1) True. My bad. In my elog entry (but not in my fit code) I forgot the impedance Z= 750Ohm (as in the fit) of the resonant circuit in front of the square root: Vn = Vdn + Z * sqrt( 2 e Idc )
2) That is exactly the point I was raising! The measured dark noise at resonance is 2x what I expect.
I also admitted that the data points were few, especially around the 3dB point.
Today I'm going to repeat the measurement with a new setup that lets me tune the light intensity more finely.
Here's another measurement of the noise of the REFL11 PD.
This time I made the fit constraining the Dark Noise. I realized that it didn't make much sense leaving it as a free coefficient: the dark noise is what it is.
Result: the transimpedance of REFL11at 11 MHz is about 4000 Ohm.
What kind of fit did you use? How are the uncertainties in the parameters obtained?
Would it be possible to write about the technique on a wiki page as you get measurements and results?
[Alberto, Koji, Rana]
The RFM network failed today. We had to reboot the frame builder anf restart all the front end following the instructions for the "Nuclear Option".
Burt-restoring to May 1st at 18:07, or April 30 18:07 made c1sosvme crash. We had to reset the front ends again and restore to April 15th at 18:07 in order to make everything work.
Everything seems fine again now.
Here's the (calibrated) transimpedance of the new REFL55 PD.
T(55.3) / T_(11.06) = 93 dB
After munching analytical models, simulations, measurements of photodiodes I think I got a better grasp of what we want from them, and how to get it. For instance I now know that we need a transimpedance of about 5000 V/A if we want them to be shot noise limited for ~mW of light power.
Adding 2-omega and f1/f2 notch filters complicates the issue, forcing to make trade-offs in the choice of the components (i.e., the Q of the notches)
Here's a better improved design of the 11Mhz PD.
This should be better. It should also have larger resonance width.
How much is the width?
The transfer function phase drops by 180 degrees in about 2MHz. Is that a good way to measure the width?
The measured transimpedance of the latest POY11 PD matches my model very well up to 100 MHz. But at about ~216MHz I have a resonance that I can't really explain.
The following is a simplified illustration of the resonant circuit:
Perhaps my model misses that resonance because it doesn't include stray capacitances.
While I was tinkering with it, i noticed a couple of things:
- the frequency of that oscillation changes by grasping with finger the last inductor of the circuit (the 55n above); that is adding inductance
- the RF probe of the scope clearly shows me the oscillation only after the 0.1u series capacitor
- adding a small capacitor in parallel to the feedback resistor of the output amplifier increases the frequency of the oscilaltion
I started putting together the components that are coint to go inside the frequency generation box. Here's how it looked like:
The single component are going to be mounted on a board that is going to sit on the bottom of the box.
I'm thinking whether to mount the components on an isolating board (like they did in GEO), or on an aluminum board.
I emailed Hartmut to know more details about his motivations on making that choice.
The choice of 100 Ohm for the isolating resistor was mainly empirical. I started with 10, then 20 and 50 until I got a sufficient suppression of the resonance. Even just 10Ohm suppressed the resonance by several tens of dB.
In that way the gain of the loop didn't change. Before that, I was also able to kill the resonance by just increasing the loop gain from 10 to 17. But, I didn't want to increase the closed-loop gain.
One thing that I tried, on Koji's suggestion, was to try to connect the RF output of the PD box to an RF amplifier to see whether shielding the output from the cable capacitance would make the resonance disappear: It did not work.
I update my old 40mUpgrade Optickle model, by adding the latest updates in the optical layout (mirror distances, main optics transmissivities, folding mirror transmissivities, etc). I also cleaned it from a lot of useless, Advanced LIGO features.
I calculated the expected power in the fields present at the main ports of the interferometer.
I repeated the calculations for both the arms-locked/arms-unlocked configurations. I used a new set of functions that I wrote which let me evaluate the field power and RF power anywhere in the IFO. (all in my SVN directory)
As in Koji's optical layout, I set the arm length to 38m and I found that at the SP port there was much more power that I woud expect at 44Mhz and 110 MHz.
