Suresh is captivating his audience with gravity waves on last Friday, March 25
Koji was unable to build his c1lst model first thing this morning. Turns out there was a bug with RCG parser that was introduced on Friday when we did the RCG updates. We talked Alex who did a quick comment fix. The diff is as follows:
--- Parser3.pm (revision 2328)
+++ Parser3.pm (working copy)
@@ -1124,8 +1124,8 @@
print "Flattening the model\n";
print "Finished flattening the model\n";
- CDS::Tree::do_on_nodes($root, \&remove_tags, 0, $root);
- print "Removed Tags\n";
+ #CDS::Tree::do_on_nodes($root, \&remove_tags, 0, $root);
+ #print "Removed Tags\n";
CDS::Tree::do_on_nodes($root, \&remove_busses, 0, $root);
This was some code to remove TAGs from the .mdl file for some reason which I do not understand at this time. I will ask tommorrow in person so I can understand the full story.
Koji then rebuilt and started the c1lst process. This is his new test version of the LSC code. We descovered (again) that when you activate too many DAQ channels (simply uncommenting them, not even recording them with activate=1 in the .ini file) that the frame builder crashes. In addition, the c1lsc machine, which the code was running on, also hard crashed.
When a channel gets added to the .ini file (or uncommented) it is sent to the framebuilder, irregardless of whether its recorded or not by the frame builder. There is only about 2 megabytes per second bandwidth per computer. In this case we were trying to do something like 200 channels * 16384 Hz * 4 bytes = 13 megabytes per second.
The maximium number of 16384 channels is roughly 30, with little to no room for anything else. In addition, test points use the same allocated memory structure, so that if you use up all the capacity with channels, you won't be able to use testpoints to that computer (or thats what Alex has led me to believe).
The daqd process then core dumped and was causing all sorts of martian network slowdowns. At the same time, the c1lsc computer crashed hard, and all of the front end processes except for the IOP on c1sus crashed.
We rebooted c1lsc, and restarted the c1sus processes using the startc1SYS scripts. However, the c1susfe.ko apparently got stuck in a wierd state. We were completely unable to damp the optics and were in general ringing them up severely. We tried debugging, including several burt restores and single path checks.
Eventually we decided to reboot the c1sus machine after a bit of debugging. After doing a burt restore after the reboot, everything started to damp and work happily. My best guess is the kernel module crashed in a bad way and remained in memory when we simply did the restart scripts.
Last Friday, we discovered a bug in the RCG where the delay part was not actually delaying. We reported this to Alex who promptly put a fix in the same day. This allowed Matt's newly proposed frequency discriminator to work properly.
It also required a checkout of the latest RCG code (revision 2328), and rebuild of the various codes. We backed up all the kernel and executables first such as mbuf.ko and awgtpman.
We did the following:
1) Log into the fb machine.
2) Go to /opt/rtcds/caltech/c1/core/advLigoRTS/src/drv/mbuf and run make. Copy the newly built mbuf.ko file to /diskless/root/modules/18.104.22.168/kernel/drivers/mbuf/mbuf.ko on the fb machine.
3) Use "sudo cp" to copy the newly built mbuf.ko file to /diskless/root/modules/22.214.171.124/kernel/drivers/mbuf/
4) Go to /cvs/cds/rtcds/caltech/c1/core/advLigoRTS/src/gds and run make.
5) Copy the newly built awgtpman executable to /opt/rtcds/caltech/c1/target/gds/bin/
6) Go to /opt/rtcds/caltech/c1/core/advLigoRTS/src/mx_stream/ and run make.
7) Copy the newly built mx_stream executable to /opt/rtcds/caltech/c1/target/fb/
I modified the c1gfd.mdl simulink model. I made a backup as c1gfd_20110325.mdl.
The first change was to use a top_names block to put everything in. The block is labeled ALS. So all the channels will now be C1:ALS-GFD_SOMETHING. This means medm channel names will need to be updated. Also, the filter modules need to be updated in foton because of this.
I then proceeded to add the suggested changes made by Matt. To avoid a divide by zero case, I added a saturation part which saturates at 1e-9 (note this is positive) and 1e9.
