I calibrated the ALS-OFFSETTER output.
I measured the FSR of cavity in unit of counts. That was 395 counts. Our cavity FSR is 3.8 MHz, so 1 count of the OFFSETTER output is 9.7 kHz.
I calibrated the ALS-OFFSETTER output.
I measured the FSR of cavity in unit of counts. That was 395 counts. Our cavity FSR is 3.8 MHz, so 1 count of the OFFSETTER output is 9.7 kHz.
Really? What cavity length did you use in the calculation?
My job right now is to characterize the green PDH loops on each arm. Today, Jenne took me around and pointed at the optics and electronics involved. She then showed me how to lock the green beams to the arms (i.e. opening the shutters until you hit a TM00 shape on the transmitted beam camera). Before lunch, the y arm was easiest to lock, and the transmitted power registered at around 0.75.
After lunch, I took a laptop and SR785 down to the y end station. I unhooked the PDH electronics and took a TF of the servo (without its boost engaged, which is how it is currently running) and noise spectrum with the servo input terminated.
I then set up things a la ELOG 8817 to try and measure the OLTF. However, at this point, getting the beam to lock on a TM00 (or something that looked like it) was kind of tough. Also, the transmitted power was quite a bit less than earlier (~0.35ish), and some higher order modes were higher than that (~0.5). Then, when I would turn on the SR785 excitation, lock would be lost shortly into the measurement, and the data that was collected looked like nonsense. Later, Koji noted that intermittent model timeouts were moving the suspensions, thus breaking the lock.
We then tried to lock the x arm green, to little success. Koji came to the conclusion that the green input pointing was not very good, as the TM00 would flash much less brightly than some of the much higher order modes.
Tomorrow, I will measure the x arm OLTF, as it doesn't face the same timeout issue that is affecting the y arm.
Yesterday, made a slew of measurements on the X-arm when locked on green. By tweaking the temperature loop offset and the green input PZT pointing, I was able to get the transmitted green to around 1.0. The PDH board gain was set to 4.0. I had trouble making swept sine measurements of the OLTF; changing the excitation amplitude for different frequency ranges would result in discontinuities in the measured TF, and there was only a pretty narrow band around the UGF that seemed to have reasonable coherence.
So, I used the SR785 as a broadband noise generator and measured the TF via dividing the spectra in regions of coherence. Specifically, I used the "pink noise" option of the SR785. I also used a SR560 as a low pass to get enough noise injected into the lower frequency range to be coherent, while not injecting so much into the higher frequencies that the mode hopped while measuring.
The servo board TF was easily fitted to a 4th order zpk model via VFIT, but I'm having trouble fitting the OLTF. (There is a feature in the servo TF that I didn't fit. This is a feature that Zach saw [ELOG 9537], and attributed to op amp instability) Plots follow. Also, while these need to be calibrated to show the real noise spectrum of the cavity motion, I'm attaching the voltage noise spectra of the error and control signals as a check that electronics/PD noise isn't dominating either signal.
What a such long pain we suffered.
After more than three years from Kiwamu's discovery, the PDH box 50kHz oscillation issue was finally solved.
This "weird peak at 50kHz" was caused by the oscillation of the voltage regulator (ON's MC7912).
As it imposed common noise almost everywhere, it screwed up transfer function measurements
like EricQ saw recently.
The negative voltage regulator (79XX) tends to get unstable than the positive counter parts (78XX).
The oscillation was removed by adding 22uF electrolytic capacitor between the output pin (pin3) and the ground pin (pin1) of MC7912.
This is indeed more than 20 times of the specification you can find in the data sheet.
With the newly repaired PDH board, I spent some time with the x arm green PDH loop. I found it SO MUCH EASIER to measure the OLTF by injecting before the servo, instead of after it. (i.e. I added a swept sine from the SR785 to the mixer output (error signal) before the servo input). This is likely because the error signal is much flatter. I used a 10mV excitation across the whole frequency range (30-100kHz).
Here's the OLTF. I'm working on fitting it and breaking it up into its constituent TFs, then making a rudimentary noise budget.
OK, the next question will be "why the loop bandwidth is so miserable?"
In other words, what is preventing us to have the bandwidth of 20~30kHz?
Your breaking down will give us the answer.
I'm not as good as a fit, but it seems that there is a loop delay of about 30 microseconds, looking at the high frequency phase. This might explain the limitation on the BW. Eric should be able to get the delay out of the fit with some care.
There was 0.2 mW green at the X end.
The doubling oven temp was changed from 37.5 to 36 degrees C
Power at green shutter 3 mW The alignment was not touched.
I found that the X end SLOW control was left on for ~15days. The output of the filter had grown to ~2e7.
This yielded the laser temperature pulled with the maximum output of the DAC.