It's not straightforward to identify unequivocally what is causing it (I have about 100 frequencies going around in the IFO), but presumably the measured power at 44MHz was from the beat between f1 an f2 (55-11=44MHz), and that at 110MHz was from the f2 first sidebands.
Here's what i found:
I found that When I set the arm length to 38.55m (the old 40m average arm length), the power at 44 and 110 MHz went significantly down. See here:
I checked the distances between all the frequencies circulating in the IFO from the closest arm resonance to them.
I found that the f2 and 2*f2 are two of the closest frequencies to the arm resonance (~80KHz). With a arm cavity finesse of 450, that shouldn't be a problem, though.
I'll keep using the numbers I got to nail down the culprit.
Anyways, now the question is: what is the design length of the arms? Because if it is really 38m rather than 38.55m, then maybe we should change it back to the old values.
... not just because we haven't locked the interferometer for quite some time. I mean, it literally stinks. The chiller's chiller is molding. Its' dripping water and there's mold all under it (Jo just confirmed: "yeah, it's mold").
Someone from Caltech maintenance just crossed the door. Hopefully he'll help us fix it.
I'll keep you updated. Stay tuned.
This is how the RF generation box might soon look like:
A dedicated wiki page shows the state of the work:
The second sideband is resonant in the arms for a cavity length of 37.9299m.
The nearest antiresonant arm lengths for f2 (55MHz) are 36.5753m and 39.2845m.
If we don't touch the ITMs, and we use the room we still have now on the end tables, we can get to 37.5m.
This is how the power spectrum at REFL would look like for perfect antiresonance:
And this is how it looks like for 37.5m:
Or, god forbid, we change the modulation frequencies...
For both sidebands to be antiresonant in the arms, the first modulation frequency has to be:
f1 = (n + 1/2) c / (2*L)
where L is the arm length and c the speed of light. For L=38m, we pick to cases: n=3, then f1a = 13.806231 MHz; n=2, then f1b = 9.861594 MHz.
If we go for f1a, then the mode cleaner half length has to change to 10.857m. If we go for f1b, the MC length goes to 15.200m. A 2 meter change from the current length either way.
And the mode cleaner would only be the first of a long list of things that would have to change. Then it would be the turn of the recycling cavities.
Kind of a big deal.
A poor lonely SR785 was found this morning roaming around in the lab in evident violation of the fundamental rule which requires all the equipment on carts to be brought back inside the lab right after use.
The people and the professors related to the case should take immediate action to repair for their misdeed.
[Alberto, Kiwamu, Kevin, Rana]
Today we tried to measured the beam shape after the MC MMT1 that Jenne installed on the BS table.
The beam scan showed a clipped spot. We tracked it down to the Farady and the MCT pickoff mirror.
The beam was getting clipped at the exit of the Faraday. But it was also clipping the edge of the MCT pick-off mirror. I moved the mirror.
Also the beam looked off-center on MC2.
We're coming back on Sunday to keep working on this.
Now things are bad.
The mode cleaner is locked and the air conditioning is full on. So the the air conditioning doesn't seem to be so important for the lock to hold.
Two days ago I opened the PSL shutter by switching the switch on the shutter driver. That caused the shutter's switch on the medm screen to work in reversed mode: open meant closed and closed meant open.
I fixed that. Now the medm screen switch state is correct.
We moved the MC-trans pick-off mirror (= the beam splitter between the input of the Faraday and the steering mirror located right after MC3). Now the beam goes through the Farady without getting clipped.
This is the list of the things that have to be done next:
To get a feel for the Capacitive Bridge problems, we setup a simple bridge using fixed (1 nF) caps on a breadboard. We used an SR830 Lock-In amplifier to drive it and readout the noise.
The measurement setup for the Capacitor Bridge Test is still sitting on one of the work benches.
Unless the experiment is supposed to continue today, the equipment shouldn't have been left on the bench. It should have been taken back to the lab.
Also the cart with HP network analyzer used for the test was left in the desk area. That shouldn't have left floating around in the desk area anyway.
The people responsible for that, are kindly invited to clean up after themselves.
BTW, latex launched this new thing for writing pdfs. doesnot require any installations. check http://docs.latexlab.org
I could not dare to share my google doc with this site...
