Today we were looking at the MC TFs and pulled out the FSS box to measure it. We took photos and removed a capacitor with only one leg.

Still, we were unable to see the weird, flat TF from 0.1-1 MHz and the bump around 1 MHz. Its not in the FSS box or the IMC servo card. So we looked around for a rogue Pomona box and found one sneakily located between the IMC and FSS box, underneath some cables next to the Thorlabs HV driver for the NPRO.

It was meant to be a 14k:140k lead filter (with a high frequency gain of unity) to give us more phase margin (see elog 4366; its been there for 3.5 years).

From the comparison below, you can see what the effect of the filter was. Neither the red nor purple TFs are what we want, but at least we've tracked down where the bump comes from. Now we have to figure out why and what to do about it.

* all of the stuff above ~1-2 MHz seems to be some kind of pickup stuff.

** notice how the elog is able to make thumbnails of PDFs now that its not Solaris!

Some TFs of the TTFSS box

Today we decided to continue to modify the TTFSS board.

The modified schematic can be found here: https://dcc.ligo.org/D1400426-v1 as part of the 40m electronics DCC Tree.

What we did

1) Modify input elliptic filter (L1, C3, C4, C5) to give zero and pole at 30 kHz and 300 kHz, respectively. L1 was replaced with a 1 kOhm resistor.  C3 was replaced with 5600 pF. C4 and C5 were removed. So the expected locations of the zero and pole were at 28.4 kHz and 256 kHz, respectively. This lead filter replaces the Pomona box, and does so without causing the terrible resonance around 1 MHz.

2) Removed the notch filters for the PC and fast path. This was done by removing L2, L3, and C52.

At this point we tested the MC locking and measured the transfer function. We successfully turned up the UGF to 170kHz and two super-boosts on.

3) Now a peak at 1.7MHz was visible and probably causing noise. We decided to revert L2 and adjusted C50 to tune the notch filter in the PC path to suppress this possible PC resonance. Again the TF was measured. We confirmed that the peak at 1.7MHz is at -7dB and not causing an oscillation. The suppression of the peak is limited by the Q of the notch. Since its in a weird feedback loop, we're not sure how to make it deeper at the moment.

4) The connection from the MC board output now goes in through the switchable Test1 input, rather than the fixed 'IN1'. The high frequency gain of this input is now ~4x higher than it was. I'm not sure that the AD829 in the MC board can drive such a small load (125 Ohms + the ~20 Ohms ON resistance of the MAX333A) very well, so perhaps we ought to up the output resistor to ~100-200 Ohms?

Also, we modified the MC Servo board: mainly changed the corner frequencies of the Super Boost stages and some random cleanup and photo taking. I lost the connecting cable from the CM to the AO input (unlabeled).

1.  The first two Super Boost stages were changed from 20k:1k to 10k:500 to give us back some phase margin and keep the same low freq gain. I don't really know what the gain requirement is for this servo here at the 40m. The poles and zeros were chosen for iLIGO so as to have the frequency noise be 10x less than the SRD at 7 kHz.
2. The third Super Boost (which we never used) was changed from 10k:500 to ~3k:150 (?) just in case we want a little more low freq gain.
3. There was some purple vestigial wiring on the back side of the board with a flying resistor; I think this was a way to put a DC offset in to the output of the board, but its not needed anymore so I removed it.

 I've updated the scripts for the MC auto locking. Due to some permissions issues or general SVN messiness, most of the scripts in there were not saved anywhere and so I've overwritten what we had before. 

After all of the electronics changes from Monday/Tuesday, the lock acquisition had to be changed a lot. The MC seems to catch on the HOM more often. So I lowered a bunch of the gains so that its less likely to hold the HOM locks.

A very nice feature of the Autolocker running on megatron is that the whole 'mcup' sequence now runs very fast and as soon as it catches the TEM00, it gets to the final state in less than 2 seconds.

I've also increased the amplitude of the MC2 tickle from 100 to 300 counts to move it through more fringes and to break the HOM locks more often. Using the 2009 MC2 Calibration of 6 nm/count, this is 1.8 microns-peak @ 0.03 Hz, which seems like a reasonable excitation.

Using this the MC has relocked several times, so its a good start. We'll have to work on tuning the settings to make things a little spicier as we move ahead.

