As I suspected, when the SR560 is operated in 1 Hz, first order LPF mode, the (electronic) transfer function has a zero at ~5kHz (!!!).
This is what allowed the PLL to be locked with this setting with UGF of ~30kHz. On the evidence of Attachment #3, there is also some flattening of the electrical TF at low frequencies when the SR560 is driving the NPRO PZT. I'm pretty sure the flattening is not a data download error but since this issue needs further investigation anyway, I'm not reading too much into it. I fit the model with LISO but since we don't have low frequency (~1Hz) data, the fit isn't great, so I'm excluding it from the plots.
We also did some PLL loop characterization. We decided that the higher output range (10Vp bs 10Vpp for the SR560) of the LB1005 controller means it is a better option for the PLL. The lock state can also be triggered remotely. It was locked with UGF ~ 60kHz, PM ~45deg.
We also measured the actuation coefficient of the NPRO laser PZT to be 4.89 +/- 0.02 MHz/V. Quoted error is (1-sigma) from the fit of the linear part of the measured transfer function to a single pole at DC with unknown gain. I used the "clean" part of the measurement that extends to lower frequencies for the fit, as can be seen from the residuals plot. Good to know that even though the LDs are dying, the PZT is still going strong :D.
Remaining loop characterization (i.e. verification of correct scaling of in loop suppression with loop gain etc.) is left to Jon.
Some other remarks:
Last night (Mar 17) I checked the PLL setup as Mott have had some difficulty to get a clean lock of the PLL setting.
Now the beating signal is much cleaner and behave straight forward. I will add some numbers such as the PD DC output, RF levels, SR560 settings...
Last night (Mar 17) I checked the PLL setup as Mott had some difficulty to get a clean lock of the PLL setting.
I also had noticed the progressive change of the aux NPRO alignment to the Farady.
I strongly agree about the need of a good and robust PLL.
By modifying the old PDH box (version 2008) eventually I was able to get a PLL robust enough for my purposes. At some point that wasn't good enough for me either.
I then decided to redisign it from scratch. I'm going to work on it. Also because of my other commitments, I'd need a few days/1 week for that. But I'd still like to take care of it. Is it more urgent than that?
We use the current PLL just now, but the renewal of the components are not immediate as it will take some time. Even so we need steady steps towards the better PLL. I appreciate your taking care of it.
I checked the setup further more.
Now I have significant fraction of beating (30%) and have huge amplitude (~9dBm).
The PLL can be much more stable now.
It looks like the PLL drifted alot over the weekend, and we couldn't get it back to 9 dBm. We switched back to the new focus wideband PD to make it easier to find the beat signal. I replaced all the electronics with the newly fixed UPDH box (#17) and we aligned it to the biggest beat frequency we could get, which ended up being -27 dBm with a -6.3V DC signal from the PD.
Locking was still elusive, so we calculated the loop gain and found the UGF is about 45 kHz, which is too high. We added a 20 dB attenuator to the RF input to suppress the gain and we think we may have locked at 0 gain. I am going to add another attenuator (~6 dB) so that we can tune the gain using the gain knob on the UPDH box.
Finally, attached is a picture of the cable that served as the smb - BNC adaptor for the DC output of the PD. The signal was dependent on the angle of the cable into the scope or multimeter. It has been destroyed so that it can never harm another innocent experiment again!
We have managed to lock the PLL to reasonable stability. The RF input is attenuated by 26 dBm and the beat signal locks very close to the carrier of the marconi (the steps on the markers of the spectrum analyzer are coarse). We can use the marconi and the local boost of the pdh box to catch the lock at 0 gain. Once the lock is on, the gain can be increased to stabilize the lock. The locked signals are shown in the first photo (the yellow is the output of the mixer and the blue is the output to the fast input of the laser. If the gain is increased too high, the error signal enters an oscillatory regime, which probably indicates we are overloading the piezo. This is shown in the second photo, the gain is being increased in time and we enter the non-constant regime around mid-way through.
