Summary:

I've attached the results from my measurements of the noise characteristics of the Y-end auxiliary PDH system.

Details:

The following spectra were measured, in the range DC-1MHz:

1. Analyzer noise floor (measured with input terminated)
2. Green REFL PD dark noise (measured with the Y-end green shutter closed)
3. Mixer noise (measured with input to mixer terminated - measured with an SR560 with a gain of 100)
4. Servo noise (measured with input to servo terminated)
5. In loop error signal (measured with green locked to Y-arm, LSC off - using monitor point on PDH box)
6. In loop control signal (measured with green locked to Y-arm, LSC off - using monitor point on PDH box)

In order to have good spectral resolution, the frequency range was divided into 5 subsections: DC-200Hz, 200Hz-3.4kHz, 3.4kHz-16.2kHz, 10kHz-100kHz, 100kHz-1MHz. The first three are measured using the SR785, while the last two ranges are measured with the Agilent network analyzer. The spectrum of the mixer output with its input terminated was quite close to the analyzer noise floor - hence, this was measured with an S560 preamplifier set to a gain of 100, and subsequently dividing the ASD by 100. To convert the Y-axis from V/rtHz to Hz/rtHz, I used two conversion factors: for the analyzer noise floor, PD dark noise, mixer noise and in-loop error signal, I made an Optickle simulation of a simple FP cavity (all parameters taken from the wiki optics page, except that I put in Yutaro's measured values for the arm loss and a modulation depth of 0.21 which I estimated as detailed here), and played around with the demodulation phase until I got an error signal that had the same qualitative shape as what I observed on an oscilloscope with the arms freely swinging (feedback to the laser PZT disabled). The number I finally used is 45.648 kHz/V (the main horns were 800mV peak-to-peak on an oscilloscope trace, results of the Optickle FP cavity simulation shown in Attachment #2 used to calibrate the X-axis). For the servo noise spectrum and in-loop control signal, I used the value of 2.43 MHz/V as determined here. 

I'm not sure what to make of the strong peaks in the mixer noise spectrum between ~60Hz and 10kHz - some of the more prominent peaks are 60Hz harmonics, but there are several peaks in between as well (these have been confusing me for some time now, they were present even when I made the measurement in this frequency range using the Agilent network analyzer. My plan is to repeat these measurements for the Xend now. 

Here's something to ponder.

Our online MCL feedforward uses perpendicular vertex T240 seismometer signals as input. When designing a feedforward filter, whether FIR Wiener or otherwise, we posit that the PSD of the best linear subtraction one can theoretically achieve is given by the coherence, via Psub = P(1-C). 

If we have more than one witness input, but they are completely uncorrelated, then this extends to Psub = P(1-C1)(1-C2). However, in reality, there are correlations between the witnesses, which would make this an overestimate of how much noise power can be subtracted. 

Now, I present the actual MCL situation. [According to Ignacio's ELOG (11584), the online performance is not far from this offline prediction]

Somehow, we are able to subtract much more noise at ~1Hz than the coherence would lead you to believe. One suspicion of mine is that the noise at 1Hz is quite nonstationary. Using median [C/P]SDs should help with this in principle, but the above was all done with medians, and using the mean is not much different.

Thinking back to one of the metrics that Eve and Koji were talking about this summer, (std(S)/mean(S), where S is the spectrogram of the signal) gives an answer of ~2.3 at that peak at 1.4Hz, which is definitely in the nonstationary regieme, but I don't have much intution into just how severe that value is.

So, what's the point of all this? We generally use coherence as a heuristic to judge whether we should bother attempting any noise subtraction in the first place, so I'm troubled by a circumstance in which there is much more subtraction to be had than coherence leads us to believe. I would like to come up with a way of predicting MISO subtraction results of nonstationary couplings more reliably.

After making sure that the upper UGFs were properly in place, I saved these settings to the SDF files. Thanks Yutaro!

To get C1PEM data back into the frames, I removed the new BLRMS blocks, recompiled, reinstalled, re-enabled it in daqd, restarted.

We still really want more headroom in our framebuilder situation. 