Today we tried the Schmitt trigger DFD, and while it works it does not improve the noise performance. At least part of our problem is coming from the discrete nature of our DFD algorithm, so I would propose that an industrious day job person codes up a new DFD which avoids switching. We can probably do this by mixing the input signal (after high-passing) with a time-delayed copy of itself... as we do now, but without the comparator. This has the disadvantage of giving an amplitude dependent output, but since we are working in the digital land we can DIVIDE. If we mix the signal with itself (without delay) to get a rectified version, and low-pass it a little, we can use this for normalization. The net result should be something like:
output = LP2[ s(t) * s(t - dt) / LP1[ s(t) * s(t) ]],
where s(t) is the high-passed input and LP is a low-pass filter. Remember not to divide by zero.
I tidied up some of the stuff that was on the SP table. The ISS box that has been sitting on there for months is now underneath the X-arm on top of the spare Marconi which is stored there.
Kiwamu and I looked at all the electronics that are currently in place for the green locking on the X-arm and have made a set of block diagrams of the rack mounted units that we should build to replace the existing ... "works of art" that sprawl around out there at the moment.
1. "ETM Green Oscillator/PDH support box". Not a great name but this would provide the local oscillator signal for the end PDH (with a controllable phase rotator) as well as the drive oscillator for the end laser PZT. Since we need to hit a frequency of 216.075kHz with a precision that Kiwamu needs to determine, we'd need to be able to tune the oscillator ... it needs to be a VCO. It'd be nice to be able to measure the output frequency so I've suggested dividing it down by N times to put it into the DAQ - maybe N = 2^7 = 128x to give a measured frequency of around 1.7kHz. Additionally this unit will sum the PDH control signal into the oscillation. This box would support the Universal PDH box that is currently at the X-end.
2. "Vertex X-arm beatnote box" - this basically takes the RF and DC signals from the beatnote PD and amplifies them. It provides a monitor for the RF signal and then converts the RF signal into a square wave in the comparator.
3. "Mixer Frequency Discriminator" - just the standard MFD setup stored in a box. For temperature stability reasons, we want to be careful about where we store this box and what it is made of. That's also the reason that this stage is separated from the X-arm beatnote box with it's high-power amps.
4. RS232 and EPICS control of the doubling ovens
5. Intensity stabilization of the End Laser
P.S. I used Google Diagrams for the pictures.
X_arm and Y_arm vs PSL comparison.
Just a quick check of the performance of the X arm and Y arm lasers in comparison to the PSL. Plotting the data from the X arm vs PSL and Y arm vs PSL on the same plot shows that the X arm vs the PSL has no observable trending of mode-hopping in the laser, while the Y arm vs the PSL does. Suspect this is due to the fact that the X arm and PSL are both Innolight lasers with essentially identical geometry and crystals and they'll tend to mode-hop at roughly the same temperatures - note that the Xarm data is rough grained resolution so it's likely that any mode-hop transitions have been skipped over. The Lightwave on the other hand is a very different beast and has a different response, so won't hop modes at the same temperatures.
Given how close the PSL is to one of the mode-transition regions where it's currently operating (32.26 degC) it might be worth considering shifting the operating temperature down one degree or so to around 31 degC? Just to give a bit more headroom. Certainly worth bearing in mind if problems are noticed in the future.
The good news is that we seem to be running in a linear region of the PSL laser with a degree or so of range before the PSL Innolight laser starts to run multi-mode. On the attached graph we are currently running the PSL at 32.26degrees (measured) which puts us in the lower left corner of the plot. The blue data is the Lightwave set temperature (taken from the display on the laser controller) and the red data is the Lightwave laser crystal measured temperature (taken from the 10V/degC calibrated diagnostic output on the back of the laser controller - between pins 2 and 4).
The other good news is that we can see the transition between the PSL laser running in one mode and running in the next mode along. The transition region has no data points because the PMC has trouble locking on the multi-mode laser output - you can tell when this is happening because, as we approach the transition the PMC transmitted power starts to drop off and comes back up again once we're into the next mode region (top left portion of the plot).
The fitted lines for the region we're operating in are:
Y_arm_Temp_meas = 0.95152*T_PSL + 3.8672
Y_arm_Temp_set = 0.87326*T_PSL + 6.9825
Just a quick update... the Lightwave laser has now been moved up to the end of the Y arm. It's also been mounted on the new mounting block and heatsinks attached with indium as the heat transfer medium.
A couple of nice piccies...
There was a 2" mirror mount among the spares on the PSL table. It has a window LW-3-2050 UV mounted in it. I
have moved it to the Y-end table. We seem to have run out of 2" mirror mounts ...
With the exception of a 2" mirror mount, I've confirmed that we have everything for the Y-end green production and mode-matching.
