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ID Date Author Type Categorydown Subject
  926   Thu Sep 4 17:03:25 2008 YoichiUpdatePSLRF oscillator noise comparison
I measured current spectra of the RF signal going to the FSS EOM.
The attachment compares the spectra between a Stanford signal generator and a Marconi.
I borrowed the Marconi from the abs. length measurement experiment temporarily.
The measurement was done using the signal going to the EOM. That means the spectra include
noise contributions from the RF amp., splitter and cables.

21.5MHz peak was not included because that would overload the ADC and I would have to use a large attenuation.
This means the measurement would be totally limited by ADC noise everywhere except for 21.5MHz.

I noticed that with the Marconi, the FSS is a little bit happier, i.e. the PC path is less loaded
(0.9Vrms with Stanford vs. 0.7Vrms with Marconi). But the difference is small.
Probably the contribution from the 77kHz harmonics in the laser light is more significant (see entry #929).
Also the peaks in the Stanford spectrum are not harmonics of 77kHz, which we see in the FSS error signal.

I returned the Marconi after the measurement to let Alberto work on the abs. length measurement.
  927   Thu Sep 4 17:12:57 2008 YoichiUpdatePSLFSS open loop TF
I changed the gain settings of the FSS servo.
Now the Common Gain is 5dB (the last night it was 2dB) and the Fast Gain is 12dB (formerly 16dB).
I measured the open loop TF with this setting (the attachment).
I also plotted the OPLTF when CG=2dB, FG=20.5dB. With this setting, the MC looses lock every 30min.

You can see that the OPLTF is smoother with FG=12dB.
When the FG is high, you can see some structure around 250kHz. This structure is reproducible.
This may be some interruption from the fast path to the PC path through a spurious coupling.
  929   Thu Sep 4 17:44:27 2008 YoichiUpdatePSLFSS error signal spectrum
Attached is a spectrum of the FSS error signal.
There are a lot of sharp peaks above 100kHz (the UGF of the servo is about 200kHz).
These are mostly harmonics of 77kHz. They are the current suspects of the FSS slew rate saturation.
I remember when I blocked the light to the PD, these peak went away. So these noises must be
in the light. But I checked it a few weeks ago. So I will re-check it later.

One possible source of the lines is a DC-DC converter in the NPRO near the crystal.
We will try to move the converter outside of the box.
  931   Fri Sep 5 08:34:03 2008 steveUpdatePSLMZ locked
The MC is happy.
The MZ can be locked if you move the slider by hand.
  937   Mon Sep 8 15:38:57 2008 YoichiConfigurationPSLPOY RF amp is back to its original task
I temporarily fixed the busted ZHL-32A RF amplifier's power connector by simply soldering a cable to the internal circuit and pulling the cable out of the box through a hole for the power connector.
So I released the POY RF amplifier from the temporary duty of serving the FSS RF distribution and put it back to the original task,
so that Rob can finally re-start working on the lock acquisition.
Now the temporarily fixed ZHL-32A is sitting next to the IOO rack along with the power supply and a Stanford signal generator.
Please be careful not to topple over the setup when you work around there. They will be there until Peter's Wentzel RF box arrives.
  951   Tue Sep 16 16:47:01 2008 peteConfigurationPSLPrototype FSS reference installed
After verifying output, I installed the new prototype 21.5 MHz FSS reference (Wenzel crystal oscillator and ZHL-2 amp). Yoichi and I successfully locked the MC, and have left the new reference in place. It's temporarily sitting on the corner of the big black optics table (AP table?).
  954   Wed Sep 17 13:43:54 2008 YoichiConfigurationPSLRC sweep going on
I'm doing a cavity sweep of the RC. Please leave the IFO untouched until the meeting is over.
  957   Wed Sep 17 15:22:31 2008 YoichiConfigurationPSLRC sweep going on

Quote:
I'm doing a cavity sweep of the RC. Please leave the IFO untouched until the meeting is over.


The measurement is still going on.
I will post an entry when it is done.
Thank you for the patience.
  958   Wed Sep 17 17:31:24 2008 YoichiUpdatePSLFSS calibration
I calibrated the reference cavity error signal with the following procedure.

(1) I disconnected the PC path BNC cable and locked the RC only using the PZT. To do so, I had to insert a 20dB attenuator
in the RF signal path going to the EOM to reduce the gain of the loop sufficiently.
The normal RF level going to the EOM is 17dBm. With the attenuator it is of course -3dBm.

(2) Using the SR785, I injected signal into the Test-IN2 (a sum-amp after the mixer) of the FSS box and measured the TF from the Ramp-IN to the IN1.
When the Ramp-In switch is off, the Ramp-IN port can be used as a test point connected to the PZT drive signal path just before the output.
There is a RC low-pass filter after the Ramp-IN. IN1 is the direct output from the mixer (before the sum-amp).
The attm1 is the measured transfer function along with the fitting by a first order LPF.
From this measurement, the DC transfer function from the applied voltage on the PZT to the error signal is determined to be 163.6 (V/V).
Since the RF level is lowered by 20dB, the cavity gain in the normal operation mode is 10 times larger (assuming that the modulation depth is
linearly proportional to the applied voltage to the EOM).

