40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  40m Log, Page 113 of 341  Not logged in ELOG logo
ID Date Authordown Type Category Subject
  12778   Tue Jan 31 18:51:07 2017 gautamUpdateSEISeismic Rainbow Strip - myths debunked

I've been suggesting that there may be something wonky with the Seismic Rainbow Striptool on the wall for the last couple of weeks. Here are a few things that were verified today.

  1. If you want to restore the StripTools in the control room, just run /opt/rtcds/caltech/c1/scripts/general/startStrip.sh. I have verified as of today that this works, and in future, any changes to channels/limits/colors of traces etc should be reflected in this script.
  2. Though some of the BLRMS bands have looked anomalous over the last few weeks, in particular the 0.3-1Hz band. The attached 120 day trend plot suggests that there hasn't been any dramatic change recently. In fact, looking on the summary pages, Rana noticed that today was an unusually low 0.3-1Hz activity day..
Attachment 1: Seis_BLRMS.png
  12780   Tue Jan 31 22:07:13 2017 gautamUpdateIMCRF AM stabilization box revamp

I've added the schematic of the RF AM stabilization board to the 40m PSL document tree, after having created a new DCC document for our 40m edits. Pictures of the board before and after modification will also be uploaded here...

  12790   Thu Feb 2 17:43:20 2017 gautamUpdatePEMEM172 mic is hooked up in the PSL

I had noticed something wonky with the microphone, but neglected to elog it. I had tested it after installation by playing a sine wave from my laptop and looking at the signal on the PSL table, it worked fine. But you can see in the attached minute trend plot that the signal characteristics changed abruptly ~half a day after installation, and never quite recovered.\


Attachment 1: Mic_broken.png
  12793   Fri Feb 3 00:36:52 2017 gautamUpdateIMCMCL Feedback - framing the problem

Rana motivated me to take a step back and reframe the objectives and approach for this project, so I am collecting some thoughts here on my understanding of it. As I write this, some things still remain unclear to me, so I am leaving these as questions here for me to think about...


  1. The PSL is locked to the IMC cavity - but at frequencies near 1 Hz, the laser frequency is forced to follow the IMC cavity length fluctuations, even though the free-running PSL frequency noise at those frequencies is lower. This excess is also imprinted on the arms when locked to the IR. We would like to improve the situation by feeding back a portion of the MC PDH error signal to the cavity length actuator to stabilize the MC cavity length at low frequencies. Moreover, we would like this loop to not imprint additional control noise in the arm control signals, which is a problem we have observed with the existing MCL loop. 
  2. The borader goal here is to use this project as a case study for designing the optimal loop and adaptive feedback. Can we come up with an algorithm, which takes
    • A model of our system (made with measured data where possible)
    • A list of our requirements (e.g. in this case, frequency noise requirements in various frequency bands, smooth crossovers between the various loops that enable locking the PSL to the IMC cavity and avoid injecting excess control noise into the plant)

and come up with the best loop that meets all our rquirements? What constitutes the "best" loop? How do we weight the relative importance of our various requirements? 

Proposed approach:

For the specific problem of making the MCL feedback loop better, the approach I have in mind right now is the following:

  1. Build a model of the 40m IMC loop. Ultimately the performance of the loop we implement will depend on the transfer function from various additive noise sources and disturbances in the feedback loop (e.g. electronics noise) to the output (i.e. laser frequency). Building an accurate model will allow us to quantify the performance of the proposed control loop, and hence, optimize it with some algorithm. I did some work on a simplistic, purely analytical model of the two MC loops (MCF and MCL), but Rana pointed out that it is better to have something more realistic for this purpose. I have inherited his Simulink models, which I will now adapt to reflect the 40m topology. 
  2. Come up with a list of requirements for the MCL controller. Some things that come to mind:
    • Reduce the arm control signal spectral amplitude below 20 Hz
    • Not increase the arm control signal spectral amplitude above 20 Hz
    • Crossover smoothly with the FSS slow temperature control loop and the MCF loop. 
    • What factor of suppression are we looking for? What is achievable? Once I build the model, it should shed some light on these..
    • Is the PMC a more stable frequency reference than the NPRO crystal at low frequencies? This measurement by Koji seems to suggest that it isn't (assuming the 1e4 product for the NPRO free-running frequency noise)..
  3. Once we have a model and a satisfactory list of requirements, design a control loop that meets these using traditional techniques, i.e. desired tracking error in the control band of 0.1-20 Hz (is this possible? The model will tell us...), gain and phase margin requirements etc. But this need not necessarily be the optimal controller that meets all of our requirements
  4. Optimize the controller - how? Can we define an objective function that, for example, rewards arm control signal suppression and penalizes injection of control noise, and just fminsearch in the [z,p,k] parameter space of the controller? Is there a smarter way to do this?
  5. Can this algorithm be adaptive, and optimize the controller to adapt to prevailing seismic conditions for example? Is this the same as saying we have a model that is accurate enough for us to predict the response of the plant to environmental disturbances? 

My immediate goal is to have the Simulink model updated.

Thoughts/comments on the above will be appreciated...

  12803   Mon Feb 6 15:18:08 2017 gautamUpdateCDSslow machine bootfest

Had to reboot c1psl, c1susaux, c1auxex, c1auxey and c1iscaux today. PMC has been relocked. ITMX didn't get stuck. According to this thread, there have been two instances in the last 10 days in which c1psl and c1susaux have failed. Since we seem to be doing this often lately, I've made a little script that uses the netcat utility to check which slow machines respond to telnet, it is located at /opt/rtcds/caltech/c1/scripts/cds/testSlowMachines.bash.

The script can be executed by ./testSlowMachines.bash.

  12804   Mon Feb 6 17:03:41 2017 gautamUpdateIMCMCL Feedback - simulink model updated

I've edited Rana's Simulink model to reflect the current IMC servo topology (to the best of my understanding). I've tried to use Transfer Function blocks wherever possible so that we can just put in the appropriate zpk model in the script that will linearize the whole loop. I've also omitted the FSS SLOW loop for now.

I've been looking through some old elogs and it looks like there have been several modifications to both the MC servo board (D040180) and the TT FSS Box (D040105). I think it is easiest just to measure these TFs since the IMC is still down, so I will set about doing that today. There is also a Pomona Box between the broadband EOM and the output of the TT FSS box, which is meant to sum in the modulation for PMC locking, about which I have not yet found anything on the elog.

So the next steps are:

  1. Measure/estimate all the unknown TFs and gains in this schematic
  2. Linearize the model, get the OLG, see if the model matches previously measured OLGs (with the MCL part disabled)
  3. Once the model is verified to be correct, look at couplings of various noise sources in the MCL part of the loop, and come up with a suitable controller.

If anyone sees something wrong with this topology, please let me know so that I can make the required changes.

Attachment 1: mc40_v1.pdf
  12806   Tue Feb 7 10:18:58 2017 gautamUpdateIMCMC REFL weirdness

A few minutes back, I glanced up at the control room StripTool and noticed that the MCREFL PD DC level had gone up from ~0 to ~0.7, even though the PSL shutter was closed. This seemed bizzare to me. Strangely, simply cycling the shutter returned the value to the expected value of 0. I wonder if this is just a CDS problem to do with c1iool0 or c1psl? (both seem to be responding to telnet though...)

Since things look to be back to normal, I am going to start with my characterization of the various TFs in the IMC FSS loop...

  12812   Wed Feb 8 19:13:02 2017 gautamUpdateIMCMCL Feedback - TF measurements

Quick summary elog, details to follow. I did the following:

  • Updated the Simulink model based on Koji's feedback. 
  • Today morning, I measured the (electronic) open-loop TFs of
    • MC Servo Board
    • FSS Fast path (PZT)
    • FSS PC Drive path
  • The summing amplifiers in the latter two paths are assumed to be broadband for the purposes of this model.

The measurements I have look reasonable. But I had a hard time trying to look at the schematic and determine what is the appropriate number and locations of poles/zeros with which to fit the measured transfer function. Koji and I spent some time trying to go through the MC Servo board schematic, but looks like the version uploaded on the 40m DCC tree doesn't have changes made to it reflected (we compared to pictures on the 40m google photos page and saw a number of component values were different). Since the deviation between fit and measurement only occurs above 1MHz (while using poles/zeros inferred from the schematic), we decided against pulling out the servo board and investigating further - but this should be done at the next opportunity. I've marked the changes we caught on a schematic and will upload it to the 40m DCC page, and we can update this when we get the chance.

So it remains to fit the other two measured TFs, and add them to the Simulink model. Then the only unknown will be the PDH discriminant, which we anyway want to characterize given that we will soon have much more modulation.  

Data + plots + fits + updated schematics to follow...


  12814   Thu Feb 9 11:22:56 2017 gautamUpdateGeneralSorensens and DIN connections at 1X1

I'd like to fix a few things at 1X1 when we plug in the new amplifier for the 29.5MHz modulation signal. 

  1. Split off separate +24 and ground wires to the green BBPD RF amplifiers and the AOM driver (they are sharing a single fuse at the moment)
  2. Tap a new +24 GND -24V set for the FSS Fast summing box - this is currently running with a bench power supply underneath the PSL table set to +/-18V, but I checked the 7815/7915 datasheets and they accept up to 35V input for a 15V output, so it should be fine to use 24V
  3. Hook up the ZHL-2A for the IMC modulation.

Steve has ordered rolls of pre-twisted wire to run from 1X1 to the PSL table, so that part can be handled later.

But at 1X1, we need to tap new paths from +/- 24V to the DIN connectors. I think it's probably fine to turn off the two Sorensens, do the wiring, and then turn them back on, but is there any procedure for how this should be done? 