This was the cause of the power reduction of the X end SHG; phase matching condition was changes as the wavelength of the IR was changed.
Once the SLOW output was reset, the green REFL was reduced from 4000cnt to 1800cnt.
Y arm green: Nothing much was disturbed. I touched the steering mirrors and brought GTRY from 0.2 to 0.9.
X arm green: The PDH lock was not very stable mostly because of the low power in green. I changed the oven temperature for the doubler to 36.4 corresponding to maximum green power. GTRX increased from 0.1 to 0.9
Both the X and Y arm green alignment were tuned on the PSL table to their respective beat PDs.
The PSL green shutter was not responding to the medm buttons. I found the PSL green shutter set to 'local' and 'N.O' (these are switches in the shutter controller). I do not see any elog and not sure as to why the controller was even touched in the first place. I set the shutter controls to 'remote' and 'N.C'.
The X and Y arms were locked successfully using ALS and the arms could be scanned and held to support IR resonance.
The same procedure as in elog 9219 was followed. In-loop noise was measured to be between 200-300 Hz rms for the lock.
ALS settings for the lock
X arm : FM 2, 3, 5, 6, 7, 8, 10 Gain = 11.0
Y arm : FM 2, 3, 5, 6, 7, 8, 10 Gain = 10.0
Nice restoration. We eventually want to make transition of the servo part from ALS to LSC model for the further handing off to the other signals.
Please proceed to it.
ETMX green power at shutter 3.6 mW at 36.35 C doubler crystal temp. [ Innolight IR settings 2.0 A, 40.83 C, 500 mW before Faraday 1/2 plate ]
ETMY green power at shutter 0.75 mW at 35.8 C doubler crytal temp. [ NPRO IR settings 1.82A, 231 mW_ display, DT 21 C, DTEC +1V, LT 40 C, LTEC 0.1V, T +41.041 ]
Alex, Gautam and Steve,
Single mode fiber 50m long is layed out into cable tray that is attached to the beam tube of the Y arm.
It goes from ETMY to PSL enclosure. It is protected at both ends with " clear- pvc, slit corrugated loom tubing " 1.5" ID
The fiber is not protected between 1Y1 and 1Y4
The X -arm fiber is in the high cable tray and it has has coupler mounts.
The Y -arm fiber is in the low cable tray and it has no coupler mounts.
The fibers are only protected at entering and exiting the trays.
We have only 68 ft spare 1.5" ID protective plastic tubing.
Nic, Jenne, EricQ, and Koji should describe the demonstartion of CESAR achieved tonight.
Q and I have started to use the ALS slow servo for the end aux lasers while locking the arms using ALS. The servo prevents us from hitting the limits of the PZT range for the end lasers and a better PDH locking.
But keeping the servo ON causes the slow output to drift away making it hard to find the beat note everytime the arm loses lock. The extensive beat note search following the unlock can be avoided by clearing history of the slow servo.
Q and I have started to...
To check the basolute frequency stability of the old monochrome HP 8591E RF Spectrum analyzer that we're using for the ALS beat readout, I hooked its 10 MHz reference output (from its rear panel) into the A channel of the SRS SR620 frequency counter. The SR620 is locked to the FS 720 Rubidium clock via the 10 MHz connections in their rear panels.
So, we can assume that this is a good absolute readout. It reads 9.999860.7 +/- 0.3 Hz. So its 139.1-139.4 Hz lower than 10 MHz. The +/- 0.3 is just a slow drift that I see over the course of 10 minutes.
So, let's say that the analyzer is low by 10 ppm, so the arm length estimates are short by ~0.4 mm. A negligible correction, so there's no need to use atomic clocks to measure our arm lengths.
Green light power decreased from 3 mW to 1 mW at the ETMX-ISCT shutter. More later.
It is actually very tricky to measure the green power at the output of the doubling crystal as the IR often leaks into the measurement.
I checked the green beam powers on the X/Y/PSL tables.
CONCLUSION: There is no green beam which exceeds 5mW anywhere in the 40m lab.
Note: The temperature of the doubling crystal at the X end was optimized to have maximum green power. It was 36.3degC and is now 36.7degC.
When the angles of the wave plates are optimized, we have 539mW input to the doubling crystal.
With the Xtal temperature of 36.7degC, where the green power is maximized, the power right after
the harmonic separator (H.S.) was 9.6mW.
Xtal temp 36.7degC ~~~
Xtal temp 36.7degC
If we believe these 4.69mW and 4.54mW are purely from the green, we have 4.8mW right after the H.S.
This corresponds to the conversion efficiency of 1.6%/W (cf. theretical number 2%/W)
By disabling the heating of the crystal, we can reduce the green light by factor of 60. But still the reading right after the H.S. was 5.3mW
Xtal temp 29.2degC ~~~
Xtal temp 29.2degC
Naively taking the difference, the green beam right after the H.S. is 4.4mW.