Tomorrow I will use this locked system to measure the PZT response (finally!).
After realigning and getting the lock today, I tried to add in the SR785 to measure the transfer function. As soon as I turn on the piezo input on the PDH box, however, the lock breaks and I cannot reacquire it. We are using an SR650 to add in the signal from the network analyzer and that has worked. We also swapped the 20 dB attenuator for a box which mimics the boost functionality (-20 dB above 100 Hz, 0 dB below 6Hz). I took some spectra with the SR750, and will get some more with the network analyzer once Alberto has finished with it.
The SR750 spectra is posted below. The SR750 only goes up to 100 kHz, so I will have to use the network analyzer to get the full range.
In this afternoon, Mott and I tried to find a beat note between two NPROs which are going to be set onto each end table for green locking.
At first time we could not find any beats. However Koji found that the current of innolight NPRO was set to half of the nominal.
Then we increased the current to the nominal of 2A, finally we succeeded in finding a beat note.
Now we are trying to lock the PLL.
P.S. we also succeeded in acquiring the lock
Stabilizing the beat note frequency using Yuta's temperature servo (see this entry)
I was able to acquire the PLL of 80MHz VCO to the real green signal.
Some more details will be posted later.
I checked the slow servo and the PLL of 80MHz VCO using the real green beat note signal.
The end laser is not locked to the cavity, so basically the beat signal represents just the frequency fluctuation of the two freely running lasers.
The PLL was happily locked to the green beat note although I haven't fedback the VCO signal to ETMX (or the temperature of the end laser).
It looks like we still need some more efforts for the frequency counter's slow servo because it increases the frequency fluctuation around 20-30mHz.
(slow servo using frequency counter)
As Yuta did before (see his entry), I plugged the output of the frequency counter to an ADC and fedback the signal to the end laser temperature via ezcaservo.
The peak height of the beat note is bigger than before due to the improvement of the PMC mode matching.
The peak height shown on the spectrum analyzer 8591E is now about -39dBm which is 9dB improvement.
The figure below is a spectra of the frequency counter's readout taken by the spectrum analyzer SR785.
When the slow temperature servo is locked, the noise around 20-30 mHz increased.
I think this is true, because I was able to see the peak slowly wobbling for a timescale of ~ 1min. when it's locked.
But this servo is still useful because it drifts by ~5MHz in ~10-20min without the servo.
Next time we will work on this slow servo using Aidan's PID control (see this entry) in order to optimize the performance.
In addition to that, I will take the same spectra by using the phase locked VCO, which provides cleaner signal.
(acquisition of the PLL)
In order to extract a frequency information more precisely than the frequency counter, we are going to employ 80MHz VCO box.
While the beat note was locked at ~ 79MHz by the slow servo, I successfully acquired the PLL to the beat signal.
However at the beginning, the PLL was easily broken by a sudden frequency step of about 5MHz/s (!!).
I turned off the low noise amplifier which currently drives the NPRO via a high-voltage amplifier, then the sudden frequency steps disappeared.
After this modification the PLL was able to keep tracking the beat signal for more than 5min.
(I was not patient enough, so I couldn't stand watching the signal more than 5min... I will hook this to an ADC)
This is the update from yesterday that Harry missed to elog.
We pulled out the first spool of the PM980 fiber yesterday and checked it using the illuminator at the SP table. Harry will be using this for all his tests and characterisation of the fiber.
The PMC alarm was on this morning. It was relocked at lower HV
The FSS_RMTEMP jumped 0.5 C so The PZT was compensating for it.
I was interested what whitening filter do we have between MC servo and ADC. The shape is in the figure below, SR provided 1 V white noise. Before the whitening filter MC_F is measured in Volts with SR and ADC (for ADC the shape is calculated using the whitening filter form):
I also wondered if FSS or PZT servo can add noise to the mode cleaner length signal and what is their gain. It should be big, as the laser's calibration is ~1 MHz/V => to account for seismic noise of 10^-6 m at 1 Hz, the voltage given to the laser should be ~ 1 V. And it is indeed the case. The gain is ~1000. I measured the coherence between MC_F and the laser fast input. It is 1 in the range measured (0.05 - 100 Hz). FSS and PZT do not add significant noise.
