reduction in reflected power (see post to follow). Confusingly, I found that a 39-Ohm resistor did the best job, which doesn't make any sense since this configuration can have a maximum impedance of 39 Ohms, barring any series resistance I don't know about. Even more befuddling is that while the resistor did the trick when put in parallel externally via T-connector, upon its being soldered in the circuit had a broadly qualitatively different response. What gives?
    • Bright side/solution: When I come back, I will choose option 1 instead, which is to just use an extra transformer to match the true impedance of the LC tank to 50 Ohms. This should circumvent all the nasty problems associated with putting an extra resistor in. 
  2. I decided to reduce the gain by roughly an order of magnitude. The variable gain stage gives 5.0 dB per turn, so this increase corresponds to about 4 extra turns, or half the range. I accomplished this by increasing the value of R14 (the resistor to ground on the inverting amplifier of the input stage) from 25 Ohms to 330 Ohms. The feedback resistor is 1 kOhm, so the gain was reduced from 41 to 4. This seemed to work at first, as I was able to turn the gain knob up to just under 5.0 before the thing became unstable, however, I noticed that this modification brought back to life our favorite gain-setting-dependent DC offset. When turning the gain down to 0.0 to best capitalize on the optical gain, the DC level was sitting so far above zero that the AC signal could not go negative.
    • Bright side/solution: These circuits have been bent beyond their job description. We have modified them to the point that we don't know what the hell is going on. We have been talking about making our own dedicated servos, and now is the time that we should think about starting. Alastair and I are now reasonably comfortable with Altium, and the topology of the filter is straightforward enough that we should be able to build our own without too much hassle. We should build in cool things like multiple-stage switchable boosts for low frequency, preferably switchable remotely via relay. These boxes can live in our NIM crate and we can finally have everything controllable from the computer.
  3. Clearly there was nothing to re-optimize, and therefore no reason to take new OLTFs.
    • Bright side/solution: We now have most of the vacuum equipment, and with any luck we will have RFPDs ready to roll by the time we get everything set back up again. We will need to re-characterize both loops again once this is done, so there's not a whole lot lost on this one.
  4. CDS is in shambles. Trends and full frames are owned by different users, we can't pull data via NDS regardless of which user runs the daemon, and it appears that the only way to get inittab to run daqd and nds is with root. Also, it appears that the installations of the various tools (DataViewer, DTT, MEDM, etc.) are incomplete on different machines in a decidedly random way. For example, DataViewer doesn't run on ws2, nor does Foton, but MEDM runs on ws2 and not on fb1. Also, all the aliases are totally screwed up.
    • Bright side/solution: There really isn't a bright side here. We have fixed the previous issues regarding an incorrect default gateway on fb0 preventing NDS requests from outside, and Alastair has replaced a corrupt hard disk, but everything else is truly, truly a mess. I don't know nearly enough about hardly any of this stuff to feel confident about fixing it on my own, though I am glad to help in any way if someone has ideas!!!
  5. This is the triumph of the week. Alastair has worked hard at figuring out how to get Altium to do things that are often uncannily close to what we actually want. We have taken the suggestions and comments on the initial design posted yesterday, and we just need to work on the physical layout of the PCB.

Merry Christmas!


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