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ID Date Authorup Type Category Subject
  13595   Wed Jan 31 22:32:11 2018 gautamUpdateALSALS signal chain + power budget

Summary:

I do not have an answer to the question "What is an appropriate gain for the IF amplifier stage in the D0902745 FET demod boards?", because of the following problems.

Deatils:

The plan is to lower the gain of the IF amplifier stage on the FET demodulator board from 100 to 10. As per Attachment #1, this will make the overall gain from RF beatnote from the Beat Mouth to the signal input to the D990694 whitening board +19dB, assuming "typical" values for the conversion loss of the mixer, and the various other passive components on the FET demod board. I've used numbers I measured a couple of weeks ago for the delay line loss and the cabling loss from the PSL table to the LSC rack. This in turn will set a limit on how much RF beat power we can handle, from the Beat Mouth. According to this power budget, if we have -5dBm of beat, we will have an input to the whitening board of ~6Vpp, which is about half its full range. The trouble is, I don't know what the transimpedance gain of the Fiber Beat PDs are. The datasheet suggests a "maximum gain" of 5e4 V/W, which presumably takes into account the InGaAs responsivity and the actual transimpedance gain. However, according to the last power budget I did inside the Beat Mouth, I had -8dBm of beat for a combined 400uW of PSL+EX light, which definitely does not add up. I've emailed the company to ask about the spec, haven't gotten anything useful yet...

The problem is further complicated by the fact that the fiber inside the Beat Mouth is NOT polarization maintaining, and so the actual relative polarizations of the arm IR light and the PSL IR light is unpredictable, and also uncontrolled. I suppose we could simply place a HWP before the fiber collimator at either end, and rotate the polarization until we get a desired amount of beat, but this still does not solve the problem of the polarization being uncontrolled.

I am going to characterize the demod board using E1100114. I am unsure as to the conversion loss of the mixer - the datasheet suggested a number of 8dB, but T1000044 suggests that the conversion loss is actually only 4dB. I figure it's best to just measure it. Would also be good to verify that the overall transfer function and noise of the IF amplifier stage match my expectation from the LISO model.

Option #1: Rana ordered 50ohm and 500ohm SMD resistors of the 0805 package size, I asked Steve to get a few more values just in case we want to twiddle with the gain of this stage further (specifically, I asked for values such that we can set it to x5, x3 and x1). But changing the feedback resistors modifies the overall TF shape - see e.g. Attachment #2. Need to also look at how the noise performance varies.

Another possibility is to turn down the gain of the IF amplifier stage to x10, retire the ZHL-3A, and use a lower gain amplifier in its place. We do have the recently acquired Teledyne amplifiers, but we would have to package it in such a way that it can be integrated into the existing Fiber ALS signal chain. This would allow us to handle significantly larger RF beatnote powers, which I expect we will have if we improve the mode matching into the fibers (provided the aforementioned polarization drift possibility doesn't hurt us too much).

A third possibility is to attenuate the power coupled into the fibers to lower the RF beatnote amplitude. I don't like this option so much because placing an ND filter or a PBS+HWP combo in the beam path is likely to screw up the mode-matching into the fiber collimator, which I have already spent so many hours trying to improve, but if it must be done, it must be done.

The correct option is of course the one that gives us the lowest ALS noise. It is not clear to me which one that is at this point.

  13596   Thu Feb 1 01:24:56 2018 gautamUpdateALSD0902745 revamp underway

I effected the change to the Audio IF preamp stage on channels 3 and 4 (Xarm and Yarm respectively) using the resistors Steve ordered (the ones Rana ordered don't have any labeling on them, and I couldn't tell the 50ohm and 500ohm ones apart except by looking at the label on the ziplock bag they came infrown, so I decided against using them). I've started a DCC page to collect photos, characterization data, and marked up schematic etc for this part. Characterization is ongoing, more to follow soon. Note that for the photo-taking, I disconnected all the on-board SMA connectors so that the cabling wouldn't block components. I have since restored them for testing purposes, and was careful to use the torque-limited SMA tightening tool when restoring the connections.

In order to test various things like conversion loss etc, I figured it would be useful to have two RF signal sources, so I scavenged the Fluke RF generator that Johannes was using from under the PSL table. In the process, I accidentally bumped the PSL interlock on the southeast corner of the PSL table. I immediately turned the NPRO back on, and relocked PMC/IMC. Everything looks normal now. Acromag may even have caught my transgression.

Quote:
 

I am going to characterize the demod board using E1100114. I am unsure as to the conversion loss of the mixer - the datasheet suggested a number of 8dB, but T1000044 suggests that the conversion loss is actually only 4dB. I figure it's best to just measure it. Would also be good to verify that the overall transfer function and noise of the IF amplifier stage match my expectation from the LISO model.

 

  13597   Thu Feb 1 15:31:12 2018 gautamUpdateALSALS signal chain + power budget

Summary:

A reasonable level of RF beatnote power for operating within the specs of the demod board is 17dBm arriving at the input to the power splitter just before the delay line.

Details:

Stuff is beginning to look clearer now that I've done some initial characterization of the demod boards. I will upload a more detailed report of the characterization on the DCC page, but important findings are:

  1. The overall conversion factor from RF to IF is ~2.3V IF per volt of RF.
    • 50ohm source connected to RF input of demod board, level = 10dBm on Marconi screen, consistent with inferred value from RF mon output.
    • LO driven at 14dBm by Fluke function generator.
    • The ratio was calculated for IF voltage input into a High-Z load.
    • So let's say we want to run at half the ADC full range of 10Vpp into the whitening board - this means we need to keep the RF input to <=11dBm. 
  2. The Teledyne amplifier has a rated maximum input voltage of 17dBm. If we want to stay 3dB below this, we can send in 14dBm into the LO input of the demod board, which is what my characterizations were done with.

The delay line has a loss of ~3dB. The power splitter has a loss of 3dB. So putting everything together, 17dBm at the input of the power splitter gives us just the right amount of RF power to have the LO input driven at 14dBm, and the IF output be ~5Vpp into a High-Z load, which is about half the ADC full range.

 

  13599   Fri Feb 2 00:26:34 2018 gautamUpdateALSD0902745 revamp underway

I saw some interesting behaviour of the Audio IF amplifier stage on the demod board today, by accident. I was testing the board for I/Q orthogonality and gain balance, when I noticed a large gain imbalance between the I and Q channels for both Board #3 and #4, which are the ones we use for the IR ALS demodulation. This puzzled me for some time, but then I realized that I had only reduced the gain of this stage from x100 to x10 for the I channel, and not for the Q channel! The surprising thing though was that the output waveform still looked like a clean sinusoid on the o'scope, and there was no evidence of the voltage clipping that is characteristic of an op-amp being driven beyond its voltage rails. The conversion factor with a preamp gain on x10 was measured today to be 2V IF / 1V RF. But this means that for a preamp stage gain of x100, we expect 20V IF / 1V RF, which is well in the saturation regime of the AD829, since the Vcc is only +/-15V. I'm guessing the diodes D2 and D3 are for overvoltage protection, but given that the pre-amp gain is x100, the input signal at the inverting input of the AD829 is only 0.2V at DC, which isn't above the forward bias voltage for the switching diode BAV99. Perhaps there is some interaction between the pre-amp and the FET demodulator that I dont understand, or I am missing something about the differential to single-ended topology that would explain this behaviour. 

I found it puzzling why the large preamp stage gain didn't hurt us with the green beat - even though the green optical beat signal was smaller than the current IR beat, a back-of-the-envelope calculation suggested that it would still have saturated the ADC with a x100 gain on the preamp. Perhaps this observation is part of the story, and there is also the unpredictable behaviour of the D990694 board for an input signal with large DC levels...


I did the following tests on this board today:

  • Check +/-15V supplies, power reg board.
  • Check DC offset on I and Q front panel output with LO driven at +10dBm, RF input terminated. Found it to be 0.
  • Checked calibration of back-panel DSUB connector monitors for LO and RF powers. Data to be uploaded, looked quite linear.
  • Checked conversion gain from RF input to IF output for two sets of LO/RF powers.
  • Measured conversion gain as a function of the IF frequency (i.e. frequency offset between LO and RF inputs, out to ~700kHz, 8 datapoints)
  • Checked orthogonality and gain balance of the I and Q outputs.
  • Measured the noise of the I and Q outputs in the audio frequency range using the SR785.

I didn't really measure the transfer function of the preamp stage after the modification because there wasn't a convenient test point and I couldn't find the high impedance FET probe for the Agilent - I wonder if somebody in WB has it? Anyways, all the tests suggested the board is operating as expected, and I now have calibrations for the back panel DSUB for LO/RF power levels, and also the conversion gain from RF to IF. I will put together a python notebook with all my measurements and upload it to the DCC page for this part. I need to double check expected noise levels from LISO to match up to the measurement.

I will now proceed to the next piece (#3?) of this puzzle, which is to understand how the D990694 which receives the signals from this unit reacts to the expected DC voltage level of ~4Vpp.


After discussion with Koji, I have also decided to look into putting together a daughter board for an alternative Audio IF preamp stage. The motivation is that for the ALS application, we expect a high DC signal level all the time (because the loop does not suppress the beat note amplitude). So we would like for the preamp stage to have the usual shape of some zero around 4Hz, a pole around 40Hz, and then the LowPass profile of the existing preamp stage (to cut out the 2f frequency product, but also to minimize the possibility of the fast AD829 going into some unpredictable regime where it oscillates). So, the desired features are:

  • Whitening (z,p) at (4Hz,40Hz) or (15Hz,150Hz) so that we have frequency dependent gain that can handle the large DC signal level expected. Need to measure noise of the actual IR beat signal to determine what the appropriate whitening shape is.
  • Low-pass above a few 100kHz to cut out 2f modulation product
  • Low-passing at input of AD829 (or just use OP27?)
  13600   Fri Feb 2 13:16:55 2018 gautamUpdateALSALS signals whitening switching

While setting up for this measurement, I noticed something odd with the whitening switching for the ALS channels. For the usual LSC channels, the whitening is set up such that switching FM1 on the MEDM screen changes a BIO bit which then enables/disables the analog whitening stage. But this feature doesn't seem to be working for the ALS channels - I terminated all 4 channels at the LSC rack, and measured the spectrum of the IN1 signals with DTT in the two settings, such that I expect to see a difference in the spectra if the whitening is enabled or disabled - FM1 enabled (expected analog whitening to be engaged) and FM1 disabled (expected analog whitening to be bypassed). But I see no difference in the spectra. I confirmed that the BIO bit switching is happening at least on the software level (i.e. the bit indicator MEDM screens indicate state toggling when FM1 is ON/OFF). But I don't know if something is amiss in the signal chain, especially since we are using Hardware channels that were previously used for AS_165 and POP_55 signals.