Unfortunately, after the measurement when I unplugged BNS connector from the laser, I misaligned PMC. For several hours I adjusted the mirrors but could not significantly improve transmitted signal. I'll return to this issue tomorrow.
I suspect that it was just unlocked when you had disconnected the cable.
There is not reflection now. It seems that it is now misaligned after the alignment work.
So what you need is "align while scanning PZT -> lock -> align".
No, no, it was unlocked after I connected the cable back. The beam was even not on the PMC. I'll try PZT -> lock -> align.
No matter how you connect/disconnect, touching the laser may cause the PMC unlocked.
At least, I don't see the PMC reflection on the PD.
This means that the beam towards the PMC is largely misaligned.
If you are not sure what is misaligned, stop touching the table.
Close the shutter of the laser on the laser housing and leave the optics as they are.
Koji was right that I misaligned everything during the alignment work. I assumed that PMC should autolock and when I saw that it did not, I thought the laser is misaligned.
What we did:
1. Aligned mirrors to get the beam on the PD PMC REFL and PMCR camera. The PSL-PMC_RFPDDC was ~800 mV.
2. We disabled PMC servo, switching it to test position and changed "DC output adjust" by 0.01 in a loop
ezcawrite "C1:PSL-PMC_RAMP" -4.50
ezcastep "C1:PSL-PMC_RAMP" "+0.01,450" -s "0.1"
ezcawrite "C1:PSL-PMC_RAMP" 0.0
ezcastep -s "0.1" -- "C1:PSL-PMC_RAMP" "-0.01,450"
3. While the script was running we adjusted the position of the beam on the far PMC mirror looking at an IR viewer. The goal is to align two steering mirrors to catch some resonances. We monitored them on the oscilloscope and on the PMCT camera.
4. We locked PMC and aligned steering mirrors.
The PMC is not happy and the ITMX UL OSEM is moving too much
I have realigned the beam pointing to PMC. The transmitted light increased from 0.74 to 0.83.
The misalignment was mainly in pitch.
The PMC pointing has changed, so MC is resonating in high order modes.
The PMC transmission was around 0.78 all day, rather than the usual 0.83ish. Rana went out to the PSL table and fixed up the PMC alignment. This should not need to be done very often, so things to check before touching the alignment are FSS / PMC settings (digital stuff). Make sure that the PC RMS (on the FSS screen) is low (at least below 2, preferably below 1), and that the FSS Fast monitor is near 5ish (not near 0 or 10).
This is a capture of PMC REFL's camera after Rana was finished. If it doesn't look this good when you finish then you are not done. Never do PMC alignment without looking at the PMC REFL camera.
The attached trend shows 80 days of PMC REFL and TRANS. The bad alignment stuff started on Sep 21-24 time period. You know who you are.
Found PMC unlocked for many hours so I relocked it. IMC relocked by itself, but the input switch seems to be flickering to fast. Also the Keep Alive bit is not flashing.
We are thinking to use the PMC signals to help us in figuring out the feedback / feedforward stuff and making it better.
Today we scoped out the PMC DAQ channels (which were never re-hooked up after the Joe/Jamie CDS upgrade 6 years ago).
There is a 4-pin LEMO connector on the front panel which gives
Both of these signals are buffered by the AD620 inst amp configured with a gain of 1. In the green scope trace, you can see that there's a ~110 MHz signal strongly evident there. In the spectrum analyzer screen shot there is a instrument noise trace and then a PMC error point trace. You can see that all the peaks are ony there when I connect to the servo board instead of a Terminator. This RF noise is mainly the higher harmonics of the 35.5 MHz modulation getting there. It seems to be in both the error and control DAQ outputs, and a question is whether or not it is also in the servo electronics.
