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  2106   Mon Feb 26 14:16:38 2018 awadeMiscSafetyWater leak in the lab

I called facilities.  They say with their new purchase approval process, and lead time on parts, that they expect the repair jobs on piping would start early next week.  

There will be a bit of manual alterations to the through hole coming down into the ceiling so we need to do some dust mitigation.  I will order some more cling wrap and salvage some of the plastic sheeting from the last episode.

Quote:

Checked the piping again this morning.  The water is just a slow drip now. The facilities people put duct tape around the pipe crack and isolated the source of water on the floor above.  Don't think its safe yet to turn the rack back on, given the proximity of the water and the quality of the patch job. 

I called facilities to find out what the status of the job was and the timelines for fixing.  They didn't have the poly pipe in stock and have reordered. The earliest they can get to starting on the job will be Monday.  The guy responsible is away today.  We should call ext 4969 (Caltech plumbing shop) early on Monday to get an update on expected completion time of the job. Until then we should redirect effort into SURF search, noise budgeting, scatter modeling, PID modeling, FSS sub-noise budget etc.

 

  2109   Wed Feb 28 10:20:40 2018 awadeDailyProgressTempCtrlHeater and temperaure sensors in cryo lab

You should check a few things.  Get a 200-300 MHz oscilloscope with a probe and look to see if the circuit has any oscillations.  This should be your first reaction to many problems: looking a little wider than the audio band can often reveal important problems that people miss. We found that the heating elements had some unexpected impedance that made our feed back unstable to the MOSFET at very high frequencies.  The solution there is to put some capacitors over the heater to dampen and maybe in some other places too. We found that very high frequency oscillations actually coupled back to the temperature sensing circuit.  You may want to check to see if you can see any pickup in your temperature sensing here circuit too.

Another thing.  From what I remember of your circuit you are transmitting the signal referenced to a common ground (rather than floated differential signal).  If you're heater is loading up a bunch of current at the table end of a common ground this will generates a potential drop between the power supply/rack and the 0 volt reference of the circuit at the table (see Ohm's law). There is a good table on the American wire gauge wiki page that gives standard resistance for different gauge wires, you can calculated expected potential difference generated by your heater circuit current from this.  Check you grounding situation.  Are you pinning one ADC pin to ground on the Acromag (maybe you shouldn't)? Are you using appropriately chunky wire to establish ground at the table? Are you committing the cardinal sin of having two or more separate paths to ground from the table? Take some time early in your experiment to ensure your grounding network is topologically like a tree rather than a fungal Mycorrhizal network. See attached figure for reference.

A source of wisdom on grounding that rana recommends is Morrison, R. (1998), Grounding and shielding techniques (4th edition), New York: Wiley (link). Craig and I have borrowed out two of the copies, any edition will do so maybe get one that is physically on the shelf or, even better, there is an electronic copy.

 

 

 

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I think I'm seeing a similar problem that y'all were when I use my heater circuit (which is I believe the same as your heater circuit, it's the one Kira and Kevin are using at the 40m). Our temperature readout circuits might be slightly different.

Basically when I have the heater on the board as my readout op amps, I get up to a few tenths of a volt jump in my temperature readout; however, even after moving this circuit to a different board and using a separate power supply, I'm getting about a millivolt shift. This is not good for <1K control. I'm also well within the limits of my power supplies, have voltage regulatorsbefore the OP amps, etc... I will try swapping out the OP amp as you did, but thought it was a pretty weird problem.

 

  2110   Wed Feb 28 11:50:21 2018 awadeMiscSafetyPipe work scheduled for lab Friday March 2nd

The Caltech plumbing shop called to say they will have all the parts they need to start work Friday morning.  There is a bit of masonary work to be done to the down pipe hole.  So there will be some dust.

I have ordered materials from McMaster for wrapping the experiment up again.  I got extra so we will have a cling-wrap-kit® for the WB labs ready to go in the future. I didn't get a tracking recipt, but the order went through around COB on the 26th.  Should be here today or tomorrow.

Quote:

I called facilities.  They say with their new purchase approval process, and lead time on parts, that they expect the repair jobs on piping would start early next week.  

There will be a bit of manual alterations to the through hole coming down into the ceiling so we need to do some dust mitigation.  I will order some more cling wrap and salvage some of the plastic sheeting from the last episode.

 

  2113   Fri Mar 2 19:33:14 2018 awadeMiscPurchasesNew Wiha 2.0-7.0 Nm torque control driver

New Torque Driver

I purchased a new torque driver for use in the CTN lab.  It is the Wiha TorqueVario 2.0-7.0 Nm (model 28655), pictured below. This is the highest value driver in their variable torque driver selection with a range that is appropriate for tightening 1/4-20 bolts on the table.  We already have the much lower range Adjustable TorqueVario 15 - 80 In/Oz (model 28501), but this range is only appropriate for very low torque applications like fastening PBS and mirrors. In the future we might like to get a mid-range driver to cover the whole range.

I also purchased a selection of Phillips and flat head driver blades to go with the driver heads because they were relatively cheap. All the Wiha blades are exchangeable between their torque tools, they will likely come in handy for a range of precision applications.

So How Much Should We Torque?

So far I've only tested the new driver qualitatively. 

The range of torques applied by humans in the lab varies widely and there isn't a lot of (good) advice out there on the optimal value.  Over torquing leads to deformation of the table, this can misalign optics in the short term and that then undergo a very slow relaxation over a long period of time.  In the worst cases the table or opto-mechanical components go past their elastic limit. An under tensioned bolt is obviously bad.  Without a strong rigid connection to the table the optic mount may be free to move in a number of lower frequency modes that otherwise wouldn't be allowed (bad).  Rana has recommended an applied torque of ~5-6 pound-inch (6.8-8.1 Nm), this is supposed to be just below the limit of most aluminum and steal plates before they go from elastic to plastic deformation.

Here is an except from LIGOX chat channel from Rana:

Most of the time, it doesn't matter. You can use whatever seems right to you. However, in situations where precision matters, you have to consider what the requirement on the fastening is: e.g. when clamping a 1/4" thick base to an optical table, we use 1/4-20 screws because that's what the table is tapped for. The screw length should be chosen so as to use all the threads in the table.

But, how much torque should be used?

If too much is used, the aluminum base will be deformed so that it is no longer in the elastic regime. Once there is significant plastic deformation, there will be slow mis-alignment of the mount.

Washers increase the total force which can be used, since it reduces the pressure on the soft aluminum given a fixed force. For the usual set of Thorlabs hardware we have the correct torque is ~5-6 ft. lbs [6.8-8.1 Nm]. Similar numbers can be found for other cases by considering what materials are being used.

Initial Qualitative Test

I tested a few different values of torque on the table for a 1/4-20 bolts (with washers) directly on the table in the ATF lab. I used the south table in the ATF lab as many of the tapped holes in the CTN lab have been damaged from over-torquing and contaminants in the treads.  

A dozen 1/4-20 bolts (with washers) were fastened with identical torque values starting at 2 Nm (see second picture). Between each torque cycle I undid the bolt under test with a regular ball driver to get a feel for force used. From their I incremented the applied torque by 0.5 Nm on each tightening cycling working up to 7 Nm. When undoing a bolt there is a kind of a 'crack'. This is the point at which the fastener goes from vertical contact friction to loose thread-only friction.

I found that the 'soft crack' point was 2.0 - 2.5 Nm.  The transition to 'hard crack' (an audible click) occurs at about 3.5 Nm.  However, interestingly, the variability between 3.5 Nm tightened bolts seemed to higher; 3 out of 12 bolts gave a softish crack.  Its likely that the particulars of washer-table-bolt surfaces may change the crack point.  I found that a torque of 4.0 Nm gave a guaranteed hard crack without seeming qualitatively excessive. The transition between regimes was a rapid one and above 4.0 Nm the friction hold was about the same, giving about the same 'crack'.

I found the recommended 6.8-8.0 Nm was very tight.  A value of 7 Nm required a very strong grip on the driver, this is the kind of torque that might only easily be applied using a T-handle driver or a long allen key. A value of 7 Nm seemed unreasonably high compared to what is usually used, this is at the upper range I've seen in various labs. 

My initial recommendation is for 3.5-4.0 Nm tightening of regular bolts for most mounts.  My usual peronsal choice is for a soft crack at around 2.5 Nm.  

Procuring Fixed Value Drivers For General Lab Use

Wiha sells fixed value torque drivers in increments of 0.5 Nm (see Wiha EasyTorque), these fit the standard blades and are reasonably priced. They also sell fixed value Wing Handles that have a compact profile.  We may want to do some scientifically rigorous tests of various post-fork and base-dogclamp combinations to see what the best objective torque value is.

 

 

  2114   Sat Mar 3 23:25:37 2018 awadeDailyProgressComputersWS3 Down

I just tried to ssh into ws3 only to find it unresponsive.  I was going to check the router but then found that the computer seemed to be off.

On closer inspection the computer seems to have some kind of power issue.  At the moment all I am seeing is a blinking amber power light on the box. One LED indicator is on on the motherboard, otherwise there are no fans or HD spinning. 

I have sequentially pulled the DVD drive, HD and RAM.  Fans won't spin up.  Various forums suggest that its either a motherboard issue or a power supply issue.  Given that we are seeing the basic amber flashing LED on the front panel I would hazard a guess that its not the power supply.

WS3 is a computer that Larry Wallace gave me that had been retired from desktop use.  Don't think this is worth days of diagnosis and, more importantly, days of down time to fix it.  I had made a clone of the computer, but that isn't much use if we need to recover to a completely different computer. For now I am going to pull ws2 from the QIL lab and attempt to get it going as the interface computer in the PSL lab.  All the important medm screens and python scripts were committed to Gitlab so we should be good. WS2 was running an old CentOS (Redhat) operation system, no out of service period.  I will switch out its HD for a fresh 250 Gb and do a fresh install of debian as a start.

Edit Sun Mar 4 00:13:37 2018: Scratch using WS2 as a stop gap machine, it won't boot from USB and the CDROM drive is busted.  We're going to have to use WS1 for now, it already has Debian installed and LIGO tools (I think).

Edit Sun Mar 4 12:48:39 2018: Last night I ended up just moving WS1 into the PSL lab.  I had previously installed Debian and all the LIGO tools (see ATF:2181) which is now come in handy.  I've changed the machine's IP to 10.0.1.34 and we can now SSH remotly using the ussual gateway address and port 22. We may want to reporpose the 'gateway' box as it is not currently in use as a ssh landing point.

  2116   Sun Mar 4 17:33:01 2018 awadeDailyProgressComputersWS1 Up

Workstation is back.  I was able to fully restore medm screens, scripts and noisebudget from Git on WS1.  Good version control is the way to go it seems.

Can't say the same for Criag's Apache noise budget stuff.  I found a SATA to USB converter and am able to mount the WS3 HD directly onto WS1.  The original HD  is intact so all the original stuff is accessible.  We just need to move it into place on the new computer.  I've left the WS3 machine HD plugged in and it is mounted in /media/controls/. 

Quote:

I just tried to ssh into ws3 only to find it unresponsive.  I was going to check the router but then found that the computer seemed to be off.

On closer inspection the computer seems to have some kind of power issue.  At the moment all I am seeing is a blinking amber power light on the box. One LED indicator is on on the motherboard, otherwise there are no fans or HD spinning. 

I have sequentially pulled the DVD drive, HD and RAM.  Fans won't spin up.  Various forums suggest that its either a motherboard issue or a power supply issue.  Given that we are seeing the basic amber flashing LED on the front panel I would hazard a guess that its not the power supply.

WS3 is a computer that Larry Wallace gave me that had been retired from desktop use.  Don't think this is worth days of diagnosis and, more importantly, days of down time to fix it.  I had made a clone of the computer, but that isn't much use if we need to recover to a completely different computer. For now I am going to pull ws2 from the QIL lab and attempt to get it going as the interface computer in the PSL lab.  All the important medm screens and python scripts were committed to Gitlab so we should be good. WS2 was running an old CentOS (Redhat) operation system, no out of service period.  I will switch out its HD for a fresh 250 Gb and do a fresh install of debian as a start.