Attachment 1: Screen_Shot_2017-02-10_at_9.01.46_AM.png
  12815   Thu Feb 9 23:35:34 2017 gautamUpdateIMCMCL Feedback - TF measurements

Here are the details as promised.

Attachment #1: Updated simulink model. Since I haven't actually run this model, all the TF blocks are annotated "???", but I will post an updated version once I have run the model (and fix some of the questionable aesthetic choices)

Attachment #2: Measured and fitted transfer functions from the "IN1" input (where the demodulated MC REFL goes) to the "SERVO" output of the MC servo board (to FSS box). As mentioned in my previous elog, I had to put in a pole (fitted to be at ~2MHz, called pole 9 in the plot) in order to get good agreement between fit an measurement up to 10MHz. I didn't bother fitting all the high frequency features. Both gain sliders on the MEDM screen ("IN1 Gain" and "VCO gain") were set to 0dB for this measurement, while the super boosts were all OFF.

Attachment #3: Measured and fitted transfer function from "TEST 1 IN" to "FAST OUT" of the FSS box. Both gains on the FSS MEDM screen ("Common gain adjust" and "fast gain adjust") were set to 0dB for this measurement. I didn't need any ad-hoc poles and zeros for this fit (i.e. I can map all the fitted poles and zeros to the schematic), but the fit starts to deviate from the measurement just below 1 MHz.. perhaps I need to add a zero above 1MHz, but I can't see why from the schematic...

Attachment #4: Measured TF from "TEST 1 IN" to "PC OUT" on the FSS box. MEDM gains were once again 0dB. I can't get a good fit to this, mainly because I can't decipher the poles and zeros for this path from the schematic (there are actually deviations from the schematic posted on the 40m DCC page in terms of component values, I will try and correct whatever I notice. I'll work on this...

Attachment #5: Data files + .fil files used to fit the data with LISO



Data + plots + fits + updated schematics to follow...

Most of the model has come together, I am not too far from matching the modelled OLG to the measured OLG. So I will now start thinking about designing the controller for the MCL part (there are a couple of TFs that have to be measured for this path).

Attachment 1: mc40_v1.pdf
Attachment 2: CMboard_OLTF_fit.pdf
Attachment 3: FSSFast_OLTF_fit.pdf
Attachment 4: PCdrive_OLTF_measured.pdf
Attachment 5: data.zip
  12816   Fri Feb 10 02:14:10 2017 gautamUpdateIMC29.5 MHz stabilizer box replacement

Lydia finished up installing the new RF amplifier, and will elog the details of the installation.

I wanted to try and measure the IMC OLG to compare against my Simulink model. So I went about performing a few checks. Summary of my findings:

  1. The amplifier seems to be working fine. I checked powers at the input, output to EOM and output to distribution box (that serves the various LOs) first with a 30dB attenuator at the input, and subsequently with the design choice of 5dB attenuator at the input. Everything seemed in order.
  2. I installed a 30 dB attenuator at the MC REFL PD input to the demod board since my (rough) calculations suggested that our modifications would have resulted in the RF beat power between carrier and sideband increasing in power by ~27dB.
  3. I then opened the PSL shutter and tried locking the IMC - with manual tweaking of the various gains, I was able to lock.
  4. But getting to this point took me a while so I couldn't get an OLG measurement in.

TBC tomorrow, I'm leaving the PSL shutter closed and the RF source off for tonight...

  12820   Fri Feb 10 18:21:21 2017 gautamUpdateIMCIMC Demod board

Rana and I spent some time looking at the IMC demod board earlier today. I will post the details shortly, but there was a label on the front panel which said that the nominal LO level to the input should be -8dBm. The new 29.5MHz routing scheme meant that the LO board was actually being driven at 0dBm (that too when the input to the RF distribution box was attenuated by 5dB).

An elog search revealed this thread, where Koji made some changes to the demod board input attenuators. Rana commented that it isn't a good idea to have the LO input be below 0dBm, so after consulting with Koji, we decided that we will

  • Remove the 5dB attenuator to the input of the distribution box such that the LO is driven at ~5dBm
  • Remove the input 10dB attenuator, first ERA-5SM amplifier, and the mini circuits power splitter from the demod board (schematic to follow).

After implementing these changes, and testing the board with a Marconi on the workbench, I found that the measured power levels (measured with an active FET probe) behave as expected, up till the ERA-5SM immediately prior to the LO (U4 and U6 on the schematic). However, the power after this amplifier (i.e. the input to the on-circuit LO, Minicircuits JMS-1H, which we want to be +17dBm), is only +16dBm. The input to these ERA-5SMs, which are only ~2years old, is -2dBm, so with the typical gain of +20dB, I should have 18dBm at their output. Moreover, increasing the input power to the board from the Marconi doesn't linearly increase the output from the ERA-5SM. Just in case, I replaced one of the ERA-5SMs, but observed the same behaviour, even though the amplifier shouldn't be near saturation (the power upstream of the ERA-5SM does scale linearly).

This needs to be investigated further, so I am leaving the demod board pulled out for now...

  12822   Sun Feb 12 01:16:57 2017 gautamUpdateIMCIMC length loop - summary of changes

29.5 MHz RF Modulation Source

  • The +13dBm from the Wenzel oscillator gets amplified to +27dBm by a ZHL-2-S. There is a 5dB attenuator on the input to the amplifier to avoid compression/saturation.
  • The amplified output goes to the EOM (+26dBm measured at the rack, no measurement done at the input to the triple-resonant circuit box yet), while a 10dB coupled part goes to the RF distribution box which splits the input into 16 equal parts. The outputs were measured to spit out +5dBm.
  • 2 of these go to the WFS demod boards - it was verified that this level of drive is okay for the comparator chips on the demod board.
  • A third output goes to the IMC Demod board. The demod board was modified so that the nominal LO input level is now +5dBm (details below).
  • The remaining outputs are all terminated with 50ohms.

IMC Demodulation Board

  • The input attenuator, amplifier and power splitter were removed.
  • Schematic with changes marked and power levels measured, along with a high-res photograph (taken with our fancy new Macro lens + LED light ring) has been uploaded to a page I made to track changes for this part on the DCC (linked to 40m document tree).
  • After making the changes, it was verified that the power levels in the signal chain were appropriate up till the input to the ERA-5SM amplifier directly before the LO. These levels were deemed appropriate, and also scaled in a predictable manner with the input power. As Koji mentions in the previous elog, the dynamically changing input impedance of the mixer makes it difficult to measure the LO level at this point, but I am satisfied that it is within ~1dBm of the nominal +17dBm the mixer wants.
  • The board was further checked for gain imbalance and orthogonality of the I and Q outputs. The graphic below show that there is negligible gain imbalance, but the relative phase between the I and Q channels is ~78 degrees (they should be 90 degrees). Of course this doesn't matter for the IMC locking as we only use the I phase signal, but presumably, we want to understand this effect and compensate for it. 

  • The label on the front panel has been updated to reflect the fact that the nominal LO input is now +5dBm
  • The demodulation phase had changed since the RF signal change was modified - Rana and I investigated this effect on Monday morning, and found that a new ~1.5m long cable was needed to route the signal from the RF distribution box to the LO input of the demod board, which I made. Subsequent modifications on the demod board meant that an extra ~10cm length was needed, so I just tacked on a short length of cable. All of the demodulated signal is now in the I output of the demod board (whereas we had been using the Q output).
  • The graphics below confirms that claim above. Note the cool feature on the digital scopes that the display persistence can be set to "infinity"!

I wanted to do a quick check to see if the observed signal levels were in agreement with tests done on the workbench with the Marconi. The mixers used, JMS-1H, have an advertised conversion loss of ~7dB (may be a little higher if we are not driving the LO at +17dBm). The Lissajous ellipse above is consistent with these values. I didn't measure powers with the MC REFL PD plugged into the demod board, but the time series plot above suggest that I should have ~0dBm power in the MC REFL PD signal at 29.5MHz for the strongest flashes (~0.3Vpp IF signal for the strong flashes). 


MC Servo Board

  • As mentioned above, we now use the I phase signal for lMC PDH locking.
  • This has resulted in an overall sign change of the servo. I have updated the MEDM screen to reflect that "MINUS" is the correct polarity now..
  • To set the various gains, I measured the OLTF for various configurations using the usual IN1/IN2 prescription on the MC Servo Board (using the Agilent analyzer). 
  • I started at 0dBm "In1 Gain", and the nominal (old) values for "VCO gain", "FSS Common Gain" and "FSS FAST gain"  and found that though I could lock the MC, I couldn't reliably turn on the boosts.
  • After some tweaking, I settled on +10dB "In1 Gain". Here, locking was much more reliable, and I was able to smoothly turn on the Super Boosts. The attached OLTF measurement suggests a UGF of ~118kHz and phase margin of a little more than 30 degrees. There is room for optimization here, since we have had UGFs closer to 200kHz in the recent past. 
  • I didn't get around to measuring the actual PZT/EOM crossover yesterday. But I did measure the OLTF for various values of the FSS gains. At the current value of +20dBm, the PC drive signal is hovering around 1.5V. This bit of optimization needs to be done more systematically. 
  • I've edited mcup and mcdown to reflect the new gains. 

Some general remarks

  • The whole point of this exercise was to increase the modulation depth for the 29.5MHz signal. 
  • By my estimate, assuming 8mrad/V modulation index for the EOM and a gain of 0.6 at 29.5 MHz in the triple resonant box, we should have 100mrad of modulation after installing the amplifier (compared to 4mrad before the change). 
  • The actual RF power at 29.5 MHz at the input/output of the triple resonant box has not yet been measured. 
  • The WFS input error signal levels have to be re-measured (so I've turned off the inputs to the digital WFS filters for now)
Attachment 1: DemodBoardOrthogonality.pdf
Attachment 2: IMC_PDH.pdf
Attachment 4: IMC_OLTF.pdf
Attachment 5: FSS_gain_comparison.pdf
  12824   Mon Feb 13 13:34:44 2017 gautamUpdateIMCIMC length loop - bad SMA cable replaced

I was a little confused why the In1 Gain had to be as high as +10dB - before the changes to the RF chain, we were using +27dB, and we expect the changes made to have increased the modulation depth by a factor of ~25, so I would have expected the new In1 Gain to be more like 0dB.