Just in case, granted access to Google docs can be revoked any time from here:
I wrote down the settings according to which I tuned the optickle model of the 40m Upgrade.
Basically I set it so that:
In this way when the carrier becomes resonant in the arms we have:
The DARM offset for DC readout is optional, and doesn't change those conditions.
I also plotted the carrier and the sideband's circulating power for both recycling cavities.
I'm attaching a file containing more detailed explanations of what I said above. It also contains the plots of field powers, and transfer functions from DARM to the dark port. I think they don't look quite right. There seems to be something wrong.
Valera thought of fixing the problem, removing the 180 degree offset on the SRM, which is what makes the sideband rather than the carrier resonant in SRC. In his model the carrier becomes resonant and the sideband anti-resonant. I don't think that is correct.
The resonant-carrier case is also included in the attachment (the plots with SRMoff=0 deg). In the plots the DARM offset is always zero.
I'm not sure why the settings are not producing the expected transfer functions.
Today I started writing the IFO modeling wiki page.
The idea is to make it a reference place where to share our modeling tools for the 40m.
i added my laptop's mac address to teh martian at port 13 today.
No personal laptop is allowed to the martian network. Only access to the General Computing Side is permitted.
Please disconnect it.
I calculated the phase shifts that the sidebands would pick up in the arms in the case we changed the arm length to 38.4m as proposed. I obtained the following values (in degrees):
phi(-f2) = 0.66; phi(-f1) = -0.71; phi(f1) = 0.71; phi(+f2) = -0.66
These are the plots with the results as I obtained from an Optickle simulation (the second zooms in around 38.4m).
These values agree with what Koji had already estimated (see elog entry 3023).
Since we can't make the arm longer than that, to increase the distance from the resonance, we would like to adjust the length of the short cavities to compensate for that. For f2 (=55MHz), 0.7 degrees correspond to about 5cm. That is about the length change that we expect to make to the design.
I simulated with Optickle the effect of changing the length of either the SRC or the PRC. The best way I found to do that, was to measure the cavity circulating power when the macroscopic lengths change.
The following plots show the effect of changing either the PRC or SRC length (left or right figure), on the circulating power of both cavities at the same time (top and bottom plots).
You can compare these with the case of perfect antiresonance as in the following plots:
It seems that the design length for the short cavities are not too bad. f1 is not optimized in the PRC, but changing the length of the cavity wold just make f2 worse in SRC.
These simulations seem to support the choice of not changing the design cavity lengths for PRC and SRC.
Of course these are only an "open loop" simulations. At the moment we don't know what would be the effect of closing the control loops. That is something I'm going to do later. It'll be part of my studies on the effects of cavity absolute length on the whole IFO.
You should have been in my lecture yesterday!
Power in the cavity is not a good index (=error signal) to judge the optimal length.
You should look at the phases of the length signals. (i.e. demodulation phase which gives you the maximum amplitude for CARM, PRC, SRC, etc)
You must move the SRC and PRC lengths at the same time.
The resonance of f1 (mostly) depends on the PRC length, but that of f2 depends on both the PRC and SRC lengths.
Right. Ultimately the phase gain inside the cavity is what we look at. Calculating that for the SBs inside PRC and SRC is actually the first thing I did.
But I kept getting very small angles. Too small, I thought. Maybe there was some problem in the way I calculated it.
Then I made a power analysis to check if the SBs were getting affected at all by that 0.7degree phase shift they're picking up in the arms.
I wanted to show the point where I am, before leaving. But, I keep working on it.
I also removed two of the AM stabilizers from the 1Y2 rack. The other one, which is currently running th MC modulations, is still in the rack, and there it is going to remain together with its distribution box.
I stored both AM stabilizers and the Stochmon box inside the RF cabinet down the East arm.
Lately I've been trying to calculate the corrections to the recycling cavity lengths that would compensate for the phase that the sidebands will pick up from the arms in the upgraded interferometer.
To do that calculation , I tried two quite different ways, although equivalent in principle. They both use the optickle model of the 40m, but the calculation is made differently.
In the first way, I looked directly at the phases of the field: phase of [input field] / [reflected field], phase of [input field at PRM] / [transmitted field at SRM].
In the second way I looked at the demodulation phases of the LSC signals.