Edit Sun Mar 4 00:13:37 2018: Scratch using WS2 as a stop gap machine, it won't boot from USB and the CDROM drive is busted.  We're going to have to use WS1 for now, it already has Debian installed and LIGO tools (I think).

Edit Sun Mar 4 12:48:39 2018: Last night I ended up just moving WS1 into the PSL lab.  I had previously installed Debian and all the LIGO tools (see ATF:2181) which is now come in handy.  I've changed the machine's IP to 10.0.1.34 and we can now SSH remotly using the ussual gateway address and port 22. We may want to reporpose the 'gateway' box as it is not currently in use as a ssh landing point.

 

  2122   Wed Mar 7 11:46:07 2018 awadeDailyProgressFSSFSS Fastmon RMS monitors for automatic gain cycling

Maybe add an optional binary engage channel to the *.ini parser (see for example PIDLocker_beta.py).  The default when no channel is given should be 'always on', otherwise it should activate the loop on a given list of soft channels. Running a bunch of scripts in tmux is fine for a while when testing but becomes a pain in the long term.

You can use the function,

def ANDChannels(chanList):  # find boolean AND of a list of binary EPICs chans
    return all([RCPID.read(ii, log=False) for ii in chanList])

to take a list of binary IOC channels and return true if all are state 1.  You can then make rmsMonitor.py 's behavior dependent on the FSS and PMC lock state as well as a medm binary switch.  We want our lockers and controls have a hierarchical structure so they have a sequence of behavior built into the logic of their rules of engagement.  If the parts have a well defined mode of engage and failure the whole system will be much more predictable and stable.  This also avoids the need for a central manager script of all the tasks. We can run all this stuff as daemons under services (in /etc/init) and let Linux do the heavy lifting of keeping it all alive.

On that note, the Marconi2023A_BeatnoteTrack.py script lost the PLL lock last night but failed to exit. Its inability to sense if its locked at zero PLL DC error signal or just way out of range is a problem.  When it does this it fails to drop the North cavity PID loop.  This morning the north cavity heater had railed at 0 watts which will take hours to recover back to stable ~100 MHz offset. We may need an out of loop frequency sensor to track wide variations of frequency.

 

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Very often in our lab our FSS boxes "ring", i.e. the EOM and PZT actuators fight each other for control of the laser frequency, instead of working together.  If the EOM actuator rails, the PZT comes stomping in trying to lock high frequency laser frequency noise, but the EOM comes back in and says, "no, it's my job", but the PZT is all like "obviously you can't do your job cause you're not strong enough", which really only makes the EOM angry, causing rail-to-rail actuation jumps and high, nonlinear noise in our FSS loops.  This is bad for our PLL autolocker, as the high noise hurts the PLL control signal and eventually causes it to lose the beatnote, and obviously bad for the beatnote ASD itself, which we are monitoring at all times on our webpage.
So today I created rmsMonitor.py, a python script which monitors the RMS of the FSS Fastmon voltage, the PZT control signal.  If the Fastmon RMS ever exceeds 250 mV, rmsMonitor.py will call awade's gaincycle.py on the offending FSS box, which brings both Common and Fast Gain values to their lowest setting, then steadily ramps them back up to where they were in a nice way such that ringing won't start up again.  In this way we can automatically eliminate ringing whenever it starts.


rmsMonitor.py lives in ~/Git/cit_ctnlab/ctn_scripts/,  and has two associated .ini files, RMSMonitor_North.ini and RMSMonitor_South.ini.  Inside the .ini files is defined the Fastmon path channel name, i.e. C3:PSL-NCAV_FSS_FASTMON for the north path, and the max rms limit, which is currently 250 mV for both paths.

To run this script on our North path, call
$ python rmsMonitor.py RMSMonitor_North.ini &
Every two seconds, the script should print out something like
C3:PSL-NCAV_FSS_FASTMON rms = 0.0647327284305 V
which is the channel name and the rms calculated for that channel in that two seconds.  Again, if rms is ever above 250 mV, it triggers gaincycle.py for that path and eliminates ringing.


These scripts are perpetually running in tmux sessions named RMSNorth and RMSSouth on acromag1.  To access the north tmux session, log onto acromag1 and run  $ tmux attach -t RMSNorth
These scripts will need to be turned off when debugging persistent FSS ringing.

 

  2135   Thu Mar 15 10:29:04 2018 awadeDailyProgressscatterAddressing 500 Hz scatter pickup

Going back the original issue of scattering, it appears that there is light being back reflected from somewhere in the post PMC path but before the reference cavities.  

Reducing number of optics after the North PMC 

I had installed a bunch of polarization optics before the north 14.75 MHz EOM in an effort to reduce RFAM (see attachement 1).  It looks like stuffing so many optics in such a small space is a bad idea.  You can see weak retro reflected beams from the wave plates and, probably, the PBS as well.  The short propagation distance makes it difficult to angle optics enough to be able to separate them from the main beam laterally to dump.  The EOM can't really be moved because the mode matching solution is a little tight for the available space.

After talking with rana and Craig yesterday it seems like the Pre Mode Cleaner (PMC) should be filtering polarization well enough when locked that the PBS and quarter-wave plate (QWP) are unnecessary. I removed all but the half-wave plate (HWP) and checked the residual polarization on transmission with a diagnostic PBS in place. I found 2 µW of power out of 1.2 mW was remaining when tuned all the way to s-pol.: this is a 1:600 extinction ratio which is about what we would expect from such a beam cube.  This measurement may be biased by the lower limit of the power meter, PBS should be giving 1:1000.  

I moved the PBS to before the PMC to clean up light out of the 21.5 MHz PMC phase modulator. The only optics in the post PMC-> EOM path are now a lens, a steering mirror and a half-wave plate (see attachment #2).  After realigning the PMC cavity and the north refcav I was able to reduce the RFAM to -55 dBm, which is good enough for now.  These slight changes in RFAM level mean that the FSS offset will need some adjustment.  I was unable to see any improvement in the beat spectrum as the beat note had drifted down to 2 MHz.  I turned the heating down a small amount and left it overnight to settle.

I didn't angle the HWP or lens by that much, this shouldn't be necessary because the PMC is a traveling wave cavity.  The elements should be pretty close to normal.  The glass beam dump should be checked to ensure it is not clipping any retro-reflected beams on the rough edge of the glass.

Clamping down the PMC

I never clamped down the PMC. It is just sitting on the ball baring points. This isn't great.

When I realized the tapped holes on the side of the base I went looking for clamps.  They are pictured in attachment #3 but they do not fit.  It turns out there were some issues with the choice of ball bearings on which the PMC sits.  The ball barrings sit over holes so that the PMC when placed will realigned exactly with its previous position on the base.  Antonio had found that the holes drilled for the ball barrings were spec'ed a little too big.  For standard increments of bearings size the closest size fits nicely over the hole but under force they actually slip down into the hole and are almost impossible to get out.  He bought the next ball bearing size up. However, this means that the clamps no longer reach the full height PMC assembly.  The assumed tolerances were made too tight on all these components, the next edition of drawings should allow for some wiggle room.

The drawings should be updated with at least 1-3 mm of range on slot cut side pieces for the clamps so there is room for changes in height due to ball bearing size.  Possibly even more, if future people want to put Viton or Sorbothane dampening into the clamping. The non-tapped holes should also be changed to through-all. Or at least drill with a narrower diameter through-all. This will help future users poke out objects that get stuck in the holes.  

For that matter the design of the clamps seems wrong.  There is a bar that goes over the top that is fixed with a slot-cut piece affixed to each side. This is intuitively wrong as the bolts all go in horizontally when the clamping force needs to be applied is downwards!  It means that the clamps are locking a vertically applied force from the sides; to bolt the PMC down you need to apply force to the bar and tighten the bolts at the same time for two different clamping bars.  The screws should have at least one vertical pair on each clamp so that tension can be applied in the same direction as the clamping force.  

PMC documents on the DCC

For future reference, here is a list of all the PMC documents on the DCC:

Evan's technical note for PMC design considerations: LIGO-T1600071.

I can't find assembly procedures on the DCC.  There was a report from one of Kate Dooley's summer students, LIGO-T1600503, that shows a jig for gluing the PZT. 

 

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Today I buzzed the table and determined there was a strong 500 Hz dirty resonance on the first steering mirror after the PMC. 
This caused me to go around tightening bolts everywhere, including the offending steering mirror and the optics around it.  This did not reduce the resonance.
I tightening the PMC REFL steering mirror as well, and this caused a misalignment onto the PMC REFL PD.  This took me a little while to figure out why the North path refused to lock.  I realigned the PMC REFL steering mirror into the PD.
After I got the North PMC locking again, the North path itself was not locking anymore.  I reranged the autolocker slow volts, but this did not help. 
Turns out the North Trans PD threshold voltage was not high enough.  This is likely because of the bolt tightening, causing some slight misalignment into the North cavity, lowering the overall circulating power in the cavity.  I lowered the autolocker threshold from 1.1 volts to 1.0 volts, and aligned the North Trans PD.  We need to rescan the North cavity to get better alignment/mode matching, but I'm gonna put this off until we replace this offending 500 Hz post-PMC steering mirror.
While I was realigning the Trans PD, I noticed that even touching the trans optics tables causes large ~1Hz oscillations in the trans voltage.  This is definitely exacerbating any scattering problem we have.  Also, the Trans PD output for both paths is "breathing", going up and down with a period of about a minute.  This is bad for our autolocker's threshold.  It's possible that we should build two periscopes for the north and south paths to eliminate these elevated tables which cause coherent oscillations on all trans optics.  We could copy Tara's front periscope design.

 

  2146   Mon Mar 19 13:09:43 2018 awadeMiscElectronics EquipmentPhase Noise for 21.5 MHz Sine Wave

Great, it looks like you've got your setup working.

A few things about eloging, though. More information is almost always better. It would be good to add a bit more about your setup so that people know what you actually did and so you can repeat it if you come back in the future to look at your posts.

Maybe you can add another post with a schematic of your experiment labeled with part numbers, frequencies, power levels etc: everything somebody else would need if they were to do the build they same setup. Elog also has the ability to include latex markup which is handy for posting a few key equations.  For example, there are a few Rigol function generators, I find it helpful in the elogs I make to explicitly include part numbers and also hyperlink those labels to the website/datasheet of those components.  You want to actually explain what you did in some detail; some people use dot points, others write full sentences and paragraphs.  The main thing is you include lots of context and useful information about what happened.

With the plot, it looks ok, but you want to increase the font sizes and include units on the y-axis.  I'm not really sure what measurement you made was. It says its a transfer function but it should be in units that make sense like rad/sqrtHz or Hz/sqrtHz. Craig is good at making plots in python maybe have a chat to him about how to make nice plots.

Quote:

The Phase Detector method was used to measure the phase noise of the 21.5 MHz Sine Wave generated by the RIGOL Waveform Function Generator. Noise measurements were taken using the SR785 across frequencies spanning 0.25 Hz to 102.4 kHz.

 

  2147   Mon Mar 19 13:29:24 2018 awadeDailyProgressOtherCavity Power fluctuations vs Temp Fluctuations

I guess going back to the wall supply would be a stupid step backwards.  We need to fix this leak, its either in the tubing or the air springs themselves.  

Can you check out what the connection to the air springs is and whether this can be connected to the standard clear tubing airlines?  We are using the standard 1/4" tubing (see Newport Pneumatic Isolator Accessories).  I'm not even sure if they are supposed to be permanently hooked up to air, the Newport air springs seem to have a Schrader valve (cars/bike valve) on the side and the description suggest that they only need to be pumped up and leveled once.   They do degrade. If that is the case then we need to assess if these older rubber diaphragms need replacing.

This is moderate priority. Scattering is top of the list.

---

We need to return this cylinder when getting a new one.  It will need to reach the low 100s, then we hit reorder on N2.