We need to calculate a mode-matching solution for the Lightwave laser so that it gives the correct beam size in the doubling crystal.
Additionally, Rana has suggested that we change the pedestals from the normal 1" diameter pedestal+fork combo to the 3/4" diameter posts and wider bases that are used on the PSL table (as shown in the attached image).
[Steve, Suresh, Larisa]
The following cables were laid today: ETMYT, ETMY, IFOPO, MC1, OMCR, AS Spare, and MC2T.
Though the paper suggested 135' for the MC2T, we used a 110'. This is too short: need at least another 15' for the MC2T.
The RCR cable wasn't crossed off on the list, but a cable exists at the RCR cable which is black and is labeled (old label, 75 ohms)
There was no indication of which length was needed for MC1, so a 95' cable was used.
This is the grand plan we talked about in the beginning of the meeting.
The following is a message from the LIGO 40m Chief Recycling Officer:
Please get up off your (Alignment Stabilization Servo)es and recycle your bottles and cans! There is a recycling bin in the control room. Recent weeks have seen an increase in number of bottles/cans thrown away in the regular garbage. This is not cool.
Yesterday during the day, Alex ran a script to fix the time stamps in the trends files we had messed up back during the daqd change overs around Feb 17th and 23rd. See this elog for more information on the trend problem.
Due to how the script runs, basically taking all the data and making a new copy with the correct time stamps, the data collected while the script was running didn't get converted over. So when he did the final copy of the corrected data, it created a several hour gap in the data from yesterday during the day time.
The original files still exist on the fb machine in /frames/trend/minute_raw_22mar2011 directory.
The 40m lab was visited by ~ 30 LSC members the end of last week.
Solid door, numbered 4 at south west corner of PSL enclosure was replaced by laser protective window.
The carpenter shop's Mark is making 4 more identical ones for the east side.
The Lightwave NPRO126 of 700mW was moved from the AP-table into the PSL-enclosure temporarily.
It's emergency shutdown switch can be seen at the center bottom picture
Succeeded in handing off the servo from the green to the red.
This time we found that the fluctuation in the IR signals became lesser as the gain of the ALS servo increased.
Therefore I increased the UGF from 40 Hz to 180 Hz to have less noise in the IR PDH signal.
Here is a preliminary plot for today's noise spectra.
The blue curve is the ALS in-loop spectrum, that corresponds to the beat fluctuation.
The red curve is an out-of-loop spectrum taken by measuring the IR PDH signal.
Since the UGF is at about 180 Hz the rms is integrated from 200 Hz.
The residual displacement noise in the IR PDH signal is now 1.2 kHz in rms.
I am going to analyze this residual noise by comparing with the differential noise that I took yesterday (see the last entry ).
- Plan for tomorrow
* Video cable session (I need ETMY_TRNAS) (team)
* Characterization of the Y end laser (Bryan / Suresh)
* LPF for the X end laser temperature control (Larisa)
* Frequency Divider (Matt)
* X end mechanical shutter (Kiwamu)
I'm going to take the easy question - What are the pink data points??
And I'm going to answer the easy question - they're additional beat frequency temperature pair positions which seem to correspond to additional lines of beat frequencies other than the three highlighted, but that we didn't feel we had enough data points to make it worthwhile fitting a curve.
It's still not entirely clear where the multiple lines come from though - we think they're due to the lasers starting to run multi-mode, but still need a bit of thought on that one to be sure...
A comparator has been installed before the MFDs (mixer-based frequency discriminator) to eliminate the effect from the amplitude fluctuation (i.e. intensity noise).
As a result we reached an rms displacement of 580 Hz or 80 pm.
As a result we reached an rms displacement of 580 Hz or 80 pm.
(differential noise measurement)
Here is the resultant plot of the usual differential noise measurement.
The measurement has been done when the both green and red lasers were locked to the X arm.
In the blue curve I used only MFD. In the black curve I used the combination of the comparator and the MFD.
Noise below 3 Hz become lower by a factor of about 4, resulting in a better rms integrated from 40 Hz.
Note that the blue and the black curve were taken while I kept the same lock.
A calibration was done by injecting a peak at 311 Hz with an amplitude of 200 cnt on the ETMX_SUS_POS path.
Yesterday Koji modified his comparator circuit such that we can take a signal after it goes thorough the comparator.
The function of this comparator is to convert a sinusoidal signal to a square wave signal so that the amplitude fluctuation doesn't affect the frequency detection in the MFD.