The first way is much simpler, especially from a computational point of view. It is the first I tried several weeks ago, but then I had abandoned because back then I thought it wasn't the correct way.
Anyway, both ways gave me the same results for the PRC length.
For the SRC length, the first way has given me a clear outcome. On the other hand, the second way has produced a less clear result.
According to these results, these would be the proposed adjustements to the cavity lengths:
dl(PRC) = -0.0266 m; dl(SRC) = 0.0612 m
I) 1st Way
a) case of arms ideal length (33.86 m)
b) case arm length = 38.40 m
II) 2nd Way
a) case of arms ideal length (33.86 m)
Tell me whether it is correct or not. Otherwise I won't be able to sleep tonight.
According to these results, these would be the proposed adjustements to the cavity lengths:
dl(PRC) = -0.0266 m; dl(SRC) = 0.612 m
Sorry. I was in a rush to go to the LIGO "all hands" meetings when I posted that elog entry, that I forgot a zero in the SRC length value. The correct values are:
dl(PRC) = -0.0266 m; dl(SRC) = 0.0612 m
The cavity absolute lengths are then:
L(PRC) = 0.5/2/f1*c - 0.0266 = 6.7466 m
L(SRC) = c/f2 + 0.0612 = 5.4798 m
where c is the speed of light; f1 = 11065399 Hz; f2 = 55326995 Hz
Today I noticed that the FE SYNC counters of c1susvme1/2 on the RFM network screen were stuck at 16384. I tried to reboot the machines to fix the problem but it didn't work.
The BS watchdog tripped off when I did that, because I had forgotten to disable it. I had to wait for a few minutes before it settled down again.
Later I also re-locked the mode cleaner. But before I could do it, Rana had to reduce the MC_L offset for me.
"They (shellfish) shall be an abomination to you; you shall not eat their flesh, but you shall regard their carcasses as an abomination." (Leviticus 11:11)
It looks like something wrong happened around the PSL front end. One of the PSL channel, C1:PSL-PMC_LOCALC, got crazy.
One of the PSL channel, C1:PSL-PMC_LOCALC, got crazy.
We found it by the donkey alarm 10 minutes ago.
The attached picture is a screen shot of the PMC medm screen.
The value of C1:PSL-PMC_LOCALC ( middle left on the picture ) shows wired characters. It returns "nan" when we do ezcaread.
Joe went to the rack and powered off / on the crate, but it still remains the same. It might be an analog issue (?)
The problem seems to be a software one.
In any case, Kiwamu and I looked at the at the PMC crystal board and demod board, in search of a possible bad connection. We found a weak connection of the RG cable going into the PD input of the demod board. The cable was bent and almost broken.
I replaced the SMA connector of the cable with a new one that I soldered in situ. Then I made sure that the connection was good and didn't have any short due to the soldering.
By looking at the reference pictures of the rack in the wiki, it turned out that the Sorensen which provides the 10V to the 1Y1 rack was on halt (red light on). It had been like that since 1.30pm today. It might have probably got disabled by a short somewhere or inadvertently by someone working nearby it.
Turning it off and on reset it. The crazy LO calibrated amplitude on the PMC screen got fixed.
Then it was again possible to lock PMC and FSS.
We also had to burtrestore the PSL computer becasue of the several reboots done on it today.
Today we measured the phase noise of the oscillator used for the FSS.
The source is a Wenzel crystal at about 21.5MHz that Peter Kalmus built some time ago.
We basically used the same technique that Frank and Megan have been using lately to measure the Marconi's phase noise.
Today we just did a quick measurement but today next week we are going to repeat it more carefully.
Attached is a plot that shows the measurement calibrated for a UGF at about 60 Hz. The noise is compared to that specified by Wenzel for their crystal.
The noise is bigger than that of the MArconi alone locked to the Rubidium standard (see elog entry). We don't know the reason for sure yet.
We'll get back to this problem next week.
I uploaded an updated optickle model of the upgrade to the SVN directory with the optickle models (here).
[Koji, Steve, Kiwamu, Alberto]
- This afternoon we installed a few new optics on the BS table: GR_PBS, GRY_SM2, GRY_SM1.
