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  15552   Tue Sep 1 15:39:04 2020 gautamUpdateElectronicsHV coil driver oscillations fixed


Increasing the compensation capacitance (470 pF now instead of 33 pF) seems to have fixed the oscillation issues associated with this circuit. However, the measured noise is in excess of the model at almost any frequency of relevance. I believe the problem is due to the way the measurement is done, and that we should re-do the measurement once the unit is packaged in a shielded environment.


Attachment #1 shows (schematically) the measurement setup. Main differences from the way I did the last round of testing are:

  1. A 20 ohm series resistor was connected between the high voltage output and ground to simulate the OSEM coil.
  2. The test was done under driven conditions (i.e. some non-zero input voltage) to better simulate conditions under which the circuit will be used.
  3. An Acromag XT1541 DAC was used to provide the input signal, to simulate more realistic operating conditions.
  4. A pomona box filter was used to block out the high voltage DC signal which would otherwise destroy the SR785.

Attachment #2 shows the measurement results:

  • Tests were done at a few different drive levels to check if there was any difference.
  • The difference between "Idrive=0mA" and "Input Grounded" traces is that in the former, the Acromag DAC was connected but putting out 0V, wheras in the latter, I shorted the input to the circuit ground.
  • Because the measurement was done at the output of the PA95, the Johnson noise of 25 kohms (~20 nV/rtHz) was manually summed in quadrature to all the measured traces.
  • The plotted spectra were collected in 3 spans, 0-200 Hz, 200Hz-1.8kHz, and 1.8kHz-14.6kHz. The input range was kept constant, so I'm not sure what to make of the discontinuity around 1.8 kHz. Maybe the comb of lines that were being picked up were distorting the spectra for lower frequencies?
  • The "Model" is only for the electronics noise of the circuit. The low-pass filtered noise of the Acromag should be totally negligible above 10 Hz, see here. The fact that there is little difference between the "Idrive=0mA" and "Input Grounded" traces further supports this claim.
  • The diodes in the Pomona box are also unlikely to be the culprit, because with this Pomona box connected to the SR785 and its input terminated with 50ohms, I don't see the comb of spectral lines.

I didn't capture the data, but viewing the high voltage output on an Oscilloscope threw up no red flags - the oscillations which were previously so evident were nowhere to be seen, so I think the capacitor switch did the trick as far as stability is concerned.

There is a large excess between measurement and model out to a few kHz, if this is really what ends up going to the suspension then this circuit is useless. However, I suspect at least part of the problem is due to close proximity to switching power supplies, judging by the comb of ~10 Hz spaced peaks. This is a frequent problem in coil driver noise measurements - previously, the culprit was a switching power supply to the Prologix GPIB box, but now a Linear AC-DC converter is used (besides, disconnecting it had no visible effect). The bench supplies providing power to the board, however, is a switching supply, maybe that is to blame? I think the KEPCO supplies providing +/-250 V are linear. I tried the usual voodoo of twisting the wires used to receive the signal, moving the SR785 away from the circuit board etc, but these measures had no visible effect either. 


The real requirement of this circuit is that the current noise above 100 Hz be <1pA/rtHz. This measurement suggests a level that is 5x too high. But the problem is likely measurement related. I think we can only make a more informed conclusion after shielding the circuit better and conducting the test in a more electromagnetically quiet environment.

Attachment 1: testSetup.pdf
Attachment 2: HVampNoise_driven.pdf
  15571   Tue Sep 15 12:20:36 2020 gautamUpdateElectronicsSR785 repaired

The unit was repaired and returned to the 40m. Now, with a DMM, I measure a DC offset value that is ~1% of the AC signal amplitude. I measured the TF of a simple 1/20 voltage divider and it looks fine. In FFT mode, the high frequency noise floor levels out around 5-7nV/rtHz when the input is terminated in 50 ohms.

I will upload the repair documents to the wiki.


The "source" output of the SR785 has a DC offset of -6.66 V. I couldn't make this up.

Attachment 1: dividerTF.pdf
  15572   Tue Sep 15 17:04:43 2020 gautamUpdateElectronicsDC adaptors delivered

These were delivered to the 40m today and are on Rana's desk


I'll order a couple of these (5 ordered for delivery on Wednesday) in case there's a hot demand for the jack / plug combo that this one has. 