While walking by the PSL table, I chanced upon the scope monitoring PMC transmission, and I noticed that the RIN was unusually high (see the scope screenshot below). We don't have the projector on the wall anymore, but it doesn't look like this has shown up in the SLOW monitor channel anyways. Disabling the MC autolocker / closing the PSL shutter had no effect. I walked over to the amplifier setup in 1X2, and noticed that the SMA cable connecting the output of the amplifier to the EOM drive was flaky. By touching the cable a little, I noticed that the trace on the scope appeared normal again. Turning off the 29.5MHz modulation source completely returned the trace to normal.


So I just made a new cable of similar length (with the double heat shrink prescription). The PMC transmission looks normal on the scope now. I also re-aligned the PMC for good measure. So presumably, we were not driving the EOM with the full +27dBm of available power. Now, the In1 Gain on the MC servo board is set to +2dB, and I changed the nominal FSS FAST gain to +18dB. The IMC OLTF now has a UGF of ~165kHz, though the phase margin is only ~27 degrees.. 


MC Servo Board

  • After some tweaking, I settled on +10dB "In1 Gain". Here, locking was much more reliable, and I was able to smoothly turn on the Super Boosts. The attached OLTF measurement suggests a UGF of ~118kHz and phase margin of a little more than 30 degrees. There is room for optimization here, since we have had UGFs closer to 200kHz in the recent past. 
  12828   Tue Feb 14 10:43:06 2017 gautamBureaucracyEquipment loanEquipment to Cryo Lab

PZT Buzzer Box (Thorlabs HV Supply + Manual + 2*PZT Buzzers) ---> Cryo Lab (Brittany + Aaron)

  12833   Wed Feb 15 23:54:13 2017 gautamUpdateIMCIMC saga continues...

Following the discussion at the meeting today, I wanted to finish up the WFS tuning and then hand over the IFO to Johannes for his loss stuff. So I did the following:

  1. First I set the dark offsets on the WFS (with PSL shutter closed). Then I hand aligned the MC to maximize transmission, centered the beam on the WFS, and set the RF offsets with the MC unlocked.
  2. Given that the demod phase for the IMC PDH demodulation board changed by |45 degrees|, I tried changing the digital demod phases in each of the WFS quadrant signals by +/- 45 degrees. Turns out +45 degrees put all the error signal into the I Phase, which is what we use for the WFS loops.
  3. Then I attempted to check the WFS loops. I estimated that we have ~25 times the modulation depth now, so I reduced the WFS1/2 P/Y gains by this factor (but left the MC2 TRANS P/Y gains as is). The loop gain seemed overall too low, so I upped the gain till I saw instability in the loop (error signals ringing up). Then I set the loop gains to 1/3 of this value - it was 0.01 before, and I found the loop behaved well (no oscillations, MC TRANS stabilized) at a gain of 0.002.

At this point, I figured I would leave the WFS in this state and observe its behaviour overnight. But abruptly, the IMC behaviour changed dramatically. I saw first that the IMC had trouble re-acquiring lock. Moreover, the PC Drive seemed saturated at 10.0V, even when there was no error signal to the MC Servo board. Looking at the MEDM screen, I noticed that the "C1-IOO_MC_SUM_MON" channel had picked up a large (~3V) DC offset, even with In1 and In2 disabled. Moreover, this phenomenon seemed completely correlated with opening/closing the PSL shutter. Johannes and I did some debugging to make sure that this wasn't a sticky button/slider issue, by disconnecting all the cables from the front panel of the servo board - but the behaviour persisted, there seemed to be some integration of the above-mentioned channel as soon as I opened the PSL shutter.



Next, I blocked first the MC REFL PD, and then each of the WFS - turns out, if the light to WFS2 was blocked and the PSL shutter opened, there was no integrating behaviour. But still, locking the MC was impossible. So I suspected that something was wrong with the LO inputs to the WFS Demod Boards. Sure enough, when I disconnected and terminated those outputs of the RF distribution box, I was able to re-lock the MC fine.

I can't explain this bizzare behaviour - why should an internal monitor channel of the MC Servo board integrate anything when the only input to it is the backplane connector (all front panel inputs physically disconnected, In1 and In2 MEDM switches off)? Also, I am not sure how my work on the WFS could have affected any hardware - I did not mess around at the 1X1 rack in the evening, and the light has been incident on the WFS heads for the past few days. The change in modulation depth shouldn't have resulted in the RF power in this chain crossing any sort of damage threshold since the measured power before the changes was at the level of -70dBm, and so should be at most -40dBm now (at the WFS demod board input). The only thing different today was that the digital inputs of the WFS servos were turned on...

So for tonight I am leaving the two outputs of the RF distribution box that serve as the LO for the WFS demod boards terminated, and have also blocked the light to both WFS with beam blocks. The IMC seems to be holding lock steady, PC drive levels look normal...

Unrelated to this work, but I have committed to the svn the updated versions of the mcup and mcdown scripts, to reflect the new gains for the autolocker...

  12834   Thu Feb 16 13:29:38 2017 gautamSummaryGeneralAlternative Calibration Scheme


Craig and I have been trying to put together a Simulink diagram of the proposed alternative calibration scheme. Each time I talk the idea over with someone, I convince myself it makes sense, but then I try and explain it to someone else and get more confused. Probably I am not even thinking about this in the right way. So I am putting what I have here for comments/suggestions.

What's the general idea?

Suppose the PSL is locked to the MC cavity, and the AUX laser is locked to the arm cavity (with sufficiently high BW). Then by driving a line in the arm cavity length, and beating the PSL and AUX lasers, we can determine how much we are modulating the arm cavity length in metres by reading out the beat frequency between the two lasers, provided the arm cavity length is precisely known.

So we need:

  1. Both lasers to be stabilized to be able to sense the line we are driving
  2. A high bandwidth PDH loop for locking the AUX laser to the arm cavity such that the AUX laser frequency is able to track the line we are driving
  3. An accurate and precise way to read out the beat frequency (the proposal here is to use an FPGA based readout)
  4. An accurate measurement of the arm length (I think we know the arm lengths to <0.1% so this shouldn't dominate any systematic error).

To be able to sense a 1kHz line being driven at 1e-16 m amplitude, I estimate we need a beat note stability of ~1mHz/rtHz at 1kHz.

Requirements and what we have currently:

  • The PSL is locked to the mode-cleaner, and the arm cavity is locked to the PSL. The former PDH loop is high BW, and so we expect the stabilized PSL to have frequency noise of ~1mHz/rtHz at about 1kHz (to be measured and confirmed)
  • The AUX laser is locked to the arm cavity with a medium-BW (~10kHz UGF) PDH servo. From past out-of-loop ALS beat measurements, I estimate the expected frequency noise of the AUX laser at 1kHz to be ~1Hz/rtHz with the current PDH setup
  • Rana suggested we "borrow" the stability of the PSL by locking the AUX laser and PSL in a high bandwidth PLL - if we want this loop to have ~300kHz BW, then we need to use an EOM as an actuator. The attached Simulink diagram (schematic representation only, though I think I have measurements of many of those transfer functions/gains anyways) shows the topology I had in mind. Perhaps I did not understand this correctly, but if we have such a loop with high gain at 1kHz, and the error signal being the beat between PSL and AUX, won't it squish the modulation we are applying @1kHz?
  • Is it feasible to instead add a parallel path to the end PDH loop with an EOM as an actuator (similar to what we do for the IMC locking)? Ideally, what we want is an end PDH loop which squishes the free-running NPRO noise to ~1mHz/rtHz at 1kHz instead of the 1Hz/rtHz we have currently. This loop would then also have negligible tracking error at 1kHz. Then, we could have a low bandwidth PLL offloading onto the temperature of the crystal to keep the beat between the two lasers hovering around the PSL frequency.


On the hardware side of things, we need:

  • Broadband EOM
  • FSS box to drive the EOM (Rana mentioned there is a spare available in the Cryo lab)

Koji and I briefly looked through the fiber inventory we have yesterday. We have some couplers (one mounted) and short (5m) patch fibers. But I think the fiber infrastructure we have in place currently is adequate - we have the AUX light brought to the PSL table, and there is a spare fiber running the other way if we want to bring the PSL IR to the end as well.

I need to also think about where we can stick the EOM in given physical constraints on the EX table and the beam diameter/aperture of EOM...

Attachment 1: AltCal.pdf
  12838   Fri Feb 17 20:10:18 2017 gautamUpdateIMCWFS servos turned back on

[Koji, gautam]

Turns out the "problem" with WFS2 and the apparent offset accumulation on the IMC Servo board is probably a slow machine problem.

Today, Koji and I looked at the situation a little more closely. This anomalous behaviour of the C1:IOO-MC_SUM channel picking up an offset seems correlated with light being incident on WFS2 head. Placing an ND filter in front of WFS 2 slowed down the rate of accumulation (though it was still present). But we also looked at the in-loop error signal on the IMC board (using the "Out 2" BNC on the front panel), and this didn't seem to show any offset accumulation. Anyways, the ability of the Autolocker doesn't seem to be affected by this change, so I am leaving the WFS servo turned on.

The new demod phases (old +45degrees) and gains (old gains *0.2) have been updated in the SDF table. It remains to see that the WFS loops don't drag the alignment over longer timescales. I will post a more detailed analysis here over the weekend...