(3) According to elog:791, the conversion factor from the voltage on the PZT to the frequency change of the NPRO is 11.172MHz/V. Combining this with the
number obtained above, we get 6.83kHz/V as the calibration factor for converting the error signal (mixer output) to the frequency at DC.
Using 38kHz cavity pole frequency, the calibration factor is plotted as a function of frequency in the attm2.

(4) I took a spectrum of the error signal of the FSS and calibrated it with the obtained calibration factor. See attm3.
The spectrum was measured by SR785. I will measure wide band spectra with an RF analyzer later.

TO DO:
1: Use the actual modulation depth difference to extrapolate the calibration factor obtained by with a low RF signal for the EOM.
The cavity sweep was already done.

2: I assumed 38kHz cavity pole. I will measure the actual cavity pole frequency by cavity ringdown.

3: Measure out-of-the-loop spectrum of the frequency noise using PMC and MC.
  959   Wed Sep 17 17:58:35 2008 YoichiConfigurationPSLRC sweep going on
The cavity sweep is done. The IFO is free now.
  967   Thu Sep 18 23:31:26 2008 ranaUpdatePSLISS: Saturating too often at nominal gain
The ISS has been saturating whenever the MC relocks and puts the gain up to +8dB. I have
lowered the gain to +1 dB for now to stop this, but we need to revisit the ISS loop and
performance. Stefan can fix it up for us as penance when he returns from the hedonism of Amsterdam.
  971   Fri Sep 19 08:09:55 2008 steveUpdatePSLpsl HEPAs turned on
I have just turned on the PSL HEPA filters at 60% operational speed.
  978   Mon Sep 22 18:54:54 2008 JenneUpdatePSLPMC transfer functions with various brick-on-top configurations
Attached below is a graphical summary of different things that I have tried putting on the PMC to reduce the noise in the loop. The motivation behind these measurements is the current inability here at the 40m to increase the UGF of the PMC. This is part of a broader ISS loop/gain/noise problem that we are having, which is causing Rob's locking efforts to have trouble. (The ISS is next on the to-do list, after we find the best configuration for the PMC, if we are still having problems). Right now, it looks like we are being limited by the gain of the PMC (as mentioned by Rana in elog #968).

Anyhow, Rana and I had noticed that piling heavy things on top of the PMC seemed to reduce the noise. What follows are the transfer functions that I took with the different items on top of the PMC, so that we can compare their effects:
  • Nothing on the PMC (like it used to be)
  • New ~14kg lead brick wrapped in copper foil on top of the PMC
  • A stack of a piece of aluminum, a chunk of steel, and then the lead brick on top of the PMC
  • The lead brick + Rob pushing on top of the PMC

Unfortunately, I need to retake the power spectra in these configurations, but from eye-balling it, as one might expect, pushing on the PMC with a hand added more noise than the nominal nothing-on-PMC configuration.

Also unfortunately, none of these configurations seems to have significantly helped our noise reduction situation. We need a new plan. Rana is currently trying out some other configurations, including just aluminum+brick.

Attached is an open loop gain TF from 100Hz - 100kHz. Below that is a zoomed-in version from 5kHz - 30kHz. As you can see more clearly in the zoomed in version, the notch that Rana put onto the board at ~14.5kHz is working, but we need to make the notch deeper, to catch more of that 14.5kHz peak. We're going to try removing the resistor or reducing it's value in the RLC filter on the board (see elog #906). Also, we see that there is a giant peak at 18.3kHz. This is probably much more limiting to our stability at this point than the 14.5kHz peak. We need to add another filter to take care of this, or find another way to reduce this peak. Note that it is present even when there is no brick on the PMC, so it is not an artifact of the new brick.
  980   Mon Sep 22 21:30:06 2008 ranaConfigurationPSLbad FSS
The MC refl power was going up and the FSS PC drive was so large that I had to turn up the FSS
common gain from 1.5 dB to 10.5 dB to get it to be better. Attached are the before (REF) and
after plots of frequency noise. Is the FSS gain really supposed to be 1.5 dB?? How did we gain
so many dB's of optical gain? Is there a loop measurement from after Peter's oscillator change?
  982   Mon Sep 22 22:24:19 2008 ranaConfigurationPSLbad FSS

Quote:
The MC refl power was going up and the FSS PC drive was so large that I had to turn up the FSS...


Looks like I bumped the PS for the 21.5 MHz test setup and changed the supply voltage of the amplifier
from +24 to +38 V. This made the amplifier go hot after a few hours and the output eventually dropped.

Yoichi and I walked out there now and it was too hot to touch. We turned it off and put it on a heat
sink to make it chill out and it came back after a few minutes. We have set the input to the amp to
be -7 dBm instead of -8 dBm after deciding that we should take into account the 1 dB loss in the cable
run and deliver a real +17 dBm to the mixer.

The right way is to calibrated the LO mon of the FSS.
  984   Tue Sep 23 11:17:59 2008 steveUpdatePSLPMC scattering spot
The PMC output side has a new madly scattering spot at chamfer 2 o'clock position
  986   Tue Sep 23 15:28:06 2008 peteConfigurationPSLnew 21.5 MHz FSS reference installed
The new 21.5 MHz FSS reference is now installed in the rack with the 7 Sorensen PS. Both outputs give 18.7 dBm. The MC seems happy.