That directory is still in a conflicted state and I leave it to Eric/Diego to figure out what's going on in there. Seems like more fallout from the nodus upgrade:

controls@chiara|MC > svn up

svn: REPORT of '/svn/!svn/vcc/default': Could not read chunk size: Secure connection truncated (https://nodus.ligo.caltech.edu:30889)

Koji and I noticed that there was a comb* of peaks in the MC and FSS at harmonics of ~37 kHz. Today I saw that this shows up (at a much reduced level) even when the input to the MC board is disconnected.

It also shows up in the PMC. At nominal gains, there is just the 37 kHz peak. After tweaking up the phase shifter settings, I was able to get PMC servo to oscillate; it then makes a comb, but the actual oscillation fundamental is 1/3 of 37 kHz (some info on Jenne from elog 978 back in 2008).

Not sure what, if anything, we do about this. It is curious that the peak shows up in the MC with a different harmonic ratio than in the PMC. Any theories?

Anyway, after some screwing around with phase and amplitude of the RF modulation for the PMC from the phase shifter screen**, I think the gain is higher in the loop and it looks like the comb is gone from the MC spectrum.

Another clue I notice is that the PCDRIVE mad times often are coincident with DC shifts in the SLOWDC. Does this mean that its a flakiness with the laser? While watching the PCDRIVE output from the TTFSS interface board on a scope, I also looked at MIXER mon. It looks like many of the high noise events are associated with a broadband noise increase from ~50-140 kHz, rather than some specific lines. Don't know if this is characteristic of all of the noisy times though.

* this 'comb' had several peaks, but seem not be precise harmonics of each other: (f3 - 3*f1)/f3 ~ 0.1%

** I think we never optimized this after changing the ERA-5 this summer, so we'd better do it next.

 *** UPDATE: the second plot show the comparison between the new quiet and noisy states. Its just a broad bump.

 * PMC + MC were unlocked when I came in.

* I fiddled around some more with the mcup/down scripts to make locking snappier. The locking was breaking the PMC lock often, so I re-enabled the MC servo board output limiter during acquisition. It is disabled in the MC UP script.

* Re-measured the MC OLG. Still OK.

* Measured the PZT / EOM crossover (aka the FAST / PC crossover) using the connectors on Koji's summing box. With the FAST gain at 18 dB, the crossover is ~10 kHz. Looks way to shallow. Plots to follow.

* I finally discovered today that the PMC PZT stroke is what's causing the main mis-alignment of the beam going to the IMC. By relocking at a few positions, I could see that the IOO QPDs have steps when the PMC relocks. So the IO beam wander is NOT due to temperature effects on the optics mounts of the PSL table. I wonder if we have a large amount of length to angle coupling or if this is the same as the OMC PZTs ?

P.S. I found that someone is using a temporary bench power supply to power the summing box between the TTFSS and the Thorlabs HV driver...whoever did this has ~48 hours to hook up the power in the right way or else Koji is going to find out and lose it and then you have to wear the Mickey Mouse hat.

http://www.arroyoinstruments.com/files/Arroyo-UsingBenchPowerSupplies_11Apr.pdf

The first attachment shows the OLG measurements with 2 different values of the fast gain (our nominal FG is 18 dB). You can see that the higher gains produce some crossover instability; when tuning the gain we notice this as an increase in the PCDRIVE rms channel.

The second attachment shows the measurement of the 'crossover'. Its really just the direct measurement of the IN1 / IN2 from the FAST summing box, so its the crossover measurement where the OLG is high.

 This afternoon, I labelled both ends of this cable, and reconnected it to the MC servo board. 

Since Rana's overhaul of the IMC, the FSS input offset had been sitting at zero. 

Over the last day or so, I had noticed the MC refl wall striptool trace looking noisier, and earlier this evening, we were suffering from a fair amount of downtime due to IMC unlocks, and failure to autolock for times on the order of ten minutes. 

While we had used ezcaservo for this in the past, I set the FSS offset manually tonight. Namely, I popped open a dataviewer trace of MC_F, and scanned the FSS offset to make MC_F go to zero. It required a good amount of offset, 4.66 V according to the FSS screen. I did this while the FSS slow servo was on, which held the FSS Fast output at zero. 

That was four hours ago; MC_F is still centered on zero, and we have not had a single IMC unlock since then. The MC refl trace is thinner too, though this may be from nighttime seismic. 