In either cases, the green power right after the oven is slightly less than 5mW.
When the angles of the wave plates are optimized, we have 287mW input to the doubling crystal.
With the Xtal temperature of 36.0degC, where the green power is maximized, the power right after
the harmonic separator (H.S.) was 0.86mW.
Xtal temp 36.0degC ~~~
Xtal temp 36.0degC
When the temperature was shifted to 39.2degC, the reading after the H.S. was 70uW. Therefore the contamination by the IR is small
in this setup and we can believe the above reading in 70uW accuracy. This 0.86mW corresponds to the conversion efficiency of 1.2%/W.
The incident IR is 80mW. We have 170uW after the H.S., which corresponds to the conversion efficiency of 2.6%/W. Maybe there is some IR contamination?
From the vacuum chamber total 1mW of green is derivered when both arms are locked and aligned.
Thus the total green power at the PSL table is less than 5mW.
The Y arm green transmission had come down to 0.3 and the green steering mirrors on the Y end table required some minor alignment adjustments to bring back transmission to around 0.75 counts.
We've been having trouble tuning the ALS DIFF matrix. Trying to see if the MC2 EXC can be cancelled in ALS DARM by adjusting the relative gains in ALSX and ALSY Phase Tracker outputs.
There's a bunch of intermittent behavior. Between different ALS locks, we get more or less cancellation. We were checking this by driving MC2 at ~100-400 Hz and checking the ALS response (with the ALS loops closed). We noticed that the X and Y readbacks were different by ~5-10 degrees and that we could not cancel this MC2 signal in DARM by more than a factor of 4-5 or so. In the middle of this, we had one lock loss and it came back up with 100x cancellation?
Attached is a PDF showing a swept sine measurement of the ALSX, ALSY, and DARM signals. You can see that there is some phase shift between the two repsonses leading to imperfect cancellation. Any ideas? Whitening filters? HOM resonance? Alignment?
So far today, I've been working with the Y-end green PDH locking. Using a SR560 to roll off the AG4395A output to take a loop measurement at the servo output, I measured the following OLG, and inferred the CLG from it. The SR560 really helped it getting good coherence without introducing a big offset that changes the optical gain, thus distorting the loop shape, etc. etc.
You would think this loop looks pretty good, 10k UGF, and 45 degrees of phase margin, gain peaking is sane, and pretty smooth slope. But, the thing still was flipping out of lock while I measured this.
I suspect shenanigans at >100k. This is motivated by the fact that I've seen some big noise in the error signal around 150k. I don't have a good noise plot right now, because I'm trying to get a scheme going where I stitch together a bunch of 1 decade spectra from the 4395, but the noise floor isn't consistent across each patch (even though the attenuation stays the same, and I confirmed I'm in "noise" mode). I'm working on a loop measurement up there, too, but I haven't been able to get the right filter/amplitude settings yet.
So, even though this plot is not totally correct (read: wrong and bad), I include it just for the sake of showing the big honking spike of noise at ~150K.
[ Rana, Jenne]
We remeasured the Yend PDH box.
When we first started, the green couldn't hold lock to the arm - it kept flickering between modes. Changing the gain of the PDH box (from 7.5 to 6.0) helped.
We measured a calibration, from our injection point to our measurement point.
The concept was that we'd take the mixer output, and put that into an SR560, and put the swept sine injection into the other input port of the '560, and use A-B. So, for this calibration, we left A unplugged, and just had the RF out of the 4395 going to input B of the '560. The 600 Ohm output of the '560 went to the error point input on the PDH box (during normal operation the mixer output is connected directly to the error point input). The SR560 was set to gain of 1, no filtering. I don't recall if we were using high range or low noise, but we tried both and didn't really see a difference between them.
We had the 4395 take that calibration out, and then we measured the closed loop gain up to 1 MHz. (Same measurement setup as above, but we connected the mixer out to the input of the SR560 to close the loop, and made sure we were locked on a TEM00 green mode.) Rana used an ipython notebook to infer the open loop gain from our measurement. Our conclusion is that we don't have nearly enough gain margin in our loop. We found the PDH box gain knob at 7.5, and we turned it down to 6.0, but the loop is still pretty borderline. We used the high impedance active probe to measure the error point monitor, since we aren't sure that that point can drive a 50 Ohm load.
We also measured the error point spectra and the control point spectra. Unfortunately, the saved data from the analyzer (no matter what is on the screen) comes out in spectrum, not spectral density. So, we need to check our conversion, but right now to get from Watts power to Volts, we do sqrt(50 ohm * data). We then need to get to spectral density, and right now we're just dividing by the square root of the bandwith that is reported in the .par file. This last step is the one we want to especially check, by perhaps putting some known amount of noise (from an SR785?) into the 4395, and checking that our calibration math returns the expected noise spectrum.