Bob did the +24 V and +15 V hookups for the amp and the Wenzel oscillator respectively, off of the din strips on the right of the rack.

I have attached two photographs. One shows the front of the box as mounted in the rack, and the other shows the inside of the box. From the second photo the circuit is apparent. Black wire coming in has ground, green has +15, and white has +24. After the switches, ground and +15 go to the Wenzel crystal oscillator, and ground and +24 go to the mini-circuits amp. There is 5 dB attenuation between the Wenzel 21.5 MHz output and the amp input. There is 3 dB attenuation between the amp output and the splitter.

The Wenzel crystal oscillator is their "streamline" model, and puts out 13.2 dBm. The amp is mini-circuits ZHL-2.
  989   Thu Sep 25 02:35:21 2008 ranaSummaryPSLFAST is moving alot
It looks like the FAST signal has started moving a lot - this is partly what inspired us to tune the SLOW loop.

Some of the spiking events happen when people go on the table or the MC loses lock. But at other times it just
spikes for no apparent reason. You can also see from the first plot (9 day 10-minute trend) that there is no
great change in DTEC so we shouldn't be worried about clogging in the NPRO head.

The second plot is a 1 day minute-trend.
  991   Thu Sep 25 10:48:29 2008 YoichiUpdatePSLFSS calibration again
I did a calibration of the FSS error signal again with a different method.
This time, I swept the laser frequency with the NPRO PZT around a resonance.
The attached figures show the transmitted light power and the PDH error signal vs the applied voltage to the PZT.
From the width of the transmitted light power peak, we can obtain the PZT voltage to the laser frequency coefficient,
i.e. the HWHM (Half Width Half Maximum) equals to the FSR (38kHz).
Once the PZT is calibrated, the PDH error signal can be calibrated by the fitting the central slope with a line.

I repeated the measurement 8 times and fitted the obtained data to get the HWHM and the slope.
The results are the following:
PZT calibration = 6.3 +/-0.1 MHz/V
PDH calibration = 6.5 +/-0.5 kHz/V

Note:
(1) The calibration coefficient (6.5kHz/V) is almost consistent with the previous value (6.83kHz/V elog:958). However, that calibration
used a considerably different value for the PZT calibration (11.172MHz/V elog:791). The discrepancy in the PZT calibration is understandable
because my previous PZT calibration was very rough. The fact that the two calibrations still agree is a mystery.

(2) In the transmitted power curve, there seems to be a slight distortion, probably due to the thermal effect.
We should reduce the power to the reference cavity to remove this effect.

(3) This measurement was done after Peter installed his RF source. The demodulation phase had not yet been optimized. We should
repeat the calibration after he optimizes the phase.

(4) I used the Tektronix oscilloscope for the measurement.
Using the ethernet-wireless converter, you can connect the scope to the network from anywhere in the lab.
No hard wire required anymore.
Then you can download the data from a web browser. It is cool !
  995   Fri Sep 26 00:19:54 2008 JenneUpdatePSLFilter-action with the PMC
Written, but not posted on 24Sept2008:



PMC adventures for this evening



Today's mission was to make more progress on increasing the bandwidth of the PMC servo.



First order of business was to improve the performance of the 14.6kHz notch that Rana put in the PMC servo board a few weeks ago to remove the 14.6kHz body mode resonance of the PMC. Looking at the zoomed in TF that I posted Monday (elog #978), we see that there is still a remnant of a peak near 14.5kHz. A first gut-reaction is that the notch is not tuned properly, that we have just missed the peak. As previously noted in the elog, the peak that we are trying to notch out is at 14.68kHz (elog #874). By unlocking the PMC and measuring the transfer function between FP2 and OutMon (OutMon is the monitor for the high voltage going to the PMC's PZT), I measure the transfer function of the notch, and find that it is notching at 14.63kHz. So we're a teensy bit off, but the Q of the notch is such that we're still getting improvement at the peak frequency. After checking that we are hitting the correct frequency, I put a short (just some wire) around R21, which is the R in the RLC notch filter, to increase the depth of the notch. At the peak frequency of 14.68kHz, we see a 2.5dB improvement of the notch. At the actual notch frequency of 14.63kHz, we see a 3.2dB increase in the depth of the notch. So, shorting R21 helped a little, but not a lot. Also, it's clear that we don't get that much more improvement by being on the resonant frequency, so there's no need to go in and tune the notch on the board.



Second order of business was to investigate the 18.34kHz peak in the transfer function. (Rana spent some time Monday night measuring this peak, and determined that it was at 18.34kHz) We decided that the best plan was to re-implement the Pomona Box notch filter that had previously existed to remove a higher frequency body mode, but tuned for the 18.34kHz mode. I am still not entirely sure what this mode is, but clearly it's a problem by about 20dB (on the TF, the next highest peak is 20dB below the 18.34kHz peak). Unfortunately, while the components should, by Matlab calculations, give me an 18.3kHz notch, I ended up with something like a 21.7kHz notch. This notch is approximately -30dB at 21.7kHz, and -20dB at 18.3kHz. I still need to take transfer functions and power spectra of the PMC servo with this new filter in place to (a) confirm that it did some good, and (b) to determine how important it is that the notch be right-on. More likely than not, I'll take the filter out and fiddle with the capacitors until I get the correct notch frequency.