  15613   Mon Oct 5 14:01:41 2020 gautamUpdateElectronicsaLIGO demod boards stuffed and delivered

We received 20pcs of stuffed demodulator boards from Screaming Circuits today. Some caveats:

  1. The AP1053 amplifiers weren't stuffed. Note that this part is no longer in standard production, and lead time for a custom run is ~half a year. I recommend stuffing R2 and using a minicircuits amplifier upstream of this board. We have 6 pcs of AP1053 in hand so we can use those for the first AS WFS, but a second WFS will require some workaround.
  2. AD8306ARZ weren't sent to Screaming Circuits. This part is used for the LO and RF signal level detection/monitoring stage, and so aren't crucial to the demodulation operation. @Chub, did we order the correct part now? They are rather pricey so maybe we can just adapt the footprint using some adaptor board?
  3. DQS-10-100 hybrid 90 degree splitters were delivered to us after the lot was sent to Screaming Circuits. We have the pieces in hand, so we can just stuff them as necessary.

I removed 1 from the group to stuff some components that weren't sent to Screaming Circuits and test the functionality on the benchtop, the remaining have been stored in a plastic box for now as shown in Attachment #1. The box has been delivered to Chub who will stuff the remaining 19 boards once I've tested the one piece.

Attachment 1: IMG_8888.JPG
  15633   Mon Oct 19 15:38:42 2020 KojiUpdateElectronicsLoan: A file binder "40m wiring diagram"

I'll bring a file binder "40m wiring diagram" to home at the next chance.
There is another one on the shelf in the control room.

(I thought I put it in my bag, but it looks like that I left it somewhere around the fax area)

  15636   Thu Oct 22 11:14:47 2020 gautamUpdateElectronicsHV coil driver packaged into 2U chassis

I packaged the HV coil driver into a 2U chassis, hoping for better shielding from pickup. There is still considerable excess noise in measurement vs model around 100 Hz, see Attachment #1. The projected displacement noise from this noise contribution is shown in Attachment #2 - I've also plotted the contribution from the 4.5kohm (planned value for fast path series resistance) for comparison. Attachment #3 has some photos of the measurement setup so if someone sees some red flags, please let me know.

  • The noise was measured with the output load connected to a 20ohm load resistor, to simulate an OSEM.
  • The input signal was driven with an Acromag, to try and mimic the actual operating conditions as closely as possible (although the fast path input was left unconnected).
  • The KEPCO switching HV power supplies were used to power the unit.

I've run out of ideas to try and make the measurement cleaner - the presence of the rather prominent power line harmonics suggests that this is still not perfect, but what more shielding can we implement? I have to make the measurement on the circuit side of the 25 kohm series resistor, so I am using some Pomona minigrabbers to clip onto the leg of the wirewound resistor (see photos in Attachment #3), so that's not great maybe, but what's the alternative?

So if this is truly the noise of the circuit, then while it's an improvement on the current situaiton, it's unsatisfying that such a simple circuit can't match the design expectations. But how do we want to proceed?

Attachment 1: HVampNoise_driven_chassis.pdf
Attachment 2: HVampNoise_dispUnits.pdf
Attachment 3: D1900163_measurementSetup.zip
  15638   Thu Oct 22 13:04:42 2020 ranaUpdateElectronicsHV coil driver packaged into 2U chassis

what is the noise level before the HV stage? i.e. how well is the acromag noise being filtered?

  15639   Thu Oct 22 22:01:53 2020 gautamUpdateElectronicsHV coil driver packaged into 2U chassis

It's not so easy to directly measure this I think, because the filtering is rather aggressive. Attachment #1 shows the measured transfer function (dots) vs the model and Attachment #2 shows the noise. I think this checks out - but I can't definitively rule out some excess noise at 100 Hz from this stage. Because the gain of the HV stage is x31, we'd need a preamp with better than 1nV/rtHz to directly measure the noise I guess. The Acromag noise model in Attachment #2 is based on a measurement I describe here.


what is the noise level before the HV stage? i.e. how well is the acromag noise being filtered?