 

Quote:

Ever since I attached the vaccan air springs to the cylinder, it has been rapidly losing air pressure.  It is now down to ~1100 psi, where on Thursday it was ~1800 psi.  At this rate we will have to order a new cylinder, and figure out how to make the air springs less leaky, as this is affecting our alignment over time.

For now, I'll relock the cavities, replace the shims, and turn off the air springs as they are causing more harm than good.  We will rely on the floating table until then.

 

  2150   Wed Mar 21 11:07:17 2018 awadeDailyProgressscatterThe Sentinels: Transmission Table Black (Green) Glass Beam Dumps

Which direction are you trying to dump? Into or out-of the can?

The scatter going inward is being deflected rather than trapped. It is still attenutated which is better. That which is reflected off the cavities/window is normally reflected back off the black (green) glass. Would we be better with just two pieces of glass (in a Vee)? Or a double trap with a Vee pointing forward and backward (an X-dump?)

 

Quote:

Scattering is a huge problem in our setup, and we aren't sure where exactly the offending scattering is coming from.  The most basic thing to do is to go through our entire optics table, find all stray beams, dump them, then see what kind of spectrum we're left with.  At that point we can try more advanced techniques, like buzzing and damping resonant optics or upconverting the scatter source out of our band.

Many stray beams are coming directly from our cavities and polluting our transmission table.  Also, some beams are trying to make their way back into the can.  These beams tend to be close to the main beam, making dumping difficult.
To aid with dumping these mutant beams, I have created what I call the Sentinels.  The Sentinels stand guard at the transmission window of the vaccan, daring any puny beams to interfere with the main beam.
Related image

 

 

  2155   Wed Mar 28 11:29:19 2018 awadeNotesscatterHigher quality vaccan windows: 40m stock of wedged windows

I just checked the 40m's stock of wedged, AR coated, optics in the pull out draws.   

It looks like there is about nine CVI W2-LW-1-1025-UV-1064-45P windows: these are 1° wedged and coated on both sides for 45 incident p-pol.  Don't think this is what we want (i.e. 45 degree polarized). Also, 1 inch might be inconveniently small to use in practice.

There is one CVI W2-LW-1-2050-C-1064-0, this is the not UV grade fused silica so should probably not be used. Also we need four.

Everything else is either coated only on one side, the wrong type of glass, wedge or coating.

 

  2158   Thu Mar 29 11:14:52 2018 awadeNotesscatterHigher quality vaccan windows: 40m stock of wedged windows

I guess BK7 is fine, we're not going to be putting high power in.  I just though UV fused silica would be better practice if someone wanted to repurpose the flange in a few years.  Seems like a minor extra cost.

Bottom line is that I don't think we have four identical of anything we can use. Should order 1.5" W2s from CVI or somewhere else?

Quote:

That was unfortunate. But why does BK7 uncompatible with the purpose? We need UV fused silica only for the high power reason, I thought.

 

  2159   Fri Mar 30 17:34:52 2018 awadeDailyProgressOtherSign flip on Laser slow control PID parameters

I've been updating the IOC .db files to include new soft channels that break the refcav shield heaters into a common heating and differential heating component.  This should improve actuation on the cavity length by symmetrizing the temperature actuation.

However, after a few channel renamings and restarts to the IOC process I've found that the cavity auto lockers were no longer achieving lock.  On closer inspection it turns out that the PIDLocker on the laser slow frequency controls was switched to the beta version of the script which has a sign flip on the actuation direction. Autolockers were working fine but the PID was switching on and promptly unlocking the cavities by driving in the wrong direction.  I've flipped the sign of all the laser slow controls Kp, Ki and Kd.  The problem is fixed and cavities are now auto locking themselves again.

  2160   Fri Mar 30 20:12:22 2018 awadeDailyProgressOtherDisabling the air springs on the vac can: Shimming and clamping

Following up on the airspring issues.  Its not clear that suspending our vacuum can on 3 Hz air springs will improving things much in terms of noise hunting for now. As there is a leak and no stable supply of air, the air springs seem more a nuisance for now.  They are varying the alignment enough to change the power transmitted and reflected by about 12% (when using the wall supply of air).  Rana has suggested that it would be much better to have a really rigid connection to the table.

Craig placed metal shims under each of the four legs of the tank.  I have now clamped the tank down very tight using L-shaped pieces (see attachment).  I used a T-driver to get the tension nice and tight.  Alignment of the cavities will not have changed much as the tank was already resting on the shims.  

 

 

Quote:

Considering I can't even lock the North cavity today because of alignment drift, I'd upgrade this to super high priority.

Looks like you need to connect the air springs via this black tubing, and not the clear 1/4'' tubing.  There are 8 of these metal screw clamps on the North side of the table and 5 on the other side.  These are surely the cause of leaking, but I don't have any good ideas for how to eliminate them.  I'll google around for ways to eliminate this black tubing from the setup so we don't have 13 leaky connections.

At this rate, the cylinder will hit low 100's later this week.  This is why I'm disconnecting the air springs and shimming up the vaccan for now. 

Quote:

I guess going back to the wall supply would be a stupid step backwards.  We need to fix this leak, its either in the tubing or the air springs themselves.  

Can you check out what the connection to the air springs is and whether this can be connected to the standard clear tubing airlines?  We are using the standard 1/4" tubing (see Newport Pneumatic Isolator Accessories).  I'm not even sure if they are supposed to be permanently hooked up to air, the Newport air springs seem to have a Schrader valve (cars/bike valve) on the side and the description suggest that they only need to be pumped up and leveled once.   They do degrade. If that is the case then we need to assess if these older rubber diaphragms need replacing.

This is moderate priority. Scattering is top of the list.

---

We need to return this cylinder when getting a new one.  It will need to reach the low 100s, then we hit reorder on N2.

 

Quote:

Ever since I attached the vaccan air springs to the cylinder, it has been rapidly losing air pressure.  It is now down to ~1100 psi, where on Thursday it was ~1800 psi.  At this rate we will have to order a new cylinder, and figure out how to make the air springs less leaky, as this is affecting our alignment over time.

For now, I'll relock the cavities, replace the shims, and turn off the air springs as they are causing more harm than good.  We will rely on the floating table until then.

 

 

 

  2163   Wed Apr 4 12:52:33 2018 awadeSummaryDrawingsDrawings: 2in windows for vacuum can flang and clamp

I've drafted up some new drawings for new view ports into our vacuum can.  We have 8 inch windows at the moment with AR coating and surface quality that could be better.  Given the criticalness of scatter we plan to replace the windows with a set of four 2 inch AR-AR coated windows; smaller windows will also reduce the exposed thermal surfaces to outside, thus improving thermal isolation of the cavity shields from the outside. 

Specifications of view port windows

Planned spec for the windows is 10-5 scratch-dig, lambda/10 flatness and AR of R<=0.25 % with a 30 arc second wedge. I've contacted a number of vendors for quotes. As we can't list prices publicly here I've made a summary page HERE with prices and specs from vendors that responded.

 

Design of flange and retaining ring

To deal with any residual beams from the surfaces of the window we will angle the windows at 3 degrees to normal by insetting them into the flange: centered beams on the optic that are normal the flange face will then be reflected at 3 degrees (52 mrad) which at a distances of roughly 20 cm will be 1 cm offset from main beam, this is plenty of space to fit in a beam dump. This design may present challenges for machining.  I'm pretty sure the groves can't just be milled, they need lathing with certain tool specifications to ensure there are no radially oriented scratches that might compromise the seal. The flange is a bulky piece, we need to ask a machinist what can be done.  The angled inset is the way to go, even if its a little harder to make.

I based these drawings partly off Johannes's design (see Cryo:1456). The particulars of their design meant that I couldn't reuse the parts, but I took cues from the design features (that made things much faster to draft). I rebuilt the flange parts and retaining ring with a more parametric design.  Within the part files dimensions are defined in a way that makes it easy to change wedging angles, groove dimensions and sinking depth of the optic.  Hopefully this will help future users adapt them to their needs quickly.  Johannes also had a good reference for view port designs Abbot & Scace in J. Vac. Sci. Technol. A 28, 573 (2010).  One thing that isn't addressed much in the literature is the extra loading applied on the inner o-rings when 1 atm of pressure is applied (vacuum).  For a 2 in^2 window area. 1 atm is about 20.9 kg of pressure (8.6 lb/in of oring). For 70 duro viton o-rings the additional compression looks to be on order of 7 %.  Provided there enough clearance is left between glass and metal, this shouldn't me much of an additional error: it just something to keep in mind for tolerances of the parts.

Drawings are attached below along with an ipython notebook used for the calculation of various dimensions (this can also be found in a gist on GitHub). A zip contains all the parts, drawings and assemblies.

The basic design criteria are for a 2.0" view port optic, wedge by 30 arc min, that is 9.35 mm on its thickest edge.  Clearance of 3/128" are made around the perimeter and 0.5 mm between the faces and metal of the flange and retaining ring.  The o-ring I selected is #130 which has ID of 1.612" and thickness (toroidal diameter) of 0.103".  With a set compression ratio of 0.72 for the o-ring it is possible to make an o-ring groove just deeper than half the o-ring thickness. An o-ring any smaller will be more than double any possible groove depth, which would be bad as would will pop out during assembly. Dimensions of the o-ring groove were chosen according to rules outlined in Abbot & Scace. I think at a 2" diameter it is a little too small to make dove tale o-ring grooves, so the inner diameter of the groove was made 2% larger than the o-ring so that it will be under a little tension while the unit is assembled.  The outer edge of the o-ring groove is 2 mm from the edge of the optic, this should be enough space for the o-ring to be clear of any edge imperfections. Calculations for all these quantities can be found in the ipython notebook.  

The rest of the dimensions were adjust accordingly to reach the correct clearances, an optic angle of 3 degrees to the front of the flange and leave 2 mm protruding on the lowest side of the optic.  The two windows are oriented about the center line of the 10" flange blank, spaced 3.0" apart.  The angle is currently set to point outwards on the outside  of the tank; this orientation might be wrong for our needs as we want to primarily dump light inside the tank and it would be easier to put the beam dumps not between the two beams.

I need to stop fiddling with this now.  It would be good if someone could look over the drawings and any raise issues. 

 

  2166   Fri Apr 6 15:57:30 2018 awadeDailyProgressOtherTuning up alignement into RefCavs

Since hard clamping down the vaccan, the alignment into the cavities has been suboptimal. 

South path was down to 45% fringe vis and north was down to 14%.  As we are using the trans PDs as reference for autolocker thresholds this is causing the FSS to unlock periodically.

I walked the south periscope so that the reflected DC voltage was V_min, Vmax = (0.292 V, 1.52  V) => Vis = 67%

Similarly the north was improved V_min, Vmax = (0.376V, 1.76V) => Vis 65%.

  2167   Mon Apr 9 20:44:41 2018 awadeDailyProgressTempCtrlBeatnote Stabilization: Configuring heaters to actuate differentially

In an effort to improve the CTN experiment's temperature control I've reconfigured the cavity heaters to operate with differential and common mode heating.  By making the actuation symmetric and elevating both cavities well above the vaccan temperature hopefully this will improve the linearity of the actuator.

The heating wire on the north path is 156.8 Ω and south is 85.6 Ω (don't know why they are different). I've configured an additional channels for the south heater (previously not hooked up).  Units of the new channel are in watts.  Two additional calcout channels were made that set the common heating value (in watts) for both channels and a difference between the heaters (in watts) that ranges from -1.1 W to +1.1 W.  After rebooting the IOC and setting the common and diff heating to produce 0 W on south and 0.7734 W on north I get roughly the same beat note frequency as before.

After the change PID feedback to the differential heater channel, it was possible to use PI feedback to drive the beat note to a set point of 100 MHz but tweaking gain values to kill oscillations was takes a very long time (still not there yet).  I reattempted to implement the relay auto tuning method from before (see PSL:2142). However, I had the same converging cycle limit to zero after a few hours.  I think that the large lag on the actuator/sensor/plant in these temperature tuning situations may rule out finding objective Kp, Ki, Kd values using relay tuning methods: the lag puts us away from the -pi first real axis crossing the nyquist curve and estimates of the plant's critical gain and frequency are bogus.  