I noticed what I thought was excessive movement of the beam spot on ITMX and ETMX on the control room monitors, and when I checked the CDS FE status overview MEDM screen, I saw that c1scx and c1asx had crashed. I ssh-ed into c1iscex and restarted both models, and then restarted fb as well. However, the DAQ-DCO_C1SCX_STATUS indicator remains red even after restarting fb (see attached screenshot). I am not sure how to fix this so I am leaving it as is for now, and the X arm looks to have settled down.

I have added a new cron job in pcdev1 at CIT using the 40m shared account. This will run the /home/40m/DetectorChar/bin/cleanarchive script one minute past midnight on the first of every month. The script removes GWsumm archive files older than 1 month old.

c1sus and c1ioo were restarted. PMC locked.
 

[ericq,gautam]

Forgot to submit this yesterday...

While we were trying to get the X-arm locked to IR using MC2, frame-builder mysteriously crashed, necessitating us having to go down to the computer and perform a hard reboot (after having closed the PSL shutter and turning all the watchdogs to "shutdown"). All the models restarted by themselves, and everything seems back to normal now..

Since there are a few hours to go before the locking efforts tonight, I've temporarily borrowed the channels used to read out the green beat frequency, and have hooked them up to the broadband IR PDs in the FOL box on the PSL table. I've used the network analyzer in the control room to roughly position the two beatnotes. I've also turned the green beat PDs back on (since the PSL shutter has to be open for the IR beat, and there is some green light falling on these PDs, but I've terminated the outputs).

So this needs to be switched back before locking efforts tonight...

With the IR beats going to the nominal ALS channels as Gautam left them, we're able to measure the free running frequency noise of the end AUX lasers. 

Specifically, the end shutters are closed, leaving the AUX lasers free running. The IR beats then consist of this free running light beating with the PSL light, and the ALS phase trackers give a calibrated frequency noise spectrum. I've stabilized the PSL light by locking the laser to the Y arm via MC2 acutation, so the free running AUX laser noise should dominate by a lot above the suspension resonances. This also has the benefit of giving me the use of the CAL'd Y arm displacement as a sanity check. 

At this point in time, it looks like the X laser is close to 10x noisier than the Y laser, though it does seem to be at the rule-of-thumb "10kHz/rtHz at 100Hz" level. 

[ericq, Gautam]

We were not able to fix the excess frequency noise of the AUX X laser by the usual laser diode current song and dance. Unfortunately, this level of noise is much too high to have any realistic chance of locking.  

We're leaving things back in the IR beat -> phase tracker state with free running AUX lasers, on the off chance that there may be anything interesting to see in the overnight data. This may be limited by our lack of automatic beatnote frequency control. (Gautam will soon implement this via digital frequency counter). I've upped the FINE_PHASE_OUT_HZ_DQ frame rate to 16k from 2k, so we can see more of the spectrum.

For the Y beat, there is the additional weird phenomenon that the beat amplitude slowly oscillates to zero over ~10 minutes, and then back up to its maximum. This makes it hard for the phase tracker servo to stay stable... I don't have a good explanation for this. 

I've set up two IPC channels that take the output from the digital frequency counters and send them to the end front-ends (via the RFM model). A summary of the steps I followed:

1. Set up two Dolphin channels in C1ALS to send the output of the frequency counter blocks to C1RFM (I initially used RFM blocks for these, but eric suggested using Dolphin IPC for the ALS->RFM branch, as they're faster.. Eric's removed the redundant channel names)
2. Set up two RFM channels in C1RFM to send the out put of the frequency counter blocks to C1SCX/Y (along with CDS monitor points to monitor the error rate and a filter module between the ALS->RFM and RFM->SCX/Y IPC blocks - I just followed what seemed to be the convention in the RFM model).
3. Set up the receiving channels in C1SCX and C1SCY
4. Re-compiled and re-started the models in the order C1ALS, C1RFM, C1SCX and C1SCY.

I've set things up such that we can select either the "PZT IN" or the frequency counter as the input to the slow servo, via means of a EPICS variable called "FC_SWITCH" (so C1:ALS-X_FC_SWITCH or C1:ALS-Y_FC_SWITCH). If this is 0, we use the default "PZT IN" signal, while setting it to 1 will change the input to the slow servo to be the frequency readout from the digital frequency counter. I've not updated the MEDM screens to reflect the two new paths yet, but will do so soon. It also remains to install appropriate filters for the servo path that takes the frequency readout as the input.