Also, we thought it would be nice to have DQ channels for the WFS error signals for analysis of the servo (rather than wait for 30 mins to grab live fine resolution spectra of the error signals with the loop On/Off). So I have added 16 DQ channels [recorded at 2048 Hz] to the c1ioo model (for the I and Q demodulated signal from each quadrant for the 8 quadrants). The "DRATE" for the c1ioo model has increased from ~200 to 410. Comparing to the "DRATE" of c1lsc, which is around 3200, we think this isn't significantly stretching the DAQ abilities of the c1ioo model...


  12840   Sat Feb 18 21:50:48 2017 gautamUpdateIMCWFS servos turned back on

Here is a comparison of the error signal spectra after increasing the IMC modulation depth, to the contribution with RF inputs / whitening inputs terminated (which I borrowed from Koji's characterization of the same in Dec 2016, these shouldn't have changed).

Some general observations:

  1. This data was taken with the WFS servos disabled, but with the IMC hand-aligned to a good state (MC_TRANS ~15,000). The error signal spectra are from the new DQ channels (but still sampled at 2048Hz, I had not implemented the change to 512Hz).
  2. The error signals seem to have increased by ~25x yes, which is consistent with how much we expect the modulation depth to have increased
  3. The bump around 1 Hz is now cleaerly visible in all 16 channels, as is the bounce peak at 16Hz (relative to Dec 2016). In general, between 0.1Hz and 5Hz, there is now a fair bit of daylight between the error signals and the electronics noise contribution. 

I will update with the in-loop error signal spectra, which should give us some idea of the loop bandwidth.

I will look into lowering the sampling rate, and how much out-of-band power is aliasing into the 0-256 Hz band and update with my findings.


Yikes. Please change the all teh WFS DQ channels sample rates from 2048 down to 512 Hz. I doubt we ever need anything about 180 Hz.

There is sometimes an issue with this: if our digital AA filters are not strong enough, the noise about above 256 Hz can alias into the 0-256 Hz band. We ought to check this quantitatively and make some elog statement about our AA filters. This issue is also seen in DTT when requesting a low frequency spectrum: DTT uses FIR filters which are sometimes not sharp enough to prevent this issue.



Attachment 1: WFS_error_noise.pdf
  12847   Thu Feb 23 10:59:53 2017 gautamUpdateCOCRC folding mirrors - coating optimization

I've now made a DCC page for the mirror specifications, all revisions should be reflected there.

Over the last couple of days, I've been playing around with Rana's coating optimization code to come up with a coating design that will work for us. The basic idea is a to use MATLAB's particle swarm constrained optimization tool to minimize an error function that is a composite of four penalties:

  1. Thermal noise - we use the proxy function from E0900068-v3 to do this
  2. Deviation from target T @1064nm, p-pol
  3. Deviation from target T @532nm, p and s-pol
  4. HR Surface field

On the AR side, I only considered 2 and 3. The weighting of these four components were set somewhat arbitrarily, but I seem to be able to get reasonable results so I am going with this for now. 

From my first pass at it, the numbers I've been able to get, for 19 layer pairs, are (along with some plots):

HR Side:

  • T = 50ppm, 1064nm p-pol
  • T = 99%, 532nm s and p-pol

       (in this picture, the substrate is to the right of layer 38)

AR Side:

  • R ~50ppm for 532nm, s and p-pol

   (substrate to the right of layer 38)

These numbers are already matching the specs we have on the DCC page currently. I am not sure how much better we can get the specs on the HR side keeping with 19 layer pairs... 

All of this data, plus the code used to generate them, is on the gitlab coatings page...


Attachment 1: PR3_R_170222_2006.pdf
Attachment 2: PR3_123_TOnoise_170222_2203.pdf
Attachment 3: PR3_123_Layers_170222_2203.pdf
Attachment 4: PR3AR_R_170222_2258.pdf
Attachment 5: PR3AR_123_Layers_170222_2258.pdf
  12859   Wed Mar 1 16:00:41 2017 gautamUpdateComputer Scripts / ProgramsMatlab R2016b installed

Since it would be nice to have the latest version of Matlab, with all its swanky new features (?), available on the control room computers and Optimus, I downloaded Matlab R2016b and activated it with the Caltech Campus license. I installed it into /cvs/cds/caltech/apps/linux64/matlab16b. Specifically, I would like to run the coating optimization code on Optimus, where I can try giving it more stringent convergence criterion to see if it converges to a better spot.

I trust that this way, we don't interfere with any of the rtcds stuff.

If I've done something illegal license-wise or if this is likely to cause havoc, please point me to what is the correct way to do this.

GV 18 Mar 2017: Though I installed this using the campus network license key, this seems to only work on Rossa. If I run it on the other control room machines/Optimus, it throws up a licensing error. I will check with Larry W. as to how to resolve this...


  12862   Wed Mar 1 23:56:09 2017 gautamUpdateIMCFront panel for 29.5 MHz amplifier box

The alignment wasn't disturbed for the photo-taking - I just re-checked that the spot is indeed incident on the MC REFL PD. MC REFL appeared dark because I had placed a physical beam block in the path to avoid accidental PSL shutter opening to send a high power beam during the photo-taking. I removed this beam block, but MC wouldn't lock. I double checked the alignment onto the MC REFL PD, and verified that it was ok.

Walking over to the 1X1, I noticed that the +24V Sorensen that should be pushing 2.9A of current when our new 29.5MHz amplifier is running, was displaying 2.4A. This suggests the amplifier is not being powered. I toggled the power switch at the back and noticed no difference in either the MC locking behaviour or the current draw from the Sorensen.

To avoid driving a possibly un-powered RF amplifier, I turned off the Marconi and the 29.5MHz source. I can't debug this anymore tonight so I'm leaving things in this state so that Lydia can check that her box works fine...


I turned the RF sources back on and opened the PSL shutter. MC REFL was dark on the camera; people were taking pictures of the PD face today so I assume it just needs to be realigned before the mode cleaner can be locked again. 


  12867   Sun Mar 5 12:41:23 2017 gautamUpdateIMCWFS servo-steppin

I've been sitting on some data for a while now which I finally got around to plotting. Here is a quick summary:

Attachment #1: I applied a step input to the offset of each of the six WFS loops and observed the step response. The 1/e time constant for all 4 WFS loops is <10s suggesting a bandwidth a little above 0.1Hz. However, the MC2 P and Y loops have a much longer time contant of ~150s. Moreover, it looks like the DC centering of the spot on the QPD isn't great - the upper two quadrants (as per the MEDM screen) have ~3x the cts of the lower pair.
I did not (yet) try increasing the gain of this loop to see if this could be mitigated. I accidentally saved this as a png, I will put up the pdf plot

Attachment #2: This is a comparison of the WFS error signals with the loops engaged (solid lines) vs disabled (dashed lines). Though these measurements were taken at slightly different times, they are consistent with the WFS loop bandwidths being ~0.1Hz.

Attachment #3: Comparison of the spectra of the testpoint channels and their DQ counterparts at the same time which are sampled at 512Hz. It does not look like there is any dramatic aliasing going on, although it is hard to tell what exactly is the order of the digital AA filter implemented by the RCG. Further investigation remains to be done... For reference, here are some notes: T1600059, T1400719

GV 7 March 2017 6pm: It looks like we use RCG v2.9.6, so it should be the latter document that is applicable. I've been going through some directories to try and find the actual C-code where the filter coeffs are defined, but have been unsuccessful so far...


I will update with the in-loop error signal spectra, which should give us some idea of the loop bandwidth.

I will look into lowering the sampling rate, and how much out-of-band power is aliasing into the 0-256 Hz band and update with my findings.


Attachment 1: WFS_stepping.png
Attachment 2: WFS_comparisons.pdf
WFS_comparisons.pdf WFS_comparisons.pdf
Attachment 3: WFSdigitalAA.pdf
WFSdigitalAA.pdf WFSdigitalAA.pdf
  12870   Mon Mar 6 14:47:49 2017 gautamUpdateSummary PagesCode status check script modified

For a few days now, the "code status" page has been telling us that the summary pages are DEAD, even though the pages themselves seemed to be generating plots. I logged into the 40m shared account on the cluster and checked the status of the condor job (with condor_q), and did not find anything odd there. I decided to consult Max, who pointed out that the script that checks the code status (/home/40m/DetectorChar/bin/checkstatus) was looking for a particular string in the log files ("gw_daily_summary"), while the recent change in the default output of condor_q meant that the string actually being written to the log files was "gw_daily_summa". This script has now been modified to look for instances of "gw_daily" instead, and so the code status indicator seems to be working again...

The execution of the summary page scripts has also been moved back to pcdev1 (from pcdev2, where it was moved to temporarily because of some technical problems with pcdev1).

  12880   Fri Mar 10 11:37:25 2017 gautamUpdateComputer Scripts / Programsloss script

This was still running at ~9.30am today morning, at which point I manually terminated it after confirming with Johannes that it was okay to do so. Judging by the StripTool traces in the control room, the mode cleaner remained locked for most of the night, there should be plenty of usable data...

Note that I re-aligned the Y-arm (to experiment further with photo-taking) at about 9.30am, so the data after this time should be disregarded...


loss map script running on Rossa that moves the beam on ETMX. Yarm was misaligned for this, most recent PIT and YAW settings were saved beforehand. This will take until late at night, I estimate 2-3 am.


  12887   Tue Mar 14 10:56:33 2017 gautamUpdateCOCRC folding mirrors - coating optimization

Rana suggested including some additional terms to the cost function to penalize high sensitivity to deviations in the layer thickness (L). So the list of terms contributing to the cost function now reads:

  1. Thermal noise - we use the proxy function from E0900068-v3 to do this
  2. Deviation from target T @1064nm, p-pol
  3. Deviation from target T @532nm, p and s-pol
  4. HR Surface field
  5. The ratio \frac{d\mathcal{T}/\mathcal{T}}{dL/L} with dL/L = 1%, evaluated at 1064nm p-pol and 532nm p and s-pol (only the latter two for the AR side)

I did not include other sensitivity terms, like sensitivity to the refractive index values for the low and high index materials (which are just taken from GWINC).