Third on the list was to lock everything back up (FSS, PMC) after my tinkering, and see what kind of gain we get. Rob and I fiddled with the PMC gain, and it looks like the servo oscillates just before we get up to the max slider gain of 30dB. Looking at the power spectra in DTT, we do not see any significant peaks that suggest oscillation, so it is likely that there is some investigation to be done at frequencies above the 7kHz that we were able to look at with DTT (which isn't surprising, since all of this work has been at 14kHz and higher).



A final note is that we see a feature around 9kHz in the transfer function, and it is not at all clear where it comes from. At this time, it does not seem to be the dominant feature preventing us from increasing the gain, but at some point if we want the bandwidth of the PMC servo to be 10kHz, we'll have to figure this one out.



Still on the PMC todo list:


  • Measure the new transfer function, see if 18.34kHz peak is reduced

  • Tune Pomona Box notch filter to 18.3kHz instead of the current 21.7kHz

  • Retake power spectra of different items on top of PMC, compare to see if there is any one configuration that it obviously better than the others.

  • Find out why the PMC still oscillates when we try to take it up to the max slider gain, and fix it.

PS, is anyone else having trouble getting to the elog from laptops on other parts of the Caltech network (but not LIGO network)? My laptop won't go to the elog, but I can get to the rest of the internet using the Caltech wireless. My computer stopped seeing the elog on Tuesday or so. Joe, do you have any inspiration? Thanks.
  1000   Fri Sep 26 18:35:17 2008 JenneUpdatePSLPMC filter is out for tuning
The PMC's new Pomona Box filter is out for tuning. I'd like to get the notch right on the 18.3kHz, rather than being off in 21.7kHz land.
  1001   Fri Sep 26 19:08:43 2008 ranaConfigurationPSLRefcav Trans: PD + Camera + Dumps
I went out to improve the Refcav trans path.

I removed all ND filters to get rid of the fringing.

I removed the anodized Al dump that was there. Black anodized Aluminum dumps are forbidden for use as
dumps in any low phase noise setup (such as our frequency stabilization cavity). The scatter was going
directly back into the cavity and making noise. For now its undumped, but Steve will find the
reflections and dump them on unblemished razor blade dumps mounted stiffly.

I will post a photo of the new setup later - the new setup is sketched on the control room markerboard.

The transPD level is now 8 V, up from its previous 3-4 V. We will probably have to also put a lens
in front of it to get the beam size down.
  1003   Mon Sep 29 01:19:40 2008 ranaSummaryPSLLaser chiller running a little hot
I looked at it some last night and my suspicion was the ISS. Whenever the ISS switch came on the FAST got a kick.

We should try to disable the MC locking and ISS and see if the FSS/PMC/MZ are stable this way. If so this may be
a problem with the ISS / Current Shunt.
  1005   Mon Sep 29 13:23:40 2008 robSummaryPSLLaser chiller running a little hot

Quote:
I looked at it some last night and my suspicion was the ISS. Whenever the ISS switch came on the FAST got a kick.

We should try to disable the MC locking and ISS and see if the FSS/PMC/MZ are stable this way. If so this may be
a problem with the ISS / Current Shunt.


My entry about the laser chiller got deleted. The PSL appears to be running with the ISS gain at -5dB, so that's good, but the
chiller is still showing 21+ degrees. It should be at twenty, so there's something causing it to run out of
headroom. We'll know more once Yoichi has inspected the ISS.

In the deleted entry I noted that the VCO (AOM driver), which is quite warm, has been moved much closer to the MOPA.
This may be putting some additional load on the chiller (doubtful given the amount of airflow with the HEPAs on,
but it's something to consider).
  1007   Mon Sep 29 15:09:36 2008 steveUpdatePSLalmost 4 yrs plot of power & temps
The water chiller is normally running 1.5 C warmer than the laser head temp.
When control room temp is stable and PEM-count_temp is stable we can expect the head temp to be stable 20.0 C

PSL-126MOPA_HTEMP is running warmer in the last ~40 days

The ifo arm thermostate temp settings were raised by 2 F on 8-11-08
  1008   Mon Sep 29 17:53:33 2008 YoichiUpdatePSLISS update
ISS has been saturating easily.
Today I opened the PSL enclosure to inspect the ISS box. Then I found that the sensor PD was disconnected from the box.
I don't know for how long it has been like this, but it is clearly bad.
I connected the PD and I was able to increase the ISS gain to 0dB (from -5dB).
When I turned off the FSS, I was able to increase the gain further up to 8dB. So the FSS must have been doing something bad to the laser intensity.
The FSS fast path did not get huge kicks when ISS was turned on as observed before. But still the FSS fast signal is wondering around about +/-0.3V.
It does not stop wondering even when the ISS is turned off (even if the CS drive cable is physically disconnected).
I will try to optimize the slow servo.

After Peter tried to optimize the demodulation phase of the FSS (see his entry), I was able to increase the ISS gain to 8dB even with the FSS running.
I haven't fully understood what is behind this behavior.