Short of making some really smart controls that can anticipate the trajectory of the temperature and make more optimal estimates for the feedback I may need to vent the can and get some low noise platinum RTD sensors on the shields.

  2168   Wed Apr 11 16:06:31 2018 awadeSummaryDrawingsDrawings: 2in windows for vacuum can flang and clamp

Some updates for 2 inch window flanges.

Errors fixed

I noticed a few small errors in the 2 inch window flange design.  I hadn't factored the clearance of the window from the metal (0.5 mm) into the depth of the o-ring grooves, so as to get exactly 0.72 compression when assembled.  Also the clearance on the retaining ring side was wrong because I had computed the angling depth on the lower side of the optic based on the optic dimensions and not on the size of the cut hole with clearance.

To fix these issues and clarify the specified clearance, in the solidworks part itself, I have made the clearance explicit with two additional revolve cuts. One is around the face and around the circumference of the optic. This way the clearance will also be parameterize in the part: this should improve adaptability of the part to other applications. Groove depth is now set at the desired compression ratio and the correct real depth of the final groove is realized by the clearance cuts.

Also I changed the angling of the two windows to point inwards on the outside of the can.  This means that ghost beams will be maximally separated on the inside of the can, making it easier to mount a pair of beam dumps either side of each cavity.

Files uploaded to DCC

I've put the drawings and assembly on DCC for better version tracking. I've attached the solid works folder below in a zip for local reference.

How it fits together

I have yet to finish checking the whole tank assembly.  For now here is a pretty animation showing port and orings.

 

  2169   Thu Apr 12 22:00:13 2018 awadeSummaryTempCtrlSummary of parameters and dimensions for thermal modeling

This is a summary reference post for parameters to do with the thermal surfaces and bodies within the vacuum can. It brings together drawings and computed dimensions so we can begin to make an actuate physical model of the thermal dynamics of our system.

Design goals

At the center of the experiment are a pair of Fabry-Pérot cavities that need to be thermally stable enough to not drift more than 100 Hz in the time it takes to take a PSD of their relative brownian driven fluctuations.  

\Delta T = \frac{\lambda}{2c\alpha} \delta \nu

https://nodus.ligo.caltech.edu:8081/CTN/1874

Overview

[Insert SW cutaway with ballons]

Cavity parameters 

Refcav parameters
Property Value
Rough dimensions ø38.1mm x 36.83 mm (9.52 mm bore through middle)
Mass 112 g
Heat capacity  82.88 J/K
Outward facing surface area 75.5 cm^2
Emissivity rough fused silica 0.75
Emissivity polished fused silica 0.93
Coefficient of thermal expansion 5.5e-7 1/K
Optical frequency temperature shift @ 1064 nm 310 Hz/µK

Cavity cylindrical heat shields

https://nodus.ligo.caltech.edu:8081/CTN/1737

  2170   Thu Apr 12 23:28:18 2018 awadeDailyProgressComputersRestarted fb4 framebuilder

I've made a number of changes to channels over the last week.  Channels haven't been logging for new shield channels and some minor channels I had renamed.  

I used Craig's scrape-and-make script for generating framebuilder .ini files (see PSL:2133). And restarted the daqd process as per PSL:2014. Looks like the new channels are up and logging.

I've been experimenting with different values of P and I in the shield controllers, the beat note seems to be settling to within 100's of kHz of the a set point but the convergence time is >12 hours.  It should be interesting it be able to see this data.

  2171   Fri Apr 13 11:34:09 2018 awadeSummaryDrawingsDrawings: 2in windows for vacuum can flang and clamp, steve's comments

I discussed the 2-in window flange design with Steve.  He had a couple of suggested changes.  I'm adding them here for reference as I update the drawings.

He says the angling of the optic into the main flange is fine, 8" 40m flanges have this design.

Changes:

  • The retaining ring should be made of Delrin, this will be more forgiving on the optic when at atmosphere if there happens to be any contact points.  Also easier/cheaper to machine.  The retaining ring is only there to hold the optic in place when not under vacuum;
  • The six #8-32 holding screws should be changed to four #10-24 s.  Orient these equidistant and with lower edge parallel to the table. No helicoil.;
  • Assembly drawing says o-ring part number #2-130 but description is is #2-223 (which is the next size class up). Choose one. The both are good in terms of groove size but maybe choose the smaller one;
  • Include screw part in assembly table;
  • It would be best practice to include a Teflon gasket on the flange side of the assembly between window and metal. Probably a thickness of 0.090".  Even if there is still an air gap left in the design dimensions. If the tolerances are a bit off then having a soft plastic surface is an good idea, it gives the optic something to rest against that won't apply hard localized stress points;
  • Place centering points on assembly drawing of the front face for two windows. Also, add center line on Detail B view; and
  • The two retaining rings are too close together.  Move spacing from 1.5" to 1.662".  This will mean the beams are not centered on the windows but will give some more space between the retaining rings.

Many of the finishing and tolerance parameters don't matter so much.  Over toleranced parts will cost more.  The only place where this might need to be careful is in the spec of the oring grove where vacuum is actually being held back, the 16 Ra in the drawing is fine.

Marco rubber and plastics have a good summary page for best practice design parameters, see: O-Ring Groove Design Directory.

  2172   Mon Apr 16 16:23:09 2018 awadeDailyProgressscatterBeam size on trans BN detector

I thought I'd have a look at how big the beam is on the current 1811 New Focus detector. Over focusing here might be a source of scatter so this is a number we should probably know.

Razor blade measurement of beam on NF1811 Trans BN detector

I borrowed one of the translation mounts mounted with razor blades from the 40m and did a quick measurement this afternoon.  

Because of the tightness of space on the transmission beat breadboard and the shape of the mount, the closest I could get the blade to the PD was about 1.0 cm.  I took a series of measurements cutting the beam and noting the transmitted DC power (in units of Volts). 

# Data: vertical sweep of razor blade 1 cm in front of post cav BN detector
ypos = np.array([6.,7.,8.,9.,10.,11.,12.,13.,14.,15.,16.,17.,18.,19.,20.]) / 1000. *25.4e-3  # In units of 1/1000s of inch converted to [m]
yPDVolt = np.array([1.74,1.86,2.64,5.10,12.9,28.2,53.4,82.8,112,132,143,148,149,150,150])  # [mV]

I fitted the integral of the Gaussian profile and plotted (see plot below).  This is a quick diagnostic measurement. Iused least squares fit, so no error analyses. Here are the fitted values:

Fitted beam center relative to zero of measurement 0.3240 mm
Fitted peak power 148.2308 mV
Fitted detector dark DC reading 1.6333 mV
Fitted beam width wz 97.3314 um

Time to make a switch?

This beam is quite small although the NF1811 detector diameter is only 0.3 mm.  Not sure how scatter scales with beam size here, is there a good reference I can look up on this?

Now might be a good time to switch to Koji's new PD.  I've managed to stabilize the beat note to 20 MHz it seems to stay within a <1 kHz (3.2 µK) range over a periods of sometime more than 6 hours.  Although, it can take 12 hours to settle down over night after a large disturbance. 

  2173   Tue Apr 17 22:11:34 2018 awadeDailyProgressBEATSome final measurments before switching out NF1811

Note: I tried to attach the plot as a pdf but there is something wrong with backend processing this particular pdf and I'm getting a 502 error.  Have attached as a zip and a png

I thought it would be a good idea to document the current state of the BN spectrum before switching out the NF1811 post cavity BN detector for Koji's newly modified resonant detector.  The new detector has an area of 2 mm x 2 mm, much bigger than the 0.3 mm x 0.3 mm of the current configuration.  The larger PD means we can increase the beam size and hopefully reduce the scatter in the vicinity of the beat setup.

To compare apples with apples, I refloated the table. Also, I turned off the hepa fans around the  lab.   The current BN is 26 MHz and PID locked using the pre-cavity BN detector (Kp = -0.005, Ki = -0.000001).  It still hadn't settled from some disturbances earlier in the day, so was oscillating around ±200 kHz range about the set point. I didn't have time to wait, so I took the measurement with Marconi slope set to 10 kHz.  PLL bandwidth was ~16 kHz with SR560 gain of 100 and Marconi on slope 10 kHz/V.  In this range previous benchmarks of Marconi noise (see PSL:1588) would suggest that the PLL noise limit is 30 mHz/rtHz.  Therefore, PLL shouldn't be the limiting noise at the moment, notwithstanding PD dark noise (which isn't really quantified in the noise budget).

When I took the first measurement I found that the some of the scatter features had been improved since the last measurement (see attached figure razor blade in trace).  The noise floor seemed to be around 0.2 Hz/rtHz with a roll up at lower frequencies.  I realized, however, that I had left the razor blade profiler from the previous day's measurement (PSL:2172) in place, just above the beam.  I retook the measurement (see figure razor blade out trace).  There were a number of peaks that popped up broadly around 70 Hz, 180 Hz and 400 Hz.  Its likely that the razor blade was deflecting and dumping some of the light that has been causing these bumps in our spectrum.

I was a little suspicious of how flat the spectrum was above the characteristic scatter hump (up to 200 Hz).  In three separate measurements I blocked the beam from the BN detector, 50 Ω terminated the PLL at the BN input and then 50 Ω terminated at the ADC input .  These traces are also included in the figure below.  It shows that the apparent limit to sensitivity is the dark noise of the NF1811 photodetector.  

It should be noted that the power has been attenuated onto the NF1811 from ~ 1 mW by 3.0 OD neutral density filter down to 1 µW.  Any power much above this generates harmonics, that we are trying to avoid. So it seems that with improved dumping around the NF1811 detector we can reduce a number of mechanical resonances scatter peaks but that there is a limit to the amount of signal to noise because we are likely to saturate the RF AC path stages inside the NF1811.  Im not sure if this is the right way to calculated but dark noise (NEP) is 2.5 pW/rtHz with 0.75 A/W respositivity and 40kΩ gain PD gain the output noise of the detector is 75 nV/rtHz, For a 16 kHz loop bandwidth (preamp gain 100, VCO slope 10 kHz) the total gain, not including the mixer, is 1e6 which would place the noise floor at about 0.1 Hz/rtHz.  I need to work through exactly the right way to calculate this that treats the noise in a physically correct way at the mixer. 

So it seems that this measurement is getting closer to our previous best measurements. Data and plotting is attached in a zip below as well as being committed to the git.ligo.org ctn_labdata repository.

  2175   Sun Apr 22 17:46:55 2018 awadeSummaryBEATCompensation for MAX4107 at G=4.5

For reference I have attached original schematic to this post.  Koji sent me the link to the original document that can be found here: https://labcit.ligo.caltech.edu/~rana/dale/Length_Sensing_and_Control/LSC_Photodiode/Version_B/D980454-01.pdf
 

This version of the document includes page 1 with the pinouts for the power connector.

Quote:

Performance of the modified photodetector unit:

Performance:

- Resonant frequency f_res = 26.0MHz
- Transimpedance gain at 26MHz is 1.27kOhm (560Ohm resistance of the resonant circuit x G=4.5 x 1/2 by 50Ohm termination)
- Input referred curent noise = 9pA/rtHz
- Shotnoise intercept current is 0.24mA

Remarks:

- There is a gain peaking at 280MHz as explained in the prev elog. If one does not like this, remove 700Ohm resistor from the max4107 stage. It will increase the amplifier gain from 4.5 to 5, and the gain peaking is reduced.

- The transimpedance gain might be still too high. Then, a shunt resistor at the location of R24 can be added to the resonant circuit. This way, the shunt resistance is seen only from the RF path. Of course, lowering the signal level has to be paid by the increase of the input referred current noise level.

 

  2176   Sun Apr 22 22:00:15 2018 awadeDailyProgressscatterSwitching out NF1811 detector for KA25MHz

Rather than beating the NF1811 dead horse any more I've switched it out for the new KA 26 MHz detector.  