What still needs to be done is to calibrate this into Hz/rtHz. To do this, we were thinking that we should look at the error point on a 'scope while the cavity is flashing.
Anyhow, here is the uncalibrated error point spectrum. Purple is a measurement up to 30kHz, with 30Hz bandwidth. Blue is a measurement up to 300kHz with 300Hz bandwidth. The gain peaking schmutz above 10kHz sucks, and we'd like to get rid of it. We also see the same peak at ~150kHz that Q saw earlier today. We were using the high impedance probe here too.
We have the data for the control point (all the data files are in /users/jenne/ALS/PDHloops/Yend_18Aug2014), but we haven't plotted it yet.
Things that need doing:
* (JCD) Think about this box's purpose in life. What kind of gain do we need? Do we need more / less than we're currently getting? NPRO freq noise is 1/f and is 10kHz/rtHz at 1Hz (this is from a plot of an iLIGO NPRO from Rana's thesis, but it's probably similar). Talk to Kiwamu; the noise budget in the paper seems to indicate that we had some kind of boost on or something. Also, if we need much more gain than we already have, we'll definitely need a different box, maybe the PDH2 box that they have over in WBridge.
* (EQ, priority 1) Measure and calibrate error point noise down to lower freq for both arms. What could we win by putting in a boost? If the residual noise is high, maybe the laser isn't good at following arm, so beatnote isn't good length info for the arm, and we can't succeed.
* (EQ, priority 2) Measure TF of PDH box, and a separate measurement of the Pomona box that is between the mixer and the error point - is that eating a bunch of phase? It's already an LC circuit which is good, but do we really want a 120kHz lowpass when our modulation frequency is roughly 200kHz? Ask ChrisW - he worked on one of these with Dmass.
* (EQ, priority 2ish) Measure TF of Xend PDH loop (unless you already have one, up to ~1MHz).
* (JCD) Make DCC tree leaf for PDH box #17. Take photos of box.
Heading to dinner, going to come back for more green fun, but here's a quick update:
Xarm Peak-to-Peak of the PDH signal in the mixer output is about 70mV when GTRX was about 0.4. The sideband-generating function generator has an output of 2V (forgot to note rms or pp)
Yarm Peak-to-Peak of the PDH signal in the mixer output is about 640uV when GTRX was about 0.71. The sideband-generating function generator has an output of 0.091V (forgot to note rms or pp)
The Yarm signal thus correspondingly has a waaay noisier trace. I would've had scope plots to show here, but the scope freaked out about how large my USB drive capacity was and refused to talk to it >:|
This suggests to me that our modulation depth for the Yarm may be much too small, and may be part of our problems with it.
Here is a plot of last night's data with both the control and the error point on the same plot, in Volts. Q is still working, so I don't have a calibration number yet to get these to Hz.
Note in the control spectrum that we have very significant 60Hz lines.
EDIT: I also added a new branch to the DCC Document Tree, and 2 leafs (one for each end). Here's the ALS PDH servo branch: E1400350
It's not so impressive yet, but here's a plot that shows (a) Rana's guess for laser frequency noise, (b) The inferred in-loop version of that noise, (c) The CARM linewidth FWHM, translated to Hz.
For (b), I take the loop that Rana and I measured last night, and I assumed that it continued on forever as 1/f toward low frequency. Then I do 1/(1+G) to get the closed loop version of the loop (which is a measurement with an artificial line tacked on the end), and multiply this with the laser freq noise, which is also totally artificial.
For (c), I do df/f = dL/L, with f = c/lambda_green, since the rest of the plot is meant to be in green frequency units.
This is my beginnings of trying to come up with a requirement for our green PDH boxes. We weren't very clear in the MultiColor paper about the nitty-gritty details (obviously), but then Kiwamu didn't expand on those details in his thesis either. He talks a lot more about the design considerations for the digital ALS loop, which isn't what I want today. I will send him an email to see if he had any notes that didn't make it into his thesis.
Reasonable amounts of time were spent bending the AG4395 to my will; i.e. figuring out the calibration things Jenne and Rana did, finding the right excitation amplitude and profile that would leave the light steadily locked, and finding the right GPIB incantation for getting spectra in PSD units instead of power units. I'm nearing completion of a newer version of AG4395 scripts that have proper units, and pseudo-log spectra (i.e. logarithmically spaced linear sweeps)
Here is too many traces on one plot showing parts of the OLTF for the x green PDH. One notable omission is the PD response (note to self:check model and bandwidth). The servo oddly seems to have a notch around 100k. My calibration for the CLG injection may not have been perfect, instead of flattening out at 0dB, I had 2dB residual. I tried to correct for it after the fact, assuming that certain regions were truly flat at 0dB, but I want to revisit it to be thorough. I found some old measurements of the Innolight PZT PM response, which claims to be in rad/V, and have included that on the plot.