To investigate what is going on in the ISS, I opened the box and inspected the circuit.
I found many innovative implementations of electric circuit components. See the attached photo. It is a three dimensional mounting of
a surface mount AD602 !
Anyway, the board is somewhat different from the schematic found in the DCC. But I roughly followed the circuit.
I will measure open loop TFs and various signals to see how we can improve the ISS.
  1010   Tue Sep 30 19:50:27 2008 JenneUpdatePSLQuicky Summary - more details later
Quicky summary for now, more details later tonight / tomorrow morning:

PMC notch: It's tuned up, but it is out, and it is staying out. It looks like the 18.3kHz junk isn't being helped by the brick, in fact the brick makes it worse. And the notch isn't enough to make the peak go away. Rana's and my conclusions about the PMC: the 18.3kHz resonance is associated with the way the PMC touches its mount. Depending on where we push (very gently, not much pressure) on the PMC, we can make the peak come and go. Also, if the PMC happens to be set nicely on its ball bearings, the peak doesn't appear. More notes on this later.

PMC's RF modulation depth: Since with the PMC's brick off, and the PMC sitting nicely on its ball bearings, we don't see any crazy oscillations, we were able to take the gain slider on the PMC screen all the way up to 30dB. To give us more range, we changed the modulation depth of the RF to 2V, from its previous value of 1V.

Phase of PMC servo: Since the phase of the PMC servo hasn't been set in a while, I eyeballed it, and set the phase to: Phase Flip = 180, Phase Slider = 4.8000 . I measured many points, and will plot a calibration curve later.

I also measured the actual value of the RF out of the PMC's LO board, when changing the RF output adjust slider. Again, will post the calibration later.

The attached PNG shows the PMC spectra from now and from Aug. 30 (ref). As you can see there's been some good reduction in the acoustic noise (red v. orange). The large change in the error signal is because of the much higher gain in the servo now. We'll have to redo this plot once Jenne measures the new UGF.
  1013   Wed Oct 1 02:47:53 2008 ranaUpdatePSLPSL ERR & LODET: Too much offset
Looks like there is an anomolous mixer offset correlated with the increase in the LO level. This may be leading to crazy offset locking in the FSS and too much coupling from ISS to FSS.
  1017   Wed Oct 1 23:05:14 2008 YoichiUpdatePSLISS RIN spectra
Stefan, Yoichi

We took relative intensity noise (RIN) spectra of the ISS error point and the monitor PD (attm1).
In-loop RIN is the sensor PD and "Out of the loop RIN" is the monitor PD.
The ISS gain slider was at 8dB in this measurement.
It looks normal. 
An open loop transfer function of the ISS loop was measured (attm2). The UGF was 22kHz with the phase margin of ~22deg.
We should increase the UGF up to ~60kHz

When we increase the gain up to 14dB, the CS saturation warning comes up in the EPICS screen.
We confirmed this by monitoring the CS drive signal with an oscilloscope.
It is the output of an AD602J, which has +/-3V output range. 
By increasing the gain of AD602J, we saw that the output signal hits the rail.
There seems to be a lot of high frequency (100kHz - a few MHz) noise, out of the control band.
We also observed that AD602J itself oscillates at about 10MHz (don't remember the exact number) when the gain is increased.
(We saw this even when the loop is off. There is no such an oscillation in the input to the AD602J).
When we took wide band spectra of the CS drive signal, we saw many large harmonics of ~180kHz. We believe these peaks are limiting
our ISS gain now (causing the CS saturation). The harmonics persisted even when we disconnected the PDs. So it is not coming from the light.
We saw the same harmonics in the power lines. They may be the switching noise of the Sorensens. 
We took spectra of those harmonics, but the netgpibdata.py somehow did not save the data from AG4395A correctly. I have to debug this.

Stefan removed DC offsets from the AD829s (many of them are used in the ISS board) by turning the pots for offset adjustment.
This eliminated the problem of getting a large DC CS feedback (observable in C1:PSL-ISS_CSDRIVE_MEAN) when the gain is increased.

During the investigation, I noticed that increasing the PMC gain too much (~22dB) caused an oscillation of the PMC loop and consequently made
the ISS saturate. When the ISS is behaving bad, we should check the PMC gain.

Currently, the ISS is running OK with the gain = 8dB. I modified the mcup script to set the ISS gain to 8dB when the MC is locked.

TO DO:
Wait for Peter's answer about spare ISS boards.
Power line filtering. 
Find the cause of AD602J oscillation (Well this is the one mounted upright. So just mounting it normally might solve the problem :-). 
  1018   Wed Oct 1 23:21:03 2008 YoichiConfigurationPSLReference cavity reflection camera
I re-centered the reference cavity reflection camera, which has been mis-aligned for a while.
I also tweaked an input steering mirror to make the alignment better. This resulted in the increase of the transmission PD voltage
from 8V to 9V.
  1023   Fri Oct 3 15:09:58 2008 robUpdatePSLFAST/SLOW