Installing detector and initial test

I found that after removing the focusing lens that was previously the last element before the BN detector, the beam size was roughly 300 µm (radius) about where the PD needed to go.  Here the photodiode size is 2 mm diameter. At this size the beam fits in about 1/3 of the diameter of the new detector area, so this looks like a perfect fit.  As a bonus we have one less optic in this critical part of the optical path.  

The new detector was mounted at 3" on two 1" diameter posts.  There are bolt holes at 1" and 2" spacing on the base of the detector but I didn't have a more solid base in stock.  This mounting should be good enough for now, but can be improved on.

The the incident beam was angled at 30 deg and when centered gave a DC output voltage of about 115 mV from a DC power of 170 µW.  This is about right for the DC path where from Koji's schematic (see PSL:2162) the DC path has transresistance of 10 Ω followed by gain of 101 (total G=1010): we should expect 129 mV, the small discrepancy could be my sloppy power measurement.  This power is down from the usual value of about 800 µW mostly because I refloated the table and didn't realign the refcav input beams. Hopefully I'll get to fixing that issue tomorrow.

The activity on the table unlocked the cavities a few times which caused the beat note PID to kick the frequency around quite a bit.  I was unable to relock the PLL tonight but the peak power as the beat note slewed across 26 MHz was about 55.2 mVrms (-12.2 dBm).  We should expect that with about 85 µW in from each path, 100% overlap, 0.75 A/W responsivity, and 1.27 kΩ gain that the beat note would be on order -2.8dBm (0.162 mVrms) in this case.  I'd say there is some optimizing to do with alignment.  Could have also missed the exact point where the BN crossed over 26 MHz.

Its going to take all night for the BN frequency PID to settle (bring on the intelligent NL controls). This will have to wait till tomorrow.

Max beat note power permissible ?

Not sure what the largest permissible beat note power can be here.  Koji went with a MAX4107 which has a slew rate of  500 V/µs. There were comments from in previous posts about slew rate (see PSL:2161). What is a good rule of thumb for slew rate of signal vs the rated response of an op amp in situations like this? 

Edit Mon Apr 23 13:47:34 2018 (awade): added detector diameter.

Quote:

no This beam is WAY too big for the PD. If the beam radius (wz) is 100 microns and the PD active area diameter is 300 microns, than you're always scattering a lot of beam off of the metal of the can. For new focus 1811, the beam radius should be ~30-50 microns.

 

  2178   Wed Apr 25 12:25:09 2018 awadeDailyProgressBEATBeam Profiling Beatnote detector

Please don't post plots in png, vector graphics only, preferably pdf with the correct transparency in the background. Here a note on plotting that summarizes some common sins: ATF:2137 

Also SI units only.  Sometimes technical drawings and other commercial technical documents and quotes are in limey units but we don't use them in the lab.

I can't really tell what is going on because of the weird units, but it looks like there isn't enough propagation distance for any meaningful change in the beam size.  

You can make a prediction of the expected beam waist size from the cavity waist (~180 µm) and by measuring the beam propagation path and taking into account the lens at output of the vacuum can. By propagating the Gaussian beam through the lens and along the beam path of the beat setup on the output you can make some predicted beam radius to compare to your measurements (in SI units, of course).  

 

 

Quote:

Today, we did the beam profiling for the beatnote detector just before the photodiode. I have attached the data taken. The z values mentioned are from a point which is 2.1 inch away from a marked line on the stage.

However, the analysis concludes that either the beam radius changes too slowly to be profiled properly with given method of measurement or something else is wrong. Attaching the the z vs w(z) plot from this data and few fit plots.

 

 

  2183   Sun May 13 21:18:55 2018 awadeDailyProgressBEATReinstalling beam splitter and beat detector

I reinstalled the beam splitter, steering mirror and PD in the transmission path.  We have the beam profiling measurement and can reposition the detector more optimally in the near future.  But for now we need to get back to the scatter hunt: specifically the 500 Hz feature that haunts the area directly around the north path pre-mode cleaner (PMC).  

Some of the beam dumps were removed as they weren't bolted down or were in the way of the beam profiling (sorry Craig).

A few things to note:

  • I have now permanently removed the black cube steering mirror mount from the beat setup.  There are visable damage marks on the mirror that make it a dubious choice.  Also the mount just has too much close proximity surfaces on the reflection side that can be potential scatter points.  
  • I replaced the south path steering mirror (previously the black cube beam mount) with a Newport 10Q20HE.1 mirror.  The surface specs are ok 10-5 scratch dig and better than lambda/10.  Its optimized to be over 99% on both polarizations but not technically ultra good HR. We're out of the really good HR reflectors so I might be time to buy some more
  • After a bit of checking around I've concluded that the first beam splitters directly after the cavity are actually 50:50 splitters optimized for 45 deg on s-pol. The reflection is used to generate the beat and transmission of these splitters is used for ISS trans PDs and CCD cameras.  There is a 99% 45 s-pol splitter after the 50:50 splitters that picks most of the light off for the ISS PDs.  Its seems like we are wasting a lot of light going to the ISS here and should actually be using p-pol to get critical coupling into the InGaAs diodes.
  • We're still using two separate raised breadboards here in this optical setup.  I couldn't find any larger 3' by 1' boards in the Bridge West labs, might need to make a purchase to get this together all on one board.
  • I looks like John Martin will be using the ISS from the ATF lab in his SURF project.  I need to get onto getting more stuff fabricated so we can build the ISS better. Maybe a good task for Anchal would be to make a SolidWorks assembly of the new board to see how it will fit together.
  • There are still a few 1/4" adjustble hight mounts in the ISS part of the board.  These should be replaced with 1" mounts as we now have them in stock.
  2185   Tue May 15 14:24:25 2018 awadeDailyProgressBEATReinstalling beam splitter and beat detector

When I orient the beam into p-pol I noticed that the beam splitters give a lot of secondary reflections.  All these mirrors need to be switched out for p-pol optimized splitters and steering mirrors. 

We'll try to get all these beam dumps back in and bolted down soon.

Quote:

If I recall correctly, I placed those beam dumps everywhere because the transmission table ghost beams had gotten out of control.  Particularly bad was out of the cavity, it seems you have left those beam dumps there but I would double check their status.  There was also reflection from the first lenses back into the cavity that I tried to have the post-cav beam dumps block, unclear how successful I was. 

Also bad was reflection from the ND filter we had on our pre-RFPD lens, it seems like you have removed this lens which is great.  Also, there was some backscatter from the third camera I had installed at the other end of the recombination BS that unites the North and South path light, seems like that is gone as well, you might dump that light if you aren't already. 

Finally, the scatter from the IR cameras was like a diffraction grating pattern that went everywhere, it might be prudent to place those black glass beam dumps with the holes in front of them.

  2186   Tue May 15 14:29:47 2018 awadeDailyProgressTempCtrlBeat drift over the last few weeks

We haven't been paying attention to the beat frequency or its control in the last few weeks.  Yesterday it was apparent that the beat had drifted to ~2 GHz.  When I tuned the differential heating to ~0.42 W with a common offset of 0.56831 W I found that this had almost no effect on the beat note.

Initially I thought maybe the shield heater driver box was broken.  On closer inspection it seems that the Acromag IOC channels had been moved to a different output channel and the IOC process was only very recently reset to enact those changes.  

I've relabeled the cables and BNC ports to make it clear which components should plug into which.  I've set the differential heating to 0.422991 W and both lasers seem to be tracking in the right direction towards a 26 MHz beat note now.  It will take a few more hours to get close enough to stabilized. Currently at 250 MHz.

  2190   Thu May 17 10:20:04 2018 awadeDailyProgressLaserPower drop south laser

There has been a distinct drop in laser power in the south path since yesterday.

It looks like the drop in power happened between GPS* 1209950712 s and 1209966050 s (18:24:54 and 22:40:32).  This was in the lab when nobody was there.

I've been tuning the temperature on the cavities to bring the beatnote into range and the current south laser slow voltage is -6.2759 V when the cavity is locked -- as an aside cavity heaters are set to 0.455 W diff heating and 0.56831 W common heating.   This is about where it was before and tuning the laser slow temperature around doesn't seem to get back to the original power levels as reported by the reflection and transmission PDs.  So it seems we are not in a mode hopping region.  I've attached a dataview screen shot showing that the prior to the drop the south trans PD was giving 4.25 V and the reflected 1.094 V (when locked).  After the change in power (after some unlock time for the cavities) the south trans PD was 3.11 V  and south refl PD was 0.488 V (when locked).

The points at which the cavity was unlocked show reflected power dropped from 3.81 V to 2.56 V.  This doesn't look great and its not clear what is going on with the laser.

For reference the current power going into each cavity is 3.1 mW for the north and 2.02 mW for the south.  Visibility is good on the south at 62% (Refl Vmin=0.592, Vmax =2.55  V)

Not clear what happen here, there is no mode cleaner or ISS applied in the south at the moment so seems like the only source of a power change would be a misalignment or a change in the laser.  Nobody has been in the lab since last night and looking around with the IR viewer there is no apparent clipping.

 

For now I think 3.1 mW for the north is a little bit too high.  I've turned down north power to 2 mW to match the south and we will watch the changes in power over the next few days in both lasers.

 

* Note: the GPS time on fb4 has now drifted by 9 days, so not clear if this is correct time, it is certainly in the early hours of this morning.

  2192   Mon May 28 17:36:58 2018 awadeDailyProgressFSSFixing FSS binary under voltage issue

Autolockers were not catching the resonances again. It turns out I had unplugged the excitation to the FSS acromag controller box that engages the binary channels.  It had been plugged back in but often fails to activate the logic because it is still being powered from a 5 V plug pack.

Decided it was time to make the switch to 9V. I installed some voltage dividers to set the excitation out to the FSS interfaces to 4.9 V with a low voltage of 0.66 V maximum (unpowered).  See PSL:2058 for the wiring, I used option D with R1 = 820 Ω  and R2 = 680 Ω.  The new wiring was tested to check the voltages were right, so I don't fry the FSS interface box again.  I also made some changes to the soft binary channels in the acromag IOC: the front medm panel Test1 and Test2 switches are now flipped (NOT operation) so they make sense from the users point of view.  Before the logic was inverted so turning TEST1 off actually activated this path in the circuit.

I also noted down the channels off the ADC card that picks off the monitors from the FSS D25 connector.  This should reduce the number of front panel BNCs as it can all be routed inside the Acromag interface controller box. These have remapped from Aidan's acromag crate into the FSS interface controller box.

  2193   Thu May 31 14:17:42 2018 awadeDailyProgressFSSFixing FSS binary under voltage issue

After switching the laser fast monitor channel from the front panel FASTMON pin to the FASTM_P/FASTM_N pins I found that the PID controllers that use this channel to adjust the laser slow frequency became unstable.  After turning down the gain the oscillations of laser temperature were very difficult to tune out.

It looks like the issue is with the fact that the FSS interface board (LIGO-D040423) low passes the monitor signal with a 0.8 Hz LP filter (200kΩ wt 1 µF cap).   The narrowed bandwidth of this 'sensor' in the PID loop limits the bandwidth of the total OLG.  We could turn down the P and up the I a little bit, but this seems less good than switching out some resistors.  

I replaced four 100 kΩ resistors R99, R100, R116 and R117 with 510 Ω resistors.  This brings the LP filter on the Fast monitor channels and mixer monitor channels up to 156 Hz: the new frequency is well above the ADC 10 Hz sampling rate but should still filter very high frequencies that are not of interest to the slow loop. I'm not sure what the ideal cut off point should be wrt digitizing signal and the loop, but this seems like a good first guess.

These modifications were made to North and South FSS interfaces boards (LIGO-D040423), serial 2010:005 and 2010:00? respectively. These changes are logged in the wiki page and with a label on the box back to this post. 

Quote:

Autolockers were not catching the resonances again. It turns out I had unplugged the excitation to the FSS acromag controller box that engages the binary channels.  It had been plugged back in but often fails to activate the logic because it is still being powered from a 5 V plug pack.