I installed it and put the beat-note signal to it. Then the output signal from the comparator box is connected to the MFDs.
The input power for the comparator circuit has been reduced to -5 dBm so that it doesn't exceeds the maximum power rate.
OK. Today we did the same type of measurement for the Y arm laser as was done for the X arm laser here: http://nodus.ligo.caltech.edu:8080/40m/3759
And attached here is a preliminary plot of the outcome - oddities with adding on the fitted equations, but they go as follows
(Red) T_yarm = 1.4435*T_PSL - 14.6222
(Blue) T_yarm = 1.4223*T_PSL - 10.9818
(Green) T_yarm = 1.3719*T_PSL - 6.3917
It's a bit of a messy plot - should tidy it up later...
It's a bit of a messy plot - should tidy it up later...
Some tasks for the daytime tomorrow.
* Beam profile measurements of the Y end laser (Suresh / Bryan)
* Taking care of CDS and the simulated plant (Jamie / Joe)
* Reconnect the X end mechanical shutter to 1X9 (Kiwamu)
* LPF for the X end temperature feedback (Larisa)
I added a new ADC channel for a DC signal from the X end green PD.
It is called C1:GCX-REFL_DC and connected to adc_0_1, which is the second channel of ADC_0.
By the way, when I tried connecting it to an ADC I found that most of the channels on the AA board on 1X9 were not working.
Since the outputs form the board are too small the circuits may have benn broken. See the picture below.
In addition to that I realized that the signal from the PDH box for the temperature actuation is limited by +/- 2V due to the range of this AA board.
In fact the signal is frequently saturated due to this small voltage range.
We have to enlarge the range of this AA board like Valera did before for the suspensions (see this entry).
- Plan for this week
* Intensity stabilization for the end green laser (Matt / Kiwamu)
* Hand off the servo from Green to Red (Matt / Kiwamu)
* Y end green locking (Suresh / Bryan) (rough schedule)
* Reconnect the X end mechanical shutter to 1X9 (Kiwamu)
* Connect the end DCPD signal to a DAC (done)
* Make a LPF in a Pomona box for the temperature (Larisa)
* Clean up and finalize the X end setup (Kiwamu)
* Make a item lists for electronics. Order the electronics. (Aidan / Kiwamu)
Bryan Barr is visiting us from Glasgow for a month. He received 40m specific safety training on Friday.
PMC TRANS/REFL on MEDM showed red values for long time.
TRANS (a.k.a C1:PSL-PSL_TRANSPD) was the issue of the EPICS db.
REFL (a.k.a. C1:PSL-PMC_RFPDDC) was not physically connected.
There was an unknown BNC connected to the PMC DC output instead of dedicated SMA cable.
So they were swapped.
Now I run the following commands to change the EPICS thresholds:
ezcawrite C1:PSL-PMC_PMCTRANSPD.LOLO 0.8
ezcawrite C1:PSL-PMC_PMCTRANSPD.LOW 0.85
ezcawrite C1:PSL-PMC_PMCTRANSPD.HIGH 0.95
ezcawrite C1:PSL-PMC_PMCTRANSPD.HIHI 1
ezcawrite C1:PSL-PMC_RFPDDC.HIHI 0.05
ezcawrite C1:PSL-PMC_RFPDDC.HIGH 0.03
ezcawrite C1:PSL-PMC_RFPDDC.LOW 0.0
ezcawrite C1:PSL-PMC_RFPDDC.LOLO 0.0
As these commands only give us the tempolary fix, /cvs/cds/caltech/target/c1psl/psl.db was accordingly modified for the permanent one.
field(DESC,"RFPDDC- RFPD DC output")
field(INP,"#C0 S32 @")
field(DESC,"PMCTRANSPD- pre-modecleaner transmitted light")
field(INP,"#C0 S10 @")
A rough time-table and the various tasks are given below:
Note: 700mW NPRO sitting on AP table (Model No: 126-1064-700, Sl No. 415) = Alberto's laser
Temperature dependence of frequency of Alberto's laser:
a) Shifting Alberto's Laser (AL) to the PSL table and setting up a beat frequency measurement between AL and PSL
b) Determining the frequency vs Temperature curve for the AL
Repositioning the optics on the Y-end table and relocating Alberto's laser ( at this point it will be rechiristened as Y-End-NPRO )
I updated our lockin simulink pieces to use the newer, more streamlined lockin piece that is currently in CDS_PARTS (with new documentation block!). It means we are no longer passing clock signals through three levels of boxes.