Tangentially related to this work: I've modified the FC library block so that it outputs frequency in MHz as opposed to Hz, just for convenience..

A few more related changes:

1. The couplers that used to sit on the green beat PDs on the PSL table have now been shifted to the IR broadband PDs in the FOL box so that I can get the IR beat frequency over to the frequency counters. The FOL box itself, along with the fibers that bring IR light to the PSL table, have been relocated to the corner of the PSL table where the green beat PDs sit because of cable length constraints.
2. I've updated the ALS slow control MEDM screen to allow for slow control of the beat frequency. The servo shape for now is essentially just an integrator with a zero at 1 Hz. The idea is to set an offset in the new filter module, which is the desired beat frequency, and let the integrator maintain this beat frequency. One thing I've not taken care of yet is automatically turning this loop off when the IMC loses lock. Screenshot of the modified MEDM screen is attached. 
3. I checked the performance by using the temperature sliders to introduce an offset. The integrator is able to bring the beat frequency back to the setpoint in a few seconds, provided the step I introduced was not two big (~20 counts, but this is a pretty large shift in beat frequency, nearly 20MHz).

To do:

1. Figure out how to deal with the IMC losing lock. I guess this is important if we want to use the IR beatnote as a diagnostic for the state of the X AUX laser.
2. Optimize the servo gains a little - I still see some ringing when I introduce an offset, this could be avoided...

Here's how we should diagnose the EX laser:

1. Compare IR RIN of laser out to 100 kHz with that of another similar NPRO.
2. Look at time series of IR beat signal with a fast scope. Are there any high frequency glitches?
3. Disconnect all of the cables to the EX laser PZT and temperature control. Does the frequency noise change?
4. Change the temperature by +/- 1 deg to move away from mode hop regions. Remeasure RIN and frequency noise and plot.

A small rat / large mouse just ran through the control room. Ugh.

I'll finish up the beat / frequency noise parts of the diagnosis tomorrow later, but I've done some investigation of the AUX X laser RIN. 

I placed a PDA255 at one of the rejected beams from the PBS on the downstream side of the IR faraday, making sure the power didn't saturate the PD. I measured the RIN on a SR785, and simultaneously looked at the signal on a 100MHz scope. 

The RIN has a very strong dependence on the laser diode current, and no noticable dependence on the crystal temperature or the presence of the PDH modulation / temperature control cables. Here are some traces, note that "nominal" current up until recently was 2.0A. 

When adjusting the diode current, a peak beings to appear in the tens of kHz, eventually noticible in the DC power trace on the scope. The point at which this occurs is not fixed.

At all times, I saw a strong intensity fluctuation at around 380-400kHz on the scope whose amplitude fluctuated a fair amount (at least 75mVrms over Vdc=6.5V, but would often be 2 or 3 times that).

I didn't look at the frequency noise while doing this, because the WiFi at the X end was too slow, I'll do more tomorrow in the daytime. 

Two mechanical and two sticdky traps were set to catch univited visitor.

Absolutely no food or food remains into inside garbage cans!!!!!!!!!!!!!!!!!!!!!!!!!

When I came in this afternoon, I saw that the PZT voltage to the PMC had railed. Following the usual procedure of turning the servo gain to zero and adjusting the DC offset, I got the PMC to relock, but the PMCR level was high and the alignment looked poor on the control room monitor. So I tweaked the input alignment on the PSL till I felt it was more reasonable. The view on the control room monitor now looks more like the usual state, and the "REFL (V)" field on the PMC MEDM screen now reads 0.02-0.03 which is the range I remember it being in nominally. 

I think there should be a scientifically based aveluation of the ETMY enclosure so we can make the ETMX better.

Meanwhile I'm counting pieces to move on with the south end table cover.

The Rubidium standard we had sent in for repair and recalibration has come back. I checked the following:

• Powered the unit on - it was locked to the internal rubidium reference within a few minutes as prescribed in the manual.
• After it had locked to the internal reference, I checked that it was able to lock to an external 1pps reference from our GPS timing unit- this too was achieved within a few minutes as prescribed in the manual. 