In the end, the mixer and PZT response make it look like getting over 10kHz bandwidth may be tough. Even finding a good higher modulation frequency to be able to scoot the LP up would leave us with the sharp slope in the PZT phase loss, and could cause bad gain peaking. Maybe it's worth thinking about a faster way of modulating the green light?
Tomorrow morning, I'll calibrate all the noise spectra I have into real units. These include:
However, looking at the floors, it occurs to me that I may have left the attenuation on the input too high, in an effort to protect the input the PDH box, which rails all the time when not locked to a 00 mode, sometimes even with the input terminated or open. It's kind of a pain that the agilent makes it really hard to see the data when you're in V/rtHz mode, because I should've caught this while measuring :/
I used a scope to capture a pdh signal happening, which will let me transform the mixer output into cavity motion. The control signal goes to the innolight PZT with a ~1MHz/V factor. Here are the uncalibrated plots, for now.
A MIST simulation tells me that the green pdh horn-to-horn displacement is about 1.2nm, or ~18kHz. I used this, along with the scope trace attached to the previous post, to calibrate the mixer output at 193419 Hz per V. (EDIT: I was a little too hasty here. What I'm really after is the slope of the zero crossing, which turns out to be almost exactly twice my earlier naïve estimate. See later post for correct spectra)
For the control signal, I assumed a flat Innolight PZT PM response of 1MHz/V. ( Under 10kHz, it is indeed flat, and this is the region where the control signal is above the servo output noise in yesterday's measurements)
Here are all of the same spectra from last night, with the above calibrations.
Going off Jenne's earlier plot, it looks like the in-loop error signal RMS is ten times bigger than the CARM linewidth.
I calibrated the control signal from Volts to Hz using the rough PZT calibration of 5MHz/V for the Yend NPRO.
For the error signal, Q said that the Yarm PDH peak-to-peak height was about a factor of 100 smaller than the Xarm, so I used a calibration of 1.9e7 Hz / V.
Then, from Q's Mist simulation including the high Xarm loss, and the plot that he posted in the control room, the CARM linewidth looks like it is about 2pm. This is the number that I have included on today's plot. Note though that yesterday I was using a linewidth of about 30pm, which I got from an Optical simulation about a year ago. I do not know why these numbers come out an order of magnitude different! The CARM linewidth is actually about 20 pm. Both Q and I failed at reading log-x plots yesterday. I have corrected this, and replotted.
Anyhow, here's the Yarm noise spectra calibrated plot:
I have emailed Kiwamu, but haven't heard back from him yet on what the original design considerations were, if he remembered us ever using a boost, etc. What this looks like to me is that we need to do some serious work to get the noise down. Maybe fixing the gain peaking and triggering the boost will get us most of the way there?
I remeasured all of the noise spectra again today, making sure the input attenuation was as low as it could safely be. I also got a snap of the y green PDH signal; it's fairly larger than I saw the other day, which is good. I used this to calibrate the error signal voltage spectra.
Here are the noise traces for each arm. During these measurements GTRX was about .6, GTRY about 1.0 The Yarm noise doesn't look so good: the error signal is just barely above the mixer+lowpass output noise, and the RMS is plauged by 60Hz lines. (Is this related to what we see in IR TRY sometimes?)
Here are the arms error signals compared directly:
We spent time trying to relieve the Yend green PDH of it troubles.
We realized that the mixer in the PDH setup (mini circuits ZAD-8+), wants 7dBm of LO to properly function. However, we use one function generators output, through a splitter, to give signals to the laser PZT and the mixer LO.
We don't want 7dBm of power hitting the laser PZT, though. The summing node that adds the servo output to the sideband signal was supposedly designed to do some of this attenuation. Rana measured that 10Vpp out of the function generator resulted in 20mVpp on the fast input to the NPRO, after the summing node. Hence, the 0.09V setting was only resulting in something like 0.2mV hitting the PZT. The PZT has something like 30 rad/V PM response, meaning we only had ~0.006 rad of modulation.
Now, the function generator is set to 2 Vpp, meaning 4 mVpp hitting the PZT, meaning ~0.12 radians of modulation. The mixer is now getting +7dBm on its LO, and the PDH traces look much cleaner. However, the PDH error signal is now something like 100mVpp, which is much bigger than the PDH board is designed for, so there is now a 10dB attenuator between the reflection PD DC block and the RF input to the mixer.
Here are screenshots of the Inmon channel (which has a gain of ~20) showing a sweep through some PDH signal, and the error signal while in green lock. Huge 60Hz harmonics are still observed.
Regarding these 60Hz issues, we need to make sure that we remove all situations where long BNCs are chained together with barrel connectors, or Ts are touching other ones. We also should glue or affix the pomona summing box to the shelf, so that its not just laying on the floor.