I've been looking into the data of Jan 06 and Jan 15 taken during daytime, as the night before we left the PRC aligned in order to allow the IFO to flash; the purpose is to find out if some flashes from the IFO could propagate back to the IMC and cause it to lose lock; I will show here a sample plot, all of the others are in the archive attached.

My impression is that these locklosses of the IMC are not caused by these flashes: the signals MC_[F/L] seem quite stable until the lock loss, and I don't see any correlation with what happens to REFLDC that could cause the lockloss (apart from its drop as a consequence of the lockloss itself); in addition, in most occasions I noticed that the FSS started to go crazy just before the lock loss, and that suggests me that the lockloss source is internal to the IMC.

I can't see anything strange happen to MC_TRANS either as long as the IMC is locked, no fluctuations or weird behaviour. I also plotted the MC_REFL_SUM channel. but it is too slow to be useful for this kind of "hunt".

The MC autolocker hasn't been so snappy recently, and has been especially fussy today. Previously, the mcup script was triggered immediately once the transmission was above a certain threshold. However, this could waste time if it was just an errant flash. Hence, I've added a 0.5 second delay and a second threshold check before mcup is triggered. 

After breaking the lock 5ish times, it does seem to come back quicker.

The X-end IR Trans path was cleaned up.

I have been investigating the Xarm ASS issue. The Xarm ASS sensors behaved not so straight forward.
I went to the X-end table and found some suspect of clipping and large misalignmnet in the IR trans path.
Facing with the usual chaos of the end table, I decided to clean-up the IR trans path.

The optical layout is now slightly better. But the table is, in general, still dirty with bunch of stray optics,
loose cables and fibers. We need more effort to make the table maintained in a professional manner.

- Removed unnecessary snaking optical path. Now the beam from the 1064/532 separator is divided by a 50-50 BS before the QPD without
any other steering mirrors. This means the spot size on the QPD was changed as well as the alignment. The spot on the QPD was aligned
with the arm aligned with the current (=not modified) ASS. This should be the right procedure as the spot must be centered on the end mirror
with the current ASS.

- After the 50-50 BS there is an HR steering mirror for the Thorlab PD.

- A VIS rejection filter was placed before the 50-50 BS. The reflection from the filter is blocked with a razor blade dump.

Important note to everyone including Steve:
The transmission of the VIS rejection filter at 1064nm is SUPER angular sensitive.
A slight tilt causes significant reduction of 1064nm light. Be careful.

- As we don't need double VIS filter, I removed the filter on the QPD.

- X-End QPD was inspected. There seemed large (+/-10%) gain difference between the segments.
They were corrected so that the values are matched when the beam is only on one segment.
The corrections were applied at C1:SUS-ETMX_QPDx_GAIN (x=1, 2, 3, or 4).

I decided to put "-20dB" filters on C1:SUS-ETMi_QPD_SUM and C1:SUS-ETMi_TRY (i = X or Y)
in order to make their gain to be reasonable (like 0.123 instead 0.000123 which is unreadable).
Jenne's normalization script reads relative values and the current gains instead of the absolute values.
Therefore the script is not affected.

[Koji, Manasa]

We cleared up some optics and optomechanics at the X end table that are not being used and moved them to the SP table. [Ed by KA: They seemed to be leftover of the other projects. I blame them]

The filters were already in the damping loops but missing the MC WFS path. I checked that these accurately cover the peaks at 16.5 Hz and 23.90 and 24.06 Hz.

Jenne and I were musing the other night that the PC drive RMS may have a "favorite" laser temperature, as controlled by the FSS Slow servo; maybe around 0.2.

I downloaded the past 30 days of mean minute trend data for MC Trans, FSS Slow and PC Drive, and took the subset of data points where transmission was more than 15k, and the FSS slow output was within 1 count of zero. (This was to exclude some outliers when it ran away to 3 for some days). This was about 76% of the data. I then made some 2D histograms, to try and suss out any correlations. 

Indeed, the FSS slow servo does like to hang out around 0.2, but this does not seem to correlate with better MC transmission nor lower PC drive.

In the following grid of plots, the diagonal plots are the 1D histograms of each variable in the selected time period. The off diagnoal elements are the 2D histograms. They're all pretty blob-y, with no clear correlation. 