Is the whitening shape such that we expect the terminated noise level to be below ADC Noise even when the whitening is engaged? I just checked the shape of the de-whitening filter, and it has -40dB gain above 150Hz, so the inverse shape should have +40dB gain. 

Quote:
 

I will now proceed to the next piece (#3?) of this puzzle, which is to understand how the D990694 which receives the signals from this unit reacts to the expected DC voltage level of ~4Vpp


gautam 2.15pm: This was a FALSE ALARM, with the inputs terminated, the electronics noise really is that low such that it is buried under ADC noise even with +40dB gain. I cranked up the flat whitening gain from 0dB to 45dB for the X channels (but left the Y channels at 0dB). Attachment #2 is the comparison. Looks like the switching works just fine.

  13604   Sat Feb 3 13:03:45 2018 gautamUpdateComputer Scripts / Programsnetgpib data missing / PROLOGIX yellow box (crocetta) not working

The netgpibdata scripts are now under git version control at /opt/rtcds/caltech/c1/scripts/general/labutils/netgpibdata. I think the idea was to make this directory a collection of useful utilities that we could then pull at various labs / at the sites.

Quote:

I could not understand why 'netgpibdata' scripts are missing in "scripts/general" folder on pianosa... Where did they go???

  13606   Mon Feb 5 14:11:01 2018 gautamUpdateALSHuge harmonics in ALS channels

I've been trying to setup for the THD measuremetn at the LSC rack for a couple of days now, but am plagued by a problem summarized in Attachment #1: there are huge harmonics present in the channel when I hook up the input to the whitening board D990694 to the output of a spare DAC channel at the LSC rack. Attachment #2 summarizes my setup. I've done the following checks in trying to debug this problem, but am no closer to solving it:

  • The problem is reminiscent of the one I experienced with the SR785 not too long ago - there the culprit was a switching power supply used for the Prologix GPIB-ethernet box.
  • Then I remembered sometime ago rana and i had identified the power supply for the Fibox at the LSC rack as a potential pollutant. But today, I confirmed that this power supply is not to blame, as I unplugged it from its powerstrip and the spectrum didn't change.
  • There are a couple of Sorensens in this LSC rack, from what it looks like, they supply power to a BIO interface box in the LSC rack. I thought we would want to keep this rack free of switching power supplies? Wasn't that the motivation behind keeping the (linear) power supplies for all the LSC rack electronics in a little separate rack along the east arm?
  • I confirmed that when the D990694 input is terminated, these harmonics are no longer present. 
  • I plugged the output of the SOS dewhite board to an oscilloscope - there is a ~20mVpp signal there even when the DAC output is set to 0, but this level seems too small to explain the ~1000 ct-pp signal that I was seeing. The whitening gains for these channels are set to 0dB.
  • I also looked at the signal in the time domain using DTT - indeed the peak-to-peak signal is a few thousand counts.
  • This isn't a problem with the particular input channel either - the behaviour can be reproduced with any of the 4 ALS input channels.

Am I missing something obvious here? I think it is impossible to do a THD measurement with the spectrum in this condition...

  13608   Mon Feb 5 22:57:28 2018 gautamUpdateALSHuge harmonics in ALS channels

Did some quick additional checks to figure out what's going on here.

  1. The SOS/dewhite board for which I didn't have a DCC number is D000316. It has a single ended output.
  2. I confirmed that the origin of this noise has to do with the ground of the aforementioned D000316 - as mentioned in my previous elog, having one end of a BNC cable plugged into the whitening board D990694 and the other end terminated in 50ohms yields a clean spectrum. But making the ground of this terminator touch the ground of the SMA connector on the D000316 makes the harmonics show up.
  3. Confirmed that the problem exists when using either the SMA or the LEMO monitor output of these D000316 boards.

So either something is busted on this board (power regulating capacitor perhaps?), or we have some kind of ground loop between electronics in the same chassis (despite the D990694 being differential input receiving). Seems like further investigation is needed. Note that the D000316 just two boards over in the same Eurocrate chassis is responsible for driving our input steering mirror Tip-Tilt suspensions. I wonder if that board too is suffering from a similarly noisy ground?

Quote:
 

Am I missing something obvious here? I think it is impossible to do a THD measurement with the spectrum in this condition...

 

  13609   Tue Feb 6 11:13:26 2018 gautamUpdateALSPossible source of ground loop identified

I think I've narrowed down the source of this ground loop. It originates from the fact that the DAC from which the signals for this board are derived sits in an expansion chassis in 1Y3, whereas the LSC electronics are all in 1Y2.

  • I pulled the board out and looked at the output of Ch8 on the oscilloscope with the board powered by a bench power supply - signal looked clean, no evidence of the noisy ~20mVpp signal I mentioned in my previous elogyes.
  • Put the board back into a different slot in the eurocrate chassis and looked at the signal from Ch8  - looked clean, so the ground of the eurocrate box itself isn't to blameyes.
  • Put the board back in its original slot and looked at the signal from Ch8 - the same noisy signal of ~20mVpp I saw yesterday was evident again no.
  • Disconnected the backplane connector which routes signals from the DAC adaptor box to D000316 board - noisy signal vanished yes.

Looking at Jamie's old elog from the time when this infrastructure was installed, there is a remark that the signal didn't look too noisy - so either this is a new problem, or the characterization back then wasn't done in detail. The main reason why I think this is non-ideal is because the tip-tilt steering mirrors sending the beam into the IFO is controlled by analogous infrastructure - I confirmed using the LEMO monitor points on the D000316 that routes signals to TT1 and TT2 that they look similarly noisy (see e.g. Attachment #1). So we are injecting some amount (about 10% of the DC level) of beam jitter into the IFO because of this noisy signal - seems non-ideal. If I understand correctly, there is no damping loops on these suspensions which would suppress this injection. 

How should we go about eliminating this ground loop?

 

Quote:
 

So either something is busted on this board (power regulating capacitor perhaps?), or we have some kind of ground loop between electronics in the same chassis (despite the D990694 being differential input receiving). Seems like further investigation is needed. Note that the D000316 just two boards over in the same Eurocrate chassis is responsible for driving our input steering mirror Tip-Tilt suspensions. I wonder if that board too is suffering from a similarly noisy ground?

 

  13612   Tue Feb 6 22:55:51 2018 gautamUpdateALSPossible source of ground loop identified

[koji, gautam]

We discussed possible solutions to this ground loop problem. Here's what we came up with:

  1. Option #1 - Configure the DAC card to receive a ground voltage reference from the same source as that which defines the LSC rack ground.
  2. Option #2 - construct an adapter that is differential-to-single ended receiving converter, which we can then tack on to these boards.
  3. Option #3 - use the D000186-revD board as the receiver for the DAC signals - this looks to have differential receiving of the DAC signals (see secret schematic).  We might want to modify the notches on these given the change in digital clock frequency 

Why do we care about this so much anyways? Koji pointed out that the tip tilt suspensions do have passive eddy current damping, but that presumably isn't very effective at frequencies in the 10Hz-1kHz range, which is where I observed the noise injection.

Note that all our SOS suspensions are also possibly being plagued by this problem - the AI board that receives signals is D000186, but not revision D I think. But perhaps for the SOS optics this isn't really a problem, as the expansion chassis and the coil driver electronics may share a common power source? 

gautam 1530 7 Feb: Judging by the footprint of the front panel connectors, I would say that the AI boards that receive signals from the DACs for our SOS suspended optics are of the Rev B variety, and so receive the DAC voltages single ended. Of course, the real test would be to look inside these boards. But they certainly look distinct from the black front panelled RevD variant linked above, which has differential inputs. Rev D uses OP27s, although rana mentioned that the LT1125 isn't the right choice and from what I remember, LT1125 is just Quad OP27...

  13613   Wed Feb 7 10:16:26 2018 gautamUpdateALSALS signal chain + power budget

After emailing the technical team at Menlo, I have uploaded the more detailed information they have given me on our wiki.

Quote:

The trouble is, I don't know what the transimpedance gain of the Fiber Beat PDs are. The datasheet suggests a "maximum gain" of 5e4 V/W, which presumably takes into account the InGaAs responsivity and the actual transimpedance gain.

 

  13616   Wed Feb 7 15:51:15 2018 gautamUpdateALSD0902745 revamp complete

Summary of my tests of the demod boards, post gain modification:

  • DC tests (supply voltage, DC offsets at I and Q outputs, power LEDs etc)
  • RF tests
    • Back panel RF and LO power level monitor calibration
    • Coupling factor from RFinput to RFmon channel
    • Conversion loss as a function of demodulated beat frequency
    • Orthogonality and gain balance test
    • Linearity of unit as a function of RF input level
    • Electronics oise in the 1-10kHz band at the IF outputs.

Everything looks within the typical performance specs outlined in E1100114, except that the measured noise levels don't quite line up with the LISO model predictions. The measurement was made with the scheme shown in Attachment #1. I didn't do a point-by-point debugging of this on the board. I have uploaded the data + notebook summarizing my characterization to the DCC page for this part. I recommend looking at the HTML version for the plots.

*I'd put up the wrong attachment, corrected it now...

Quote:

I will put together a python notebook with all my measurements and upload it to the DCC page for this part. I need to double check expected noise levels from LISO to match up to the measurement.


gautam 9 Feb 2018 9pm: Adding a useful quote here from the LISO manual (pg28). I think if I add the Johnson noise from the output impedance of the mixer (assumed as 50ohms, I get better agreement between the measured and observed noises (although the variance between the 4 channels is still puzzling). The other possible explanation is small variations in the voltage noise at the various mixer output ports. Could we also be seeing the cyclostationary shot noise difference between the I and Q channels? 

For the computation of noise, the distinction between uinput and iinput is ignored, since no input signal is assumed. The source-impedance given in the uinput or iinput instruction is assumed to be connected from the input node to ground. It will affect the gain of noise contributions from their source to the output. The impedance itself is considerednoise-free, i.e. no Johnson noise is computedfor it. If you want to compute the source impedance’s Johnson noise, you must explicitly enter it as a resistor.