There is still some arbitrariness in how I chose to weight the relative contributions to the cost function, but after some playing around, I think I have a solution that I think will work. Here are the spectral reflectivity and layer thickness plots for the HR and AR sides respectively. 

HR side: for a 1% increase in the thickness of all layers, the transmission changes by 5% @ 1064nm p-pol and 0.5% @ 532nm s and p-pol


AR sidefor a 1% change in the thickness of all layers, the transmission changes by <0.5% @ 532nm s and p-pol

  (substrate to the right of layer 38)

I've also checked that we need 19 layer pairs to meet the spec requirements, running the code with fewer layer pairs leads to (in particular) large deviations from the target value of 50ppm @ 1064nm p-pol.

Do these look reasonable? 


Attachment 1: PR3_R_170313_1701.pdf
Attachment 2: PR3AR_123_Layers_170313_1701.pdf
Attachment 3: PR3AR_R_170313_1752.pdf
Attachment 4: PR3AR_123_Layers_170313_1752.pdf
  12891   Fri Mar 17 14:49:09 2017 gautamUpdateLSCMCREFL condition pictures

I did a quick measurement of the beam size on the MC REFL PD today morning. I disabled the MC autolocker while this measurement was in progress. The measurement set up was as follows:

This way I was able to get right up to the heat sink - so this is approximately 2cm away from the active area of the PD. I could also measure the beam size in both the horizontal and vertical directions.

The measured and fitted data are:


The beam size is ~0.4mm in diameter, while the active area of the photodiode is 2mm in diameter according to the datasheet. So the beam is ~5x smaller than the active area of the PD. I couldn't find anything in the datasheet about what the damage threshold is in terms of incident optical power, but there is ~100mW on th MC REFL PD when the MC is unlocked, which corresponds to a peak intensity of ~1.7 W / mm^2...

Even though no optics were intentionally touched for this measurement, I quickly verified that the spot is centered on the MC REFL PD by looking at the DC output of the PD, and then re-enabled the autolocker.

Attachment 2: MCREFL_X.pdf
Attachment 3: MCREFL_Y.pdf
  12893   Mon Mar 20 11:18:58 2017 gautamUpdateCDSNo internet connectivity on control room machines

There is no internet connectivity on any of the control room machines. 

I have been trying to debug by tracing the cabling situation in the rack in the office area, and will update if/when this problem has been resolved. I had last come into the lab on Saturday and there was no problem then. There 40m wireless network servicing the office area seems to work fine.


  12894   Mon Mar 20 14:39:44 2017 gautamUpdateCDSNo internet connectivity on control room machines

Koji diagnosed that the NAT router was to blame for this problem. I simply power cycled this router, and now the connectivity has been restored. 

It was possible to log into nodus and then to pianosa - and it was also possible to log into the various control room machines once logged into nodus. However, the outward packets seemed to not get transmitted. Anyways, power cycling the NAT Router unit seems to have done the job.


There is no internet connectivity on any of the control room machines. 

I have been trying to debug by tracing the cabling situation in the rack in the office area, and will update if/when this problem has been resolved. I had last come into the lab on Saturday and there was no problem then. There 40m wireless network servicing the office area seems to work fine.



  12896   Tue Mar 21 15:13:44 2017 gautamUpdateIMCIMC input beam mode matching

[valera, gautam]

Last night, Valera and I looked into two aspects of the IMC:

  1. How can we accurately set the offset at the error point of the PDH servo such that we lock to the true center of the resonance?
  2. What's up with the large common mode offset on the WFS?

I will post a more detailed elog about last night's work, but Valera also thought it might be a good idea to try and improve the mode-matching into the IMC. I couldn't find anything on the wiki/elog about the mode matching situation on the PSL table, so I quickly went over yesterday to measure some lengths. From looking at the MCREFL DC levels when the mode cleaner is locked (~0.37V) and unlocked (~5.7V), the current mode matching efficiency seems to be about 88%, so there is definitely some headroom for improvement.

Here is my cartoon of the situation on the PSL table. All lengths are measured in mm, and I would say correct to +/- 5 mm, so there could be considerable error here...

  (L1 : f=+200mm. L2: f=-150mm. L3:  f=+400mm)

I extracted the lengths from the edge of the PSL table to IM1 and MC1 from (what I think are) the latest CAD drawings on the DCC. I then put all this into an a la mode script [Attachment #5] - I assumed a waist of 370um at the PMC output mirror, and a waist of 1.78mm at MC1. I neglected the passage through the in-vac Faraday, EOM and BS1 (on the sketch above) and the MC1 substrate. I was able to achieve a theoretical mode-matching efficiency of 1 by just moving the positions of L2 and L3. 

Given that there are probably errors of the order 0.5cm in the lengths on the PSL table, and also the in-vacuum distance to MC1, I figured it would be ideal to just move one lens and see if we can improve the efficiency. It looks like it may be more effective to move L2 than L3. The plot on the right shows that the sensitivity is approximately equal to the positioning of L2 and L3. Judging by this plot, looks like w.r.t. the coordinates in this plot, we are somewhere around (0.02,-0.02).

It looks like if we want to do this, moving L2 (f = -150mm) may be the best way to go.

Attachment 2: IMC_ModeMatch.pdf
Attachment 3: singleLensSensitivity.pdf
Attachment 4: sensitivity.pdf
Attachment 5: IMCmodeMatch.m
close all
clear all

%Create a beamPath object
InpPath = beamPath;
%Add components - for a first pass, ignore Faraday and HWPs, so only
%mirrors and lenses..
... 115 more lines ...
  12897   Tue Mar 21 21:21:58 2017 gautamUpdateIOOWFS filter banks updated

The arrangement of filters in the WFS loop filter banks have been altered, Rana will update with details of the motivation behind these changes. Here is how the screen looks now:

I have updated the C1IOO SDF table, and also the mcwfson script to reflect these changes. The latter has been svn committed.

  12898   Tue Mar 21 21:59:48 2017 gautamUpdateIMCIMC input beam mode matching

[valera, gautam]

We implemented the plan outlined in the previous elog. The visibility (Pmax-Pmin)/(Pmax+Pmin) calculated with the MC REFL PD levels with the MC locked/unlocked is now ~96% (up from 88%yes). The MC REFL DC level in lock is now ~0.12V (compared to 0.4V). Assuming a modulation depth of 0.1 @ 29.5MHz, about 25% of this (i.e. 0.03V) is from sideband light.

The procedure followed was (see sketch in previous elog for various optic labels):

  1. Move L2 back (towards PMC) by ~2cm.
  2. Walk the beam using M3 and M4 to minimize MCREFL, re-lock IMC, run WFS. 
  3. Move L3 back (towards PMC) by ~2cm.
  4. Repeat steps 2 and 3, the latter with smaller steps, monitor MCREFL DC level.

We could probably tweak the fine positioning of L2 and L3 and improve the efficiency a little more, but the primary objective here was to see if there was any effect on the large common mode offset on the WFS demodulated "SUM" output. Unfortunately, we saw no effect.

Here are two photos of the relevant section of the PSL table before (left) and after (right) our work there:


  12899   Wed Mar 22 00:33:00 2017 gautamUpdateIMCIMC length offset nulling

[valera, gautam]

Motivation: see this elog

I was fiddling around for a few days trying to implement the method outlined in this paper to null this offset - I will post a separate elog about my efforts but Valera pointed out that we could try injecting an AF modulation at the IN2 input of the MC Servo Board. Last night, we hooked up an SR function generator (f = 312Hz, A = 0.01Vpp, IN2 gain = -5dB) to the unused BNC IN2 input of the MC Servo board. To avoid any additional offsets from the AO path during this measurement, I disconnected the LEMO cable (it is labelled).

We looked at the spectrum of the MC transmission around 312Hz and also 2*f = 624Hz. As a result of this modulation, we expect in the transmitted power, dP/P, a 2f term with amplitude ~(X_mod/X_0)^2 and a term at f with amplitude ~(X_offset * X_mod / X_0^2) - I may have missed out some numerical factors of order 1. So the latter should vanish if the offset at the error point is truly zero and the lock-point is the center of the resonance. Last night, we found that an offset in the range of -0.25 V to -0.19 V nulled this peak in the DTT spectrum. Today, the number was -0.05V. So the true offset seems to vary from lock to lock. Here are spectra around f=312Hz for a few different values of the offset slider (the center of the resonance seems to be -0.05V on the MEDM slider at this time).

Do these numbers make sense? Some time ago, I had pulled out the MC Servo board to find out what exactly is going on at this offset summing point. The MEDM slider goes from -10V to 10V, and by measuring the voltage at TP5 (see schematic below), I found that there is a 1/40 scaling factor between what is actually applied and the number on the MEDM slider (so for example, the numbers in the legend in the above plot have to be divided by 40). I've modified the MC Servo Board MEDM screen to reflect this. When I had pulled the board out, I noticed that in addition to the offset voltage applied via the backplane connector, there was also a potentiometer (R50 in the schematic below). I had nulled the voltage at TP5 using this potentiometer, but I guess drifts of ~5mV are possible. 