Last night during locking, for no apparent reason (no common mode), the PSL FAST/SLOW loop starting going just a little
nutz. Attached is a two day plot. The noisy period started around 11-ish last night.
  1034   Wed Oct 8 19:17:55 2008 YoichiConfigurationPSLLaser power is slowly recovering
This afternoon we (rich, steve, yoichi) shutdown the laser for the DC-DC converter installation.
(we decided not to do so. Detail will be posted soon.)
After we turned on the laser again,the laser power has been recovering but very slowly.
At the time of writing, the laser power is 2.6W (MOPA_AMPMON).
I think it is because the chiller temparature has not yet settled down (it went up to 25C and slowly coming down, now at 22C).
It will take some hours until the power fully comes back.
Right now I confirmed that at least the MC locks.
  1035   Wed Oct 8 21:26:20 2008 YoichiUpdatePSLAttempt to replace the DC-DC converter (aborted)
Rich, Steve, Yoichi

We opened the MOPA box and inspected our NPRO.
We concluded that this NPRO is different from the ones at the sites.
At the sites, the NPROs have a connector on the board which accepts the output of the DC-DC converter.
Rich's replacement DC-DC converter has a matching connector to it. So replacement of the DC-DC converter is easy.
In our NPRO, there is no such a connector found. The cables coming from the external power supply are directly soldered
on to the PCB (see attm1).

We have to take out the PCB in order to work on it.
As shown in the second picture, there is a D-SUB connector sticking out of the box through the rear panel.
In addition, the PCB is connected to the metal box containing the crystal with an IDE style connector.
This means the PCB is tightly constrained.
To take out the PCB without applying too much stress on it, we have to take off the rear panel.
To do so, we have to remove the screws on the bottom of the NPRO box. That means we have to move the NPRO.
We did not want to do so, because it will screw up the alignment to the amplifier.

The model number of the DC-DC converter looks like NMH0512-something.
According to the datasheet of NMH0512S, the switching frequency is typically 95kHz. We saw 77kHz harmonics in the FSS error signal.
I'm not sure if this is the culprit. I will try to measure the EMI from this guy later.
  1037   Wed Oct 8 23:18:23 2008 YoichiUpdatePSLCorrelation between the Sorensen switching noise and the FSS error signal
I took some spectra and coherence function of the FSS error signal and the +24V Sorensen power line.
The first plot shows spectra of the two signals. Looks like Sorensen is not responsible for most of the lines
in the FSS error signal.
The coherence function between the two signals supports it (second plot).
Slight coherence can be seen at 23kHz and 98.4kHz but not significant.

I will check the coherence of the power line with the ISS signal next.
  1042   Mon Oct 13 11:32:50 2008 YoixhiUpdatePSLMOPA is in trouble now
Steve, Alberto, Yoichi

A quick update.
The MOPA output went down to zero on Sunday early morning (00:28 AM).
We found that the NPRO beam is mis-aligned on the power monitoring PD (126MON).
We don't know yet if it is also mis-aligned to the power amplifier (PA) because the mechanical shutter is not working (always closed).
Most likely the beam is not aligned to the PA.
A mystery is that although the beam is terribly (more than a half inch) missing the monitor PD, the beam still goes through two faradays.
Another mystery is that the NPRO output power is now increased to 600mW.

The power drop was a very fast phenomenon (less than 1/16 sec).
We are trying to figure out what happened.
The first step is to fix the mechanical shutter. We have a spare in our hand.
  1043   Mon Oct 13 13:51:49 2008 peteConfigurationPSLattempt to measure FSS ref phase
On Friday I began a measurement of the FSS reference phase. The setup involves the following:
+ turn off the 166 MHz LO (top signal generator on 1Y2 rack)
+ bring FSS LO 21.5 MHz to the 166 MHz delay line phase shifter, and back out the phase shifter with a second length of cable
+ add a length of cable to the RF 21.5 MHz in preparation for measuring FSS IN2 as a function of delay

Trouble locking the FSS, and ran out of time before the measurement could be performed.
  1044   Mon Oct 13 13:56:03 2008 YoichiUpdatePSLMOPA is not that much in trouble now
The problem was found to be all to do with the shutter.
The shutter started to work again, after a while, apparently for no clear reason.
The alignment to the PA was actually not screwed, and the MOPA output is now slowly increasing.
We figured that the 126MON PD has been mis-aligned for a long time. It was just picking the
scattered light from the output of the PA. So when the shutter is closed, it is natural that 126MON also goes down to zero.
It is a bit difficult to center the beam on the PD because there is not much room for moving the PD.
However, Alberto came up with a configuration (flip the PD and reflect back the beam with a mirror to the PD), which seems to
be feasible. We will do this modification when the MOPA is confirmed to be ok.

Here is more detail about the shutter problem:
The shutter is controlled by the MOPA power supply. There are three ways to command the power supply.
The switch on the front panel of the power supply, the EPICS switch (through a XYCOM XY220), and the interlock.
The ribbon cable from the power supply's back is connected to J1 of the cross connect. The pin 59 of the cable is the one
controlling the shutter. It is then routed to J12 pin 36. The interlock and a XYCOM switch are both connected to this
pin.
Now what happened was, on the way tracking down those cables, I pushed some connectors, especially the ones on the XYCOM.
After that, I was able to open the shutter from the EPICS button.
Steve and Alberto tried the EPICS button many times in the morning without success.
My guess is that it was some malfunctioning of the XY220 accidentally fixed by my pushing of the cables.
But I cannot exclude the possibility of the interlock malfunctioning.