In any case, I am happy with this level of agreement, so I am going to stick this 1U chassis back in its rack with the primary aim of measuring a spectrum of the beatnote, so that I have some idea of what kind of whitening filter shape is useful for the ALS signals. May need to pull it out again for actually implementing the daughter board idea though... I have updated DCC page with LISO source, and also the updated python notebooks.

  13618   Wed Feb 7 17:01:25 2018 gautamUpdatePEMPID test plan

[kira, gautam]

We did a survey of the lab today to figure out some of the logistics for the PID control test for the seismometer can. Kira will upload sketches/photos from our survey. Kira tells me we need

  • +/- 15V for the temperature sensors
  • +/- 20V, 5A for heater circuit (to be confirmed by Kira after looking at voltage regulator datasheet for dropout voltage)
  • 2 ADC channels for temperature sensing (one inside can and one outside)
  • 1 DAC channel for controlling MOSFET

There are no DAC channels available in the c1ioo rack. In fact, there is a misleading SCSI cable labelled "c1ioo DAC0" that comes into the rack 1X3 - tracing it back to its other end, it goes into the c1ioo expansion chassis - but there are no DAC cards in there, and so this cable is not actually transporting any signals!

So I recommend moving the whole setup to the X end (which is the can's real home anyways). We plan to set it up without the seismometer inside for a start, to make sure we don't accidentally fry it. We have sufficient ADC and DAC channels available there (see Attachments #1 and #2, we also checked hardware), and also Sorensens to power the heater circuit / temperature sensing circuit. Do we want to hook up the Heater part of this setup to the Sorensens, which also power everything else in the rack? Or do we want to use the old RefCav heater power supply instead, to keep this high-current draw path isolated from the rest of our electronics?

If this looks okay (after pics are uploaded), we will implement these changes (hardware + software) tomorrow.

-----

I have attached the sketch of the whole system (attachment 3) with all the connections and inputs that we will need. Attachment 4 is the rack with the ADC and DAC channels labeled. Attachment 5 is the space where we could set up the can and have the wires go over the top and to the rack.

  13620   Thu Feb 8 00:01:08 2018 gautamUpdateCDSVertex FEs all crashed

I was poking around at the LSC rack to try and set up a temporary arrangement whereby I take the signals from the DAC differentially and route them to the D990694 differentially. The situation is complicated by the fact that, afaik, we don't have any break out boards for the DIN96 connectors on the back of all our Eurocrate cards (or indeed for many of the other funky connecters we have like IDE/IDC 10,50 etc etc). I've asked Steve to look into ordering a few of these. So I tried to put together a hacky solution with an expansion card and an IDC64 connector. I must have accidentally shorted a pair of DAC pins or something, because all models on the c1lsc FE crashedindecision. On attempting to restart them (c1lsc was still ssh-able), the usual issue of all vertex FEs crashing happened. It required several iterations of me walking into the lab to hard-reboot FEs, but everything is back green now, and I see the AS beam on the camera so the input pointing of the TTs is roughly back where it was. Y arm TEM00 flashes are also seen. I'm not going to re-align the IFO tonight. Maybe I'll stick to using a function generator for the THD tests, probably routing non AI-ed signals directly is as bad as any timing asynchronicity between funcGen and DAQ system...

  13621   Thu Feb 8 00:33:20 2018 gautamUpdateALSD990694 characterization / THD measurement plan

I decided to try doing the THD measurement with a function generator. Did some quick trials tonight to verify that the measurement plan works. Note that for the test, I turned off the z=15,p=150 whitening filter - I'm driving a signal at ~100Hz and should have plenty of oomph to be seen above ADC noise.

  1. Checked for ground loops - seem to be fine, see black trace on Attachment #1 which was taken with the FnGen hooked up to the input, but not putting out any signal
  2. Spectrum with 1Vpp sine wave @ ~103Hz. The various harmonic peaks are visible, and though I've not paid attention to bin width etc, the largest harmonics are ~1000x smaller than the main peak, and so the THD is ~1ppm, which is in the ballpark of what the datasheet tells us to expect around 100Hz for a gain of ~10 (=20dB). The actual gain was set at 0dB (so all opAmps in the quad bypassed)

I'm going to work on putting together some code that gives me a quick readback on the measured THD, and then do the test for real with different amplitude input signal and whitening gain settings.

**Matlab has a thd function, but to the best of my googling, can't find a scipy.signal analog.


To remind myself of the problem, summarize some of the discussion Koji and I had on the actual problem via email, and in case I've totally misunderstood the problem:

  1. The "Variable Gain" feature on the D990694 boards is achieved by 4 single gain stages cascaded together in series, with the ability to engage/bypass each stage individually.
  2. The 4 gain stages are constructed using the 4 OpAmps in a quad LT1125 IC, each in standard non-inverting configuration.
  3. The switches unfortunately are on the output side of each op-amp. This means that even if a stage is bypassed, the signal reaches the input pin of the OpAmp.
  4. For proper operation, in closed-loop, the differential voltage between the input pins of the OpAmp are 0.
  5. But this may require the OpAmp to source more current than it can (just using Ohms law and the values of the resistors in the feedback path).
  6. As a result, a large differential voltage develops between the input pins of the OpAmp.
  7. The LT1125 is not rated to operate in such conditions (this is what Hartmut was saying in the ilog linked earlier in this thread).
  8. Part of the internal protection mechanism to prevent damage to the IC in such operating conditions is a pair of diodes between the input pins of the OpAmp.
  9. When a large differential voltage develops between the input pins of the OpAmp, the diodes act to short the two to bring them to the same potential (minus whatever small drop there is across the diodes). Actually, according to the datasheet, when the differential voltage between the input pins exceeds 1.4V, the input current must be limited to 25mA, to avoid damaging the protection diodes? If so, we may already have damaged a bunch of these amplifiers.
  10. While the LT1125 IC is protected in this condition, the infinite input impedance of the OpAmp is reduced to the resistance between the inverting input and ground. The output voltage may still be saturated, but the output current draw is within what the IC can supply.
  11. As a result, Ohms law means that the preceeding stage is overdrawn for current. This is clearly not ideal.
  12. Another possible problem is that there is some sort of interaction between the 4 opamps in the quad IC, which means that even if one stage is overdrawn for current, all of them may be affected.
  13. The Advanced LIGO version of this board addresses #11 and #12 by (i) placing a series resistor between the input signal and the non-inverting input of the opamp, and (ii) using single opamp ICs instead of a quad, respectively.

So my question is - should we just cut the PCB trace and add this series resistance for the 4 ALS signal channels, and THEN measure the THD? Since the DC voltage level of the ALS signal is expected to be of the order of a few volts, we know we are going to be in the problematic regime where #11 and #12 become issues.

  13623   Thu Feb 8 12:00:09 2018 gautamUpdateALSD990694 is NOT differential receiving

Correcting a mistake in my earlier elog: the D990694 is NOT differential receiving, it is single ended receiving via the front panel SMA connectors. The aLIGO version of the whitening board, D1001530 has an additional differential-to-single-ended input stage, though it uses the LT1125 to implement this stage. So the possibility of ground loops on all channels using this board will exist even after the planned change to install series resistance to avoid current overloading the preceeding stage.

Quote:
 

So either something is busted on this board (power regulating capacitor perhaps?), or we have some kind of ground loop between electronics in the same chassis (despite the D990694 being differential input receiving). Seems like further investigation is needed. Note that the D000316 just two boards over in the same Eurocrate chassis is responsible for driving our input steering mirror Tip-Tilt suspensions. I wonder if that board too is suffering from a similarly noisy ground?

 

  13625   Thu Feb 8 13:13:14 2018 gautamUpdateALSD990694 pulled out

After labeling all cables, I pulled out one of the D990694s in the LSC rack (the one used for the ALS X signals, it is Rev-B1, S/N 118 according to the sticker on it).

Took some photos before cutting anything. Attachments #1-3 are my cutting plans (shown for 1 channel, plan is to do it for both ALS channels coming into this board). #1 & #2 are meant to show the physical locations of the cuts, and #3 is the corresponding location on the schematic. These are the most convenient locations I could identify on the board for this operation.

I don't know what the purpose of resistors R196, R197, R198 are. I'm assuming it has something to do with the way the ADG333ABR switches. The aLIGO board uses a different switch (MAX4659EUA+), and doesn't have an analogous resistor (though from what I can tell, it too is a CMOS SPDT switch just like the ADG333ABR, just has a lower ON resistance of 25ohm vs 45ohm for the ADG333ABR).

As for the actual resistance to be used: Let's say we don't have signals > 5V coming into this board. Then using 301ohms (as in the aLIGO boards) in series means the peak current draw will be <20mA, which sounds like a reasonable number to me. Larger series resistance is better, but I guess then the contribution of the current noise of the OpAmp keeps increasing.

  13627   Thu Feb 8 18:10:36 2018 gautamUpdateALSD990694 pulled out

This is proving much more challenging than I thought - while Cut #1 was easy to identify and execute, my initial plan for Cut #2 seems to not have isolated the input of the second opamp (as judged by DMM continuity). Koji pointed out that this is actually not a robust test, as the switches are in an undefined state while I am doing these tests with the board unpowered. It seems rather complicated to do a test with the board powered out here in the office area though - and I'd rather not desolder the 16 and 20 pin ICs to get a better look at the tracks. This PCB seems to be multilayered, and I don't have a good idea for what the hidden tracks may be. Does anyone know of a secret place where there is a schematic for the PCB layout of this board? The DCC page only has the electrical schematic drawings, and I can't find anything useful on the elog/wiki/old ilog on a keyword search for this DCC document number. The track layout also is not identical for all channels. So I'm holding off on exploratory cuts.

*I've asked Ben Abbott/Mike Pedraza about this and they are having a look in Dale Ouimette's old drives to see if they can dig up the Altium/Protel files.

  13628   Fri Feb 9 13:37:44 2018 gautamUpdateALSTHD measurement trial

I quickly put together some code that calculates the THD from CDS data and generates a plot (see e.g. Attachment #1).

Algorithm is:

  1. Get data (for now, offile file, but can be readily adapted to download data live or from NDS).
  2. Compute power spectrum using scipy.signal.periodogram. I use a Kaiser window with beta=38 based on some cursory googling, and do 10 averages (i.e. nfft is total length / 10), and set the scaling to "spectrum" so as to directly get a power spectrum as opposed to a spectral density.
  3. Find fundamental (assumed highest peak) and N harmonics using scipy.signal.find_peaks_cwt. I downsample 16k data to 2k for speed. A 120second time series takes ~5 seconds.
  4. Compute THD as \mathrm{THD} = \frac{\sqrt{\sum_{i=2}^{N}\mathrm{V}_i^2}}{V_1}where V_i denotes an rms voltage, or in the case of a power spectrum, just the y-axis value.