I also attach a close up of the servo board in the region of the post-mixer LC low pass filtering. I think its supposed to be 4th order cutoff at 1 MHz, but maybe the caps are busted or there's a way for the RF from the mixer to bypass the filters and get into the main servo path?
In the medium term, we probably want to use the new PDH servo that Rich is making. Need to buy/make a HV driver to use, but that should be easy.
The C1IOO frontend machine that resides in 1X1/1X2 has 2 ADCs, ADC0 and ADC1. The latter has 28 out of 32 channels unused at the moment, so I decided to use this for setting up fast channels for the PMC DAQ. On the RTCDS side of things, the PSL namespace block lives in the C1ALS model. I made the following modifications to it:
The PSL namespace block in C1ALS looks like this now:
I then tried hooking up the DAQ signals from the PMC servo board to the ADC via the 1U generic ADC interface chassis in 1X2 - this has 4pin LEMO inputs corresponding to 2 differential input channels. I used J6 (corresponding to ADC channels 10 and 11) for the PMC_ERR and PMC_CTRL respectively. I was a little confused about the status of the 4 pin LEMO output on the front panel of the PMC servo board. According to the DCC page for the modified 40m servo board, the DAQ outputs are wired to the backplane connector instead of the 4 pin LEMO. But looking at photographs on the same DCC page, there are wires soldered on the rear-side of the PCB from the 4-pin LEMO to the backplane connector. Also, I believe the measurements made by Rana in the preceeding elog were made via the front panel LEMO. In any case, I decided to use the single pin LEMO monitor points on the front panel as a preliminary test. The uncalibrated spectra with ADC terminated, IMC unlocked and IMC locked look like:
So it looks like at the very least, we want to add some gain to the AD620 instrumentation amplifiers to better match the input range of the ADC. We also want to make the PZT voltage monitor path AC coupled. My plan then is the following:
I will update with a circuit diagram with proposed changes shortly.
The PCB layout is such that I think using components with leads is easier rather than SMD components.
If this sounds like a reasonable plan, I will pull out the servo card from the eurocrate and implement these changes today evening...
What you have drawn looks good to me: the cut should be between TP3 and pin3 of the AD620. This should maintain the DC coupled respons for the single-pin LEMO and backplane EPICS monitors.
We want to use the PMC signal down to low frequencies, so the filter on the input of the AD620 should have a low frequency cutoff, but we should take care not to spoil the noise of the AD620 with a high impedance resistor.
It has a noise of 100 nV/rHz and 1 pA/rHz at 1 Hz. If you use 47 uF and 10 kOhm, you'll get fc = 1/2/pi/R/C ~ 0.3 Hz so that would be OK.
I made some changes to the DAQ path on the PMC servo board, as per the plan posted earlier in this thread. Summary of changes:
Details + photos + calibration of DAQ channels to follow. The PMC and IMC both seem to remain stably locked after this work.
The EOM upstream of the PMC is used as the phase corrector for the FSS/IMC servo. It is also used to apply the 35.5 MHz PDH RF sidebands for the PMC locking. There is a Pomona box which is used to merge the two signals onto a single cable for the EOM.
Does this circuit make sense to anyone?
I have calibrated the PMC LO Mon (C1:PSL-PMC_LODET) on the PMC's EPICS screen, by inputting different RF LO levels into the LO input of the PMC servo board.
Since the RF output adjust slider on the PMC's Phase Shifter screen doesn't do a whole lot (see elog 1471), I used a combination of attenuators and the slider to achieve different LO levels. I measured the level of the attenuated RF out of the LO board using the 4395A in spectrum analyzer mode, with the units in dBm, with 50dB attenuation to make it stop complaining about being overloaded. For each row in the table I measured the RF level using the 4395, then plugged the cable back into the PMC servo board to get the EPICS screen's reading.
The last 2 columns of the table below are the 'settings' I used to get the given RF LO level.