Attachment 1: DACnoiseFilterGain.pdf
Attachment 2: DACnoiseFilterNoises.pdf
  15640   Fri Oct 23 09:03:43 2020 anchalUpdateElectronicsHV coil driver packaged into 2U chassis

Andrew made a battery-powered 0.7 nVrtHz input-referred noise pre-amplifier for gain of 200. That might help you.


we'd need a preamp with better than 1nV/rtHz to directly measure the noise I guess.

RXA: 0.7 nV is OK if you're not interested in low noise measurements. Otherwise, we have the transformer coupled pre-amp from SRS which does 0.15 nV/rHz and the Rai Weiss FET amp which has 0.35 nV for high impedance sources.

  15674   Thu Nov 12 14:31:27 2020 gautamUpdateElectronicsSR560s in need of repair/battery replacement

I had to go through five SR560s in the lab yesterday evening to find one that had the expected 4 nV/rtHz input noise and worked on battery power. To confirm that the batteries were charged, I left 4 of them plugged in overnight. Today, I confirmed that the little indicator light on the back is in "Maintain" and not "Charge". However, when I unplug the power cord, they immediately turn off.

One of the units has a large DC output offset voltage even when the input is terminated (though it is not present with the input itself set to "GND" rather than DC/AC). Do we want to send this in for repair? Can we replace the batteries ourselves?

Attachment 1: IMG_8947.jpg
  15675   Thu Nov 12 14:55:35 2020 gautamUpdateElectronicsMore systematic noise characterization


I now think the excess noise in this circuit could be coming from the KEPCO switching power supply (in fact, the supplies are linear, and specd for a voltage ripple at the level of <0.002% of the output - this is pretty good I think, hard to find much better).


All component references are w.r.t. the schematic. For this test, I decided to stuff a fresh channel on the board, with new components, just to rule out some funky behavior of the channel I had already stuffed. I decoupled the HV amplifier stage and the Acromag DAC noise filtering stages by leaving R3 open. Then, I shorted the non-inverting input of the PA95 (i.e. TP3) to GND, with a jumper cable. Then I measured the noise at TP5, using the AC coupling pomona box (although in principle, there is no need for this as the DC voltage should be zero, but I opted to use it just in case). The characteristic bump in the spectra at ~100Hz-1kHz was still evident, see the bottom row of Attachment #1. The expected voltage noise in this configuration, according to my SPICE model, is ~10 nV/rtHz, see the analysis note.

As a second test, I decided to measure the voltage noise of the power supply - there isn't a convenient monitor point on the circuit to directly probe the +/- HV supply rails (I didn't want any exposed HV conductors on the PCB) - so I measured the voltage noise at the 3-pin connector supplying power to the 2U chassis (i.e. the circuit itself was disconnected for this measurement, I'm measuring the noise of the supply itself). The output is supposedly differential - so I used the SR785 input "Float" mode, and used the Pomona AC coupling box once again to block the large DC voltage and avoid damage to the SR785. The results are summarized in the top row of Attachment #1.

The shape of the spectra suggests to me that the power supply noise is polluting the output noise - Koji suggested measuring the coherence between the channels, I'll try and do this in a safe way but I'm hesitant to use hacky clips for the High Voltage. The PA95 datasheet says nothing about its PSRR, and seems like the Spice model doesn't include it either. It would seem that a PSRR of <60dB at 100 Hz would explain the excess noise seen in the output. Typically, for other Op-Amps, the PSRR falls off as 1/f. The CMRR (which is distinct from the PSRR) is spec'd at 98 dB at DC, and for other OpAmps, I've seen that the CMRR is typically higher than the PSRR. I'm trying to make a case here that it's not unreasonable if the PA95 has a PSRR <= 60dB @100 Hz.

So what are the possible coupling mechanisms and how can we mitigate it?