However, I am still having trouble setting up a serial communications link with the FS725 with a USB-serial adaptor - I've tried with a Raspberry Pi and my Mac (using screen to try and connect), and also using one of the old Windows laptops lying around on which I was able to install the native software supplied by SRS (still using the USB-serial adaptor to establish connection though). Could it be that the unit is incompatible with the USB-serial adaptor? I had specifically indicated in the repair request that this was also a problem. In any case, this doesn't seem to be crucial, though it would have been nice for diagnostics purposes in the future...

I've stored the repaired FS725 inside the electronics cabinet (marked "Eletronics Modules") for now (the other unit was returned to Antonio in W. Bridge some weeks ago). 

https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=24321

This LHO log indicates that EPICS slow down could be due to NFS activity. Could we make some trend of NFS activity on Chiara and then see if it correlates with EPICS flatlines?

I wonder if our EPICS issues frequency is correlated to the Chiara install.

The EOM upstream of the PMC is used as the phase corrector for the FSS/IMC servo. It is also used to apply the 35.5 MHz PDH RF sidebands for the PMC locking. There is a Pomona box which is used to merge the two signals onto a single cable for the EOM.

Does this circuit make sense to anyone?

Fire alarm went off several minutes ago. Talked to security and they said there was no fire. It beeped twice again just now. No one has been working on the IFO today.

PMC locked manually and PRM sus damping restored.

Suspensions are recovered after 4.2 Mag earth quake. No obvoius sign of damage.

Suspensions recovered after 4.4 Mag EQ

We changed the password for controls on nodus this afternoon. We also zeroed out the authorized_keys file and then added back in the couple that we want in there for automatic backups / detchar.

Also did the recommended Ubuntu updates on there. Everything seems to be going OK so far. We think nothing on the interferometer side cares about the nodus password.

We also decided to dis-allow personal laptops on the new Martian router (to be installed soon).

Summary:

I redid this measurement and have now determined the actuator gain to be 4.61 +/- 0.10 MHz/V. This is now pretty consistent with the expected value of ~5MHz/V as reported here.

Details:

I made the following changes to the old methodology:

1. Instead of integrating around the excitation frequency, I am now just taking the ratio of peak heights (phase tracker output / error signal monitor) to determine the actuator gain.
2. I had wrongly assumed that the phase tracker output was calibrated to green Hz and not IR Hz, so I was dividing by two where this was not necessary. I think this explains why my previous measurement yielded an answer approximately half the expected value.

I also took spectra of the phase tracker output and error signal to make sure I was choosing my excitation frequencies in regions where there were no peaks already present (Attachment #1).

The scatter of measured actuator gains at various excitation frequencies is shown in Attachment #2.

Summary:

I've re-measured the noise breakdown for the Y-end AUX PDH system. Spectra are attached. I've also measured the OLTF of the PDH loop, from which the UGF appears to be ~8.5kHz. 

Discussion:

As Eric and Koji pointed out, the spectra uploaded here were clearly wrong as there were breaks in the spectra between decades of frequency. I redid the measurements, this time being extra careful about impedance mismatch effects. All measurements were made from the monitor points on the PDH box, which according to the schematic found here, have an output impedance of 49.9 ohms. So for all measurements made using the SR785 which has an input impedance of 1Mohm, or those which had an SR560 in the measurement chain (also high input impedance), I terminated the input with a 50ohm terminator so as to be able to directly match up spectra measured using the two different analyzers. I'm also using my more recent measurement of the actuator gain of the AUX laser to convert the control signal from V/rtHz to Hz/rtHz in the plotted spectra. 

As a further check, I locked the IR to the Y-arm by actuating on MC2, and took the spectrum of the Y-arm mirror motion using the C1CAL model. We expect this to match up well with the in-loop control signal at low frequencies. However, though the shapes seem consistent in Attachment #2 (light orange and brown curves), I seem to be off by a factor of 5- not sure why. In converting the Y-arm mirror motion spectrum from m/rtHz to Hz/rtHz, I multiplied the measured spectrum by , which I think is the correct conversion factor (FSR/(0.5*wavelength))?

[ericq, Gautam]

We set out to lock a marconi to the IR fiber beat of PSL + AUX X to measure some frequency noise, and failed.

In short, the Marconi's 1.6MHz max external FM isn't enough oomph to stabilize the PLL error signal. It's actually evident on the Agilent that the beat moves around a few times more than that, which I should've noticed sooner... We could briefly "lock" the PLL for a few tenths of a second, but weren't able to get a spectrum from this.