I conducted a trial on the Y arm ALS channel whitening board (while the X arm counterpart is still undergoing surgery). With the whitening gain set to 0dB, and a 1Vpp input signal (so nothing should be saturated), I measure a THD of ~0.08% according to the above formula. Seems rather high - the LT1125 datasheet tells us to expect <0.001% THD+N at ~100Hz for a closed loop gain of ~10. I can only assume that the digitization process somehow introduces more THD? Of course the FoM we care about is what happens to this number as we increase the gain.

Quote:
 

I'm going to work on putting together some code that gives me a quick readback on the measured THD, and then do the test for real with different amplitude input signal and whitening gain settings.

 

  13632   Tue Feb 13 22:35:21 2018 gautamUpdateElectronicsPSL table power supply cleanup

The main motivation for this work is that I want +15VDC power available on the PSL table to hookup the Teledyne box that Koji made a week ago and do some noise measurements on my revised IR ALS signal chain. But I think this is a good opportunity to effect a number of changes I've been wanting to do for a while.

Tomorrow, Steve and I will do the following:

  1. Fix the AOM driver power cabling that I broke.
  2. Make the AOM +24VDC power supply independent - right now it is shared between the AOM driver and the two ZHL-3-A amplifiers.
  3. Tap an independent +24VDC power supply from 1X1 for the ZHL-3A amplifiers (I guess one power supply and fuse is sufficient for both amplifiers since they are in the same box).
  4. Tap an independent +15VDC power supply for the Teledyne box.
  5. Tap an independent +15VDC power supply for the little fan on the back of the PSL controller, that is currently powered by a bench supply (+12VDC, but it's just a fan, so +15VDC or +10VDC will do just fine, and these are the Sorensen levels we have).
  6. Tap an independent +/-24VDC power supply for the FSS summing box. Right now it is being powered by a bench supply under the PSL table. The indicated supply voltage on the box is +/-18V. But according to the schematic, this +/-18V get regulated down to +/- 15V, so we may as well use +/-24V which is available from the Sorensens in 1X1 (there is no +/-18VDC Sorensen there). The datasheets for the 7815 and 7915 ICs suggest that this will be just fine.
  7. Where possible, make at least 1 spare outlet for each supply voltage available at 1X1, such that in future, tapping extra supply points won't be such a huge pain.

So in summary, we will need, at 1X1, (at least, including 1 spare for future work):

  • New +24VDC connections ------ 3x
  • New -24VDC connections ------- 2x
  • New +15VDC connections ------ 3x
  13633   Wed Feb 14 17:49:22 2018 gautamUpdateElectronicsPSL table power supply cleanup

[steve, gautam]

We completed this work today. Need to clean up a little (i.e. coil excess cable lengths, remove unused cables etc), which we will do tomorrow. All connections have been made at the DIN rail end, but the fuses have not been inserted yet, so there is no voltage reaching the PSL table on any of the newly laid out cables. We also need to establish two +15VDC connections at the DIN rail side. I may establish this later in the evening, as the main point of this work was to get the Teledyne signal path operational. Setting up these DIN connectors is actually a huge pain, we tried to setup a few extra ports for the voltages we used today so that in future, life is easier for whoever wants to pipe DC power to the PSL table. The rule is, however, to re-establish the same number of open ports for each voltage as was available when you started.

For the ZHL-3A, Teledyne, and AOM driver cables, we used 18AWG, 2 conductor, twisted wire, while for the PSL fan we used 20AWG. For the FSS box, we decided to use the 3 conductor 24AWG twisted wire. I believe that these wire gauge choices are appropriate given the expected current in each of these paths.

Pictures + further details tomorrow.

gautam @ 1030pm: there was some mistake with the +15V wiring we did in the evening (the PSL fan and Teledyne cables were plugged into the wrong DIN terminal blocks). I fixed this, and also routed +15VDC to the newly installed set of terminal blocks for this purpose (since we had run out of +15VDC ports at 1X1). After checking voltages at both 1X1 and on the PSL table, I hooked up

  1. FSS Summing box
  2. Teledyne amplifier
  3. ZHL-3A amplifiers

to their newly laid out power supplies. IMC locks so looks like the FSS box is doing fine yes. So we can recover one bench power supply from under the PSL table on the east side. I didn't hook up the AOM driver just now because of some accessibility issues, and I'd also like to do an ALS beat spectrum measurement if possible.

  13635   Fri Feb 16 01:09:55 2018 gautamUpdateALSEX green locking duty cycle

I have been puzzled as to why the duty cycle of the EX green locks are much less than that of the EY NPRO. If anything, the PDH loop has higher bandwidth and comparable stability margins at the X end than at the Y end. I hypothesize that this is because the EX laser (Innolight 1W Mephisto) has actuation PZT coefficient 1MHz/V, while the EY laser (Lightwave 125/126) has 5MHz/V. I figure the EX laser is sometimes just not able to keep up with the DC Xarm cavity length drift. To test this hypothesis, I disabled the LSC locking for the Xarm, and enabled the SLOW (temperature of NPRO crystal) control on the EX laser. The logic is that this provides relief for the PZT path and prevents the PDH servo from saturating and losing lock. Already, the green lock has held longer than at any point tonight (>60mins). I'm going to leave it in this state overnight and see how long the lock holds. The slow servo path has a limiter set to 100 counts so should be fine to leave it on. The next test will be to repeat this test with LSC mode ON, as I guess this will enhance the DC arm cavity length drift (it will be forced to follow MCL).

Why do I care about this at all? If at some point we want to do arm feedforward, I thought the green PDH error signal is a great target signal for the Wiener filter calculations. So I'd like to keep the green locked to the arm for extended periods of time. Arm feedforward should help in lock acquisiton if we have reduced actuation range due to increased series resistances in the coil drivers.

As an aside - I noticed that the SLOW path has no digital low pass filter - I think I remember someone saying that since the NPRO controller itself has an in-built low pass filter, a digital one isn't necessary. But as this elog points out, the situation may not be so straightforward. For now, I just put in some arbitrary low pass filter with corner at 5Hz. Seems like a nice simple problem for optimal loop shaping...


gautam noon CNY2018: Looks like the green has been stably locked for over 8 hours (see Attachment #1), and the slow servo doesn't look to have railed. Note that 100 cts ~=30mV. For an actuation coefficient of 1GHz/V, this is ~30MHz, which is well above the PZT range of 10V-->10MHz (whereas the EY laser, by virtue of its higher actuation coefficient, has 5 times this range, i.e. 50MHz). Supports my hypothesis.

  13636   Fri Feb 16 01:34:40 2018 gautamUpdateALSD0902745 in-situ testing

Having implemented the changes to the audio amplifier stage, I re-installed this unit at the LSC rack, and did some testing. The motivation was to determine the shape of the ALS error signal spectrum, so that I can design a whitening preamp accordingly. Attachment #1 is the measurement I've been after. The measurement was taken with EX NPRO PDH locked to the arm via green, and Xarm locked to MC via POX. Slow temperature relief servo for EX NPRO was ON. Here are the details:

  1. Mode-matching into the BeatMouth PSL light fiber had deteriorated dramatically - it was ~1mW out of 4.4mW. I spent 5 mins getting it back to 3.2mW (72% efficiency) and then moved on... I am a little surprised the drift was so large, but perhaps, it's not surprising given that there has been a lot of work on and around the PSL table in the last couple of weeks. There is a 300mm focusing lens after the last steering mirror so the effect of any alignment drifts should be attenuated, I don't really understand why this happened. Anyways, perhaps a more intelligent telescope design would avoid this sort of problem.
  2. I removed the ND filter in the PSL pickoff to BeatMouth path (this was not responsible for the reduced power mentioned in #1). I verified that the total power reaching the photodiode was well below its rated damage threshold of 2mW (right now, there is ~620uW). I will update the BeatMouth schematic accordingly, but I think there will be more changes as we improve mode matching into the fibers at the end.
  3. Hooked up the output of the fiber PD to the Teledyne amp, routed the latters output to the LSC rack. Measured RF electrical power at various places. In summary, ~6dBm of beat reaches the splitter at the LSC rack. This is plenty.
  4. The main finding tonight was discovered by accident.
    • For the longest time, I was scratching my head over why the beat note amplitude, as monitored on the control room SA (I restored it to the control room from under the ITMX optical table where Koji had temporarily stored it for his tests on the PSL table) was drifting by ~10-15dB!
    • So each time, having convinced myself that the power levels made sense, I would come back to the control room to make a measurement, but then would see the beat signal level fluctuate slowly but with considerable amplitudeindecision.
    • The cause - See Attachment #2. There is a length of fiber on the PSL table that is unshielded to the BeatMouth. While plugging in RF cables to the BeatMouth, I found that accidentally brushing the fiber lightly with my arm dramatically changed the beat amplitude as monitored on a scope.
    • For now, I've "strain relieved" this fiber as best as I could, we should really fix this in a better way. This observation leads me to suspect that many of the peaky features seen in Attachment #1 are actually coupling in at this same fiber...
    • The beat note amplitude has been stable since, in the ~90 mins while I've been making plots/elogs.
    • Surely this is a consequence of differential polarization drift between the PSL and EX beams?
  5. There are prominent powerline harmonics in these signals - how can we eliminate these? The transmission line from PSL table to LSC rack already has a BALUN at its output to connect the signal to the unbalanced input of the demod board.
  6. Not sure what to make of the numerous peaks in the LO driven, RF terminated trace.
  7. The location of the lowest point in the bucket also doesn't quite match previous measured out-of-loop ALS noise - we seem to have the lowest frequency noise at 150-200Hz, but in these plots its more like 400Hz.

Conclusion: In the current configuration, with x10 gain on the demodulated signals, we barely have SNR of 10 at ~500Hz. I think the generic whitening scheme of 2 zeros @15Hz, 2poles@150Hz will work just fine. The point is to integrate this whitening with the preamp stage, so we can just go straight into an AA board and then the ADC (sending this signal into D990694 and doing the whitening there won't help with the SNR). Next task is to construct a test daughter board that can do this...