Decided it was time to make the switch to 9V. I installed some voltage dividers to set the excitation out to the FSS interfaces to 4.9 V with a low voltage of 0.66 V maximum (unpowered).  See PSL:2058 for the wiring, I used option D with R1 = 820 Ω  and R2 = 680 Ω.  The new wiring was tested to check the voltages were right, so I don't fry the FSS interface box again.  I also made some changes to the soft binary channels in the acromag IOC: the front medm panel Test1 and Test2 switches are now flipped (NOT operation) so they make sense from the users point of view.  Before the logic was inverted so turning TEST1 off actually activated this path in the circuit.

I also noted down the channels off the ADC card that picks off the monitors from the FSS D25 connector.  This should reduce the number of front panel BNCs as it can all be routed inside the Acromag interface controller box. These have remapped from Aidan's acromag crate into the FSS interface controller box.

 

  2197   Tue Jun 5 16:35:54 2018 awadeDailyProgressFSSMonitor channels overdrawing current in output buffers

After switching the FSS monitor channels to the Interface Controller box (with monitor read backs coming through the D25 connector) I found that the returned monitor signals were not working as expected. 

The XT1221 card installed in the interface controller box only gave positive voltages that latched near zero.  I think there is an issue with this particular unit. I've switch it out for a brand new card and have labeled in appropriately.

After switching I found that there were two further issues. The outputs were noisy and railed and latched to zero when driven above about 4.0 V.  The first of these issues was a grounding one: the Acromag XT1221 inputs are differential and need to be grounded on one side for differential inputs. I fixed this and gave all the channels a common ground referenced to the common ground of the rack.  After this the signal was a lot less noisy but railed and then dropped to zero when driven above 4.0 V.  Turns out the final buffer stage of the signals feed back through the D25 cable only has 50 Ω resistance in series (with the return pin giving a further 50 Ω).  It seems that the buffer is being overdrawn in current.  I switched out R101, R105, R118, R130 50 Ω for 1 kΩ.

In addition to the above changes I discovered last night that I had gotten the polarity of the wiring wrong in the acromag inputs.  The positive input was being sinked to ground, which is kind of bad and I don't really know how I was seeing any signal at all.  I've fixed this and now see the full range of voltages with very little noise. 

I'm not sure about the choice of 1 kΩ in series with the output (unity gain) buffer.  The ADCs are high impedance at the intput (100 kΩ) so it should be fine.  It does mean that there isn't a situation, say a short, where the output stage buffer will be drawn more than 15 mA.

Quote:

After switching the laser fast monitor channel from the front panel FASTMON pin to the FASTM_P/FASTM_N pins I found that the PID controllers that use this channel to adjust the laser slow frequency became unstable.  After turning down the gain the oscillations of laser temperature were very difficult to tune out.

It looks like the issue is with the fact that the FSS interface board (LIGO-D040423) low passes the monitor signal with a 0.8 Hz LP filter (200kΩ wt 1 µF cap).   The narrowed bandwidth of this 'sensor' in the PID loop limits the bandwidth of the total OLG.  We could turn down the P and up the I a little bit, but this seems less good than switching out some resistors.  

I replaced four 100 kΩ resistors R99, R100, R116 and R117 with 510 Ω resistors.  This brings the LP filter on the Fast monitor channels and mixer monitor channels up to 156 Hz: the new frequency is well above the ADC 10 Hz sampling rate but should still filter very high frequencies that are not of interest to the slow loop. I'm not sure what the ideal cut off point should be wrt digitizing signal and the loop, but this seems like a good first guess.

These modifications were made to North and South FSS interfaces boards (LIGO-D040423), serial 2010:005 and 2010:00? respectively. These changes are logged in the wiki page and with a label on the box back to this post. 

Quote:

Autolockers were not catching the resonances again. It turns out I had unplugged the excitation to the FSS acromag controller box that engages the binary channels.  It had been plugged back in but often fails to activate the logic because it is still being powered from a 5 V plug pack.

Decided it was time to make the switch to 9V. I installed some voltage dividers to set the excitation out to the FSS interfaces to 4.9 V with a low voltage of 0.66 V maximum (unpowered).  See PSL:2058 for the wiring, I used option D with R1 = 820 Ω  and R2 = 680 Ω.  The new wiring was tested to check the voltages were right, so I don't fry the FSS interface box again.  I also made some changes to the soft binary channels in the acromag IOC: the front medm panel Test1 and Test2 switches are now flipped (NOT operation) so they make sense from the users point of view.  Before the logic was inverted so turning TEST1 off actually activated this path in the circuit.

I also noted down the channels off the ADC card that picks off the monitors from the FSS D25 connector.  This should reduce the number of front panel BNCs as it can all be routed inside the Acromag interface controller box. These have remapped from Aidan's acromag crate into the FSS interface controller box.

 

 

  2200   Thu Jun 7 17:18:03 2018 awadeDailyProgressComputersfb4 framebuilder downtime and time slippage

Rebooted the framebuilder fb4 on Tuesday May 29.  I didn't realize that it had hung on reboot.  There was a period of 24 hour after this in which no data was logged for the lab.

Also the framebuilder clock has reset back to 1980.  I can't really find documentation on the wiki or elogs on how this was configured sans ADCs.  I guess we'll just have to go Back to The Future and just use 80's dates for now. 

  2208   Fri Jun 22 00:35:31 2018 awadeHowToComputersRunning parallel tasks on Caltech LIGO cluster

Shruti and I are running various training routines for the machine learning and non-linear controls.  It can be hard to guess the best learning rates, random action injection rates and other hyperparameters of the NN and tensorflow optimizations.  Although the best approach is to work intuitively on simple examples and then scale up, the optimization and rates of learning can be a little opaque.  At some stage we will want to throw a bunch of computing power at systematically narrowing down what works and what doesn't.

We basically want to spin up a bunch of training trials to test a range of hyperparameters without having to wait a full day for turn around for each iteration through a list of values. Running tensorflow based training on GPU might offer speedup on each step but won't necessarily help if it isn't a well parallelized problem. Its not clear to me that, for instance, the baselines deepq minibatching will work faster if we simultaneously draw samples from the buffer and do the gradient decent in parallel with between-graph replication.  At the end of each training episode the outcomes of each separate minibatch gradients have to be combined (that seems non-trivial) and then redistributed across the GPU (which sounds like it will have some hefty overhead as we scale up).  Managing this kind of parallelizing seems too far down the rabbit-hole of optimization science for our investigations.

I've been poking the people over at the jupyterhub LIGO chat channel about running parallelized clusters from notebooks.  LIGO is now running python notebooks on the LDG at http://jupyter.ligo.caltech.edu (and test server http://jupyter2.ligo.caltech.edu).  These can now launch a cluster of n nodes directly from the jupyter gui and we can use the ipyparallel python module to run parallel tasks directly from jupyter.  The only problem is that it ships with a generic virtualenv for the python3 kernel that doesn't include our gym or baselines environments from OpenAI.  We've also made modifications to these packages making them even more propritory.  Furthurmore, there is a problem with ipyparallel clusters, we've found that they won't launch worker engines unless the version of python is exactly the same. The juputer notebook kernals that we are using are python 3.5.4 and the workers are something like python 3.6

As a workaround we can launch our own ipcluster cluster on the ldas-pcdev14 headnode (or ldas-pcdev5) and connect easly directly from jupyter notebooks. ipyparallel manages all of the scheduling and we can launch over 20 learning runs simultaneously and/or schedule a longer list to run. This is relativlty easy to do and doesn't involve much hackery. 

I've got this working from within a python notebook (attached) and have documented the steps needed to get it running.  The actual worker nodes work just a little slower (15% slower maybe) than our Macbook Pros.  The advanage is that we now have more scope to make a bunch of parrallel trials and to detach from those instances.

Edit (awade) Sun Jun 24 00:24:20 2018: fixed issue where tensorflow graph is kept somehow as a global variable by baselines.

 

  2212   Sat Jul 21 14:53:16 2018 awadeDailyProgressPMCNew modifications to the PMC north board

This circuit just doesn't do what it says it should do.  Need to inject waveform at FP1test and probe at each stage.  Then compare against LISO model.  If something is busted we need to know at which point we are getting this extra zero in the response. There is a lot not great about this particular board but it should just have a flat response above 488 Hz.

I don't think AD602 is in the LISO library.  Should be able to add it as some kind of hack op amp with fixed gain, 100nV/rtHz of noise (with 10 Hz corner) and some appropriate current noise with corner of 1 kHz.  Maybe check the AD602 datasheet.

We need to clean this up this week or do something drastic like replacing electronics with minicircuits or removing MC altogether.  For now we need to move onto solving bigger problems like the ISS, thermal stability, PLL readout noise 

Also, on the ISS do you now have a prototype working servo circuit and photodetector?

Quote:

We earlier found that the intended LPF isn't working so we thought of this external LPF idea. So I checked today the PZT input with LCR meter to see if it is in good condition or not. It gave C = 406.4nF, L=60.6 mH and R = 31kOhm. From the spec sheet, the C value looks 20% below the rated value but the spec value has uncertainity of +- 15%, so maybe our PZT is still good.

With the measured values, I calculated again (fitted using LISO) what good value of output resistor would make it closest to 10Hz pole. The value came out to be 37.1k Ohm. I have replaced this output resistor with 39k Ohm now. I'm attaching updated schematic for future reference.

keywords for search: PMC North Driver Board Schematic

Quote:

should probably put back the 30k resistor. It makes a low pass filter with the PZT capacitance (not the cable capacitance). This is to prevent shorting of the HV drive.

 

 

  2223   Wed Aug 1 17:31:19 2018 awadeDailyProgressISSFinal Design for ISS Servo v3.2

AD743 is discontinued.  We have a few in stock, and there are still a few SOIC-16 version that you can buy. 

But better to design with a op amp that is replaceable into the future.

Quote:

I checked with some replacements of opamp at stage 1 to reduce the noise due to current noise. It seems AD743 is the best balanced opamp for this location. Attached is the circuit schematic of this final version v3.2 , with LISO analysis and also the analysis done to reach to this opamp.

 

  2231   Sat Sep 8 17:34:54 2018 awadeMiscComputersWS1 CPU fan dead

WS1, the main computer in the PSL lab died last night.  On reboot bios screen says that CPU fan died.  

I opened the computer up and had a look.  Bearings on main CPU fan were a bit stiff.  I wiggled them a bit and it now spins, with some noise, when booted.  

I'll order a replacement CPU fan KDE1209PTVX 12V, 7.0 W and replace pronto.

Main CPU fan
Main CPU fan, bearings nearing end of life.

---

Also there was no backup of this computer.  Almost all the stuff is kept in Git version control, but we should get these computers back on scripted backups.

 

---

Edit Sat Sep 8 18:28:19 2018: Rebooted and restarted frequency counter. Beat note has drifted off to 750 MHz.  Will take a while to bring it back in.

Fan has been reordered, will arive Friday next week.

  2234   Mon Sep 17 14:08:56 2018 awadeMiscComputersWS1 CPU fan replaced

Found that WS1 had died again.  CPU fan seized up again.

Replaced and computer restarted fine. WS1 up and running normally.

Quote:

WS1, the main computer in the PSL lab died last night.  On reboot bios screen says that CPU fan died.  

I opened the computer up and had a look.  Bearings on main CPU fan were a bit stiff.  I wiggled them a bit and it now spins, with some noise, when booted.  

I'll order a replacement CPU fan KDE1209PTVX 12V, 7.0 W and replace pronto.

Main CPU fan
Main CPU fan, bearings nearing end of life.

---

Also there was no backup of this computer.  Almost all the stuff is kept in Git version control, but we should get these computers back on scripted backups.

 

---

Edit Sat Sep 8 18:28:19 2018: Rebooted and restarted frequency counter. Beat note has drifted off to 750 MHz.  Will take a while to bring it back in.

Fan has been reordered, will arive Friday next week.

 

  2238   Mon Sep 24 12:49:20 2018 awadeDailyProgressFSSModifying TTFSS box LP elliptical filters

After we improved the performance of the PMC locking -- that was degraded because of excessive RF getting past the demodulation stage, see PSL:2228) -- I though I'd have a look at the TTFSS boxes.  It turns out that the FSS has a elliptical LP with a notch tuned to 21 MHz.  This had not been modified since the box was made, meaning that for the period of 14.75 MHz modulation it was sub-optimal.