We also tried using the digital phase tracker temperature servo for some help at ~DC; this worked to the extent that we didn't have to twiddle the Marconi carrier frequency to stay on top of the fringes as the beat wandered, but it didn't otherwise stabilize the beat enough to make a difference in locking the PLL.

I suppose one more thing to try is to lock the PSL laser itself to each AUX laser in turn via PLL, and look for different / excess noise.

The Green and IR beat electronics are a in a little bit of disarray at the moment, but it's not like anyone else is going to be using them for the time being...

Our janitor confirmed that Q was not hallucinating about this animal. The dropping size indicating a good size one in the IFO room.

One of the mechanical traps moved from the control room to the east arm, close to the " machine shop " door.

I'm going to get more traps.

The problem here is that the MC displacement noise is leading to large frequency excursions of the PSL beam. Options

1. Feed back the low frequency PLL control signal to the MC2 length to suppress the excursion required by the Marconi. This is better than driving the laser, since the drive to the laser would be squashed by the MC locking loop.
2. Put the beat signal through a divider? Don't know if this makes the Marconi more able to handle it.
3. Turn on the MCL path. this will make the low frequency MC error signal go to the MC length, thereby reducing the low frequency feedback to the NPRO.

Please look around when working close to these five locations. Use flashlights or leave lights on.

These mechanial traps are HAZARDOUS !

No visitors or tours till Monday, Jan 11  2016

Turning on the MCL path (in addition to the MCL FF we always have on) let me lock the PLL for multiple seconds, but low frequency excursions still break it in the end. I was able to briefly observe a level of ~50Hz/rtHz at 1kHz, which may or may not be real. Tomorrow we'll send the PLL control signal to MC2, which should lock it up just fine and give us time to twiddle laser diode current, measure the PLL loop shape, etc. 

In the modern times, people use glue traps to catch rats instead of springs. They are less hazardous to people and don't spread rat fluid on the floor.

IFO restored after 4.5 Mag Banning, Ca earthquake.

Projector light bulb ordered

The new wifi router, a Netgear R6400, has been installed, next to the old one which is disconnected (but not yet removed). 

Same SSID, and I've added only the wireless MAC addresses of viviana, paoloa and asia, the three thinkpads inside.

Qualitatively, dataviewer at the X end seems pretty snappy. I'll do some more quantitative comparison of the two routers at some point soon. I will update the wiki, too.

[ericq, Gautam]

Brief summary of tonights work:

• Locked Marconi to AUX X vs PSL beat at around 320MHz, PSL shutter closed (i.e. both lasers free running)
• Measured control signal spectrum at various laser diode currents, crystal temperatures. Oddly, spectra remained consistent across these variables. 
• Measured OLG of PLL to calibrate into open-loop frequency noise of the beat, found UGF ~30kHz

Our "requirement" for the end laser is as follows: We expect to (and have in the past) achieved ALS sensitivity of 1Hz/rtHz at 100 Hz. If the end PDH loop is 1/f from 100Hz-10kHz, then we have 40dB of supression at 100Hz, meaning the free running AUX laser noise should be no more than 100Hz/rtHz at 100Hz.

So, if we expect both the PSL and AUX lasers to have this performance when free running, we would get the green curve below. We do not. 

I'll post more details about the exact currents, temperatures and include calibrated plots for the >30kHz range later. Here's the OLG for kicks. 

The puzzle continues...

I found some reference for computing "multicoherence," which should properly estimate the potential MISO subtraction potential in situations where the witness channels themselves have nontrivial coherence. Specifically, I followed the derivations in LIGO-P990002. The underlying math is related to principal component analysis (PCA) or gram-schmidt orthogonalization. 

This produced the following results, wherein the Wiener subtraction is still below what the coherences predict. 

I've attached the data and code that produced this plot. 

Here is some of the promised data. As mentioned, changing diode current and crystal temperature didn't have much effect on the frequency noise spectrum; but the spectrum itself does seem too high for our needs. 

At each temperature, we started measuring the spectrum at 1.8A, and stepped the current up, hoping to reach 2.0 A.