 

  13639   Fri Feb 16 22:15:30 2018 gautamUpdateGeneralc1mcs model restarted

c1mcs had died for some reason. Looking at dmesg, I see:

[769312.996875] c1mcsepics[1140]: segfault at 7f5000000012 ip 00007f50ea8ded8f sp 00007f50e9f53a10 error 4 in libc-2.19.so[7f50ea865000+1a1000]

None of the other EPICS processes died. Not sure what to make of this. I was at the PSL table working, and had closed the PSL shutter to avoid MC autolocker trying to keep the MC locked while I was mucking about, but this shouldn't have had any effect on an EPICS process?

Anyway, I just logged into c1sus, stopped and restarted the model. IMC locks fine now.

  13640   Fri Feb 16 22:19:07 2018 gautamUpdateGeneralFibel ALS input polarization tuning

After discussing with Koji, I decided to try and align the input beam polarization at the PSL fiber coupler to one of the special axes of the PM fiber. The motivation is to try and narrow down the source of the large RF beatnote amplitude drift I noticed and reported last night.

The setup for doing so is shown in Attachment #1 - essentially, I setup one of the newly purchased couplers in a mount, set up a PBS, and placed two photodiodes at the S and P ports of the PBS. The idea is to rotate the input coupler in its mount, thereby maximizing the PER (monitored on two Thorlabs PDA520s - I didn't check the gain balance of them). 

I spent ~30mins doing some preliminary trials just now, and, I was able to achieve a PER of ~1/20. But I think much better numbers were reported in this SURF project (although I'm not entirely sure I understand that measurement). I will spend a little more time tweaking the alignment. The procedure is tricky as at some point, simply rotating the mount reduces the mode-matching efficiency into the fiber so much that it is not possible to get a meaningful PER measurement from the photodiodes. I'm adjouring for now, more to follow...

  13641   Mon Feb 19 14:27:25 2018 gautamUpdateGeneralFibel ALS input polarization tuning

Summary:

Current configuration of PSL free-space to fiber coupling is:

  • 3.25 mW / 4.55mW (~71%) coupling efficiency, both numbers measured with Ophir power meter, Filter OFF
  • \mathrm{PER} \doteq \frac{\mathrm{P_{fast}}}{\mathrm{P_{slow}}} (I choose to define it in this way as opposed to the reciprocal) of 75 (~19dB). The uncertainty in this number is large (see discussion), but I am confident that we have >10dB, which while isn't as good as can be, is sufficient for the main motivation behind this work.

Motivation:

I had noticed that the RF beat amplitude was fluctuating by up to 20dBm as viewed on the control room analyzer. As detailed in my earlier elog, I suspected this to be because of random polarization drift between the PSL and EX fields incident on the Fiber coupled PDs. Since I am confident the problem is optical (as opposed to something funny in the electronics), we'd like to be able to isolate which of the many fiber segments is dominating the contribution to this random polarization drift.

Some useful references:

  1. General writeup about how PM fibers work and PER. Gives maximum achievable PERs for a given misalignment of incident beam relative to one of the two birefringent axes.
  2. Another similar writeup. This one put me onto the usefulness of the alignment keys on the fibers.
  3. Thorlabs PM980 specs - this tells us about the orientation of the two axes for the kind of fibers we use.

Procedure and details:

  • The principle of operation behind polarization maintaining (PM) fibers is that intentional birefringence is introduced along two perpendicular axes in the fiber.
  • As a result of which light propagates with different phase velocity along these axes.
  • For an arbitrary incident field with E-field components along both axes, it is almost impossible to predict the output polarization as we do not know the length of propagation along each axis to sufficient precision (it is also uncontrolled w.r.t. environmental fluctuations). So even if you launch linear polarization into the fiber, it is most likely that the output polarization state will be elliptical.
  • But if we align the incident, linear polarization along one of the two axes, then we can accurately predict the polarizaiton at the output, to the extent that the fiber doesn't couple power in the two axes during propagation. I cant find a spec for the isolation between axes for the fiber we use, but the specs I could find for other fiber manufacturers suggest that this number is >30dB, so I think the assumption is a fair one.
  • A useful piece of information is that the alignment key on the fibers gives us information about the orientation of the birefringent axes inside the fiber. For the Thorlabs fibers, it seems that the alignment key lines up with the stress-inducing rods inside the fiber (i.e. the slow axis). I confirmed this by looking at the fiber with the fiber scope.
  • The PSL pickoff beam I am using for this setup is from the transmission of the PBS after the Faraday. So this field should have relatively pure P-polarization.
  • The way I have set up the fiber on the PSL table, the fast axis of the fiber corresponds to P-polarization (i.e. E field oscillates parallel to the plane of the optical table). Actually, it was this alignment that I tweaked in this work.
  • Using the information about the alignment key defining polarization axes on the fiber, I also set up the output fiber coupler such that the fast axis lined up as near parallel to the plane of the optical table as possible. In this way, the beam incident on the PBS at the output of my setup should be pure P-polarizaiton if I setup my input alignment into the fiber well.
  • I tweaked the rotation of the Fiber mount at the input coupler to maximize the ratio of P_p / P_s, as measured by the pair of PDs at the output. 
  • As #1 of my listed references details, you need to align the incident linear polarization to one of the two birefringent axes to closer than 6 degrees to achieve a PER of >20dB. While this sounds like a pretty relaxed requirement, in practise, it is about as good as we can hope to achieve with the mounts we have, as there is no feature that allows us to lock the rotational degree of freedom once we have optimized the alignment. Any kind of makeshift arrangement like taping the rotating part to the mount is also flaky, as during the taping, we may ruin the alignment.
  • Attachment #1 shows the result of my alignment optimization - the ratio P_p / P_s is about 75.
  • The uncertainty on the above number is large. Possible sources of error:
    • Output coupler is not really aligned such that fast axis corresponds to P-polarizaiton for the output PBS.
    • The two photodiodes' gain balance was not checked.
    • The polarization content of the input beam was not checked.
    • The PBS at the output could be slightly misaligned relative to the S/P polarization directions defined by the tabletop.
    • The PBS extinction ratio was not checked.
  • But anyways, this is a definite improvement on the situation before. And despite the large uncertainty, I am confident that P_p / P_s is better than 10dB.
  • Moreover, Steve and I installed protective tubing on the lengths of fiber that were unprotected on the PSL table, this should help in reducing stress induced polarization drifts in the fiber, at least in these sections of fiber.
  • So I think the next step is to monitor the stability of the RF beatnote amplitude after these improvements. At some point, we need to repeat this procedure for the EX and EY fibers as well.
  • If the large drifts are still seen, the only thing we can exclude as a result of this work is the section of fiber from the PSL light coupler to the beat mouth.
  13643   Tue Feb 20 21:14:59 2018 gautamUpdateCDSRFM network errors

I wanted to lock the single arm POX/POY config to do some tests on the BeatMouth. But I was unable to.

  • I tracked the problem down to the fact that the TRX and TRY triggers weren't getting piped correctly to the LSC model
  • In fact, all RFM channels from the end machines were showing error rates of 16384/sec (i.e. every sample).
  • After watchdogging ETMX, I tried restarting just the c1scx model - this promptly took down the whole c1iscex machine.
  • Then I tried the same with c1iscey - this time the models restarted successfully without the c1iscey machine crashing, but the RFM errors persisted for the c1scy channels.
  • I walked down to EX and hard rebooted c1iscex.
  • c1iscex came back online, and I ssh-ed in and did rtcds start --all.
  • This brought all the models back online, and the RFM errors on both c1iscex and c1iscey channels vanished.

Not sure what to make of all this, but I can lock the arms now.

  13644   Tue Feb 20 23:08:27 2018 gautamUpdateALSD0902745 in-situ testing

Attachment #1 shows the ALS noise measurement today. Main differences from the spectrum posted last week is that

  1. I have tried to align the input polarization axis (p-pol) to the fast axis of the fiber, and believe I have done it to ~75dB.
  2. Steve and I installed some protective tubing for the vertical lengths of fiber going into the beat mouth.
  3. Today, I decided to measure the noise at the differential rear panel outputs rather than the single-ended front panel outputs. For the test, I used a DB25 breakout board and some pomona mini-grabber to BNC clips to connect to the SR785.

For comparison, I have plotted alongside today's measurement (left column) the measurement from last week (right column).

Conclusions:

  • The clear daylight between red and green traces in the left column give me confidence that I am measuring real laser frequency noise in the red trace. It even has the right shape considering the bandwidth of the EX PDH servo.
  • The installation of protective tubing doesn't seem to have reduced the heights of any of the peaks in the red traces. I hypothesize that some of these are acoustic coupling to the fiber. But if so, either the way we installed the protective tubing doesn't help a whole lot, or the location of the coupling is elsewhere.
  • Judging by the control room analyzer, there doesn't seem to be as large drifts in the RF beat amplitude tonight (yes) as I saw the last couple of times I was testing the BeatMouth®. For a more quantitative study, I'm gonna make a voltage divider so that the ~10V output I get at the rear panel power monitor output (for a LO level of ~0dBm, which is what I have) can be routed to some ADC channel. I'm thinking I'll use the Y ALS channels which are currently open while ALS is under work.
  • Still have to make preamp prototype daughter board with the right whitening shape... This test suggests to me that I should also make the output differential sending...
  13645   Wed Feb 21 00:04:27 2018 gautamUpdateElectronicsTemporary RF power monitor setup

I made a voltage divider using a 20.47kohm and 1.07kohm (both values measured with a DMM). The whole thing is packaged inside a Pomona box I found lying around on the Electronics bench. I have hooked it up to the ALSY_I channel and will leave it so overnight. The INMON of this channel isn't DQed, but for this test, the 16Hz EPICS data will suffice. I've locked the EX laser to the arm, enabled slow temperature servo to allow overnight lock (hopefully) and disabled LSC mode (as locking the arm to the MC tends to break the green lock)

To convert the INMON counts to RF power, I will use (based on my earlier calibration of this monitor channel, see DCC document for the demod chassis).

\mathrm{P_{RF}} (\mathrm{dBm}) = \frac{19.13 \times \frac{cts}{1638.4} - 10.23}{0.12}


1AM update: Attachment #1 shows that the RF amplitude has been relatively stable (less than 10% of nominal value variation) over the course of the last hour or so. Even though there is some low frequency drift over timescales of ~20mins, no evidence of the wild ~20dB amplitude changes I saw last week. The signs are encouraging...

overnight update: See Attachment #2 - looking at the past 11 hours of second trend data during which the arm stayed locked, there actually seems to have been more significant variation in the beatnote amplitude. Swings of up to 6dBm are seen on a ~20min timescale, while there is also some longer term drift over 12 hours by a couple of dBm. There is probably a systematic error in the Y-axis, as I measured the RF power at the input of the power splitter at the LSC rack to be ~3dBm, so I expect something closer to 0dBm to be the LO input power which is what I am monitoring. So further debugging is required - I think I'll start by aligning the X fiber coupled beam to one of the fiber's special axes.