- We pulled up the cables so that we had more freedom to move one of the cable towers farther South.
- Then we re-leveled the table. PRM OSEMs were adjusted to be nominal insertions.
- Koji released the earthquake stops on BS but the readout of the OSEMs was apparently frozen on the MEDM screens.
Initially we thought it was a software problem. a nuclear reboot didn't solve it. We spent the following three hours investigating the cause.
Eventually it turned out that the earthquake stops on BS weren't actually fully released.
We opened the tank and accessed to BS. Releasing the earthquake stops in full solved the issue. The OSEMs readout went back to normal.
I just restarted the elog after I found it down a few minutes ago.
I finished assembling the frequency generation unit for the upgrade. I tested it through to check that the power levels are as expected at the various connection (see attached png, showing in black the design power values, and in red the measured ones).
Because of some modifications made on the design along the construction, I have to recalculate the SNR along the lines.
I can now start to measure phase noise and distortion harmonics.
A document with a description of the design and the results of the characterization measurements will be available in the end.
A few weeks ago, on Jul 24, Rana and I measured the phase noise of the FSS frequency box (aka the 'Kalmus Box'). See elog entry 3286.
That time, for some reason, we measured a phase noise higher than we expected; higher than that of the Marconi.
I repeated the measurement today using the SR785 spectrum analyzer. Here is the result:
(The measurement of July 24 on the plot was not corrected for the loop gain. The UGF was at about 30 Hz)
To make sure that my measurement procedure was correct, I also measured the combined phase noise of two Marconis. I then confirmed the consistency of that with what already measured by other people in the past (i.e. Rana elog entry 823 in the ATF elog).
This time the noise seemed reasonable; closer to the Marconi's phase noise, as we would expect. I don't know why it was so bad on July 24.
The shoulder in the Marconi-to-Marconi measurement between 80Hz and 800Hz is probably due to the phase noise of the other Marconi, the one used as LO.
I'm going to repeat the measurement connecting the setup to the DAQ, and locking the Marconi to the Rubidium standard.
Ultimately, the goal is to measure the phase noise of the new Sideband Frequency Generation Box of the 40m Upgrade.
Today I put the FSS frequency box back into the 1Y1 rack.
To power it on, I turned on the 24V and 15V Sorensen switches in the same rack.
The PMC crystal board in the same rack should not be affected (it runs with 10V), but, to make sure it was not powered, I disconnected it from its crate. Since the board was disconnected from the EOM for the PSL table's upgrade, I wanted to avoid having the RF output floating.
We just have to remember to plug it back in, when we need it again.
I just turned on the other Sorensen's too in 1Y1.
I measured the phase noise of the LO output of the FSS box from the DAQ. I'm attaching the results.
As we expected, the measurement is limited by the internal phase noise of the Marconi.
The measurement was done as shown in this diagram.
The differences between this setup and the one used previously is the lack of the 50 Ohm terminator in the mixer output and
that the SR560 readout with the G=100 should come before the first SR560 via T, so as not to be spoiled by the high noise of the G=1 SR560.
I removed the 50 Ohm in-line terminator when I did the measurement with the SR785. The for some reason I was getting more noise, so I removed it.
Now I put it back in and I did the measurement with the DAQ. I also moved the SR560 that amplifies the signal for the DAQ, Tee'ing it with the input of the in-loop SR560.
Now the setup looks like this:
And the phase noise that I measure is this:
Comparing it with the phase noise measured with the previous setup (see entry 3506), you can see that the noise effectively is reduced by about a factor of 2 above 10 Hz.
I found this very interesting German maker of cool cable cutting tools. It's called Jokari.
We should keep it as a reference for the future if we want to buy something like that, ie RF coax cable cutting knives.
The Netgear Network Switch in the top shelf of Nodus' rack has a broken fan. It is the one interfaced to the Martian network.
The fan must have broken and it is has now started to produce a loud noise. It's like a truck was parked in the room with the engine running.
Also the other network switch, just below the Netgear, has one of its two fans broken. It is the one interfaced with the General Computer Side.
I tried to knock them to make the noise stop, but nothing happened.
We should consider trying to fix them. Although that would mean disconnecting all the computers.