Quote:
Steve, Alberto, Yoichi

A quick update.
The MOPA output went down to zero on Sunday early morning (00:28 AM).
We found that the NPRO beam is mis-aligned on the power monitoring PD (126MON).
We don't know yet if it is also mis-aligned to the power amplifier (PA) because the mechanical shutter is not working (always closed).
Most likely the beam is not aligned to the PA.
A mystery is that although the beam is terribly (more than a half inch) missing the monitor PD, the beam still goes through two faradays.
Another mystery is that the NPRO output power is now increased to 600mW.

The power drop was a very fast phenomenon (less than 1/16 sec).
We are trying to figure out what happened.
The first step is to fix the mechanical shutter. We have a spare in our hand.
  1045   Mon Oct 13 18:59:39 2008 YoichiUpdatePSLNPRO EMI and FSS error signal correlation
I made a simple loop antenna to measure the electro-magnetic inteference (EMI) around the master oscillator NPRO.

The first plot shows the comparison of the FSS error signal with the EMI measured when the antenna was put next to the NPRO (the MOPA box was opened).
There are harmonics of 78.1kHz which are present in both spectra. It is probably coming from the DC-DC converter in the NPRO board.

The second plot is the same spectra when the antenna was put far from the NPRO (just outside of the PSL enclosure).
The 78.1kHz harmonics are gone. So these are very likely to be coming from the NPRO.

The third plot shows the coherence functions between the signal from the antenna and the FSS error signal.
When the antenna was put near the NPRO, there is a strong coherence seen around 78.2kHz, whereas there is no strong coherence
when the antenna is far away from the NPRO.
This is a strong evidence that the 78.2(or 78.1)kHz harmonics is coming from the NPRO itself.

There are many peaks other than 78.1kHz harmonics in the FSS error signal spectrum. For most of them you can also find corresponding peaks in the EMI spectrum.
We have to hunt down those peaks to avoid the slew-rate saturation of the FSS.
  1046   Tue Oct 14 14:19:36 2008 peteConfigurationPSLFSS ref phase
Today I made several measurements which should yield the optimized phase for the FSS 21.5 MHz reference. I made two sets of measurements, using the 166 MHz phase delay shifter. For each phase value I made 5 measurements of a 500 kHz injection into test2 at 1 Vpp, with the 4195 spectrum analyzer on in1 with the high impedance probe, 51 points, a 10 kHz range. It was surprisingly noisy. I will make plots using matlab to find the maximum, and hope for consistent results between the two sets of measurements. If it is too noisy or inconsistent I will repeat the measurement at ~800 kHz.

Once I find the phase which yields peak amplitude in in1, I will measure the relative phase between LO and RF going in to the FSS, measure the speed of light in RG58 cable, and construct a new cable which will implement the desired relative phase.
  1048   Tue Oct 14 19:24:34 2008 YoichiConfigurationPSLFSS light power reduced
Rana, Yoichi

To change the gain distribution in the FSS, Rana reduced the VCO power for the AOM to reduce the light incident to the reference cavity.
Now the transmitted power of the RC is 2.3V compared to 6.5V before.
The FSS common gain can be increased to 5dB. I haven't changed the normal gain for this slider, so the mcup script will still set
the common gain to 1.5dB after an MC lock.
With this change, we take some gain from the optical part and give more gain in the electronics.
This might relieve the slew rate limit problem if it is happening in an early stage of the electronics.
  1049   Wed Oct 15 17:40:50 2008 ranaUpdatePSLPMC Offset adjusted
I set the PMC servo input offset: closed the MOPA shutter, zeroed the mixer output with the offset slider,
relocked everything, and set the nominal to the new value of -6 V.
  1050   Wed Oct 15 22:07:52 2008 peteConfigurationPSLFSS ref phase measurements
Optimizing the FSS LO/RF phase at 500 kHz, above the servo band, proved to be noisy and those measurements were useless. Today I repeated
the measurement at 35 kHz and got good signal to noise. I've attached a plot of the 35 kHz peak in dBm as measured at IN2 by SR785, with
an injection into TEST2 at 35 kHz with 0.2 Vpp, as a function of delay in ns given by the delay phase shifter normally used by the 166 MHz.
I fit the bottom (quadratic) portion of this curve, and found an optimum delay of 25.8 ns, which can be implemented as 25.81 ns on the phase
shift box (25 + 1/2 + 1/4 + 1/16). This is an uncalibrated number and meaningless.. For all these measurements a very long SMA cable
(did not measure) was inserted on the RF output of the 21.5 MHz reference box. The actual phase difference depends on these cable lengths
which I didn't measure.

To determine the actual phase difference I compared the LO and RF input points with the 25.81 ns delay, using a scope with poor man's
averaging (33 manual triggers and recording the phase measurements). The phase difference was 8.24 degrees with an error on the mean of 3.4%,
with the LO having the longer effective cable (cable plus delay from the phase delay box). As a sanity check I set the phase delay box
to 20 ns and re-measured, and found 49.5 degrees. (1/21.5 MHz) * (49.5-8.24)/360 = 5.3 ns, which is about the difference between 20 ns
and 25.81 ns. I did the same with 0 ns dialed in, and found a difference of 21.5 ns (I expected 25.8 ns). So it is possible that the
phase delay box isn't too precise.