When the new mixers that Steve ordered come in (tomorrow hopefully), I'll put in a Level 13 mixer in place of the current Level 23 mixer that we have. Also, Rana suggested increasing the gain on the op-amp which is read out as the LO Mon so that 13dBm looks like 1V. To do this, it looks like I'll need to increase the gain by ~80.
We found that the PMC LO level was fluctuating in a strage way (it was not stable but had many clitches like an exponential decay), we suspected the infamous PMC LO level decay. In fact, in June 2014 when Rana recalibrated the LO level, the number on the medm screen (C1:PSL-PMC_LO_CALC) was about 11dBm. However, today it was about 6dBm. So we decided to jump in to the 1X1 rack.
The LO and PC outputs of the PMC Crystal module (D980353) were measured to be 6.2dBm and 13.3dBm. Rana reported in ELOG 10160 that it was measured to be 11.5dBm. So apparently the LO level decayed. Unfortunately, there was no record of the PC output level. In any case, we decided to pull the module for the replacement of ERA-5 chips.
Once we opened the box we found that the board was covered by some greasy material. The ERA-5 chip on the LO chain seemed unreasonably brittle. It was destryed during desoldering. We also replaced the ERA-5 chip in the PC chain, just in case. The board was cleaned by the defluxing liquids.
Taking an advatage of this chance, the SMA cables around the PMC were checked. By removing some of the heat shrinks, suspicious broken shields of the connectors were found. We provided additional solder to repair them.
After the repair, the LO and PC output levels became icreased to 17.0dBm(!) and 13.8dBm, respectively. (Victory)
This LO level is way too much compared to Rana's value. The MEDM LO power adj has little effect and the adj range was 16dBm~17dBm. Therefore we moved the slider to 10, which yields 16dBm out, and added a 5dB attenuator. The measured LO level after the attenuator was measured to be 11.2dBm.
Locking of the PMC was tried and immediately acquired the lock. However, we noticed that the nomoinal gain of 10dB cause the oscillation of the servo. As we already adjusted the LO level to recover the nominal value, we suspeced that the modulation depth could be larger than before. We left the gain at 0dB that doesn't cause the oscillation. It should be noted that the demodulation phase and the openloop gain were optimized. This should be done in the day time as soon as possible.
When the PMC LO repair was completed, the transmission of the PMC got decreased to 0.700V. The input alignment has been adjusted and the transmission level of 0.739V has been recovered.
The IMC lock stretch is not stable as before yet. Therefore, there would still be the issue somewhere else.
I think the IMC locking was somewhat improved. Still it is not solid as long time before.
Before the PMC fix (attachment 1)
After the PMC fix (attachment 2)
- PMC loop inspection / phase check / spectral measurements
- PMC / IMC interaction
- IMC loop check
Let's order a pair of 35.5 MHz Wenzel for this guy and package like Rich has done for the WB low noise oscillators.
WE're only sending 6 dBm into it now and its using a 13 dBm mixer. Bad for PMC stability.
Also, if anyone has pix of the servo card, please add them to the DCC page for the PMC.
Back in 2009, Jenne replaced the PMC board mixer with a Level 13 one. Today I noticed that the LO level on the PMC screen was showing a LO level of ~5-10 dBm and fluctuating a lot. I think that it is related to the well known failure of the Mini-Circuits ERA-5SM amplifier which is on the D000419-A schematic (PMC Frequency Reference Card). The Hanford one was dying for 12 years and we found it in late 2008. If we don't have any in the blue bin, we should ask Steve to order 10 of them.
The attached trend shows 2000 days of hour trend of the PMC LODET channel. The big break in 2009 is when Jenne changed the mixer and then attenuated the input by 3 dB. The slow decay since then is the dying amplifier I guess.
Since the LOCALC channel was not in the trend, I added it to the C0EDCU file tonight and restarted the FB DAQD process. Its now in the dataviewer list.
I went out and took out the 3 dB attenuator between the LO card and the PMC Mixer. The LO monitor now reads 14.9 dBm (??!!). The SRA-3MH mixer data sheet claims that the mixer works fine with an LO between 10 and 16 dBm, so I'll leave it as is. After we get the ERA-5, lets fix the LODET monitor by upping its gain and recalibrating the channel.