In the attached plot you can see that the MC REFL fluctuations started getting larger on Feb 24 just after midnight. Its been bad ever since. What happened that night or the afternoon of Feb 23?
The WFS DC spot positions were far off (~0.9), so I unlocked the IMC and aligned the spots on there using the nearby steering mirrors - lets see if this helps.

Also, these mounts should be improved. Steve, can you please prepare 5 mounts with the Thorlabs BA2 or BA3 base, the 3/4" diameter steel posts, and the Polanski steel mirror mounts? We should replace the mirror mounts for the 1" diameter mirrors during the daytime next week to reduce drift.

I was asked by Koji to point out where a schematic of the triple resonant circuit is.
It seems that I had posted a schematic of what currently is installed (see elog 4562 from almost 4 yrs ago!).

(Some transfomer story)
Then I immediately noticed that it did not show two components which were wideband RF transformers. In order to get an effective turns ratio of 1:9.8 (as indicated in the schematic) from a CoilCrfat's transformer kit in the electronics table, I had put two more transformers in series to a PWB1040L which is shown in the schematic. If I am not mistaken, this PWB1040L must be followed by a PWB1015L and PWB-16-AL in the order from the input side to the EOM side. This gives an impedance ratio of 96 or an effective turns ratio of sqrt(96) = 9.8.

(An upgrade plan document)

Also, if one wants to review and/or upgrade the circuit, this document may be helpful:
https://wiki-40m.ligo.caltech.edu/Electronics/Multi_Resonant_EOM?action=AttachFile&do=get&target=design_EOM.pdf
This is a document that I wrote some time ago describing how I wanted to make the circuit better. Apparently I did not get a chance to do it.

MC Refl alignment follow up: the alignment from last night seems still good today. We should keep an  on the MC WFS DC spots and not let them get beyond 0.5.

The autolocker was struggling to lock the IMC. I disabled the autolocker and locked the IMC manually. It seems happy right now. 

With PMC trans at 0.717 counts, the IMC trans sum is ~15230.

Polaris mounts ordered.

Are the two  visible small srews holding the adapter plate only?

If yes, it is the weakest point of the IOO path.

I've done a few things to start waking up the IFO after it's week of conference-vacation.

PMC trans was at 0.679, aligned the input to the PMC, now it's up at 0.786.

MC transmission was very low, mostly from low PMC transmission.  Anyhow, MC locked, WFS relieved so that it will re-acquire faster.

Many of the optics had drifted away. AS port had no fringing, and almost every optic was far away from it's driftmon set val.  While putting the optics back to their driftmon spots, I noticed that some of the cds.servos had incorrect gain.  Previously, I had just been using the ETMX servo, which had the correct gain, but the ITMs needed smaller gain, and some of the optics needed the gain to be negative rather than positive.  So, now the script ..../scripts/SUS/DRIFT_MON/MoveOpticToMatchDriftMon.py has individually defined gains for the cds.servo. 

Next up (after lunch) will be locking an aligning the arms.  I still don't have MICH fringing at the AS port, so I suspect that the ASS will move some of the optics somewhat significantly (perhaps the input tip tilts, which I don't have DRIFT_MON for?)

If the optics moved such amount, could you check the PD alignment once the optics are aligned?

X arm ASS is having some issues. ITMX oplev was recentered with ITMX in a good hand-aligned state. 

The martian wifi network wasn't showing up, so I power cycled the wifi router. Seems to be fine now. 

To achive the same beam height each components needs their specific post height.

We have 2.625" tall, 3/4" OD SS posts for Polaris K1 mirror mounts: 20 pieces

Ordered Newport LH-1 lens mounts with axis height 1.0

The PMC is seated on 3 SS balls and it is free to move. I'm sure it will move in an earthquake. Not much, because the input and output K1 mirror frame will act as an earthquake stop.Atm2

Are there a touch of super glue on the balls? No, but there are V grooves at the bottom and on the top of each ball.Atm3

I've been fiddling with the mode cleaner and green beat box today, to try and get an absolute frequency calibration for MC2 motion. The AC measurements have all turned out weird, I get fractional power laws instead of the 1/f^2 that we expect from the MC2 pendulum. At DC, I get a rough number of 15 green kHz per MC2 count, but this translates to ~7e-10 m/count which is in contrast to the 6e-9 m/count from 2009. I will meditate on this a bit. 