  1. Use better power supply - I'm not sure how this spec of 10-50 uV/rtHz from the power supply lines up in the general scheme of things, is this already very good? Or can a linear power supply deliver better performance? Assuming the PSRR at 100 Hz is 60 dB and falls off as 1/f, we'd need a supply that is ~10x quieter at all frequencies if this is indeed the mechanism.
  2. Better grounding? To deliver the bipolar voltage rails, I used two unipolar supplies. The outputs are supposedly floating, so I connected the "-" input of the +300 V supply to the "+" input of the -300 V supply. I think this is the right thing to do, but maybe this is somehow polluting the measurement?
  3. Additional bypass capacitors? I use 0.1 uF, 700V DC ceramic capacitors as bypass capacitors close to the leads of the PA95, as is recommended in the datasheet. Can adding a 10uF capacitor in parallel provide better filtering? I'm not sure if one with compatible footprint and voltage rating is readily available, I'll look around.

What do the analog electronics experts think? I may be completely off the rails and imagining things here.

Update 2130: I measured the coherence between the positive supply rail and the output, under the same conditions (i.e. HV stage isolated, input shorted to ground). See Attachment #2 - the coherence does mirror the "bump" seen in the output voltage noise - but the coherence is. only 0.1,  even with 100 averages, suggesting the coupling is not directly linear - anyways, I think it's worth it to try adding some extra decoupling, I'm sourcing the HV 10uF capacitors now.

Attachment 1: powerSupplyNoise.pdf
Attachment 2: coherence.pdf
  15676   Thu Nov 12 15:40:42 2020 KojiUpdateElectronicsMore systematic noise characterization

Yes. The datasheet has a recommendation circuit with 10uF caps. Companies are careful to show reproducible, reliably functional circuit examples on datasheets. So, if the caps are there you should try to replicate the design.


Additional bypass capacitors? I use 0.1 uF, 700V DC ceramic capacitors as bypass capacitors close to the leads of the PA95, as is recommended in the datasheet. Can adding a 10uF capacitor in parallel provide better filtering? I'm not sure if one with compatible footprint and voltage rating is readily available, I'll look around.

  15677   Mon Nov 16 00:02:34 2020 ranaUpdateElectronicsMore systematic noise characterization

true. also try to choose a cap with a goow high frequency response. In the Electronics Noise book by Ott there's some graph about this. I bet you good do a Bing search and also find something more modern. Basically we want to make sure that the self resonance is not happening at low frequencies. Might be tought to find one with a good HF response, a high voltage rating, and > 1uF.


Yes. The datasheet has a recommendation circuit with 10uF caps. Companies are careful to show reproducible, reliably functional circuit examples on datasheets. So, if the caps are there you should try to replicate the design.


Additional bypass capacitors? I use 0.1 uF, 700V DC ceramic capacitors as bypass capacitors close to the leads of the PA95, as is recommended in the datasheet. Can adding a 10uF capacitor in parallel provide better filtering? I'm not sure if one with compatible footprint and voltage rating is readily available, I'll look around.

  15679   Tue Nov 17 00:26:32 2020 ranaUpdateElectronicsSR560s in need of repair/battery replacement

yes, both problems can be fixed. Usually we just order some spare lead-acid batteries from SRS (Steve may have some spare ones somewhere). The DC offset often comes from a busted FET input. I bought 50 of those at one point - they're obsolete. Its also possible to replace the input stage with any old FET pair.

I'll handle the one with the offset if you leave it on my desk.

  15699   Thu Dec 3 10:46:39 2020 gautamUpdateElectronicsDC power strip requirements

Since we will have several new 1U / 2U aLIGO style electronics chassis installed in the racks, it is desirable to have a more compact power distribution solution than the fusable terminal blocks we use currently. 

  • The power strips come in 2 varieties, 18 V and 24 V. The difference is in the Dsub connector that is used - the 18 V variant has 3 pins / 3 sockets, while the 24V version uses a hybrid of 2 pins / 1 socket (and the mirror on the mating connector).
  • Each strip can accommodate 24 individual chassis. It is unlikely that we will have >24 chassis in any collection of racks, so per area (e.g. EX/EY/IOO/SUS), one each of the 18V and 24V strips should be sufficient. We can even migrate our Acromag chassis to be powered via these strips.
  • Details about the power strip may be found here.