The first step is
The second uptick (In Nov 14, 2013) is when I removed a 3 dB attenuator from the LO line. Don't know why the decay accelerates after that.
I try to measure the linewidth of the PMC by ramping the PMC PZT.
I do it by connecting a triangular shape signal to FP Test 1 on the PMC servo front panel (I know, it is probably better to connect it to DC EXT. next time.) and turn the servo gain to a minimum.
Attachment 1 shows the PMC transmission PD as the PZT is swept with the EOM connected and when it is disconnected. It shows the PMC over more than 1 free spectral range.
For some reason, I cannot seem to be able to find the 35MHz sidebands which I want to use to calibrate the PZT scan. I made sure that the EOM is driven by a 35MHz signal using the scope. I also made sure that the PMC cannot to lock without the EOM connected.
I am probably doing something silly.
Turns out the 35MHz sidebands are way too weak to resolve from the resonance when doing a PZT scan.
I connect the IFR2023B function generator on the PSL table to the EOM instead of the FSS box and set it to generate 150MHz at 13dbm.
To observe the resulting weak sideband I place a PDA55 at the peak-off path from the transmission of the PMC where there is much more light than the transmission of the PMC head mirror. Whoever is using this path there is a PD blocking it right now.
I do a PZT scan by connecting a triangular signal to the EXT DC on the PMC servo with and without the EOM (Attachment 1). A weak sideband can clearly be spotted now.
Using the above 150MHz sideband calibration I can find the roundtrip time to be 1.55ns.
I take a high-resolution scan of a resonance peak and fit it to a Lorentzian (Attachment 2) and find a roundtrip loss of 1.3%.
Using the above results the cavity decay time is 119ns.
We should investigate what's going on with the ringdown measurements.
For the ringdowns, I suggest you replicate the setup I had - infrastructurally, this was quite robust, and the main problem I had was that I couldn't extinguish the beam completely. Now that we have the 1st order beam, it should be easy.
There are fewer lies on this screen now. For reference, the details of the electronics modifications made are in this elog.
I think many of the readbacks on the PMC MEDM screen are now bogus and misleading since the PMC RF upgrade that Gautam did awhile ago. We ought to fix the screen and clearly label which readbacks and actuators are no longer valid.
I added a clock to the PMC medm screen.
I made a backup of the original file in the same directory and named it *.bk20090805
Mode Power (V)
BE 0.36 **Bull's Eye mode is TEM02 + TEM20. This can be fixed by lens adjustment.
This afternoon we tried to improve the mode matching of the beam to the PMC. To do that we tuned the positions of the two lenses on the PSL table that come before the PMC.
We moved the first lens back an forth the without noticing any improvement on the PMC transmitted and reflected power. Then we moved the first backwards by about one cm (the order is set according to how the beam propagates). That made the things worse so we moved also the second lens in the same direction so that the distance in between the two didn't change significantly. After that, and some more adjustments on the steering mirrors all we could gain was about 0.2V on the PMC transmission.
We suspect that after the problems with the laser chiller of two months ago, the beam size changed and so the mode matching optics is not adequate anymore.
We have to replace the mode matching lenses with other ones.
Quick entry, details to follow in the AM tomorrow.
Here are the details:
The updated schematic with changes made, along with some pictures, have been uploaded to the DCC page...
Quick entry, details to follow in the AM tomorrow.
What's the reasoning behind setting the the gain to this new value? i.e. why do these 'margins' determine what the gain should be?
The PMC PZT capacitance was measured.
- Turn off the HV supplies. Disconnect HV OUT cable.
- Make sure the cable is discharged.
- Measure the capacity at the cable end with an impedance meter.
=> The PMC PZT capacitance at the cable end was measured to be 222nF
Combined with the output impedance of 63.3kOhm, the LPF pole is at 11.3Hz