In any case, while working at the IOO rack, I tuned the 11MHz modulation frequency, as was done in ELOGs 9324 and 10314, by minimizing one of the beats of the 11MHz and 29.5MHz sidebands. 

The new modulation frequency / current IMC FSR is 11.066209 +- 1 Hz, which is a only a few ppm change from the tuning from last July.

This implies a IMC round trip length of 27.090800m +- 2um.

Attached is a plot showing the beat of 55-29.5 going down as I changed the marconi frequency. 

I found that FSS SLOW servo is not engaged. Is this intentional test to keep the NPRO temp constant?
This is making the FSS Fast unhappy (~ -7.5V right now).

Yes, I had turned it off while looking for the PSL/X AUX beat, and forgot to turn it back on.

I will post an elog with more detail this evening, but I found a temperature which restored the X green beatnote at its nominal amplitude (-30dBm) with no mode hops within +-1 IR beat GHz, and offloaded the slow offset slider to the X-end laser crystal dial. I will look for the Y beatnote after dinner. 

Currently the control room analyzer is hooked up to recieve the Y IR and green beats; no X signals. 

We spent an hour today to put the AOM back in the beam before the PMC and verified that the diffraction is working.

1. The fuse holder was missing from the rack. We inserted a 5A fuse. We expect that the quiesscent draw is < 0.5 A. The power is from the +24V Sorensen supply.
2. The alignment was tricky, but we optimized it as well as we could in translation and the RZ direction. Its a fixed mount still.
3. We noticed that according to the datasheet, the polarization is wrong! It wants S-Pol light and we're giving it P-Pol. How come no one noticed this? We expect that the efficiency is reduced because of this. We (Steve) need to brainstorm what kind of mount we can use there to mount it at 90 deg to the plane of the table.
4. The lens after the AOM has f = +400 mm. The distance from the AOM to the lens is ~800-900 mm so its not so terrible. However, if someone were to put the AOM halfway between the turning mirrors there, the beam diffraction would be canceled.
5. The AOM input impedance seems to be 50 Ohm as advertised. The previous Koji entry claim of 25 Ohm is mysterious. We checked the Ohmage by sending a signal into the AM input of the AOM using the DS345 which as a 50 Ohm output. 1 Vpp from the DS345 made 1 Vpp on the input of the AM input as measured by Oscope connected by T with high impedance setting.
6. With 0.5 V offset and a 1 Vpp signal, we get ~20-25% modulation of the power.
7. We have left it running with a 4444.4 Hz modulation and a small amplitude. This is to see if we can use this to measure the cavity poles of the MC and the arms.
8. We noticed some hash on the Teed input monitor. It was backstreaming of the RF drive. Whoever uses this thing in an ISS feedback ought to make sure to put an RF choke between the servo and this AOM driver.

We also removed a 50/50 pickoff mirror which was used to take one of the NPRO -> EOM polarizer reject beams and send it across the table into a floppy dump. Its now hitting a closer floppy dump. Let's stop using these crappy anodized aluminum flappers anywhere, Steve.

We also noticed that the PMC REFL path uses a W1 from CVI to send the PMC reflection to the REFL RFPD. The dim beam from the AR coated surface is being used rather than the bright beam from the uncoated surface. Ooops. Steve, can you please order another W1 for 1064 from CVI, but get it with a 2-3 deg wedge angle? This one has a wedge which is too small.

after re-aligning the beam into the PMC, I touched up the steering mirros into the IOO QPDs so that the beams are now centered again. Please don't adjust these references without prior authorization and training.

This plot shows the 10-minute trends for these QPDs over the last 400 days.

Our noise cancellation SURFS will be doing online subtraction on the mode cleaner length, among other things. 

I made a measurement of the MC2 actuator transfer function by injecting noise from 1-100Hz into LSC_MC2_EXC for about 15 minutes, then estimating the TF from MC2_OUT to IOO_MC_L with CSD/PSD. The inverse of this TF will be applied to their Wiener target data to give us the direct subtration filter we want. 

I figured I would post the results here for posterity. The last time this seems to have been done is in ELOG 5900. There are some differences found here, the effective Q of the 1Hz pendulum resonance seems lower, and the behavior above 20Hz has definitely changed. 

IIR fits will be done by one of the SURFs to be used in their Wiener filter calculations. 

Data attached!