Discussion on calibration of offset slider in Hz/V:

I've yet to do a rigorous calibration of this slider into Hz, but looking at the spectrum of the transmitted intensity at 2f, we estimated the coefficient (X_mod/X_0) ~ 3e-3 for an offset of 0.2V. dP/P ~1 when the applied modulation equals the linewidth of the cavity, which is 3.6kHz. So 0.2V of offset slider corresponds to ~ 10Hz frequency offset. In other words, I estimate the slider calibration to be 50Hz/V. So with the full range of +/- 10V, we should be able to scan ~1kHz of frequency offset. What does this imply about the variation of the offset slider value that removes the peak at 1f between locks? As mentioned above, this variation is ~0.2V over a day - with the calibration mentioned above, this corresponds to a change in cavity length of ~10um, which seems reasonable to me...

So how did all of this tie in with WFS SUM offsets? We did the following:

  • After nulling the length offset using the procedure detailed above, we noticed non-zero offsets on both WFS1 and WFS2 "I" SUM outputs
  • So we set the dark offsets and RF offsets for the WFS, with no light incident on the WFS (PSL shutter closed). 
  • Re-locking the IMC and closing the WFS loops, we noticed that WFS2 SUM offset was still hovering around 0, but WFS1 SUM offset was ~ -2000cts.
  • Looking at some trends on dataviewer, this offset seems to drift around over a few days timescale by a few thousand counts - for example, the WFS1 offset today was +2000cts. Moreover, the WFS1 offset seems to drift around by ~factor of 3 times as much as WFS2 offset in the 24 hour period I looked up (plot to follow)...
  • Misaligned MC2 and looked at the sum offset with just the single bounce beam off MC1 onto the WFS

I neglected to screenshot the StripTool from the times we were doing these trials but I have the times, I will pull up some dataviewer plots and upload them here tomorrow...

Attachment 1: offsetInvestigation.pdf
Attachment 2: offset_summing_amp.pdf
  12900   Wed Mar 22 16:58:25 2017 gautamUpdateIMCWFS sensing matrix measurements

I've taken a bunch of transfer function measurements from the MC ASC PIT and YAW channels to the WFS error signals using the same set of DTT templates Koji used while characterizing the WFS loops a couple of months ago, before the IMC RF changes. Analysis is underway and I will post the results here shortly...

As an aside, Rana had added 10dB and 20dB gains to all of the WFS filter banks yesterday. I tried engaging the 10dB gains on the two MC2_TRANS PD loops, and this did not seem to induce any instability. I stepped both loops and saw that as expected, the 1/e times for both of these loops is about 45 seconds now (compared to ~150 seconds at the nominal gain). These have been running all day today, and the IMC seems well behaved, so I am going to leave these on for now... Jacking up the gain on the MC2_TRANS_QPD loops by 20dB induced instability - same story for the 4 WFS loops with 10dB additional gain...

  12901   Thu Mar 23 01:44:53 2017 gautamUpdateIMCWFS sensing matrix measurements

Thanks to Koji's nice MATLAB script using DttData functions, I was able to quickly analyze the TF data. Essentially, this measurement was a repetition of what was done here. The difference is that the modulation depth has been increased by ~25x compared to that measurement from December 2016. Here are the measured TFs (before accounting for the 1/f^2 normalization) for the various quadrants and the PIT/YAW channels:


The plots above are just to illustrate that the measurement was clean between the band over which the averaging will be done to compute the TF amplitude - i.e. 7-15Hz. The full summary of TF amplitudes, standard deviations, and the sensing matrix in the style of the referenced elog (the actual excel spreadsheet is Attachment #4, minus some of the graphics Koji had on his excel sheet):

Inverting those matrices, we get the matrices that diagonalize the sensor-actuator chain:


\begin{pmatrix} -0.00518 & -0.00305 & -639.6\\ 0.00354 & -0.00281 & 198.8\\ 0.00102 & 0.00672 & -756.6 \end{pmatrix}


\begin{pmatrix} 0.00523 & -0.00276 & -856.7\\ 0.000318 & 0.00010 & -366.4\\ 0.00039 & -0.00548 & -851.9 \end{pmatrix}

I will try implementing these matrices tomorrow and take a look at the step responses of the loops - the idea is that perhaps the system wasn't optimally diagonalized before and perhaps we can now improve the bandwidths of all the loops.


Attachment 1: IMC_WFS_segment_TF.pdf
Attachment 2: IMC_WFS_channels_TF.pdf
Attachment 3: TFsummary.pdf
Attachment 4: IMC_WFS_170322.xlsx.zip
  12903   Thu Mar 23 23:38:58 2017 gautamUpdateIMCMC SUS damping gains stepped down

I've reduced the gains of the damping on all 3 MC SUS by a factor of 3 for overnight observation as part of the ongoing feedforward noise cancellation investigations. I will return them to the nominal values tomorrow morning.

  12904   Fri Mar 24 11:26:57 2017 gautamUpdateIMCMC SUS damping gains restored

I've restored the damping loop gains to their nominal values. Analysis of the coherence between MCL and seismometer channels under this reduced gain setting is underway, results to follow.

  12905   Fri Mar 24 12:21:27 2017 gautamSummaryIOOMCL / MCF / Calibration

I repeated this measurement as follows:

  1. Added a filter in the MC_F filterbank (FM9) to account for the Pomona box between the PZT control signal and the laser PZT (pole@2.9Hz). So the filter bank at the time of TF measurement looks like this:
  2. Measured TF from driving MC2 (with C1:SUS-MC2_MCL_OUT channel) to C1:IOO-MC_F, which is the output of the above filter bank. The response is the expected 1/f^2 shape of the free optic
  3. From this transfer function, the magnitude is 0.0316 ct/ct. Using the value of 6nm/ct for the MC2 actuator gain that I found in a previous elog entry, I calibrated the MC_F output into Hz using the calibration factor 3.95MHz/ct (FM10 in the above filterbank).

Here is a calibrated MC_F spectrum:

RXA: I've added this plot of the free-running noise of the Lightwave NPRO which is probably similar to our Innolight Mephisto. Seems like the laser is quieter than MC_F everywhere below 100 Hz.

Attachment 2: MCF_cal.pdf
Attachment 3: MCFTF_mag.pdf
Attachment 4: MCFTF_phase.pdf
Attachment 5: MCFTF_coh.pdf
Attachment 6: FreqNoiseReq.pdf
  12906   Fri Mar 24 19:04:18 2017 gautamUpdateIMCSeismic feedforward and WFS

[valera, gautam]

On Wednesday at the meeting, we were discussing why we aren't able to achieve more seismic feedforward subtraction in MCL. We spent some time thinking about this yesterday, and this elog is meant to be a summary of the stuff we tried. 

  1. We let the WFS loops run for a while and settle, and then turned the input gain down to zero so that the integrators held the outputs to the suspension at a "good" alignment. If the WFS loop bandwidth is ~0.1 Hz, then they aren't helping us at 1Hz anyways. We then looked at coherence between the seismometer signals in this state compared to when the WFS loops were running, and noticed negligible difference. It doesn't seem like the WFS loops are injecting noise into MCL at ~1Hz.
  2. We decided agains implementing the WFS sensing matrix I measured on Wednesday evening, as we found that the relative magnitudes of the matrix elements are virtually the same as in Koji's measurement back in December 2016. But looking at matrix elements like MC1P->WFS1P compared to MC3P->WFS1P - there is a difference of a factor of ~3. Why should there be? The response should be completely symmetric to MC1 and MC3?
  3. While looking at the OSEM channels (i.e. SUSPIT_IN1_DQ, SUSYAW_IN1_DQ etc) for each of the MC optics, we noticed a dramatic difference between MC1 (factor of ~10 higher) and the other two MC optics.
  4. Looking at coherence between MCL and the seismometer channels, we felt that there is less coherence at low frequencies (1Hz and lower) now than there was back in January when I took a measurement. However, there was coherence between the OSEM signals and the seismometers - so it doesn't look like the seismometer is to blame. To make an apples-to-apples comparison, I compared the MCL and Seismometer channel spectra from January to now (for the latter, at two different settings of the damping loop gains on the MC suspensions), and also the maximum predicted achievable subtraction (using EricQs frequency domain multicoherence tool). The two changes I can think of since January are that the MC1 satellite box has been interchanged with the SRM satellite box, and the IMC servo gains have been reallocated since the RF upgrade. My findings are summarized in attachments #1 and #2.

The seismometer spectra look similar enough to be explained by time of day variations, so perhaps the culprit is MC1. The ambient MCL spectrum is almost an order of magnitude higher above 4Hz now, with the nominal damping loop gains, as compared to back in January. I think the damping loops on MC1 need to be tweaked.


Attachment 1: MCL_comparison.pdf
Attachment 2: seis_comparison.pdf
  12916   Wed Mar 29 11:41:19 2017 gautamUpdatePSLPMC DAQ assay for feed-forward integration

The C1IOO frontend machine that resides in 1X1/1X2 has 2 ADCs, ADC0 and ADC1. The latter has 28 out of 32 channels unused at the moment, so I decided to use this for setting up fast channels for the PMC DAQ. On the RTCDS side of things, the PSL namespace block lives in the C1ALS model. I made the following modifications to it:

  1. Added channels for the PMC DAQ
  2. Added CDS filters for both the newly added PMC DAQ channels and the existing FSS DAQ channels, so that we can calibrate these into physical units
  3. Changed the names of the existing FSS channels from FSS_MIXER and FSS_NPRO to FSS_ERR and FSS_CTRL. The latter is still a bit ambiguous, but I felt that FSS_CM_BOARD_CTRL was too long. 
  4. Added DQ channels for the new PMC channels. These are recording at 16K at the moment, but since we have the fast testpoints courtesy of the CDS filter modules for diagnostics, perhaps the DQ channels need only be recorded at 2K?