  13646   Wed Feb 21 12:17:04 2018 gautamUpdateCDSLO Power mon channels added to c1lsc

To make this setup more permanent, I modified the c1lsc model to pipe the LO power monitor signals from the Demod chassis to unused channels ADC_0_25 (X channel LO) and ADC_0_26 (Y channel LO) in the c1lsc model. I also added a couple of CDS filter blocks inside the "ALS" namespace block in c1lsc so as to allow for calibration from counts to dBm. I didn't add any DQ channels for now as I think the slow EPICS records will be sufficient for diagnostics. It is kind of overkill to use the fast channels for DC voltage monitoring, but until we have acromag channels readily accessible at 1Y2, this will do.

Modified model compiled and installed successfully, though I have yet to restart it given that I'll likely have to do a major reboot of all vertex FEs frown

  13648   Thu Feb 22 00:09:11 2018 gautamUpdateALSD0902745 in-situ testing

I thought a little bit about the design of the preamp we want for the demodulated ALS signals today. The requirements are:

  1. DC gain that doesn't cause ADC saturation.
  2. Audio frequency gain that allows the measured beat signal spectrum to be at least 20dB the ADC noise level.
  3. Electronics noise such that the measured beat signal spectrum is at least 20dB above the input-referred noise of this amplifier.
  4. Low pass filtering at the input to the differential receiving stages, such that the 2f product from the demodulation doesn't drive the AD829 crazy. For now, I've preserved the second-order inductor based LPF from the original board, but if this proves challenging to get working, we can always just go for a first-order RC LPF. One challenge may be to find a 2.2uH inductor that is compatible with prototype PCB boards...
  5. Differential sending, since this seems to be definitively the lower noise option compared to the single-ended output (see yesterday's measurement). The plan is to use an aLIGO AA board that has differential receiving and sending, and then connect directly to the differential receiving ADC.

Attachment #3 shows a design I think will work (for now it's a whiteboard sketch, I''ll make this a computer graphic tomorrow). I have basically retained the differential sending and receiving capabilities of the existing Audio I/F amplifier, but have incorporated some whitening gain with a pole at ~150Hz and zero at ~15Hz. I've preserved the DC gain of 10, which seems to have worked well in my tests in the last week or so. Attachments #1 and #2 show the liso modelled characteristics. Liso does not support input-referred noise measurements for differential voltage inputs, so I had to calculate that curve manually - I suspect there is some subtlety I am missing, as if I plot the input referred noise out to higher frequencies, it blows up quite dramatically.

Next step is to actually make a prototype of this. I am wondering if we need a second stage of whitening, as in the current config, we only get 20dB gain at 150Hz relative to DC. Yesterday's beat spectrum measurement shows that we can expect the frequency noise of the ALS signal at ~100Hz to be at the level of ~1uV/rtHz, but this is is around the ADC noise level? If so, 20dB of whitening gain may be sufficient?

Quote:

Still have to make preamp prototype daughter board with the right whitening shape... This test suggests to me that I should also make the output differential sending...


*Side note: I was wondering why we need the differential receiving stage, followed by a difference amplifier, and then a differential sending stage. After discussing with Koji, we think this is to suppress any common-mode noise from the mixer outputs.

  13655   Sun Feb 25 00:03:12 2018 gautamUpdateALSDaughter board prototyping

Using one of the prototype PCB boards given to me by Johannes, I put together v1 of this board and tested it. 

Attachment #1 - Schematic with stages grouped by function and labelled. 

Attachment #2 - Measured vs modelled Transfer function.

Attachment #3 - Measured vs modelled noise. Measurement shown only between positive output and ground, the other port is basically the same. I will update this attachment to reflect the expected signal level in comparison to the noise, but suffice it to say that given the measured input referred noise, we will have plenty of SNR between 0.1Hz and 10kHz. The single stage of whitening should also be sufficient to amplify the signal above ADC noise in the same frequency band

Attachment #4 - Positive output as viewed on a fast (300 MHz) scope using a Tektronix x1 voltage probe.

Attachment #5 - Daughter board noise with measured ALS noise overlaid (the gain of x10 on the existing audio pre-amp has been divided out). 

Comments:

  • I may have overlooked the GBW of the OP27 in the design - specifically, the negative feedback is wired for gain x100 at high frequencies, and so the input signal should be filtered above 8MHz/100 ~80kHz. But the LC poles are at ~500kHz. I wonder if the small deviation seen between modelled and masured TFs is reflecting this. Practically, the easier fix is to add a feedback capacitor that rolls off the gain at high frequencies. 300pF WIMA should do the trick, and we have these in stock.
  • I don't understand why the modelled response starts to roll off around 5kHz, even though the poles of the LC filter at the input stage are at 500kHz. This happens because at low frequencies, the 1.5uH inductor is basically a short - so the RC divider at the input of the Op27 has a pole at 1/2/pi/R/C ~5kHz for R=499, C = 68nF.
  • I am not sure what to make of the peaky comb seen in Attachment #3, but I'm pretty sure it's electronic pickup from something. The GPIB adapter power suppy is not to blame. The peaks are 10 Hz spaced.
  • From Attachment #4, I don't suspect any opamp oscillations given that the signal seen is tiny, but I don't know what amplitude is characteristic of an oscillating op amp, so I am not entirely confident about this conclusion. 
  • Initially while thinking about the design, I was trying to think of making the design generic enough that we could use these signals for high-bandwidth ALS control (a.k.a. Fast ALS) but in the current incarnation, no consideration was given to minimizing phase lag at high frequencies. 
  • Putting the PCB board together was more painful than I imagined as the board is configured for 4 single op amps whereas my design requires 5 - so I needed to do some trace cutting surgery. Rather than make 3 more of these, I'm just going to finish the characterization, and if the design looks good, we can get some custom PCBs printed.
  • Power decoupling caps (47nF) are added to all op amp power pins, but is not shown in the schematic.

Given the overall good agreement between model and measurement, I am going to test this with the actual RF beat. For this test, we will need a differential receiving AA board to interface the output of the daughter board with the ADC input

Quote:

Next step is to actually make a prototype of this.

  13658   Tue Feb 27 21:10:45 2018 gautamUpdateALSDaughter board testing

I thought a little bit about the next steps in testing the daughter board. The idea is to install this into the existing 1U chassis and tap the differential output from the FET Mixers as inputs to the daughter board. Looking at the D0902745 schematic, I think the best way to do this is to simply remove L3, L4, C10, C11, C15 and C16. I will then use the pads for L3 and L4 to pipe the differential output of the FET mixer to the differential input of the daughter board. 

The daughter board takes care of whitening the ALS signal.

Then we need to pipe the differential output of the daughter board into the differential input of a differential receiving AA board. Koji and Johannes surveyed the available stockpile from the WB workshop. The best option seems to be to use the available v5 of D070081 and install 4 of them into a 1U chassis unit (also available from WB EE shop). The v5s can be upgraded to v6 by replacing the set of input and output buffer OpAmps with AD8622, as per the revision history notes. Koji ordered 100pcs of these today. 

The input to the proposed 1U chassis housing these 8 AA boards (each with 8 channels) is a DB9 connector. The aLIGO demod board chassis that we use to demodulate the ALS signals has a nice DB25 output connector that supplies all the differential I and Q demodulated signals. But since we will install a daughter board, we will hae to hack together some connector solution anyways. I propose using a DB9 connector to pipe the outputs of the daughter board to the inputs of the AA board. Space is tight in the LSC rack, but I think we have space for a 1U chassis (see Attachment #3).

Finally - how to interface the AA board with the ADC? Koji and I discussed options, and seems like the least painful way will be to install a new ADC in the c1lsc expansion chassis in 1Y3. I checked the computer hardware cabinet and there seems to be 1 spare general standards 16bit ADC in there (see Attachment #1). Its health/providence is unknown. But Koji and I will test it after the meeting tomorrow. I also have another ADC card that Jamie and I removed from c1ioo sometime ago. I have labelled it as "GPIO0 LED RED", though I don't remember exactly what the problem was and can't find any elog about it. Incidentally, there are also 2 spare DAC cards available in the cabinet, although their health/rpovidence too is unknown. There are sufficient free slots in the c1lsc expansion chassis (see Attachment #2 though we will need a LIGO ADC adaptor card). Then we can just change the input ADC channels for the ALS signals in the c1lsc model.

In the short term, while the hardware for this plan is being put together, I can test the uncalibrated noise performance of the demod + daughter board combo (uncalibrated because I will make a measurement of voltage noise with an SR785 as opposed to frequency noise). A second daughter board will also need to be assembled - I'm just going to do it on another prototyping board as figuring out how to use Altium will probably take me longer. There is also the matter of fine tuning the polarization axes alignment of the input to the EX fiber coupler.

  13661   Wed Feb 28 19:13:25 2018 gautamUpdateALSADC test for differential receiving in c1lsc

[koji, gautam]

we did a bunch of tests to figure out the feasibility of the plan I outlined last night. Bottom line is: we appear to have a working 64 channel ADC (but with differential receiving that means 32 channels). But we need an aLIGO ADC adaptor card (I'm not sure of the DCC number but I think it is D0902006). See attached screenshot where we managed to add an ADC block to the IOP model on c1lsc, and it recognizes the additional ADC. The firmware on the (newly installed) working card is much newer than that on the existing card inside the expansion chassis (see Attachment #1).