In order to use the piece, you need to right click on it after copying from CDS_PARTS and go to Link Options->Disable Link. This forces the .mdl to save all the relevant information about the block rather than just a pointer to the library. I talked with Rolf and Alex today and we discussed setting up another model file, non-library format for putting generically useful user blocks into, rather than using the CDS_PARTS library .mdl.
The BS, ITMX, ITMY, PRM, SRM, ETMX, ETMY now have working lockins, with the input matrix to them having the 2nd input coming from LSC_IN, the 3rd from the oplev pitch, and the 4th from oplev yaw.
This necessitated a few name changes in the medm screens. I also changed the lockin clock on/off switch to a direct amplitude entry, which turns green when a non-zero value is entered.
Currently, the Mode cleaner optic suspension screens have white lockins on them. I started modifying a new set of screens just for them, and will modify the generate_master_screens. Unfortunately, this requires modifying two sets of suspension screens going forward - the main interferometer optics and the MC optics.
The reason for using Alberto's laser is that some amount of work has already gone into characterising its phase noise. Ref elog entry 2788
We use Alberto's laser for the Y end Green Locking.
Which laser are we going to use, Alberto's laser or MOPA laser ?
Just for a record. We got 4 new laser pointers (2 greens, 1 blue, and 1 green and red combination). Don't lose them.
They reside in a bucket on the SP table, where IR viewers and sensor cards also reside.
Prior to the works on the Y end setup I propose to perform the temperature scan business like Koji and Suresh did before (see this entry).
This business will allow us to easily find a beatnote at 532nm after the installation on the Y end.
I guess the right persons for this work are Bryan and Suresh.
Bryan will have a safety guidance from Steve in this after noon. So after that they can start working on it.
/* - - - coarse plan - - - */
* remove Alberto's laser from the AS table
* setup Alberto's laser on the PSL table
* put some stuff such as lenses, mirrors and etc. (Use the IR beam picked off after the doubling crystal for the main laser source)
* mode matching
Which laser are we going to use, Alberto's laser or MOPA laser ?
Steve pointed out to me today he couldn't get trends for his PEM slow channels like C1:PEM-count_full.
I experimented a bit and found for long time requests (over 20 days), it would produce minute trends up to the current time, but only if they started far enough back. So the data was being written, but something was causing a problem for dataviewer/NDS to find it.
On further investigation it looks to be some incorrect time stamps at several points in the last few months are causing the problems. Basically when Alex and I made mistakes in the GPS time stamp settings for the frame builder (daqd) code, the wrong time got written for hours to the raw minute trend data files.
So Alex is going to be running a script to go through the roughly 180 gigabytes of affected trend data to write new files with the correct time stamps. Once it done, we'll move the files over. We'll probably lose a few hours worth of recent trend data, depending on how quickly the scripts run, but after which minute trends should work as they are supposed to.
4. The blue Output Filters section has been changed to agree with the new filter of matrices row, column labeling. My fault for not testing it and realizing it was broken. The change was made in /opt/rtcds/caltech/c1/medm/master/C1SUS_DEFAULTNAME.adl and then ,/generate_master_screens.py was run, updating all the screens.
5. I have swapped the logic for the sensor filter banks (ULSEN, URSEN, etc). It now sends a "1" to the Binary Output board controlling the OSEM analog whitening when the FM1 filter is ON. This has been done for all the suspensions (BS, ITMX,ITMY, SRM, PRM, MC1, MC2,MC3, ITMX, ITMY).
I am also updating the first sensor filter banks for the BS, ITMX, ITMY, SRM, PRM,MC1,MC2,MC3, called "3:30", to match the Y and X ends.
8. I can't find any documentation on how to get a momentary button press to toggle states. I could stick a filter bank in and use the on/off feature of that part, but that feels like a silly hack. I've decided for the moment to split the TM offset button into 2, one for ON, one for OFF. I'll put in on the list of things to have added to the RCG code (either a method, or documentation if it already exists).
EDIT: TM offset still doesn't work. Will worry about it next week.
9. Fixed a connection in SPY/SPX models where the side senor path that was missing a constant to a modulo block.
I did some work on the ETMY real and Sim.
It seems like there is still a problem with the input whitening filters. I believe the Xycom logic is set such that the analog whitening of the OSEM signals is turned ON only when the FM1 is turned OFF. Joe has got to fix this (and elog it) so that we can damp the suspension correctly. For now, the damping of the ETMY and the SETMY require different servo gains and signs, probably because of this.