The PSL namespace block in C1ALS looks like this now:

I then tried hooking up the DAQ signals from the PMC servo board to the ADC via the 1U generic ADC interface chassis in 1X2 - this has 4pin LEMO inputs corresponding to 2 differential input channels. I used J6 (corresponding to ADC channels 10 and 11) for the PMC_ERR and PMC_CTRL respectively. I was a little confused about the status of the 4 pin LEMO output on the front panel of the PMC servo board. According to the DCC page for the modified 40m servo board, the DAQ outputs are wired to the backplane connector instead of the 4 pin LEMO. But looking at photographs on the same DCC page, there are wires soldered on the rear-side of the PCB from the 4-pin LEMO to the backplane connector. Also, I believe the measurements made by Rana in the preceeding elog were made via the front panel LEMO. In any case, I decided to use the single pin LEMO monitor points on the front panel as a preliminary test. The uncalibrated spectra with ADC terminated, IMC unlocked and IMC locked look like:

So it looks like at the very least, we want to add some gain to the AD620 instrumentation amplifiers to better match the input range of the ADC. We also want to make the PZT voltage monitor path AC coupled. My plan then is the following:

  1. Figure out what is going on with the 4-pin LEMO connector on the front panel - is it connected to the DAQ monitor points or not?
  2. Ground pin 5 of U15 (this has already been done by Koji for U14 according to the DCC page)
  3. Add a resistor between pins 1 and 8 of U14 and U15 to get some gain. According to the datasheet, a 1k resistor will give a gain of 50, which for U15 will mean that we undo the existing 1/50 attenuation. Of course we need to AC couple this path first by adding a capacitor in series with R14. 
  4. Figure out where the RF harmonics are coming from and what is the best way to attenuate them.

I will update with a circuit diagram with proposed changes shortly.

Proposed changes:

  1. Cut PCB trace between R14 and R13, install capacitor - what is is correct type of capacitor to use here? I figured installing a series capacitor after the resistive divider, to the input of the instrumentation amplifier avoids the need for a HV capacitor, so we can use a 1uF WIMA capacitor.
  2. Add gains to U14 and U15 (error and control signal monitors respectively). Based on the uncalibrated spectra attached, I think we should go for a gain of ~50 for U15 (1kohm between pins 1 and 8), and a gain of ~200 for U14 (250ohms between pins 1 and 8).

The PCB layout is such that I think using components with leads is easier rather than SMD components.

If this sounds like a reasonable plan, I will pull out the servo card from the eurocrate and implement these changes today evening...

Attachment 2: PMCcheckout.pdf
Attachment 3: D980352-A-40m_151119.pdf
  12918   Thu Mar 30 00:16:09 2017 gautamUpdatePSLPSL NPRO PZT calibration

As part of the ongoing effort to try and calibrate the PMC DAQ channels into physical units, I tried to get a calibration for the PSL NPRO PZT actuator gain. In order to do this, I selected "Blank" on the PMC servo MEDM screen such that there was no feedback signal to the PMC PZT for length control. Then I used the summing box right before the  PSL PZT to inject a ~1Hz triangular wave, 4Vpp. This was sufficient to sweep the NPRO frequency over 70MHz such that both sidebands and the carrier go through resonances in the PMC cavity. I then simultaneously monitored the applied triangular wave voltage and the PMC error signal (using the single pin LEMO connector on the front panel) on an oscilloscope. Analysis is underway, but a quick look at one measurement suggests a PZT actuator gain of ~1.44MHz/V, which is close to what we expect for the Innolight NPROs. The idea is to use this calibration to convert the DQ channels into physical units. 

Details + plots + error analysis to follow...

  12924   Mon Apr 3 17:09:47 2017 gautamUpdateCDSC1PSL burt-restored

When I came in this morning, Steve had re-locked the PMC and IMC - but I could see a ~1Hz intensity fluctuation on the PMC REFL video monitor. I unlocked the PMC and tried to re-lock it, but couldn't using the usual prescription of turning the servo gain down and moving the DC bias slider around. I checked the status of the slow machines - all were responding to pings and could be telnet'ed into, so that didn't seem to be the problem. In the past, this sort of behaviour was characteristic of the infamous "sticky slider" problem - so I simply burt-restored c1psl using a snapshot from 29 March, after which I could easily re-lock the PMC. The transmitted light level looked normal on the scope on the PSL table, and the PMC REFL video monitor also look normal now.

  12925   Mon Apr 3 17:25:13 2017 gautamUpdatePSLPSL NPRO PZT calibration


By sweeping the laser frequency and looking at the PMC PDH error signal, I have determined the 2W Mephisto Innolight PZT actuator gain to be 1.47 +/- 0.04 MHz/V


  1. Re-aligned the input beam into the PMC to maximize transmission level on the oscilloscope on the PSL table to 0.73V.
  2. Disabled control signal from IMC servo to PSL. 
  3. Unlocked the PMC and disabled the loop by selecting "BLANK" on the PMC MEDM screen.
  4. Connected a 0.381 Hz 5Vpp triangular wave with SR function generator to the "SUM" input of the Fast I/F box just before the PSL PZT input. These params were chosen considering the Pomona box just before the NPRO has a corner at 2.9Hz, and also to sweep the voltage to the NPRO PZT over the full 150V permitted by the Thorlabs HV amplifier unit. Monitored the voltage to the Thorlabs HV amp from the "AFTER SUM" monitor point on the same box. Monitored the PMC PDH error signal using the single-pin LEMO monitor point on the PMC servo board (call this Vmon). Both of these signals were monitored using a Tektronix digital O'scope.
  5. Downloaded the data using ethernet.
  6. Fit a line to the voltage applied to the NPRO PZT - I assumed the actual voltage being applied to the PZT is 15*Vmon, the pre-factor being what the Thorlabs HV amplifier outputs. The zero crossings of the sideband resonances in the PDH error signal are separated by 2*fmod (separated by fmod from the carrier resonance, fmod = 35.5MHz assumed). With this information, the x-axis of the sweeps can be converted to Hz, from which we get the PZT actuator gain in MHz/V. 

An example of the data used to calculate the actuator gain (left), and the spread of the calculated actuator gain (right - error bars calculated assuming 5e-4 s uncertainty in the sideband zero-crossing interval, and using the error in the slope of the linear fit to the sweep voltage):

This will now allow calibration of the PMC DAQ channels into Hz.

GV 4 April - The y-axis of the lower plot in Attachment #1 has mis-labelled units. It should be [V], not [MHz/V].

Attachment 1: PDHerr.pdf
Attachment 2: NPROcalib.pdf
  12926   Mon Apr 3 23:07:09 2017 gautamUpdatePSLPMC DAQ assay for feed-forward integration

I made some changes to the DAQ path on the PMC servo board, as per the plan posted earlier in this thread. Summary of changes:

  1. AC coupling PMC control signal path using 2 x 47uF metal film capacitors (in parallel)
  2. Grounding pin 5 of U15
  3. Adding gain to U14 (gain of ~500) and U15 (gain of ~50)

Details + photos + calibration of DAQ channels to follow. The PMC and IMC both seem to remain stably locked after this work.

  12929   Wed Apr 5 16:05:47 2017 gautamUpdateGeneralNB code checkout

[evan, gautam]

We spent some time trying to get the noise-budgeting code running today. I guess eventually we want this to be usable on the workstations so we cloned the git repo into /ligo/svncommon. The main objective was to see if we had all the dependencies for getting this code running already installed. The way Evan has set the code up is with a bunch of dictionaries for each of the noise curves we are interested in - so we just commented out everything that required real IFO data. We also commented out all the gwpy stuff, since (if I remember right) we want to be using nds2 to get the data. 

Running the code with just the gwinc curves produces the plots it is supposed to, so it looks like we have all the dependencies required. It now remains to integrate actual IFO data, I will try and set up the infrastructure for this using the archived frame data from the 2016 DRFPMI locks..

  12936   Mon Apr 10 15:37:11 2017 gautamUpdateCOCRC folding mirrors - v3 of specs uploaded

Koji and I have been going over these calculations again before we send a list of revised requirements to Ramin. I've uploaded v3 of the specs to the DCC page. Here is a summary of important changes.

  1. Change in RoC specification - I condensed the mode-matching information previously in 8 plots into the following 2 plots. Between tangential and saggital planes, the harmonic mean was taken. Between X and Y cavities, the arithmetic mean was taken. Considering the information in the following plots, we decided to change the spec RoC from 600 +/- 50m to 1000 +/- 150m. The required sensitivity in sag measurement is similar to the previous case, so I think this should be feasible.

    Why this change? From the phase map information at  /users/public_html/40m_phasemap/40m_TTI gather that we have 2 G&H mirrors, one with curvature ~ -700m and the other with curvature ~ -500m. An elog search suggests that the installed PR2 has RoC ~ -700m, so this choice of RoC for PR3 should give us the best chance of achieving optimal modematching between the RCs and arms as per the plots below.

  2. Cavity stability checks - these plots confirm that the cavity remains stable for this choice of RoC on PR3...
  3. Coating design - I've been playing around with the code and my understanding of the situation is as follows. to really hit low AR of 10s of ppms, we need many dielectric layer pairs. But by adding more pairs, we essentially become more susceptible to errors in layer thickness etc, so that even though the code may tell us we can achieve R_AR(532nm) < 50ppm, the minima is pretty sharp so even small perturbations can lead to much higher R of the order of a few percent. On the HR side, we need a large number of layer pairs to achieve T_HR(1064nm)=50ppm. Anyways, the MC studies suggest that for the HR coating design, with 19 layer pairs, we can be fairly certain of T_HR(1064nm)<100ppm and R_HR(532nm)>97% for both polarizations, which seems reasonable. In order to make the R_HR(532nm) less susceptible to errors, we need to reduce the number of layer pairs, but then it becomes difficult to achieve the 50ppm T_HR(1064nm) requirement. Now, I tried using very few layer pairs on the AR side - the best result seems to be with 3 layer pairs, for which we get R_AR(532nm)<1% and T_AR(1064nm)>95%, both numbers seem reasonable to me. In the spectral reflectivity, we also see that the minima are much broader than with large number of layer pairs.