The concrete next step is to go fiddle with things, and see if we can get the 60Hz noise to go away, then measure the PDH loop and noises again. Hopefully, this should make the ALS much more reliable.
I found that the barrel of one the BNC to BNC connectors used for getting the output of the PDH servo box to the laser controller was touching the ETMY chamber. When I held it away, all of the 60Hz harmonics disappeared from the mixer output spectrum; this was pretty repeatable. This inspired me to replace the refl PD and PZT signal cables (which were 2 and 3 cables stitched together, respectively) with 20' long BNCs. I also cleaned up a lot of the routing of signal and power cables in the little rack, and moved the big T->DC Block->Attenuator combo off of the panel mount, because I didn't like how it was wiggling. It and the summing pomona box are sitting on top of the PDH box and function generator, instead of hanging freely.
All of the 60Hz harmonics were banished afterwards, and the green locked happily.
This required me touching the Y end table, to remove the old cable and its cable ties, and putting the new one in. I don't think I did anything immediately apparently bad; the green and IR transmissions both are within nominal ranges.
I haven't had luck measuring the CLG yet, which I wanted to do to get and set the UGF before measuring the noises. However, here is a scope trace of the in-lock error signal, which compares quite favorably to the trace posted in the previous post; the scope indicates that the signal has 1/3 of the RMS that it did before I replaced the cables.
I hope to measure up the current status after I get back from dinner.
Yesterday I measured the spectra and OLTF of the Y-Arm green PDH, after the LO touch-up and 60Hz hunt from last week. I also went to lower frequencies with the SR785, but forgot to take some of the background spectra down there, so I don't have the full breakdown plots yet. Nevertheless, here is the improvement in the PDH error signal:
I also measured the OLTF (SR785 injection at the error signal, Auto level ref 5mV at channel 2, 10mV/s source ramping, 50mV max output)
As you can see, we have tons of phase margin. Flipping the local boost switch had no visible effect on the OLTF; we should change it to something that puts this surplus of phase to good use, and squash the error signal even more. Putting an integrator at 5kHz should still leave about 45 degrees phase margin at 10k. I've started making a LISO model of the PDH board from the DCC drawing, and then I'll inspect the boards individually to make sure I catch the homegrown modifications.
Data, and code used to generate the plots is attached.
Quick post of plots and data; I'll fill in more detail tonight.
TL;DR: I pulled both green PDH boxes and made LISO models, compared TFs and noise levels.
Pictures of X and Y boards, respectively
TF comparison to LISO. (Normalized to coincide at 1Hz)
Noise comparison to LISO
All data, EAGLE schematics, LISO source and plots in the attached zip.
I had noticed in the past, that the digital control signal monitor for the X end would saturate well before the ADC should saturate (C1:ALS-X_SLOW_SERVO_IN1, which is from the "output mon" BNC on the box). It turns out that there is some odd saturation happening inside the box itself.
In this scope trace, the servo input is being driven with a 0.02Vpp, 0.1Hz sine wave, gain knob at 1.0. This is bad.
Evan and I poked around the board, and discover that for some reason currently unknown to us, the variable gain amplifier (AD8336) can't reach its negative rail, despite the +-12V arriving safely at its power supply pins.
I also realized that the LF356 in the integrator stage in this box had been replaced with a LT1792 by Kiwamu in ELOG 4373. I've updated my schematic, and will upload both boxes' schematics to the DCC page Jenne created for them. (D1400293 and D1400294)
I've been having trouble locking the X - green for the past few hours. Has there been some configuration change down there that anyone knows about?
I'm thinking that perhaps I need to replace the SHG crystal or perhaps remove the PZT alignment mirrors perhaps. Another possibility is that the NPRO down there is going bad. I'll start swapping the Y-end NPRO for the X-end one and see if that makes things better.
I had pulled out both X and Y servo boxes for inspection, put the Y box back, soldered in a missing op amp power capacitor on the X end box, and had not yet put back the X end box yet because of the saturation issue I was looking into. Otherwise nothing was changed at the ends; I didn't open the tables at all, or touch laser/SHG settings, just unplugged the servo boxes.
I narrowed down the saturation point in the X green PDH box to the preamp inside the AD8336, but there is still no clear answer as to why it's happening.
As per Jenne's request, I put the X end PDH box back for tonight's work. It locks, but we have an artificially low actuation range. With SR785, I confirmed a PDH UGF around 5k. Higher than that, and I couldn't reliably measure the UGF due to SR560 saturations. The analyzer is not currently in the loop.
Both arms lock to green, but I haven't looked at beatnotes today.
What monitor point is being plotted here? Or is it a scope probe output?