Eric downloaded MC2 to MCL transfer function data (H) as well as its inverse, MCL to MC2 (Hinv). He also downloaded new MCL and MC2 data.

I used vectfit to fit the MC2 to MCL transfer function, 

The ZPK parameters for this fit were,

Zeros              1278.36719876674 + 0.00000000000000i
                   -100.753249679343 + 0.00000000000000i
                   -18.6014192997845 + 13.0294910760217i
                   -18.6014192997845 - 13.0294910760217i

Poles              -1.11035771175328 + 7.03549674098987i
                   -1.11035771175328 - 7.03549674098987i
                   -18.8655320274072 + 0.00000000000000i
                   -690.294337433234 + 0.00000000000000i

Gain               0.00207206036014220

Using the above vectfit model, I filtered the raw MC2 signal to get 'MCL'. The PSD's of the raw MCL data and the filtered MC2 result is shown below,

The lack of accuracy of the transfer function at replicating MCL at frequencies lower than 0.7Hz is expected, the vectfit model I generated fails to follow accurately the raw transfer function data. My question: Does it matter? My guess: Probably not. In order to mitigate seismic noise from the mode cleaner we are mainly concerened with the 1-3 Hz region.

I also used vectfit to fit the transfer function for MCL to MC2,

This one was harder to fit accurately for some reason, I could do it with four pairs of zeros and poles but it took some preweighting.

The ZPK parameters for the above fit were, 

Zeros              0.173068278283995 + 0.00000000000000i
                   0.995140531040529 + 0.0268079821980457i
                   0.995140531040529 - 0.0268079821980457i
                   0.894476816129099 + 0.00000000000000i

Poles              -19.9566906920707 + 18.0649464375308i
                   -19.9566906920707 - 18.0649464375308i
                   -109.275971483008 + 0.00000000000000i
                   -1791.88947801703 + 0.00000000000000i

Gain               1237.46417532120

Similarly, using this ZPK model, I filtered the MCL signal to get 'MC2'. I plotted the PSD for the MC2 signal and the filtered MCL to get,

Again, the lack of accuracy of the filtered MC2 at replicating MCL below 0.7 Hz and above 12 Hz is due to the inverse transfer function failing to converge in these ranges.

Rana pointed out that another way to mitigate seismic motion at in the mode cleaner would be to look at the YAW and PITCH output  channels of the WFS sensors that control the angular alignment of the mode cleaner. 

I downloaded 45 mins of data from the following two channels:

C1:IOO-WFS1_YAW_OUT_DQ

C1:IOO-WFS1_PIT_OUT_DQ

And did some quick offline Wiener filtering with no preweighting, the results are shown in the PSD's below,

and

I'm quite surprised at the Wiener subtraction obtained for the YAW signal, it required no preweighting and there is about an order of magnitude improvement in our region of interest, 1-3 Hz. The PIT channel didn't do so bad either.

I'm attaching a SISO IIR Wiener filter here for reference purposes that will go online either tonight or tomorrow evening. This is a first test to convince myself that I can get this to work, MISO IIR filters are close to being ready and will soon be employed. 

This Wiener filter uses the STS-X channel as a witness and MCL as target. The bode plot for the filter is shown below,

The performance of the FIR and IIR Wiener filters and the ammount of subtraction achive for MCL is shown below,

Output from quack to be loaded with foton: filter.zip

K bye.

Last night we finally got some online subtraction going. The filter used is described in the post this eLOG is @eLOG 11488. 

The results were as follow:

The filter worked as expected when subtracting noise out of MCL,

There is about a factor of 6 subtraction at the ~3Hz resonant peak. The static IIR filter predicted a factor of 6-7 subtraction of this peak as well.

The 1.2 Hz resenonant feature improved by a factor of 3. This should improve quite drastically when I implement the y-channel of the T240 seismo.

There is some high frequency noise being injected, not very noticeable, but present. 

We then took a look at the power in the MC when the filter was on,

The power being transmitted in the cavity was not as stable as with the feedforward on. We believe that the filter is not at fault for this as Eric mentioned to me that the MC2 actuator lacked some sort of compensation that I need to understand a bit better.

YARM was then locked when the filter was on and we took a look at how it was doing. There was stationary sound arising from the locking of the YARM, leading us to believe that the filter might have injected some noise in the signal. IT DID.