I did a quick walkaround of the lab and the electronics rack today. I estimate that we will need 5 units of the 24 V and 5 units of the 18 V power strips. Each end will need 1 each of 18 V and 24 V strips. The 1Y1/1Y2/1Y3 (LSC/OMC/BHD sus) area will be served by 1 each 18 V and 24 V. The 1X1/1X2 (IOO) area will be served by 1 each 18 V and 24 V. The 1X5/1X6 (SUS Shadow sensor / Coil driver) area will be served by 1 each of 18 V and 24 V.  So I think we should get 7 pcs of each to have 2 spares.

Most of the chassis which will be installed in large numbers (AA, AI, whitening) supports 24V DC input. A few units, like the WFS interface head, OMC driver, OMC QPD interface, require 18V. It is not so clear what the input voltage for the Satellite box and Coil Drivers should be. For the former, an unregulated tap-off of the supply voltage is used to power the LT1021 reference and a transistor that is used to generate the LED drive current for the OSEMs. For the latter, the OPA544 high current opamp used to drive the coil current has its supply rails powered by again, an unregulated tap-off of the supply voltage. Doesn't seem like a great idea to drive any ICs with the unregulated switching supply voltage from a noise point of view, particularly given the recent experience with the HV coil driver testing and the PSRR, but I think it's a bit late in the game to do anything about this. The datasheet specs ~50 dB of PSRR on the negative rail, but we have a couple of decoupling caps close to the IC and this IC is itself in a feedback loop with the low noise AD8671 IC so maybe this won't be much of an issue.

For the purposes of this discussion, I think both Satellite Amp and Coil Driver chassis can be driven with +/- 24 V DC.

On a side note - after the upgrade will the "Satellite Amplifiers" be in the racks, and not close to the flange as they currently are? Or are we gonna have some mini racks next to the chambers? Not sure what the config is at the sites, and if the circuits are designed to drive long cables.

  15700   Thu Dec 3 11:02:35 2020 ranaUpdateElectronicsElectrical LO signal for AS WFS

looks good to me.

The thing I usually look for is how much the downstream system (mixers, etc) can perturb the main oscillator. i.e. we don't want mixer in one chain to reflect back and disturb the EOM chain. But since our demods have amplifiers on the LO side we're pretty immune to that.

  15709   Fri Dec 4 19:23:40 2020 KojiUpdateElectronicsAA/AI board testing ongoing

I have the setup built for the AA/AI board testing around the PD testing area. Please let me leave it like that for a week or so.

12/4 TF Tested 5 PCBs
12/6 TF Tested 19 PCBs (12min/PCB) - found 1 failure (S2001479 CH1) -> Fixed 12/11
12/8 TF Tested 16 PCBs (12min/PCB)
       PSD Tested 4 PCBs (11min/PCB)
12/11 TF Tested 10 PCBs + 1 fixed channel (All channels checked)
       PSD Tested 10 PCBs (11min/PCB)
12/14 PSD Tested 4 PCBs (6.5min/PCB) fixed noise issue of 2 ch, TF issue of 1 ch
12/15 PSD Tested 32 PCBs (6.5min/PCB) fixed noise issue of 1ch
Temp dependence measurement
Crosstalk measurement


  15735   Tue Dec 15 12:38:41 2020 gautamUpdateElectronicsDC power strip

I installed a DC power strip (24 V variant, 12 outlets available) on the NW upright of the 1X1 rack. This is for the AS WFS. Seems to work, all outlets get +/- 24 V DC.

The FSS_RMTEMP channel is very noisy after this work. I'll look into it, but probably some Acromag grounding issue.

In the afternoon, Jordan and I also laid out 4x SMA LMR240 cables and 1x DB15 M/F cable from 1X2 to the NE corner of the AP table via the overhead cable trays.

  15741   Sat Dec 19 20:24:25 2020 gautamUpdateElectronicsWFS hardware install

I installed 4 chassis in the rack 1X2 (characterization on the E-bench was deemed satisfactory, I will upload the analysis later). I ran out of hardware to make power cables so only 2 of them are powered right now (1 32ch AA chassis and 1 WFS head interface). The current limit on the +24V Sorensens was raised to allow for similar margin to the limit with the increased current draw.