Previous state of TTFSS box's RF LP filtering

I modeled the TF from the output of the mixer (U2 in D0901894-v1)  to the input of the servo (U3 in LIGO-D040105 rev C).  The model is compared below with the measured TF injected from Test1 to TP1 on the servo board.  As plotted below, for both north and south paths, it is in ok agreement to within the tolerances of the components. Note that one discrepancy with how I measured the TF is that I injected into Test1 without 22 Ω in series.  This doesn't seem to make a big difference.  The bump at 50 MHz in the LISO model is an artifact of the AD829 but doesn't seem to turn up in the measured TF.

 

Modifying the Elliptical filter

The original circuit is illustrated below:

Schematic modeling RF LP filter in TTFSS box (modeling the virtual ground of the
ad829 op amp as ground)

Here a basic LP filter is formed between the C13 and C14 and L3.  The combination of C12 and L3 form the notch.

For the best LP filter at the modulation frequency we want to lower the first (lowest) poles of the filter to as low as possible without lowering the required capacitance of C12 so low that parasitic capacitance is a concern for the notch.  I've attached a notebook below, this includes some pyliso and widget sliders that where values can be varied to find the best combination. Also plotted below is a contour plot of the notch frequency as a function of the inductor (L3) and the notching capacitor in parallel (C12), this is also in the notebook.

Notch frequency as a function of inductor and capacitor choices.  Green point marks the closest whole values that give 36 MHz, the blue box is the ±5% uncertainity in the values for both of these components. Thermal drift is so small that box for this error is too small to display.

For 36 MHz a good combination looks like L3 = 750 nH, C13 = C14 = 220 pF, C12 = 26 pF (green point labeled in above plot).  Absolute tolerance of these components are 5% with a thermal drift of 30 ppm/K for the ceramic capacitors and +25 to 125 ppm/K for the ceramic core inductor (see see cap spec sheet and coil craft ceramic core spec sheet for 1206cs series). A box in the above plot shows the ±5% error level on components showing the range of potential error in the notch frequency due to manufacture variations.

The South TTFSS RF board was modified with L3 changed from 1200 nF to 750 nF and C12 being changed from 47 pF to a 18 + 1.5 + 1.5 + 1.0 pF = 22.0 pF stack (i.e. parallel).  These values were initially guessed and then trial and error was used to swap out small value caps until the notch was centered on 36 MHz. The bottom of the dip was an attenuation of -58 dB (plotted below)

The North TTFSS RF board was modified with L3 changed from 1200 nF to 750 nF and C12 being changed from 47 pF to a 18 + 3.9 + 1.5 + 1.0 pF = 24.4 pF in parallel. This gave a notch at 37 MHz with a dip that was also -58 dB. The measured transfer function is plotted below.

This is a high(ish) Q narrow peak so we don't want it to drift too much. As mentioned above the thermal drift is 30 ppm/K for caps and 125 ppm/K for the inductor (worst case).  The LISO model indicates that this corresponds to a drift of 0.5 kHz/K for the cap and 2.2 kHz/K for the inductor: this is a tolerable margin of error. Thermal drift, even for large 10's K variations in box temperature, is not a concern.

The LISO model indicated that we could squeeze some more attenuation by shifting C13 and C14 from 220 pF to a larger value.  But when I tried 1 nF (for both) I found that there was some weird 1/f roll up below 200 kHz when I measured the TF (rather than flat). I got the same even when I tried 440 pF.   Maybe its some impedance matching thing and the resistors need to be modified; I wanted to get the FSS back up and working so I didn't look further into this. C13 and C14 were left at 220 pF for both North and South TTFSS boxes.

It also looks like the real measured TF have a broader notch than the LISO model.  There is probably some stuff that isn't modeled in the simple LISO model.  This relaxes the requirments for the notch tuning a little.

TTFSS field boxes were reinstalled on the table, ready for the demodulation phase to be tuned for the new frequencies.

---

Data is commited into https://git.ligo.org/cit-ctnlab/ctn_labdata/tree/master/data/20180925_FSS_EllipticFilterRetuned_TF and zipped and attached below.

  2245   Fri Oct 5 12:42:29 2018 awadeDailyProgressPDPMC reflection PD DC path fix

The DC path of the north PMC resonant reflection photodetector was not showing any signal.

Some work on the North PMC resonant reflection phododetector:

  • Drop in voltage accross R4 is 0.9 V, its 22 Ω so that would be a current of 40 mA: way too much current when dark.
  • I looked inside and there was a lot of blobbed solder joints R4 and C8, C33 was also not fully contacted on the ground side. I wicked all the excess solder off and retouched all the joints that didn't look great. These modifications didn't fix the problem
  • Checked voltage supplies and DC levels in the DC path.  The first OP27 (U5) is showing 1 V on the input but 2 V on the output.  This should be a buffering stage: the op amp is busted. I removed U5 and replaced with new.  Pulled of the pin5 pad but this is not used anyway.  The dark PD doesn't show any voltage drop accross R4 anymore, this is good.
  • Modified the gain of the final DC output stage OP27 (U6).  It is now R16=1 kΩ, R11=10 Ω so gain of that stage is x101 and the total DC transimpedance gain is 2222 Ω.  
  2246   Mon Oct 8 10:22:09 2018 awadeSummaryRFSome notes on reflectionless RF filters for demodulation electronics

Edit Thu Oct 25 14:57:57 2018 (awade): Note that the LISO models here have wrong units.  They are supposed to be in units of power but were not converted from V/V.  Things listed as dB are correct but multiply the exponent of magnitude plots by two. This will be corrected in future posts.


I came across two interesting papers by Matt Morgan et al. on the subject of reflectionless RF filters.   

It seems like there wasn't a lot of thought put into optimal RF filter designs before this point that properly terminated the stop band elements. The Morgan filters are essentially diplexing filters arraignments, that has been done before. However, in the case of these reflectionless filters, there are additional symmetries that lower the inductor and capacitor values, reduce the requirements on component Q and widen the component tolerance requirements in general. The matched component design means that all circuit parts can be drawn from the same batch, improving the manufacture variation errors and also reducing the sensitivity to temperature. Together this means that we can more readly make custum filters with off-the-shelf discreet components with more confidence they will preform as designed and be relativity imune to thermal variations.

Comparisons to the equivalent Butterworth and Chebyshev designs suggest that it performs just slightly worst than Chebyshev but with a better complex gain slope, this means flatter phase in the passband and better stability of the filter for mismatch with component variation. It seems like these reflectionless filter structures are more directly comparable to the inverse Chebyshev filters or the elliptic (Cauer filter) topologies. These have minimal or no pass band ripple at the expense of slower roll off in the stop band some stronger stop band dips.  The bonus is that they are properly terminated at all frequencies and absorb all stopband by design.

The shape of the pass band response is much closer to the current elliptical filter installed in that there is a strong dip designed to coincide with the PDH modulation frequency. The dip corresponds to the first pole of the filter.  As documented previously in PSL: 2238 the stop band attenuation of the TTFSS elliptical filters is not much better than 14 dB. However, the attentuation at the critical modulation frequency is 58 dB and that is what really matters.  The problem with the current TTFSS RF demodulation design is that it does not have proper dumping the the stop band, much of the rejected ω and 2ω frequency components are reflected strait back into the mixer.  

The naive wisdom is that we add a terminating resistor at the input of the RF LP filter to quell backreflections to the mixer.  This is probably good enough for many configurations where we just don't care that much.  However, 50 Ω to ground isn't strictly impedance matched accross the whole frequency band and messes up the impedance in the passband. What we want is something that optimally dumps the higher order RF terms and presents the mixer with a well defined impedance at its output* accross the whole band: that way the mixer is loaded properly to its design specs.

Below is an schematic of a first order reflectionless filter with component values selected to achieve the pole dip at 36 MHz.

Here the optimal choice for passive component values is:

L = \frac{Z_0}{\omega_\textrm{pole}}

C = \frac{1}{Z_0\omega_\textrm{pole}}

R = Z_0

where Z_0 is the input terminating impedance of the filter.  It is also implicitly assumed that the output terminating impedance is matched to this Z_0 value.

I modeled the worst case filter design variations by performing a Monty Carlo simulation of the ideal circuit with LISO.  I Assumed a standard deviation of compoent values of 5.0% (this is what the spec sheets claim and set each component to normal random sample for each component about its ideal design value. Below is a plot of 500 samples (thin low alfa lines) along with computed median and 1σ band. In reality the component variation will be much less as they are drawn from the same batch and the manufactures probably leave a margin of error in the absolute value of components that they can deliver to.

First order reflectionless filter modeled with Monty-Carlo random error of components 5% std each component.

What the model shows is that the tolerance to component variation is probably ok.  It doesn't show the impact of component Q on filter performace (especially about the deep notch around the pole is what we are really interested in).  I'm working on getting PySpice to simulate this with the coilcraft spice models.

Here also is the input impedance of the filter as a function of frequency:

This seems less good, as can be seen from the actuall mismatch for individual samples.  The real variation of inductors and caps drawn from the same batch probably isn't this bad.

 


*Its not clear to me what the output impedance of a naked mixer is supposed to be. In the TTFSS design a 22 Ω resistor is in series with a MAX333A switching chip before the RF filter (these 333A's have ~ 30 Ω series resistance): so it seems like that design assume that you need to add resistance in serieis to get to 50 Ω termination.  

Also attached is the notebook used to compute the plots in this post using pyliso.

 

Edit Thu Feb 21 19:30:48 2019 (awade): fixing unrendered laxex

  2249   Fri Oct 12 14:41:17 2018 awadeSummaryEnvironmentTemperature logging PSL lab (Sep-Oct 2018)

Temperature data in the ATF (QIL) lab has been collected for a month now.  Data is sampled at 0.1 s intervals and saved on WS2. I'm not attaching the full data here (its about 4 Gb/month in csv format) but the minute trend is included along side the plots below.

Plotted below is the hour trend from September 11th up until October 13th 2018* (UTC).  

The Thermostate trace shows temperature as measured at the AC control sensor.  EnvMon is the themerature at the center of the table (sensor is mounted to table directly). 

 

*Note that these sensors were never calibrated for their absolute offset (but they should be pretty close).  Drift of the transducer box is unknown.  It uses a LT1128 op amp to convert AD590 sense current into voltage.  This is not the right choices.  Also the resistors are the lower grade radial type thick film, which is also not the best choice for immunity to circuit induced drift.

  2256   Wed Nov 21 19:33:23 2018 awadeMiscElectronics EquipmentSR785 (SN46950)

The main SR785 (Serial number 46950) has a dead screen. 

I power cycled the unit and also checked that the screen brightness wasn't set to zero.

Getting it serviced at Stanford Research Systems can be expensive.  In the past they have just sent the CRT and it is easy to replace ourselves.  I'll put in a request for a quote and orgainize for the screen to be replaced later this week.

  2262   Fri Dec 7 11:26:30 2018 awadeMiscDAQFrame builder back up

Framebuilder has been down for months now.  It looks like I've been able to get it up and going again albeit with the same issues of drifting clock from before.

I restarted the TCS lab framebuild computer (fb4) that is supposed to be recording all our PSL lab channels.  It seems that the daqd process was hung on something. After poking around a bit I found that a large proportion of channels have been renamed, maybe requests to dead channels was overwhelming it.  I used Craig's python channelFramebuilderConfigFileCreator.py script (see PSL:2133) to regenerate all channels into a .ini file used by cds, copied the output to them to /opt/rtcds/caltech/c4/chans/daq/C3CTN.ini and restarted.  

Framebuild is now back and recording all channels (except modecleaner channels) with a UTC time that is 2h 20 m too slow.  Otherwise we are now collecting data that will be useful in assesing goodness of vacuum can temperature control.