At 47.5 C, we were able to scan the current from 1.8 to 2.0 A without much problem. At 49.0C, the laser mode would hop away above 1.95A. At 50.4C it would hop away above 1.85A. The spectra were not seen to change when physically disconnecting the PZT actuation BNC from the rear of the laser. 

The flattening out at the upper end is likely due to the SR560 output noise. I foolishly neglected to record the output spectrum of it, but with the marconi external modulation set to 3.2MHz/V, the few Hz/rtHz above 20k translates to a signal on the order of uV/rtHz, which seems reasonable. 

Data and code attached. 

The next step is to compare this data with the same measurement with the PSL and the AUX laser on the PSL table (or the end Y laser). If these show a lot lower noise level, we can say 1) the x-end laser is malfunctioning and 2) the y-end and AUX laser on the PSL are well low noise.

Here are some results from measuring the PSL / AUX Y beat. 

With the Y end laser, I was able to lock the PLL with a lower actuation range (1.6MHz/V), and with the PSL in both the free-running and MCL locked configurations. (In the latter, I had to do a bit of human-turning-knob servo to keep the control signal from running away). I also took a spectrum with the marconi detuned from the beat frequency, to estimate the noise from the PD+mixer+SR560. 

It looks like the AUX X laser is about 3 times noisier than the Y, though the Y laser looks more like a 10^5 noise-frequency product, whereas I thought we needed 10^4. 

Gautam is investigating the PSL / AUX PSL beat with Koji's setup now. 

Unless this is the limit from the way you guys set up the PLL, it seems like there's no difference between the two lasers that's of any import. So then the locking problem has been something else all along - perhaps its noise in the X-PDF lock somehow? PDH box oscillations?

I configured a new wifi bridge for a GPIB Instruments.

The some facts are described on https://wiki-40m.ligo.caltech.edu/Network

The setting up wasn't so straight forward. I added more details there as a linked page.
One thing I had to do with the martian wifi router was that I had to separate the name of SSIDs for 2GHz and 5GHz networks.

Now the data download from Agilent is super fast!
The first establishing the connection takes the most of the time, and the data transfer takes literary nothing.

controls@pianosa|netgpibdata > time ./netgpibdata -i 192.168.113.167 -d AG4395A -a 10 -f meas01
Connecting to host 192.168.113.167, GPIB 10...
done.
Data will be written into meas01.dat.
Parameters will be written into meas01.par.
Writing measurement data to file...
Writing to the parameter file.

real    0m4.056s
user    0m0.068s
sys    0m0.020s

I took spectra (attached) with the same actuation range (3.2 MHz/V) for the AUX X+PSL and AUX Y+PSL combinations (PSL shutter closed) just to keep things consistent. It looks like there is hardly any difference between the two combinations - could the apparent factor of 3 worse performance of the X end laser have been due to different actuation ranges on the Marconi? 

I've not managed to take a spectrum for the proposed replacement Lightwave laser on the PSL table, though with Eric's help, I've managed to find the beatnote (at a temperature of 53.0195 degrees). I had to do some minor alignment tweaking for this purpose on the PSL table - the only optics I touched were the ones in the pink beam path in attachments 1 and 2 in this elog (the setup used to make the measurement is also qualitatively similar to attachment 3 in the same elog, except for the fact that we are feeding back to the Marconi and not the laser - a detailed sketch with specific components used will be put up later). I'll try and measure the frequency noise of this laser as well over the weekend and put up some spectra. 

With regards to possibly switching out the Lightwave on the PSL table for the (faulty?) Innolight at the X end, I've verified the following:

• The beam-height from the Lightwave on the mount it is currently sitting on is the same as that from the Innolight on the X end table.
• There is sufficient space on the X end table to house the Lightwave laser+mount

It remains to characterize the beam coming out from the Lightwave laser and do a mode matching calculation to see if we can use the same optics currently in place (with slight rearrangement) to realize a satisfactory mode-matching solution - I've obtained a beam profiler to do this from Liyuan and have the software setup, but have yet to do the beam scan - the plan is to do this on the SP table, but we've put off moving the Lightwave laser off the PSL table until we (i) establish conclusively that the X end laser is malfunctioning and (ii) check the frequency nosie of the Lightwave relative to the Aux lasers currently at the ends. 

