I was aware of a problem on those units since I acquired the data. Then it wasn't totally clear to me which were the units of the data as downloaded from the Agilent 4395A, and, in part, still isn't.
It's clear that the data was in units of spectrum, an not spectral density: in between the two there is a division by the bandwidth (100KHz, in this case). Correcting for that, one gets the following plot for the FSS PD:
Although the reason why I was hesitating to elog this other plot is that it looks like there's still a discrepancy of about 0.5dBm between what one reads on the display of the spectrum analyzer and the data values downloaded from it.
However I well know that, I should have just posted it, including my reserves about that possible offset (as I'm doing now).
This evening we measured the noise spectrum of the reference cavity PD used in the FSS loop. From that we estimated the transimpedance and found that the PD is shot-noise limited. We also found a big AM oscillation in correspondence of the FSS modulation sideband which we later attenuated at least in part.
These are the dark noise spectrum that I measured on the 11MHz and 55MHz PD prototypes I modified.
The plots take into account the 50Ohm input impedance of the spectrum analyzer (that is, the nosie is divided by 2).
With an estimated transimpedance of about 300Ohm, I would expect to have 2-3nV/rtHz at all frequencies except for the resonant frequencies of each PD. At those resonances I would expect to have ~15nV/rtHz (cfr elog entry 2760).
I have to figure out what are the sources of such noises.
There's several more of the this vintage in one of the last cabinets down the new Y-arm.
Hold on, did the arms get re-baptized?
After adding an inductor L=100uH and a resistor R=10Ohm in parallel after the OP547A opamp that provide the bias for the photodiode of REFL11, the noise at low frequency that I had observed, was significantly reduced.
See this plot:
A closer inspection of the should at 11MHz in the noise spectrum, showed some harmonics on it, spaced with about 200KHz. Closing the RF cage and the box lid made them disappear. See next plot:
The full noise spectrum looks like this:
A big bump is present at ~275MHz. it could important if it also shows up on the shot noise spectrum.
This morning the pencil soldering iron of our Weller WD2000M Soldering Station suddenly stopped working and got cold after I turned the station on. The unit's display is showing a message that says "TIP". i checked out the manual, but it doesn't say anything about that. I don't know what it means. Perhaps burned tip?
Before asking Steve to buy a new one, I emailed Weller about the problem.
This morning, at about 12 Koji found all the front-ends down.
At 1:45pm rebooted ISCEX, ISCEY, SOSVME, SUSVME1, SUSVME2, LSC, ASC, ISCAUX
Then I burtestored ISCEX, ISCEY, ISCAUX to April 2nd, 23:07.
The front-ends are now up and running again.
Lately I've been trying to sort out the problem of the discrepancy that I noticed between the values read on the spectrum analyzer's display and what we get with the GPIB interface.
It turns out that the discrepancy originates from the two data vector that the display and the GPIB interface acquire. Whereas the display shows data in "RAW" format, the GPIB interface, for the way the netgpibdata script is written, acquires the so called "error-corrected data". That is the GPIB downloaded data is postprocessed and corrected for some internal calibration factors of the instrument.
I noticed that someone, that wasn't me, has edited the wiki page about the netgpibdata under my name saying:
* A4395 Spectrum Units
Independetly by which unites are displayed by the A4395 spectrum analyzer on the screen, the data is saved in Watts/rtHz"
# gnd n22
# | |
# Rip Rw2
# | | |\
# nt- Rsi-n2- - - C2 - n3 - - - - | \
# | | | | |4106>-- n5 - Rs -- no
# iinput Rd L1 L2 R24 n6- | / | |
# |- nin- | | | | | |/ | Rload
# Cd n7 R22 gnd | | |
# | | | | - - - R8 - - gnd
# gnd R1 gnd R7
# | |
# gnd gnd
What??? I don't see any gray trace of Rs in the plot. What are you talking about?
Anyway, if you are true, the circuit is bad as the noise should only be dominated by the thermal noise of the resonant circuit.
From the measurements of the 11 MHz RFPD at 11Mhz I estimated a transimpedance of about 750 Ohms. (See attached plot.)
The fit shown in the plot is: Vn = Vdn + sqrt(2*e*Idc) ; Vn=noise; Vdn=darknoise; e=electron charge; Idc=dc photocurrent
The estimate from the fit is 3-4 times off from my analsys of the circuit and from any LISO simulation. Likely at RF the contributions of the parassitic components of each element make a big difference. I'm going to improve the LISO model to account for that.
The problem of the factor of 2 in the data turned out to be not a real one. Assuming that the dark noise at resonance is just Johnson's noise from the resonant circuit transimpedance underestimates the dark noise by 100%.
Putting my hands ahead, I know I could have taken more measurements around the 3dB point, but the 40m needs the PDs soon.
Something must be wrong.
1. Physical Unit is wrong for the second term of "Vn = Vdn + Sqrt(2 e Idc)"
2. Why does the fit go below the dark noise?
3. "Dark noise 4 +/- NaN nV/rtHz" I can not accept this fitting.
Also apparently the data points are not enough.
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.
I can't find the DELL laptop anywhere in the lab. Does anyone know where it is?
Also one of the two netbooks is missing.
All the details and data will be included in the wiki page (and so also the results for AS55). Here I just show the comparison of the transfer functions that I measured and that I modeled.
I applied an approximate calibration to the data so that all the measurements would refer to the transfer function of Vout / PD Photocurrent. Here's how they look like. (also the calibration will be explained in the wiki)
The ratio between the amplitude of the 55Mhz modulation over the 11MHz is ~ 90dB
The electronics TF doesn't provide a faithful reproduction of the optical response.
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.