Details:

  • Watchdogged all optics because we expected messing around with c1lsc to take down all the vertex FEs. Actually, only c1sus was killed, c1ioo survived.
  • Closed PSL shutter. Shut down c1lsc FE machine.
  • Started out by checking the functionality of the two ADC cards I found.
  • Turns out the one Jamie and I removed from c1ioo ~6months ago is indeed broken in some way, as we couldn't get it to work.
  • Took us a while to figure out that we require the adaptor board and a working ADC card to get the realtime model to run properly. A useful document in understanding the IO expansion chassis is this one.
  • Another subtlety is that the ADC card we installed today (photo in previous elog) is somewhat different from the ones installed in c1sus and c1lsc expansion chassis. But a similar one is installed at the Y-end at least. Point is, this ADC card seems to need an external power supply via a 4-pin Molex connector to work properly.
  • We borrowed an adapter card from c1iscey expansion chassis (after first shutting down the machine).
  • It seems like a RED GPIO0 LED on these ADC cards isn't indicative of a fault.
  • Added an ADC part from CDS_PARTS library. Added an ADC selector bus and an "MADC" block that sets up the 64k testpoints as well as the EPICS readbacks.
  • We were able to see sensible numbers (i.e. ~0 since there is no input to the ADC) on these readback channels.
  • To restore everything, we first shutdown c1lsc, then restored the adaptor card back to c1iscey, and then rebooted c1iscey, c1lsc and c1sus. Recompiled c1x04 with the added ADC block removed as it would otherwise complain due to the absence of an adapter board.
  • Did rtcds restart <model> on all machines to bring back all models that were killed. This went smoothly.
  • IMC and Yarm locked smooth.

Note that we have left the working ADC card inside the c1lsc expansion chassis. Plan is to give Rolf the faulty ADC card and at the same time ask him for a working adapter board.


Unrelated to this work: we have also scavenged 4 pcs of v2 of the differential receiving AA board from WB EE shop, along with a 1U chassis for the same. These are under my desk at the 40m for the moment. We will need to re-stuff these with appropriate OpAmps (and also maybe change some Rs and Cs) to make this board the same as v6, which is the version currently in use.

  13662   Wed Feb 28 21:14:34 2018 gautamSummaryPEMChannel admin

Since we decided to use the Acromag for readback of the temperature sensor for Kira's seismometer temperature control, I enabled logging of the channel Johannes had reserved for this purpose last week. Kira has made the physical connection of a temperature sensor to the BNC input for this channel - it reads back -2.92 V right now, which is around what I remember it being when Kira was doing her benchtop tests. I edited C0EDCU.ini to enable logging of this channel at 16 Hz. Presumably, a study of the ADC noise of the Acromag at low frequencies has to be made to ensure appropriate whitening (if any) can be added. Channel name is C1:PEM-SEIS_EX_TEMP_MON. Similarly, there is C1:PEM_SEIS_EX_TEMP_CTRL which is meant to be the control channel for the servoing. Calibration of the temperature sensor readback into temperature units remains. It also remains to be verified if we can have these slow EPICS channels integrated with a fast control model, or if the PID temperature control will be purely custom-script based as we have for the FSS slow loop.

I removed the fast channels I had setup temporarily in c1als. Recompilation and restart of the model went smoothly.

Quote:
 I then made a "PEM" namespace block inside the c1als model, and placed a single CDS filter module inside it (this can be used for calibration purposes). The filter module is named "C1:PEM-SEIS_EX_TEMP", and has the usual CDSfilt channels available. I DQ'ed the output of the filter module (@256 Hz, probably too high, but I'm holding off on a recompile for now). Recompilation and model restart of c1als went smoothly. 
 
  13663   Fri Mar 2 01:45:06 2018 gautamUpdateALSnew look ALS electronics

I spent today making another daughter board (so that we can use the new scheme for I and Q for one arm), testing it (i.e. measuring noise and TF and comparing to LISO model), and arranging all of this inside the 1U demod chassis. To accommodate everything inside, I decided to remove the 2 unused demod units from inside the box. I then drilled a few holes, installed the daughter boards on some standoffs, removed the capacitors and inductors as I outlined yesterday, and routed input and output signals to/from the daughter board. The outputs are routed to a D-sub on the rear panel. More details + better photo + results of testing the combined demod+daughter board signal chain tomorrow...

  13664   Mon Mar 5 10:13:21 2018 gautamUpdateVACvacuum health

In Steve's absence, I've tried to keep an eye on the health of the vacuum system. From Attachment #1, the pressure of the main volume seems stable, no red flags there. I also don't here any anomalously loud sounds near the vacuum pumps. I've changed the N2 cylinders that keep V1 open twice, on Wednesday and Sunday of last week. So in summary, the vacuum system looks fine based on all the metrics I know of.

  13665   Mon Mar 5 11:58:24 2018 gautamUpdateElectronicsThree opamps walked onto an AA board

For testing the new IR ALS noise, we had decided that we would like to use the differential output of the demodulated ALS beat signal, as opposed to a single-ended output, as measurements suggested the former to be a lower noise configuration than the latter. For this purpose, Koji and I acquired a couple of old AA boards from the WB electronics shop. These are however, rev2 of the board, whereas the latest version is v6. The main difference between v2 and v6 is that (i) the THS4131 instrumentation amplifier has the Vocm pin grounded in v6 but is floating in v2 and (ii) the buffer opamps are AD8622 in v6 but are AD8672 in v2. But in fact, the boards we have are stuffed with AD8682

I talked to Rich on Friday, and he seemed to think the AD8672 didn't have any issues noise-wise, the main reason they changed it was because its power consumption was high, and was causing overheating when several of these 1U chassis were packed closely together in an electronics rack. But the AD8682, which is what we have, has comparable power consumption to the AD8622. It is however a JFET opamp, and the voltage noise is a bit higher than the AD8622. 

I am sure there is a way to LISO model a differential output opamp like the THS4131, but I thought I'd simulate the noise in LTSPICE instead. But I couldn't get that to work. So instead, I just measured the transfer function and noise of a single channel, for which Koji had expertly hacked together a custom shorting of the THS4131 Vocm pin to ground. Attachments #1 and #2 show the measurement. All looks good. Note that the phase is 180 at DC because I had hooked up the input signal opposite to what it should have been. The voltage noise of the differential outputs (each measured w.r.t. ground, with both inputs shorted to ground by a short patch cable) at 10 Hz is <100nV/rtHz, and the ADC noise is expected to be ~1uV/rtHz, so I think this is fine.

Conclusion: I think for the ALS test, we can just use the AA board in this config without worrying too much about replacing the buffer stage opamps, even though we've ordered 100pcs of AD8622.


Addendum 7 Mar 2018 11am: As per this document, the output noise of the AA board should be <75nV/rtHz from 10 Hz-50 kHz. So maybe the AD8682 noise is a little high after all. I've gotten the LTSpice model working now, will post the comparison of modelled output noise for various combinations here shortly.

  13666   Mon Mar 5 17:27:34 2018 gautamUpdateALSnew look ALS electronics - characterization

I did a quick test of the noise of the new ALS electronics with the X arm ALS. Attachment #1 shows the results - but something looks off in the measurement, especially the "LO driven, RF terminated" trace. I will have to defer further testing to tomorrow. Of course the real test is to digitize these signals and look at the spectrum of the phase tracker output, but I wanted a voltage noise comparison first. Also, note that I have NOT undone the whitening TFs of (z,p) = (15,150) on these traces. I wonder if these noisy signals (particularly the 10Hz multiple harmonics) are an artefact of measurement, or if something is wonky in the daughter board circuits themselves. I am measuring these with the help of a DB9 breakout board and some pomona minigrabbers. Reagrdless, the sort of ripple seen in the olive green trace for the I channel wasn't present when I did the same test with RF signal generators out on the electronics workbench, so I am inclined to think that this isn't a problem with the circuit. I'm measuring with the SR785 with the "A" input setting, but with the ground set to "Float". I need to look into what the difference is between this mode, and the "A-B" mode. At first glance, both seem to be equivalent differential measurements, but I wonder if there is some subtlety w.r.t. pickup noise.

Perhaps I can repeat the test at the output of the AA board. I looked into whether there is a spare +/- 24V DC power supply available at the LSC rack, to power the 1U AA chassis, but didn't see anything there.

  13667   Wed Mar 7 12:04:14 2018 gautamUpdateElectronicsThree opamps walked onto an AA board

Here are the plots. Comments:

  1. Measurement and model agree quite well yes.
  2. Of the 3 OpAmps, the ones installed seem to be the noisiest (per model)
  3. Despite #2, I don't think it is critical to replace the buffer opamps as we only win by ~10nV/rtHz in the 300-10kHz range.
  4. I don't understand the spec given in T070146. It says the noise everywhere between 10Hz-50kHz should be <75nV/rtHz. But even the model suggests that at 10Hz, the noise is ~250nV/rtHz for any choice of buffer opamp, so that's a factor of 3 difference which seems large. Maybe I made a mistake in the model but the agreement between measurement and model for the AD8682 choice gives me confidence in the simulation. LTSpice files used are in Attachment #3. Could also be an artefact of the way I made the measurement - between an output and ground instead of differentially...

I like LTspice for such modeling - the GUI is nice to have (though I personally think that typing out a nodal file a la LISO is faster), and compared to LISO, I think that the LTspice infrastructure is a bit more versatile in terms of effects that can be modeled. We can also easily download SPICE models for OpAmps from manufacturers and simply add them to the library, rather than manually type out parameters in opamp.lib for LISO. But the version available for Mac is somewhat pared down in terms of the UI, so I had to struggle a bit to find the correct syntax for the various simulation commands. The format of the exported data is also not as amenable to python plotting as LISO output files, but i'm nitpicking...

Quote:

I've gotten the LTSpice model working now, will post the comparison of modelled output noise for various combinations here shortly.

 

  13668   Thu Mar 8 00:40:25 2018 gautamUpdateALSnew look ALS electronics - characterization

I am almost ready for a digital test of the new ALS electronics. Today, Koji and I spent some time tapping new +/-24VDC DIN terminal blocks at the LSC rack to facilitate the installation of the 1U differential receiving AA chassis (separate elog entry). The missing piece of the puzzle now is the timing adapter card. I opted against trying a test tonight as I am having some trouble bringing c1lsc back online.

Incidentally, a repeat of the voltage noise measurement of the X arm ALS beat looked much cleaner today, see Attachment #1 - I don't have a good hypothesis as to why sometimes the signal has several harmonics at 10Hz multiples, and sometimes it looks just as expected. The problem may be more systematically debuggable once the signals are being digitally acquired.

  13669   Thu Mar 8 01:10:22 2018 gautamUpdateGeneralCDS recovery after work at LSC rack

This required multiple hard reboots, but seems like all the RT models are back for now. The only indicator I can't explain is the red DC field on c1oaf. Also, the SUS model seems to be overclocking more frequently than usual, though I can't be sure. The "timing" field of this model's state word is RED, while the other models all seem fine. Not sure what could be going on.

Will debug further tomorrow, when I probably will have to do all this again as I'll need to recompile c1lsc for the ALS electronics test with the new ADC card from the differential AA board.