If this saturation is in the uPDH-X but not in the uPDH-Y, then just replace the VGA chip. Because these things have fixed attenuation inside, they often can't go the rails even when the chip is new.
In any case, we need to make a fix to get this box on the air in a fixed state before tomorrow evening.
The traces were from the front panel output BNCs, but the VGA preamp exhibited this asymmetric saturation at its output.
In any case, I tried to replace the Xend box's AD8336 with a new one, and in doing so, did some irreparable damage to the traces on the board I was not able to get a new AD8336 into the board. There are some ATF ELOGs where Zach found the AD8336 noise to be bad at low frequencies (link), and its form factor is totally unsuitable for any design that may involve hand modification, since it doesn't even have legs, just tiny little pads. I suggest we never use it for anything in the future.
Instead, I've hacked on a little daughter board with an OP27 as an inverting op-amp with the gain resistor on the front panel as its feedback resistor, which can swing from 0 to x20 gain (the old gain setting was around 15dB=~x6). I've checked out the TF and output noise, and they look ok. The board can output both rails as well.
I don't really like this as a long term solution, but I didn't want to leave things in a totally broken state when I left for dinner.
Just a quick note, plots and data will come tomorrow:
I grabbed an unused uPDH board from the ATF (thanks Zach!), and re-stuffed almost the entire thing to match Jenne's latest schematic for the y end box. I also threw some 22uF caps on the regulators, as Koji did with the previous box, to eliminate some oscillations up in the high 10s of kHz. I replaced the tragedy of a box that I created on Wednesday with this new box. The arm locks pretty stably with the boost on, 30 degrees of phase margin with 10kHz UGF, and locks pretty darn reliably.
Now we should now have two nicely boosted PDH loops. I'll do a noise/loop breakdown again in the upcoming days.
I measured the noise spectra and loop TF of the green PDH with the newly stuffed board. Unfortunately, I never took the noise below 100Hz of the previous box, so we can't see what has happened to the overall RMS, or more specifically, the RMS due to the pendulum resonance. All of these plots are in the boosted state, as that is how we intend to use the box.
Here is the loop, which does not have quite as much margin as the y-arm, but 10dB of gain peaking is probably ok, since the RMS at 10s of kHz is not so important to ALS. (OL measured, CL inferred) We see the 1/f shape from 1k to 50k or so, and 1/f^2 under 1k, as desired.
Comparing in the in loop error signals, we see the effect from the increased gain from 100Hz to 10kHz. (Here is where I regret not looking at the low frequency spectrum two weeks ago)
Finally, here is the noise breakdown.
The error signal RMS is now dominated by the 1Hz peak. We have talked about using digital feedback for this, since we have the PDH error signal coming into an ADC, and can sum in a DAC signal into the servo output. This also lets us intelligently trigger a sub-10Hz boost once the PDH box locks itself. With a good boost, we maybe could bring the in-loop RMS of the error signal to under 1kHz.
Something odd that Rana brought to my attention, however, is that my measurement and calibration indicates an RMS of ~5kHz, but the cavity pole should be something like 18kHz. If this is true, how can we be seeing stable power? This maybe means that my calibration is too many Hz per Volt.
I performed the calibration by creating a MIST model of the arm, and generating the PDH error signal on a demodulated PD, I then find the slope of Hz per arbitrary error signal unit. Then, looking at a scope trace, I match up the horn-to-horn voltage to the horn-to-horn arbitrary error signal units, which lets me finally find Hz per error signal volt.
However, there is some qualitative difference in the shape between the simulated and observed error signals, namely, that the outer horns are larger than the inner horns in the real signal.
Does this matter? Is there something in my simulation that I can correct that would give a more accurate calibration?
Data, plots, code, attached.
What modulation depth are you using for the simulation? I have never seen a real measurement of that in our elog for the end-PDH systems.
I also disbelieve your RMS calculations. It looks like in the 1.5-0.5 Hz band we're picking up 50 kHz of frequency noise even though the 1 Hz peak is only 80 Hz/rHz, even though math says "80 * sqrt(1) = 80".
Take a look at:
I used a modulation depth of 0.3, which, if I recall correctly, is what we aimed for on the Y-arm when we adjusted the LO signal there. However, this is probably not the case for the X arm.
In any case, I found the bug in my RMS calculation. (I had forgotten to flip the x array in addition to the y array for the right-to-left integration, and had uneven bin spacing, so the integration bandwidths weren't correct...)
Here are the updated plots. The properly evaluated RMS is ~600Hz, which seems to mostly come in around 10k, so we may want to turn down the gain for less gain peaking in that region.
600 Hz seems ~OK. From the measured reflectivities for 532 nm, the green Finesse = 108. So the green cavity pole should be 18.3 kHz given an arm length of 37.8 m.