To fix the time drift issues we need to get GPS stablized timing waveform into the daq

Todo list:

  • Run 10 MHz backlink from cryo lab to TCS lab
  • Aquire function generator to generate 65kHz (32 KHz?) waveform to feed into framebuilder
  • Electronics hardware to connect into ADC in fb4
  • Configure fb4 to stablize to GPS standard

Someone else needs to do this.  Maybe a secondary project for Gupta.

 

  2266   Wed Dec 19 11:01:00 2018 awadeDailyProgressTempCtrlProposed circuit for cavity temperature sensor Part 2

Missing current excitation to RTD?

I don't know if I'm missing it, or if you've yet to add it, but can't see current excitation into the RTD.  I remember you were looking into REF200 and some other voltage-> current highly stable sources (PSL:2264),  that part will be key to actually getting a high precision temperature readout.  I didn't understand how the op amp stage in your schematic (PSL:2264) worked; it looks like you pinned the inverting input to ground but then also connected the output pin for feedback.  Doesn't really make sense to me. 

Do you want something more like this: technical note. There they use the op amp to bootstrap the voltage reference so that the actual applied voltage rises to whatever is necessary to get the design current fixed through the reference resistor AND sink it through the load. The point of the that design is that the resistance of the sensor and its cabling does not affect the current: current is set only by the reference resistor. The stablity of the excitation current will hinge on the goodness of the voltage reference and that of the reference resistor.  

Also, as a side note, you were proposing a 5V reference.  I thought I read somewhere that the standard buried zenor reference in these types of voltage references were actually typically 7 V and the 5 V and 10 V versions some how stepped up/down using a precision resistor network buried within the chip.  My understanding was that 7V references were the best because there was no additional drift.  If the specs of the MAX6325 are good, then maybe its well compensated and its nothing to worry about.  It might be worth checking if that series of chips has other discreet​ voltage options that perform better.

Add some impedance to output of OP27s

Its a good idea to add about 100 Ω (or more) of resistance to output of OP27s. Otherwise poor little chips are going to overdrawn in current for low impedance loads. Probably won't break them, but it will produce bogus voltages. The acromags are 10 kΩ impedance but you can never predict if someone is going to plug in a 50 Ω oscilloscope at some point.  Check what max current of OP27 is and adjust so that at full range you wont be overdrawing with 50 Ω load.  

How many wires?

RTD is pinned to ground on one side.  Not sure what your intending for the wiring scheme here? 3 wires, 4 wires.  Ideally you want to excite current through two wires and read back voltage drop across another two.  But if you're pinning one side to ground then you've already effectively reduced it to a three wire scheme: ok but not best.  I guess you want to think about how this scheme is going to minimize your uncertainty/sensitivity with respects to wiring (see maybe this). Four wires is best.  Be best.

Also are we going to miss out on the excellent CMRR of the instrument amplifier if it isn't a truly differential readout on the front end of the circuit?

Best RTD nominal resistance

You've looked at a bunch of numbers for choice of RTD resistance @ 25 C.  From the numbers we looked at 1 kΩ seemed like the best for order 100 µA excitation (this is the break point with Johnson noise). It might be nice to have a plot or a table that shows the tradeoff off of sensing SNR (for various noise sources) for different choices of RTD nominal resistance, i.e. 100 Ω, 1 kΩ, 5 kΩ, 10 kΩ.   The excitation current and resistance determine the tradeoff points and the right choice is the one that gets below/close to the nominal input referred noise of available pre-amp stages. That would be good for framing a good science reason for choosing a particular RTD (with a particular slope).

So many channels

Still not sure you need so many channels.  If we already have a good idea of the operating range, then a one high precision channel will do.  We are almost certainly not going to need all that dynamic range and changing resistors is easy.

 

Quote:

The second stage of the amplifier for temperature sensor would be subtractor circuits using OP27G.

  • PFA the circuit schematic consisting of the instrumentation amplifier and different circuits for acromag channels.
  • There would be 1 low-resolution channel with a range from about 15 ^\circ C to 95 ^\circ C with a response: 0.238053435115*T - 13.167938931297705
  • Rest would high-resolution channels with a smaller range. These channels would give us about 1mV/mK slope so that we can read with mK precision with 16 bit \pm 10 V range.
  • Channel 1: From 15 ^\circ C to 35 ^\circ C with a response: 0.952875*T - 22.5
  • Channel 2: From 35  ^\circ C to 55 ^\circ C with a response: 0.943758551683*T - 43.329447115384596
  • Channel 3: From 55  ^\circ C to 75 ^\circ C with a response: 0.955040625*T - 61.9375
  • Channel 4: From 75  ^\circ C to 95 ^\circ C with a response: 0.9429496875*T - 79.74466991341995

I'm attaching calculations done in the jupyter notebook as zip file.

 

  2273   Fri Jan 4 19:04:29 2019 awadeDailyProgressRFAMTime to look at RF AM and RIN

That roll up in the HF range to 3 kHz hump looks suspect. 

PLL?

We need to check that we're actually implementing the PLL properly and unwrapping the BN PSD properly from the actuation signal.  You could end up from a result like this if the UGF was, say, ~ 1 kHz: then the PSD * (marconi slope) approximation will no longer be true and would look more like PSD * (marconi slope + 1/Mix*SR560).  As demodulation for PLL frequency gives a 1/f in closed loop (from freq->V_out), this would give a roll up directly proportional to frequency around and above UGF when inverting. We should go back and check again that the transfer function measured yesterday of the PLL had a UGF of >60 kHz. You should remeasure the PLL open loop gain and post it on the elog. Make sure you use a small excitation signal and play around with the cycle averaging settings to smooth out noise for a nice clean trace. Also a schematic of exactly where signals are injected, values of gains, slopes and mixer conversion efficiencies (spec sheet value will do) and RF power from beat note detector.  

You'll also way to check that you're not saturating the NF1811.  You don't want weird artifacts from not being linear in response at RF... slew rate limit etc. Details of internals of NF1811 can be found in the PSL Electronics Wiki.

Just Marconi noise?

Also see figure 4.8 of Tara's thesis.  I'm having trouble making out the colours but there is peaking there in the marconi frequency noise at around 3 kHz. This plot places the 10 kHz/V modulation slope as being clear by at least an order of magnitude.  Always be suspicious. Are our macnoni's still performing?  Add this to the moderate-to-low-priorities list of things to check.  

RF AM again?

The BN PSD doesn't extend high enough to compare fully to all the others but if we find that the PLL -> BN is correct then maybe this is an old problem related to excess AM from the 36 MHz and 37 MHz EOMs producting AM and PM.  See PSL:2083 and related backlinks.  In particular we want to know if the laser RIN has some coherence with the BN.  See what Evan did in PSL:1524.  Before screwing around with optimizing the AM (maybe a 1-2 day task) figure out how you can use the transmission signals to get a RIN of both cavities and make a coherence plot.  This would be your smoking gun.  After that maybe look into whether you should sink time into optimizing RF AM and ISS. 

Checking RIN and coherence with the beat note is maybe a 1/2 day task once everything is locked up and working.

Also, put those thermal hats back on those EOMs.  Its winter, have a heart.

 

  2278   Thu Jan 10 11:38:53 2019 awadeDailyProgressRFAMTime to look at RF AM and RIN

Is this the resonant EOM or a BBEOM driven with an external circuit? The driving requirements are different, if you overdrive you'll get a bunch of higher harmonics.

Maybe check the modulation depth, make sure it isn't too excessive.  Beta=0.3 should be enough. Error signal Vpp = V_\textrm{pp} = G\times4\sqrt{P_cP_s} = G\times 2P_0\beta (where P0 is the incident power, and G is the PD + mixer gain. Just make sure beta it isn't excessive in producing significant higher order components.  

You want to figure out if the 29.5 MHz harmonic feature is an electrical cross coupling or optical. Check your cable routing, are you getting any cross coupling pickup from collocated coaxial lines?  Pretty sure we got the mode cleaner PDH demodulation stuff right, but check mixer is right power level with 4th order LP filter + 50 ohm termination on input side of the LP filter with a RF compatible T-junction.   

Also, looking back at the design documents for the PMC, it looks like the nominal design frequency considered was ~30 MHz modulation.  Have a look Evan's earlier​ calculations, especially with respect to HOM suppression.  It looks like for 30 MHz the 60 MHz harmonic is in a relatively low transmission point for HOM. If the issues are optical artifacts transmitted it might be HOM... you can always up the frequency to something closer to 30 MHz.

Take all of the above with a grain of salt.  Consider how much time to sink into it based on whether you think its actually driving a noise source in the system.  Eliminating this 14.75 MHz harmonic may be in the basket of diminishing returns for your time.  The 36 MHz and 37 Mhz AM components are a much bigger concern for FSS related coupling of RIN into frequency noise.

Quote:

Preliminary testing

I hooked up an HP8560E to RF port of the South FSS Refl RFPD. I unlocked laser from the cavity and brought it to a place which seemed far away from any mode of the cavity.

I was hoping to see a peak at 37 MHz which is PDH modulation frequency of FSS on this path. To my surprise, signal level was ~-80 dBm only at 37 MHz. On zooming out, I saw a 29.5 MHz peak instead of about ~-55dBm. @9.5 MHz is second harmonic of the modulation frequency for South PMC PDH loop. This seemed weird as sidebands of EOM before PMC should not reach after PMC. So I have taken noted some preliminary numbers for future. This isn't complete analysis:

Freq Offset Peak Frequency (MHz) Peak Value (dBm)
-5.9975 0 -11
-5.9975 29.5 -55
-5.9975 37 -60
-5.9875 0 -11
-5.9875 29.5 -55
-5.9875 37 -60
-6.0275 0 -11
-6.0275 29.5 -55
-6.0275 37 -70

All data was taken with a span of 50 MHz. Note how on changing the frequency offset, I got a offset point where 37 MHz Peak almost disappears but the 29.5 MHz peak is present irrespective of the frequency. Need to investigate more.

 

  2282   Sat Jan 12 13:11:02 2019 awadeDailyProgressRFAMTime to look at beatnote after RF AM and RIN tuneup

Nice.  

It would be a good time to retake the BN + cross spectrum with RIN to see if these improvements squash the 3 kHz hump.  You'll probably want to recheck the RFAM levels immediately before retaking the BN in case there has been any drift.  When plotting the RIN you should include a trace that quantifies the dark noise of the PDs and the shot noise. The traces are not much use if they are limited by either of these, the elog readers would benefit from having such traces as a bench mark.

A good thing to do, as well, would be to resurrect the AEOM power controls in both paths.  You can then use a swept sine to take a transfer function from intensity noise to beat note directly without just relying on the residual RIN of the laser*.  This will help you model the expected real TF from intensity noise to frequency noise (for the noise budget) and help justify the design choices of the ISS (how far along is the ISS BTW?).  It also lets you know the susceptibility of each path to RIN, regardless of the performance differences between the two laser.  Maybe its also possible to fine tune the offsetting on the FSS PDH controls by watching the transfer function live and adjusting the DC offset to minimize Intensity->FSS->Freq Noise conversion.  The offsets and gains on the FSS loop are now more or less arbitrary, this is something you can try next time you tweak up these loops.

THe AEOMs should be configured with s-polarized light (vertical) going into the modulator with a quarter-wave plate, followed by a PBS at the output.  Adjust the quarter-wave plate to get yourself to 50:50 splitting, this gives you the maximum W/V slope with close to linear response. You'll need to adjust upstream powers accordingly to give you 1.5 mW at the input of the RefCavs. Be aware of how hard you are driving the modulators so that you are sure that it is in the linear regime.  Terminate AEOMs when not in use. 


* It would also be nice to have a tap off somewhere (<10%) to be able to sample the pre-cavity RIN.  Given that any BS/ wedge you insert will misaligned the beams it will be a 0.5 day exercise to realign cavities etc, thus mid-low priority.

 

Quote:

I think I found the culprit. the polarizer behind the EOM (PMC loop EOM) was a multi-order one and hence was extremely sensitive to temperature. I replaced it with a zero order half wavepleate and realigned the EOM to minimise the RF AM to about -70 dBm. The PMC has got misaligned in this procedure, so tomorrow I'll align it back and see if our changes made a difference.

 

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