Finally, to determine the length of cable needed to implement 8.24 degrees of phase at 21.5 MHz with RG58 cable, I made some phase measurements
using the FSS reference box and mismatched cables. I used three cable lengths (93 cm, 140.5 cm, and 169.5 cm ) and two mismatched pairs with
dL of 29 and 76.5 cm. For each pair I took average of 20 measurements, finding 22.54 degrees mean for the dL=29 cm pair (0.78 degrees/cm, or
a speed of light of 1.0e10 cm/s, or 10.6 cm of cable length on the LO) and 43.57 degrees mean for dL=76.5 cm pair (0.57 degrees/cm, or a speed
of light of 1.4e10 cm/s, or 14.5 cm of cable length on the LO). I expected more precise agreement.

Maybe the 21.5 MHz reference box is not zero phase between the outputs. This could be easily tested. It might be interesting to repeat this
measurement with a few more dL values.
  1051   Thu Oct 16 09:44:49 2008 YoichiUpdatePSLBad cable for FSS
Yesterday arount 1:30PM, we lost the LO signal for the FSS.
I found it was caused by a bad cable connecting from the peter's RF oscillator box to the LO of the FSS.
I temporarily replaced it with a BNC cable of comparable length.
  1052   Thu Oct 16 09:47:49 2008 YoichiConfigurationPSLFSS ref phase measurements

Quote:
I fit the bottom (quadratic) portion of this curve, and found an optimum delay of 25.8 ns, which can be implemented as 25.81 ns on the phase shift box (25 + 1/2 + 1/4 + 1/16).


The gain of the loop is sinusoidally dependent on the phase delay. So the fit will be better with a function like this: 1/(1+G*sin(dphi + const)).
  1053   Thu Oct 16 13:12:58 2008 peteConfigurationPSLphase between FSS reference outputs
I verified the phase between the FSS reference outputs (used for LO and RF) using matched BNC cables. I measured 0.95 degree (average of 12 scope measurements).
  1054   Thu Oct 16 16:26:26 2008 peteConfigurationPSLFSS phase matching cable installed
RG 405 cable has a solid teflon dielectric, and a velocity factor of 0.69. To get the 8.2 degrees of additional phase on the LO output at 21.5 MHz then requires 22 cm of cable. I made a cable that ended up being 21 cm long after I'd gained some experience putting on the connector. It gives a phase difference between LO and RF of about 10 degrees. It is currently installed.
  1061   Mon Oct 20 20:50:09 2008 YoichiConfigurationPSLFSS board chip replacement
A quick update.

I changed two AD797s on the FSS board to AD829s to mitigate the 50MHz oscillation, which I plan to report later.
For some reason, the PA85 was broken after the replacement. So I had to replace it with a spare one too.
Right now the FSS is back and working. The oscillation is gone.
However, the maximum achievable gain is still about the same as before.
  1063   Tue Oct 21 16:17:45 2008 YoichiUpdatePSLAD797 Oscillation in the FSS board
I checked each op-amp's output in the FSS board to see if any indication of slew-rate saturation can be found.
PA85, which was the most suspicious one, actually has a very large slew rate limit (1000V/usec).
Its output swing was about 5V/usec. So PA85 was ok in terms of slew rate.
However, I found that an AD797 used at the first stage of the PC path was oscillating by itself, i.e. even without the loop closed.
The frequency was about 50MHz and the amplitude was large enough to reach the slew rate limit of this chip (the steepest slope was 30V/usec whereas the slew
rate limit of AD797 is 20V/usec).

I replaced it and another AD797 right after the oscillating one with AD829s. Just replacing the chips caused oscillation of AD829.
It was because there were no phase compensation capacitors connected to the pin 5 of AD829s.
Since the PCB was designed for AD797, there is no pattern for compensation caps. So I ended up putting Mica capacitors (47pF) across the pin 5 and the nearest ground point.
It worked and the oscillation stopped.

As I reported in an earlier elog, stopping the oscillation did not solve the problem of low FSS bandwidth.
  1064   Tue Oct 21 17:52:30 2008 ranaSummaryPSLFSS Photo: early October
This is a photo of the FSS board before Yoichi did his surgery - it was taken with the D40 in macro mode, sitting on the big Gorilla pod.
  1078   Thu Oct 23 20:47:28 2008 peteConfigurationPSLFSS LO calibration for MEDM
Today I took a quick series of measurements to calibrate the FSS LO power measurement in the MEDM. This was done by using the spec.an. to measure the 21.5 MHz peak in dBm at the LO input to the FSS box on the PSL table, and recording the MEDM value, for attenuations applied at the FSS REF box output ranging from -5 dBm to -30 dBm.

I measured the loss due to the BNC cable I used, which was (19.66-19.50) dBm. I accounted for this and plotted ln(MEDM) vs. dBm on the attached plot. A linear fit of this gives the CALC field of a calc record for the IOC db:
6.29*LOGE(A)+5.36

Since no one knew how to do this nonlinear conversion in EPICS I will describe how to do it in detail tomorrow. It is simple, although it requires power cycling the scipe3 bunch (typing "reboot" or "ctl-x" at the command prompt took it down, but it did not come back). I did power cycle those computers a few times today.
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