600 Hz of green frequency noise means that we would get 38 pm RMS of arm mirror motion. We should assumed a peak/RMS factor of 10, so this would allow us to get to ~0.4 nm CARM offset.
However, its better than that. What we really care about for ALS is the amount of this green frequency noise which is put onto the arm. With an ALS feedback bandwidth of 100 Hz, my eyeball estimate say that the contribution from green PDH error will be ~100 Hz RMS, since we don't care too much about the 10 kHz stuff. So this seems good enough for now; let's figure out what's up with PDH-Y and get back to locking.
These are plots and notes from last week's PDH adventures.
For the PDH servo box re-design, we wanted to think a little bit about what we actually wanted out of the box.
* We want the zero of the main transfer function to be at the same frequency as the cavity pole for green, which is about 18kHz.
* We want the boost to suppress noise at a few hundred Hz. We don't need super-duper low-frequency boost, nor do we want it. We'd like to leave the boost on all the time.
* Wanted to get rid of 10dB attenuator on PD input, so needed to lower the overall gain.
* We acknowledge that the gain of the raw error signal times the PZT response is very high, so no matter what, we will have to have a low-gain servo, even perhaps have the servo shape be less than unity gain.
---> We reduced the gain of the first amplification stage from a gain of 20 to a gain of 3.
---> Made the boost stage have a DC gain of 1. Pole at 75 Hz and Zero at 1.6kHz to give suppression at a few hundred Hz. Boost is *not* a pure integrator, so that we can leave it on. (If we required triggering anyway, we would have made it a pure integrator).
---> In transfer function stage, put zero at 17.7kHz to match cavity pole. Pole of servo was going to be at 20 Hz, but we wanted a little more gain, so we lowered it to 2 Hz.
Here is the final measured servo box transfer function for the Yend box (with an arbitrary gain knob setting):
Once installed, I set the gain knob for the Yend at 4.0, which gave an overall UGF of about 10kHz. Then I measured the loop:
I also measured the error point and the control point, and compared them to Q's measurements in elog 10430.
In order to see what we might expect for a contribution to ALS noise, I looked at the error point spectra and lowpassed it with a pole at 200Hz. I do this because the PDH error is like sensor noise for the ALS, but the ALS UGF is around 200 Hz, so noise at frequencies higher than that will be suppressed like 1/f. So, I lowpass the error signal, then look at the RMS, and see that we should be pretty happy with our result. I include also the Xend error spectrum, as measured and reported by Q in elog 10460.
Summary: Cannot find beatnotes between the arms and PSL.
I wanted to measure the ALS out of loop noise before putting stuff on the PSL table for frequency offset locking.
But I was not able to find the beat notes between the arms and PSL green. All I could find while scanning through the end laser temperatures is the beatnote between the X and Y green.
EricQ says that he spent some time yesterday and could not find the beatnotes as well.
Debugging and still could not find:
1. Checked the FSS slow actuator. This was close to zero ~0.003
2. Checked the green alignment on the PSL table. Everything seems fine.
3. Checked the actual PSL laser temperature. It was 31.28deg and not very far from when it was last set at 31.33deg elog.
4. Also checked the end laser temperatures. Both the lasers are ~40deg (where I could see the beatnote between the arms). Based on the plot here and here , we are very much in the regime where there should be a beatnote between the PSL and the arms.
I have been looking at the X-end ALS setup.
I was playing with the control bandwidth to see the effect to the phase tracker output (i.e. ALS err).
For this test the arm was locked with the IR and the green beat note was used as the monitor.
From the shape of the error signal, the UGF of the green PDH was ~10kHz. When I increased the gain
to make the servo peaky, actually the floor level of the ALS err became WORSE. I did not see any improvement
anywhere. So, high residual error RMS cause some broadband noise in the ALS??? This should be checked.
Then when the UGF was lowered to 3kHz, I could see some bump at 3kHz showed up in the ALS error.
I didn't see the change of the PSD below 1kHz. So, more supression of the green PDH does not help
to improve the ALS error?
Then, I started to play with the phase tracker. It seems that someone already added the LF booster
to the phase tracker servo. I checked the phase tracker error and confirmed it is well supressed.
Further integrator does not help to reduce the phase tracker error.
For the next thing I started to change the offset of the phase tracker. This actually changes
the ALS error level! The attached plot shows the dependence of the ALS error PSD on the phase tracker
output. At the time of this measurement, the offset of -10 exhibited the best noise level.
This was, indeed, factor of 3~5 improvement compared to the zero offset case below 100Hz.
I'm afraid that this offset changes the beat frequency as I had the best noise level at the offset of -5
with a different lock streatch. We should look at this more carefully. If the beat freq changes the offset,
this give us another reason to fix the beat frequency (i.e. we need the frequency control loop.
= Today's ALSX error would have not been the usual low noise state.
We should recover the nominal state of the ALS and make the same test =