  13670   Thu Mar 8 14:41:25 2018 gautamUpdateGeneralCDS recovery after work at LSC rack

As I had found before, restarting the c1oaf model fixed the DC error. There is however still a pesky red indicator light on the "ADC0" in c1oaf. Trying to open up the ADC MEDM screen to investigate this further leads to the blank screen on the bottom right of Attachment #1. Probably has something to do with the fact that the model has an ADC block (because every model needs one?) but no signals are actually being piped to the model directly from the ADC.

Another observation, though I don't have any hypothesis as to why this was happening: on the c1sus machine, the c1sus model would frequently overclock, and then eventually, crash. I observed this behaviour at least 3 times between last night and now. The other models seemed fine though, in fact, IMC stayed locked. Why should this have been the case? It remains to be seen if this was somehow connected to the red DC indicator on c1oaf, though why should this be the case? Isn't the DC just concerned with writing data to frames? Any sort of IPC should be independent? Attachment #2 shows that there's been a definite increase in the maximum time on c1sus clock-cycle since yesterday (it's a 10 day minute trend plot of the model clock cycle timing and also the maximum time). Why? Koji and I did switch off all the Sorensens at the LSC rack for about 30mins, but why should this affect anything at 1X6? There are no red lights in either the c1lsc or c1sus expansion chassis. Curiously, the PRM also seems to be glitchy - as I'm sitting in the control room, I see a spot flashing across vertically on the REFL CRT monitor sporadically. Note that nominally, with PRM misaligned, the REFL CRT should be dark. dmesg on c1sus doesn't shed any light on the issue.

Seems like some high level voodoo indecision.


Edit 330pm: The model just crashed again. dmesg rather unhelpfully just says "ADC timeout". Unclear how to debug further. See Attachment #3.

Quote:

This required multiple hard reboots, but seems like all the RT models are back for now. The only indicator I can't explain is the red DC field on c1oaf. Also, the SUS model seems to be overclocking more frequently than usual, though I can't be sure. The "timing" field of this model's state word is RED, while the other models all seem fine. Not sure what could be going on.

Will debug further tomorrow, when I probably will have to do all this again as I'll need to recompile c1lsc for the ALS electronics test with the new ADC card from the differential AA board.

  13671   Thu Mar 8 15:23:16 2018 gautamUpdateElectronicsNew DC power ports at c1lsc

[Koji, Gautam]

Yesterday, we installed some new DIN rail connectors at the LSC rack to provide 3 new outputs each for +24V DC and -24V DC. The main motivation was to facilitate the installation and powering of the differential receiving AA board. The regulators used inside the 1U chassis actually claims a dropout voltage of 0.5V and outputs 14V nominally, so a +/-15V DC supply would've perhaps been sufficient, but we decided to leave a bit more margin, and unfortunately, there are no +/-18V DC KEPCO linear power supplies to the LSC rack. Procedure:

  1. Prepared a bunch of DIN rail connectors with tinned, daisy-chained wires in the office area. Checked continuity and isolation with DMM.
  2. Checked that the two Sorensens at the bottom of the LSC rack were powering the RF distribution box and nothing else at the LSC rack.
  3. Walked over to the little rack housing all the KEPCO DC power supplies that supply DC voltages to the LSC rack. After checking that the labelled voltage and current values were correct, we turned them off, first +/-5V, then +/-15V (2 sets), and finally +/-24V.
  4. Installed the pre-assembled DIN connectors on the side rail at the LSC rack (we had to remove the side panel for the rack to do this work).
    • We used the ports supplying power to the ALS 1U demod chassis (+/-24V DC) to tap these voltages to our newly installed connectors.
    • The interconnecting wires are rather thick gauge, and especially for the ground wire, we found it impossible to push in our tap-off wire into the "correct/hot" side of the DIN blocks. So we had to use the other side instead. I'll upload a picture shortly which will make this more clear.
    • Checked continuity and isolation with DMM.
    • Turned the KEPCOs back on in reverse order to how they were turned off.
    • Measured voltages on the hot side of the DIN blocks, confirmed that they were as expected.
  5. Prepared a 12AWG aLIGO style power cable to connect to the 1U chassis. A reel of this cabling, with yellow shielding, is located ~halfway along under the EW arm. Koji prepared the actual connector and housed it in a DSUB shell as per aLIGO wiring color scheme.
  6. Installed the power cabling to one set of our 3 newly installed +/-24V DC power supplies.
  7. Inserted fuses into the hot DIN blocks, measured voltage at connector end of our newly installed power cable. At first, I forgot to check if the fuse blocks had fuses inside, but after this was rectified, voltages were as expected yes.

The c1lsc frontend models crashed for some reason during this procedure. Now the c1sus frontend model is also behaving weirdly. It is unclear to me if/how this work would have led to these problems, but the temporal correlation (but not causation?) is undeniable.

  13672   Thu Mar 8 18:15:42 2018 gautamUpdateGeneralCDS recovery after work at LSC rack

I was forced into a simultaneous power-cycle rebooting of the three vertex FEs just now. I took the opportunity to completely disconnect the c1sus expansion chassis from all power and then restart it.

Everything is back up right now, and the weird timing issues I noticed in the sus model seem to be gone now (I'll need a longer baseline to be sure and I'll post a trend of the CPU timing tomorrow). It's disconcerting that apparently the only way to get everything back up and running is the nuclear option of power-cycling all FE related electronics. I was considering borrowing an ADC adapter card from the Y end and measuring the calibrated IR ALS noise with the digital system, but if I'm going to have to go through this whole dance each time I do a model recompile on c1lsc (which I'm going to have to in order to get the extra ADC recognized), I'm wondering if it's just better to wait till we get the new adapter cards we ordered. I think I'm going to work on tuning the input coupling into the fiber at EX in the next couple of days instead.

Quote:
 

Seems like some high level voodoo indecision.


Edit 330pm: The model just crashed again. dmesg rather unhelpfully just says "ADC timeout". Unclear how to debug further. See Attachment #3.

 

  13673   Thu Mar 8 19:38:37 2018 gautamUpdateALSdigital unwhitening of daughter board

I made a LISO fit of the measured TF of the daughter board, so that I can digitally invert the daughter board whitening. Results attached. (Inverse) Filters have been uploaded to the ALS X Foton filter banks.

  13674   Thu Mar 8 23:50:27 2018 gautamUpdateALSFirst look at new ALS electronics
  • Locked single arms, dither aligned, and saved offsets to EPICS (slow) sliders in anticipation of having to reboot all vertex FEs.
  • Shutdown ETMY watchdog, stopped all models on c1iscey, and shutdown that frontend.
  • Walked down to Y-end, powered of c1iscey expansion chassis, and removed the ADC adaptor card.
  • Stopped all models on c1lsc. Shutdown watchdog on all optics in anticipation of c1sus model failing. Shutdown the c1lsc frontend.
  • Powered off the c1lsc expansion chassis. Installed the borrowed adapter card from c1iscey in c1lsc expansion chassis. Connected it to the "spare" ADC card Koji and I had installed in c1lsc expansion chassis last Wednesday.
  • Connected differential output of demod board to differential input of AA chassis. Connected SCSI connector from output of AA chassis to the newly installed adapter card.
  • Powered the c1lsc expansion chassis back on. Then powered c1lsc FE on.
  • Walking back out to the control room, I saw that all vertex FEs had crashed. I had to go back in and hard-reboot c1sus.
  • Before bringing back any models, I backed up the existing c1lsc model, and then modified c1x04 and c1lsc to use the newly acquired ALS signals for the X arm ALS signal chain.
  • Restarted all vertex FE models. Everything came back up smooth. DC light is still red on c1oaf but I didn't bother trying to rectify it tonight for these tests.
  • Reset appropriate LSC offsets with PSL shutter closed. Locked X arm on IR. Reset phase tracker servo gain for X arm ALS. Engaged slow temperature servo on EX laser.

Then I looked at  the spectrum, see Attachment #1. Disappointingly, it looks like the arm PDH servo is dominating the noise, and NOT unsuppressed EX laser frequency noise,. Not sure why this is so, and I'm feeling too tired to debug this tonight. But encouragingly, the performance of the new ALS signal chain looks very promising. Once I tune up the X arm loop, I'm confident that the ALS noise will be at least as good as the reference trace.

I am leaving c1iscey shutdown until this is fixed. So ETMY is not available for the moment.

Random factoid: Trying to print a DTT trace with LaTeX in the label text on pianosa causes the DTT window to completely crash - so if you dont save the .xml file, you lose your measurement.

Quote:

I made a LISO fit of the measured TF of the daughter board, so that I can digitally invert the daughter board whitening. Results attached. (Inverse) Filters have been uploaded to the ALS X Foton filter banks.

 

  13675   Fri Mar 9 01:07:01 2018 gautamUpdateALSFirst look at new ALS electronics

[koji, gautam]

I was going to head out but then it occurred to me that I could do another simple test, which is to try and lock the X arm on ALS error signal (i.e. actuate on MC length to keep the beat between EX laser and PSL fixed, while the EX frequency is following the Xarm length). Comparing the in loop (i.e. ALS) error signal with the out-of-loop sensor (i.e. POX), it seems like POX is noisy. The curves were lined up by eye, by scaling the blue curve to match the red at the ~16Hz peaks. This supports my hypothesis in the previous elog. On the downside, could be anything. Electronics in the POX chain? The demod unit itself? Will look into it more tomorrow..

As an aside, controlling the arm with ALS error signal worked quite well, and the lock was maintained for ~1 hour.

  13678   Mon Mar 12 13:58:37 2018 gautamUpdateGeneralprojector light bulb blown

Bulb went out ~10am today. Looks like the lifetime of this bulb was <100 days.

Steve: bulb is arriving next week

Quote:

Bulb  is replaced.

  13679   Mon Mar 12 22:08:31 2018 gautamUpdateALSNoisy POX

[kevin, gautam]

we tested my noisy POX hypothesis tonight. By locking the single arm with POX, the arm length is forced to follow PSL frequency, which is itself slaved to IMC length. From Attachment #1, there is no coherence between the arm control signal and MC_F. This suggests to me that the excess noise I am seeing in the arm control signal above 30 Hz is not originating from the PSL. It also seems unlikely that at >30Hz, anything mechanical is to blame. So I am sticking with the hypothesis that something is wonky with POX. For reference, a known "normal" arm control signal spectrum looks like the red curve in this elog.

 

ELOG V3.1.3-