The filter injected nasty high frequency noise at YARM from 11 Hz and on. This is to be expected since the filter did not roll off to zero at high frequencies. Implementing a 1/f rolloff should mitigate some of the injected noise.

 Also, as one can see above, subtraction by around a factor of 2 or so, was induced by the mode cleaner feedforward subtraction.

Today I finished fitting the transfer function to a vectfit model for seismometers T240_X and T240_Y, and then used these to filter noise online from the mode cleaner. 

The Bode plot for T240_X is in figure 1, and T240_Y is in figure 2. I made sure to weight the edges of the fit so that no DC coupling or excessive injection of high frequency noise occurs at the edges of the fit.

I used C1:IOO-MC_L_DQ as the first channel I filtered, with C1:IOO-MC_L_DQ(RMS) for RMS data. I took reference data first, without my filter on. I then turned the filter on and took data from the same channel again. The filtered data, plotted in red, subtracted from the reference and did not inject noise anywhere in the mode cleaner. 

I also looked at C1:LSC-YARM_OUT_DQ and C1:LSC-YARM_OUT_DQ(RMS) for its RMS to see if noise was being injected into the Y-Arm when my filter was implemented. I took reference data here also, shown in blue, and compared it to data taken with the filter on. My filter, in pink, subtracted from the Y-Arm and injected no noise in the region up to 10 Hz, and only minimal noise at frequencies ~80 Hz. Frequencies this high are noisy and difficult to filter anyways, so the noise injection was minimal in the Y-Arm. 

In my previous elog:11492, I stated that in order to improve the subtraction and reduce the injection of high frequency noise we want the filter's magnitude to have a 1/f rolloff.

I implemented this scheme on the filter SISO filter previously analyzed. The results are shown below.

The filters bode plot:

The nice 1/f rollof is the main change here. Everything else remained pretty much the same.

The predicted FIR and IIR subtractions:

Everything looks right but that hump at 8 Hz. I used 8 pairs of poles/zeros to get this subtraction.

The online MCL subtraction:

This looks better than I expected. One has to keep in mind that I ran this at 1 AM. I wonder how well this filter will do during the noisier hours of the day. The RMS at high frequencies doesn't look great, there will definitely be noise being injected into the YARM signal at high frequencies.

Measuring the YARM signal:

There is still noise being injected on YARM but it is definitely much better than the previous filter. I'm thinking about doing some IIR subtraction on the arms now to see if I can get rid of the noise that is being injected that way, but before I embark on that quest I will rething my prefiltering.

The plot below shows the ratio of the unfiltered versus filtered ASDs for the FIR and IIR subtraction predictions as well as for the measured online IIR subtraction. Positive dB means better subtraction.

Last night I performed some MISO FF on MCL using the T240-X and T240-Y as witnesses. Here are the results:

Filter:

T240-X

T240-Y

Training data + Predicted FIR and IIR subtraction:

Online subtraction results:

MCL

YARM

Subtraction performace:

Last night, I also worked on a "better" (an improvement, I thought) of the MISO Wiener filter (T240-X and T240-Y witnesses) IIR filter. The FF results are shown below:

Filter:

T240-X

T240-Y

Training data + Predicted FIR and IIR subtraction:

Online subtraction results:

MCL

YARM

Subtraction performace:

 Although the predicted FIR and IIR results are "better" than the previous MISO filter, the subtraction performance for this filter is marginally better if not worse (both peak at 15 dB, in slightly different regions). 

Since I will need to do transfer function measurements in order to implement FF for the arms and the MC2's yaw and pitch channels, I decided to practice this by replicating the transfer function measurement Eric did for MC2 to MCL. I followed his procedure and the data that I aquired for the TF looked as shown below,

About five minutes of data were taken (0.05 Hz resolution, 25 averages) by injecting noise from 1 to 100 Hz. The TF coherence looked as below,

Yesterday I adjusted the preweighting of my IIR fit to the transfer function of MC2, and also managed to reduce the number of poles and zeros from 8 to 6, giving a smoother rolloff. The bode plots are pictured here:

The predicted IIR subtraction was very close to the predicted FIR subtraction, so I thought these coefficients would lead to a better online filter.

However, the actual subtraction of the MCL was not as good and noise was injected into the Y arm.