Remaining work:

  1. Make 2 more power cables for ISC whitening chassis and quad demod chassis.
  2. Make a 2x 4pin LEMO-->DB9 cable to digitize the FSS and PMC diagnostic channels with the new AA chassis. If RnD cables has a very short turnaround time, might be worth it to give this to them as well.
  3. Connect ADC1 on c1ioo machine to new AA chassis (transfer SCSI cable from existing AA unit to the new one). This will necessarily involve some model changes as well.
  4. Make a short cable to connect 55 MHz output from RFsource box to the LO input on the quad demod chassis.
  5. Install the WFS head on the AS table at a suitable location. Probably will need a focusing lens as well. 
  6. Connect WFS head to the signal processing electronics (the cables were already laid out by Jordan and I).
  7. Make the necessary CDS model changes (WFS filters, matrices, servos etc). I personally don't see the need for a new model but if anyone feels strongly about separating the IMC WFS and AS WFS we can set up another model.
  8. Commission the system.

While I definitely bumped various cables, I don't seem to have done any lasting damage to the CDS system (the RFM errors remain of course).

  15773   Wed Jan 20 10:13:06 2021 gautamUpdateElectronicsHV Power supply bypassing


Installing 10uF bypass capacitors on the High Voltage power supply line for the HV coil driver circuit doesn't improve the noise. The excess bump around a few hundred Hz is still present. How do we want to proceed? 


  • The setup is schematically shown in Attachment #1.
  • Physically, the capacitors were packaged into a box, see Attachment #2.
  • This box is installed between the HVPS and the 2U chassis in which the circuit is housed, see Attachment #3.
  • I measured the noise, (using the same setup as shown here to avoid exposing the SR785 input to any high voltage), for a variety of drive currents. To make a direct comparison, I took two sets of measurements today, one with the decoupling box installed and one without.
  • The results are shown in Attachment #4. You can see there is barely any difference between the two cases. I've also plotted the expected noise per a model, and the measured Johnson noise of one of the 25kohm resistors being used (Ohmite, wirewound). I just stuck the two legs of the resistor into the SR785 and measured the differential voltage noise. There is a slight excess in the measured Johnson noise compared to what we would expect from the Fluctuation Dissipation theorem, not sure if this is something to be worried about or if it's just some measurement artefact.


So what do we do about this circuit? For the production version, I can make room on the PCB to install two 10uF film capacitors on the board itself, though that's unlikely to help. I think we've established that 

  1. The excess noise is not due to the Acromag or the input Acromag noise filtering stage of the circuit, since the excess is present even when the input to the HV stage is isolated and shorted to ground.
  2. There was some evidence of coherence between the supply rails and the output of the HV stage (with input isolated and shorted to ground). The coherence had the "right shape" to explain the excess noise, but the maximum value was only ~0.1 (could have been because I was not measuring directly at the PA95's supply rail pins due to space constraints).
  3. The impedance of 10uF at 100Hz is ~150 ohms. idk what the impedance of the supply pins of the PA95 are at this frequency (this will determine the coupling of ripples in the HVPS output to the PA95 itself.

Do we have any better bipolar HV supply that I can use to see if that makes any difference? I don't want to use the WFS supplies as it's not very convenient for testing.

Not really related directly to this work but since we have been talking about current requirements, I attach the output of the current determining script as Attachment #5. For the most part, having 220ohm resistances on the new HAM-A coil driver boards will lead to ~half the DAC range being eaten up for the slow alignment bias. For things like MC1/MC3, this is fine. But for PRM/SRM/BS, we may need to use 100ohms. Chub has ordered all manner of resistances so we should have plenty of choices to pick from.

Attachment 1: bypassCaps.pdf
Attachment 2: IMG_9079.jpg
Attachment 3: IMG_9078.jpg
Attachment 4: HVampNoise_driven_chassis.pdf
Attachment 5: printCoilCurrents.pdf
  15786   Mon Feb 1 12:30:21 2021 gautamUpdateElectronicsMore careful characterization


  1. Swapping out the KEPCO HV supplies (linear) I was using for a pair of HP6209s I borrowed from Rich has improved the noise performance somewhat.
  2. However, there is still an excess relative to the model. I confirmed that this excess originates from the PA95 part of the circuit (see details).
  3. The bypass capacitors don't seem to have any effect on the measured ripple from these HP6209s. Maybe they're internally fitted with some 10uF or similar bypass caps?
  4. The production version of this board, with several improvements (after discussions with Koji and Rich), are on the DCC. They're being fabbed right now and will arrive in ~1 week for more bench testing. 