    First row below is for the HR side, second row is for the AR side. For the MC studies, I perturbed the layer thicknesses and refractive indices by 1%, and the angle of incidence by 5%.

If there are no objections, I would like to send this version of the specs to Ramin and get his feedback. Specifically, I have assumed values for the refractive indices of SiOand Ta2O5 from google, Garilynn tells me that we should get these values from Ramin. Then we can run the code again if necessary, but these MC studies already suggest this coating design is robust to small changes in assumed values of the parameters...

Attachment 1: PRC_modematch.pdf
Attachment 2: SRC_modematch.pdf
Attachment 3: TMS_PRC.pdf
Attachment 4: TMS_SRC.pdf
Attachment 5: PR3_HR_spectralRefl.pdf
Attachment 6: PR3_HR_MC_CDF_revised.pdf
Attachment 7: PR3_AR_spectralRefl_new.pdf
Attachment 8: PR3_AR_MC_CDF_new.pdf
  12939   Tue Apr 11 00:38:37 2017 gautamUpdatePSLPMC demod moved off servo board

As discussed at the Wednesday meeting last week, I tried moving the demodulation of the PMC error signal off the PMC servo board, by using some minicircuits components. This is just a quick summary elog, more details to follow tomorrow.

  • I used the Mini Circuits ZAD-6+. This is a level 7 mixer, and the LO board puts out ~16dBm, so I replaced the existing 3dB attenuator between the LO board and the input to the PMC servo board with a 9dB attenuator.
  • On the RF side, I retained the 35.5 MHz bandpass filter on the PD input.
  • On the IF output, I used an in-line 50ohm terminator in series with a minicircuits BLP1.9+ low pass filter
  • The mixer output was routed to the FP1 test input of the servo board
  • After some twiddling with the demod phase MEDM screen, I was able to lock the PMC. I've not done a thorough characterization of the loop with the current configuration, this will be done tomorrow. But the PMC and IMC have been stably locked for the last couple of hours...

During the course of this work, I noticed that there was a 35.5MHz line (at ~-55dBm) in the 4-pin LEMO DAQ outputs even when all other inputs to the servo board were terminated. So it seems like this pickup is not coming from the RFPD or demod path. The LO board has a shield enclosure similar to what we have on the LSC demod boards, but perhaps this shield does not enclose the full RF path, and there is some residual pickup between the two cards in close proximity in the Eurocrate?

On the bright side, with this demod setup, the higher harmonic peaks seem to be significantly suppressed.

In particular, the 3x35.5 MHz peak which was very prominent when I looked at these spectra with the nominal demod setup, seems to be much suppressed. 

I'm leaving the PMC servo in this configuration (off servo board demodulation using minicircuits parts) overnight.

Attachment 1: PMC_Ctrl_spec.pdf
  12940   Wed Apr 12 00:36:53 2017 gautamUpdatePSLPMC demod moved off servo board

Here is a more detailed comparison of the spectra of the signals at the front panel DAQ LEMO output, measured with the Agilent analyzer. I've left the scale linear, it looks like when the demodulation is done on the servo board, the 1x, 3x and 5x harmonics of the 35.5MHz modulation are clearly visible. I also plut in a plot of the spectra when both the PD and LO inputs to the servo board are terminated (and so the PMC is unlocked), but with the HV In and OUT of the servo board still connected. In this case, the higher harmonics vanish, but a 35.5MHz peak of ~-50dBm remains. Since this is present with no input to the servo board, this must be direct pickup from the nearby LO board? 

In any case, it looks like many of the harmonics that are present with the nominal demod setup either vanish or are much more suppressed when the error signal demodulation is done off the servo board yes.

Further down the signal chain, I had noticed sometime last week that the ADC signals for the PMC DAQ channels I set up seemed to saturate around 4000 counts. Rana mentioned that the ADC interface box with LEMO connectors on the front is powered with +/-5V. Valera and co. had simply increased the suppy voltage sometime ago to get around this problem, so I did something similar, and increased the supply voltage to +/- 15V. I then confirmed that the ADC doesn't get saturated by driving the input with a +/-5V signal. So now the amplified AD620 signals from the PMC servo board are better matched to the ADC range. 

Here is an uncalibrated spectrum (taken with IMC locked), compared to the current ADC noise and signal levels before the AD620s were given gain.

I now need to think a little about what exactly the control scheme would be if the PMC is used as a reference for the IMC over some frequency range...


Attachment 1: PMC_digitalSpec.pdf
Attachment 2: PMC_DAQ_spectra.pdf
  12944   Tue Apr 18 01:01:03 2017 gautamUpdatePSLPMC OLTF measured, DAQ channels calibrated

Quick entry, details to follow in the AM tomorrow.

  • I calibrated the PMC DAQ channels into physical units - there now exists in the filter modules  cts2m and cts2Hz filter modules, though of course only one must be used at a time
  • Finally measured the PMC OLTF, after moving the PMC PDH error signal demodulation off the servo board - I used the same procedure as Koji when he made the modifications to the PMC servo board, I will put up the algebra here tomorrow. Turns out the previously nominal servo gain of +10dB on the MEDM sliders was a little low, the new nominal gain is +20dB, and has been updated on the MEDM screen.


  • Put up the modified schematic on the 40m DCC tree Done April 18 10pm
  • Check calibration by comparing inferred PMC cavity displacement from error point and control point spectra, using the measured OLTF
  • Finish up looking at multicoherence with MCL and various witness channel combinations


Attachment 1: PMCspectra_calibrated.pdf
  12945   Tue Apr 18 16:10:00 2017 gautamUpdatePSLPMC OLTF measured, DAQ channels calibrated

Here are the details:

  1. PMC OLTF:
    • the procedure used was identical to what Koji describes in this entry.
    • I used the SR785 to take the measurement.
    • MEDM gain slider was at +20dB 
    • I used the two single pin LEMO front panel monitor points to make the measurement. 
    • Mix_out_mon was CH2A, HV_out_mon was CH1A on the SR785
    • A = CH2A/CH1A with the SR785 excitation applied to the EXT_DC single pin LEMO input on the front panel. I used an excitation amplitude of 15mV
    • B = CH2A/CH1A without any excitation
    • Couple of lines of loop algebra tells us that the OLTF is given by the ratio A/B. The plot below lines up fairly well with what Koji measured here, UGF is ~3.3kHz with a phase margin of ~60degrees, and comparable gain margin at ~28kHz. As noted by Koji, the feature at ~8kHz prevents further increase of the servo gain. I've updated the nominal gain on the PMC MEDM screen accordingly... I couldn't figure out how to easily extract Koji's modelled OLTF so I didn't overlay that here... Overlaid is the model OLTF. No great care was taken in analyzing the goodness of the agreement with the model and measurement by looking at residuals etc, except that the feature that was previously at 28.8kHz now seems to have migrated to about 33.5 kHz. I'm not sure what to make of that. 
  2. PMC DAQ calibration:
    • The calibration was done using the swept cavity, the procedure is basically the same as described by Koji in this elog.
    • The procedure was slightly complicated by the fact that I added gain to the AD620 buffers that provide the DAQ signals. So simply sweeping the cavity saturates the AD620 very quickly.
    • To workaround this, I first hooked up the un-amplified single pin LEMO front panel monitor points to the DAQ channels using some of the available BNC-LEMO patch cables.
    • I then did the swept cavity measurement, and recorded the error and control signals fron the single pin LEMO front panel monitor points. Sweep signal was applied to EXT_DC input on front panel.
    • In the nominal DAQ setup however, we have the amplification on the AD620. I measured this amplification factor by hooking up the single pin LEMO monitor point, along with its corresponding AD620 amplified counterpart, to an SR785 and measuring the transfer function. For the PMC_ERR channel, the AD620 gain is ~53.7dB (i.e. approx 484x). For the PMC_CTRL channel, the AD620 gain is ~33.6dB (i.e. approx 48x). These numbers match up well with what I would expect given the resistors I installed on the PMC board between pins 1 and 8 of the AD620. These gains are digitally undone in the corresponding filter modules, FM1.
    • To calibrate the time axis into frequency, I located the zero crossings of the sidebands and equated the interval to 2 x fmod. For the PMC servo, fmod = 35.5MHz. I used ~1Hz triangle wave, 2Vpp to do the sweep. The resulting slope was 1.7026 GHz/s.
    • The linear part of the PDH error signal for the carrier resonance was fitted with a line. It had a slope of 1.5*10^6 cts/s.
    • The round trip length of the PMC cavity was assumed to be 0.4095m as per Koji's previous entry. This allows us to calibrate the swept cavity motion from Hz to m. The number is 1.4534 * 10^-15 m/Hz. I guess we could confirm this by sweeping the cavity with the DC bias slider through the full range of 0-250V, but we only have a slow readback of the PMC reflection (and no readback of the PMC transmission).
    • Putting the last three numbers together, I get the PMC_ERR signal calibration as 1.6496 pm/ct. This is the number in the "cts2m" filter module (FM10).
    • An analogous procedure was done to calibrate the control signal slope: from the sweep, I got 4617 cts/s, which corresponds to 2.7117*10^-6 cts/Hz. Using the FSR to convert into cts/m, I get for PMC_CTRL, 535.96 pm/ct. This is the number in the "cts2m" filter module (FM10).
    • For convenience, I also added "cts2Hz" calibration filters in FM9 in the corresponding filter modules. 

The updated schematic with changes made, along with some pictures, have been uploaded to the DCC page...


Quick entry, details to follow in the AM tomorrow.​


Attachment 1: PMC_OLTF_170418.pdf
ELOG V3.1.3-