The final comparison of the subtraction factors between the online and offline data showed that the preweighting, while it improved the offline subtraction, needs more work to improve the online subtraction also.

Today I tried and doubly-improved SISO FF filter on MCL. This filter has a stronger rolloff than the previous SISO filters I have tried. The rolloff most definelty helped towards reducing the ammount of noise being injected into YARM. Below is the usual stuff:

Filter:

T240-X (SISO)

Training data + Predicted FIR and IIR subtraction:

 

Online subtraction results:

MCL

YARM

MCL TRANS

 

 

 

Subtraction performace:

• When plotting the subtraction performance, we mainly care about the 0.5 - 10 Hz band, so we care about the RMS in this band. Don't integrate over the whole band.
• When calculating the Wiener filter, you must use the pre-weighting so as to not let the Wiener residual be dominated by the out of band signals. We don't want the filter to try to do anything outside of the 0.5 - 10 Hz band.
• Somehow, we want to assign a penalty for the filter to have high frequency gain. We do NOT want to slap on an ad-hoc low pass filter. The point of the Wiener filtering is to make the optimum.
• What is the reason for the poor filter performance from 0.5 - 2 Hz ? If we use the frequency domain (Dmass) subtraction technique, we can do better, so there's some inefficiency in this process.
• we're getting too much of the 3 Hz stack mode coupling into MCL. I think this means that our damping filters should be using RG around the suspension eigenmodes rather than just simple velocity damping. We had this years ago, but it caused some weird interaction with the angular loops...to be puzzled out.

PMC needed to be locked manually.

A little more information about the IMC loop tweak...

I increased the overall IMC loop gain by 4dB, and decreased the FAST gain (which determines the PZT/EOM crossover) by 3dB. This changed the AO transfer function from the blue trace to the green trace in the first plot. This changed the CARM loop open loop TF shape from the unfortunate blue shape to the more pleasing green shape in the second plot. The red trace is the addition of one super boost. 

Oddly, these transfer functions look a bit different than what I measured in March (ELOG 11167), which itself differed from the shaping done December of 2014 (ELOG 10841). 

I haven't yet attempted any 1F handoff of the PRMI since relocking, but back when Jenne and I did so in April, the lock was definitely less stable. My suspicion is that we may need more CARM supression; we never computed the loop gain requirement that ensures that the residual CARM fluctuations witnessed by, say, REFL55 are small enough to use as a reliable PRMI sensor.

I should be able to come up with this with data from last night. 

Earlier today I constructed yet another SISO filter for MCL. The one thing that stands out about this filter is its strong roll off . This prevents high frequency noise injection into YARM. The caviat, filter performance suffered  quite a bit, but there is subtraction going on.

I have realized that Vectfit lacks the ability of constraining the fits it produces, (AC coupling, rolloff, etc) even with very nitpicky weighting. So the way I used vectfit to produce this filter will be explained in a future eLOG, I think it might be promising. 

Anyways, the usual plots are shown below. 

Filter:

T240-X (SISO)

Training data + Predicted FIR and IIR subtraction:

 

Online subtraction results:(High freq. stuff shown for noise injection evaluation of the filter)

MCL

 

YARM

 

 

 

Subtraction performace:

 

This is my final SISO Wiener filter for MCL that uses the T240-X seismo as its witness.

The main difference between this filter and the one on elog:11532 is the actual 1/f rolloff this filter pocesses. My last filter had a pair of complex zeroes at 2kHz, that gave the filter some unusual behavior at high frequencies, thanks Vectfit. This filter has 10 poles and 8 zeroes, something Vectfit doesn't allow for and needs to be done manually.

The nice thing about this filter is the fact that Eric and I turned this filter on during his 40 min PRFPMI lock last night, Spectra for this is coming soon.

This filter lives on the static Wiener path on the OAF machine, MCL to MC2, filter bank 7.

Anyways, the usual plots are shown below. 

Filter:

T240-X (SISO)

Well, green looks better than blue, but it makes the PCDRIVE go high, which means its starting to saturate the EOM drive. So we can't just maximize the phase margin in the PZT/EOM crossover. We have to take into account the EOM drive spectrum and its RMS.

Also, your gain bump seems suspicious. See my TF measurements of the crossover in December. Maybe you were saturating the EOM in your TF ?

Lets find out what's happening with FSS servos over in Bridge and then modify ours to be less unstable.