Power supply bypassing [updated 10pm]:

As mentioned earlier in this thread, I prepared a box with two 10uF, 1kV rated capacitors to bypass the high-voltage rails (see inset in the plot), to see if that improves the performance. However, in measuring the voltage ripple directly with the SR785 (no load connected), I don't see any significant difference whether the decoupling caps are connected or not, see Attachment #1. For this, and all other HV measurements made, I used this box to protect the SR785. One hypothesis is that this box itself is somehow introducting the excess noise, maybe because of leakage currents of the diode pair going into the 1Mohm SR785 input impedance, but I can't find any spec for this, and anyway, these diodes should be at ground potential once the transient has settled and the DC blocking capacitor has charged to its final value.

Note that the 10uF caps have an ESR of 7.2 mOhms. The HP6209 has a source impedance "<20mOhm" when operated as a CV source, per the datasheet. So perhaps this isn't so surprising? The same datasheet suggests the source impedance is 500 mOhms from 1kHz to 100 kHz, so we should see some improvement there, but I only measured out to 2 kHz, and I didn't take much effort to reduce these crazy peaks so maybe they are polluting the measurement out there. There must also be some continuous change of impedance, it cannot be <20 mOhm until 1 kHz and then suddenly increase to 500 mOhms. Anyways, for this particular circuit, the nosie DC-1kHz is what is important so I don't see a need to beat this horse more. 

Simplified circuit testing:

I decided to see if I can recover the spec'd voltage noise curve from the PA95 datasheet. For this, I configured the PA95 as a simple G=31 non-inverting amplifier (by not stuffing the 15 uF capacitor in the feedback path). Then, with the input grounded, I measured the output voltage noise on the circuit side of the 25kohm resistor (see inset in Attachment #2). To be consistent, I used the DC blocking box for this measurement as well, even though the output of the PA95 under these test conditions is 0V. Once again, there is considerable excess around ~100 Hz relative to a SPICE model. On the basis of this test, I think it is fair to say that the problem is with the PA95 itself. As far as I can tell, I am doing everything by the book, in terms of having gain > 10, using a sufficiently large compensaiton cap, HV rail decoupling etc etc. Note that the PA95 is a FET input opamp, so the effects of input current noise should be negligible. The datasheet doesn't provide the frequency dependence, but if this is just shot noise of the 1200 pA input bias current (for 300 V rails, per the spec), this is totally negligible, as confirmed by LTspice.

In the spirit of going step-by-step, I then added the feedback capacitor, and still, measured noise in excess of what I would expect from my model + SR785 measurement noise.

Integrated circuit testing:

After the above simplified test, I stuffed a full channel as designed, and tested the noise for various drive currents. To best simulate the operating conditions, an Acromag XT1541 was used to set the DC voltage that determines the drive current through the 25 kohm resistor. The measurements were made on the circuit side of this resistor (I connected a 20ohm resistor to ground to simulate the OSEM). As shown in Attachment #3, the noise with these HP6209 supplies is significantly better than what I saw with the KEPCO supplies, lending further credence to the hypothesis that insufficient PSRR is the root of the problem here. I've added subplots in a few different units - to be honest, I think that reaching this level of measured displacement noise at the 40m at 100 Hz would already be pretty impressive.

So what's next?

The main design change is that a passive R-C-R (4k-3uF-20k) replaces the single 25kohm resistor at the output of the PA95. 

  • This allows similar current drive range.
  • But adds an LPF to filter out the observerd excess noise at 100 Hz. 