The area around the Marconi is in a little disarray at the moment with a bunch of cables, SR560s, analyzers etc - I didn't want to disconnect the measurement setup till we're done with it. I have however turned both IR beat PDs on the PSL table off, and have reconnected the Marconi output to the Frequency Generation Unit and have set the carrier back to 11.066209MHz, +13dBm. 

EDIT 01/12/2016 6PM: I've updated the plots of the in-loop spectra such that they are calibrated throughout the entire domain now. I did so by inferring the closed-loop transfer function (G/(1-G)) from the measured open-loop transfer function (G), and then fitting the inferred TF using vectfit4 (2 poles). The spectra were calibrated by multiplying the measured spectra by the magnitude of the fitted analytic TF at the frequency of interest.

EricQ brought back one of the Marconis that was borrowed by the Cryo lab to the 40m today (it is a 2023B - the Marconi used for all previous measurements in this thread was 2023A). Koji had suggested investigating the frequency noise injected into the PLL by the Marconi, and I spent some time investigating this today. We tried to mimic the measurement setup used for the earlier measurements as closely as possible. One Marconi was used as a signal source, the other as the LO for the PLL loop. All measurements were done with the carrier on the signal Marconi set to 310MHz (since all our previous measurements were done around this value). We synced the two Marconis by means of the "Frequency Standard" BNC connector on the rear panel (having selected the appropriate In/Out configurations digitally first). Two combinations were investigated - with either Marconi as LO and signal source. For each combination, I adjusted the FM gain on the Marconi (D in the plot legends) and the overall control gain on the SR560 (G in the plot legends) such that their product remained approximately constant. I measured the PLL OLG at each pair to make sure the loop shape was the same throughout all trials. Here are the descriptions of the attached plots:

Attachment #1: 2023A as LO, 2023B as source, measured OLGs

Measured OLG for the various combinations of FM gain and SR560 gain tested. The UGF is approximately 30kHz for all combinations - the exceptions being D 1.6MHz, G=1e4 and D=3.2MHz, G=1e4. I took the latter two measurements just because these end up being the limiting values of D for different carrier frequencies on the Marconi.

Attachment #2: 2023A as LO, 2023B as source, measured spectra of control signal (uncalibrated above 30kHz)

I took the spectra down to 2Hz, in two ranges, and these are the stitched versions. 

Attachment #3: 2023B as LO, 2023A as source, measured OLGs

Attachment #4: 2023B as LO, 2023A as source, measured spectra of control signal (uncalibrated above 30kHz)

So it appears that there is some difference between the two Marconis? Also, if the frequency noise ASD-frequency product is 10^4 for a healthy NPRO, these plots suggest that we should perhaps operate at a lower value of D than the 3.2MHz/V we have been using thus far? 

As a quick trial, I also took quick spectra of the PLL control signals for the PSL+Aux X and PSL+Aux Y beat signals, with the 2023B as the LO (Attachment #5). The other difference is that I have plotted the spectrum down to 1 Hz (they are uncalibrated above 30Hz). The PSL+Y combination actually looks like what I would expect for an NPRO (for example, see page 2 of the datasheet of the Innolight Mephisto) particularly at lower frequencies - not sure what to make of the PSL+X combination. Also, I noticed that the amplitude of the PSL+Y beatnote was going through some large-amplitude (beat-note fluctuates between -8dBm and -20dBm) but low frequency (period ~10mins) oscillations. This has been observed before, not sure why its happening though. 

More investigations to be done later tonight.

Gautam will soon follow up with detailed analysis, but here is a brief summary of some of our activities and findings.

• Two Marconis were beat together in various ways, we figured the noise added by turning on external modulation didn't make us happy. 
• I locked the AUX X laser to the PSL via PZT. I'm more likely to believe we're seeing real broadband laser noise in this configuration; locking the the PSL laser to the IMC brought the noise down in a reasonable way. The PLL bandwidth was a smidge over 100k.
• We saw a factor ~6 increase in noise when changing the diode current from 1.8 to 1.96A. We'll be following this up at more temperatures and currents soon. 
• Gautam will verify the AUX X laser PZT calibration tomorrow, and post calibrated spectra of this increase. 

Please note that there is a long BNC cable still laid out from the IOO rack area to the X end table; watch your step!

PMC locked and ETMX suspension damping restored.

Air condition maintenance is happening. It should be done by 10am