John has changed the NDS2 code and restarted it on Mafalda. The issue is that it goes off the rails everytime the DAQD is restarted on FB because of filename convention war between GDS and CDS.
Until this is resolved, please make sure to restart the NDS2 process on Mafalda everytime you restart DAQD by doing this:
pkill -KILL nds2
The FM1 filter module change for XXSEN was propagated to the ETMX suspension. This was a change from a 30,100:3 with a DC gain of 1 to a 3:30 which just compensates the hardware filter.
The good gains for the Sim damping were found to be: 100 for ETMX_SUSPOS, 0.1 ETMX_SUSPIT, and 0.1 ETMX_SUSYAW, ETMX_SUSSIDE is -70. Gains much higher tended to saturate the simulated coils (i.e. hitting 10V limit) and then had to have the histories cleared for the RESPONSE matrix.
These seem to work to damp the real ETMX as well.
New Focus Servo Controller has just arrived. We have 25 days to evaluate this product.
It will have to be shipped back to the vendor on April 4, 2011 the latest in order to get full refund.
While fixing up some medm screens and getting spectra of the simulated plant, I realized that the naming convention for the Matrices of Filter banks was backwards when compared to that of the normal matrices (and the rest of the world). The naming was incorrectly column, row.
This has several ramifications:
1) I had to change the suspensions screens for the TO_COIL output filters.
2) I had to change the filters for the suspension with regards to the TO_COIL output filters so they go in the correct filter banks.
3) Burt restores to times previous March 11th around noon, will put your TO_COIL output filters in a funny state that will need to be fixed.
4) The simplant RESPONSE filters had to be moved to the correct filter banks.
5) If you have some model I'm not aware of that uses the FiltMuxMatrix piece, it is going to correctly build now, but you're going to have to move filters you may have created with foton.
Ridiculous and hacky. Digital stabilization removed as well as the old "leave a pile of equipment on a stool" strategy.
We used a a BNC cable to send a pickoff of the beam before the recombination to the end via an SR560.
We want to increase gain in the lower frequencies, so a circuit must be designed (a passive low pass filter).
First, measurements were taken at the X arm for impedance and capacitance, which were 104.5kOhms and 84.7pF respectively. Kiwamu decided to make the circuit resemble a voltage divider for ease of calculation, such that Vout/Vin would be a ratio of some values of the equivalent circuit reactance values. After a few algebra mistakes, this Vout/Vin value was simplified in terms of the R, C measured and the R', C' that would be needed to complete the circuit.
Since the measured C was very small and the measure R was fairly high, the simplified form allowed us to pick values of R' and C' that would make the critical frequency occur at 0.1Hz: set the R' resistance to 1MOhm and C' capacitance to 10uF, which would yield a gain ~1.
With these values a circuit we can start actually making the circuit.
Aidan: Joe and I have added a channel that takes the DC output from the vertex beatnote PD and sends it, via RFM, to a DAC at the ETMX end. Immediately before the output is a filter C1GCX_AMP_CTRL. The output of the DAC is connected to the CURRENT LASER DIODE modulation input on the back of the Innolight driver. This will modulate the current by 0.1A/V input.
We should be able to modulate the green laser on the end now and stabilize the intensity of the amplitude on the beatnote PD at the vertex. (In this configuration, the ampltiude noise of the PSL laser will be injected onto the end laser - we may want to revisit that).
Joe's comments on model change:
I added a RFM connection at the output of the C1:GCV-XARM_BEAT_DC filter in the c1gcv model. The RFM connection is called: C1:GCV-SCX_ETMX_AMP_CTRL.
This RFM connection goes to the c1scx model and into Kiwamu's GCX box, which uses top_names. There's a filter inside called AMP_CTRL, so the full filter name is C1:GCX-AMP_CTRL. The output then goes to the 7th DAC output.
There are 3 standard techniques to reduce this effect:
1) Stabilize the end laser by sensing the green light coming into the PSL before recombination and feeding back with SR560 (this is the only one that you should try at first).
The reason that I chose this PD is that, apparently, the green light coming from the cavity is clipped when it is picked off for its DC PD.
2) Moving to the center of the MFD fringe via ETM steps.
3) Auto-alignment of the beam to the arm.
We can modify the freq divider circuit to make it a comparator.
The next steps we should do are :
We are limited by the intensity noise of the X arm transmitted green light.