Let's see if this fixes the issue. Not that I've also added a pair of input protection diodes to the input of the PA95 in the new design. The idea is that this would protect the (expensive) PA95 IC from, for example, the unit being powered with the +/- 18V rail but not the +/- 300 V rail. As I type this, however, I wonder if the leakage current noise of these diodes would be a problem. Once again, the datasheet doesn't provide any frequency dependence, but if it's just the shot noise of the 1nA expected when the diodes are not reverse biased (which is the case when the PA95 is operating normally since both inputs are at nearly the same potential), the level is ~20 fA/rtHz, comparable to the input current noise of the PA95, so not expected to be an issue. In the worst case, the PCB layout allows for this component to just be omitted. 

Attachment 1: HVPS.pdf
Attachment 2: HV_testckt.pdf
Attachment 3: totalNoise.pdf
  15798   Wed Feb 10 14:14:58 2021 gautamUpdateElectronicsCustom cables received

We received the custom cables to test the new suspension electronics. They are under my desk. So we are ready.

This batch was a small one - the company says that they can make molded cables if we have a minimum order, something to consider I gues.s.

Update 1900 11 Feb: I verified that the pin outs of the cables are as we intended (for one set of each type of cable). Because this was a small order, the connectors have metal shells, and so for cable #2 (sat box to flange), the two shells are shorted to each other. I can't verify if the shield is isolated from the shell on J5 without cutting open the cable. One thing that occurred to me is that we should give pins 5,8,11 on J4 and 16,20,24 on J5 (respectively) unique identifiers. They should only be shorted to GND on the circuit board itself. To be fixed for the next iteration. I uploaded some photos here.

I was unable to measure the capacitance of the cable using the LCR meter, and didn't opt to try any other method.

Attachment 1: satWiring.pdf
  15802   Wed Feb 10 21:14:03 2021 gautamUpdateElectronicsProduction version of the HV coil driver tested


I did what I consider to be a comprehensive set of tests on the production version of the high voltage coil driver circuit. I think the performance is now satisfactory and the circuit is ready for the production build. Barring objections from anyone, I will ask Chub to place an order for components to stuff the 4 necessary units + 1 spare on Friday, 12 Feb (so that people have a full day to comment). A big thanks to Chub and the folks at JLCPCB for dealing with my incessant order requests and patiently supporting this build and letting me turn this around in 10 days - hopefully this is the end of this particular saga.

Schematic is here. All references to component designations are for v4 of the schematic.

Important design changes:

  1. All I/O to this board will be via D9 connectors. This will allow bypassing the high voltage stage in future suspensions while retaining the same cable config in the suspension drive, if that is desired. Some re-arrangement of the grouping of coils was also done for consistency with the planned upgrade.
  2. Differential receiving for the input signal from the Acromag. The nominal quad opamp is LT1125 but if we find noise issues (which I didn't), the OP497 has compatible PCB footprint.
  3. Added input protection dual diode D6 to protect the PA95 as recommended in the datasheet. This should protect the IC if (for example) the HV line isn't plugged in but the Acromag input is non-zero.
  4. Increased the feedback resistance from 30kohms to 12kohms. R16 through R21 are now 20k, while the old design had 5k. The purpose is to reduce the current demand in the feedback path, hopefully this opens up the number of DCPS we can use. To keep the overall gain of 31, the resistor R15 was upped from 1kohms to 4kohms.
  5. Feedback capacitance reduced from 15 uF to 3 uF. This was largely for space saving / ease of layout on the PCB. The resulting corner frequency is increased slightly from 0.35 Hz to 0.45 Hz but this doesn't have any imapct on the performance of the circuit at frequencies of interest (1/2/pi/R/C had R=30k, C=15uF, now R=120k, C=3uF).
  6. Added an R-C-R network at the output of the PA95, before the fast actuation signal is summed and sent to the OSEM coil.
    • This is probably the most important change, noise-performance wise.
    • The purpose of the network is to passively filter out the excess noise we saw at ~100 Hz (the corner from the 4kohm resistor + 3uF cap is at ~13 Hz, so factor of 10 filtering at 100 Hz, which was deemed sufficient, see earlier elogs in the thread). 
    • The Johnson noise contribution of the 20 kohm resistor is slightly higher than the original design which had a 25 kohm series resistor (but no R-C-R passive filter at the output of the PA95). But once again, this was deemed to have negligible effect on the performance at frequencies of interest to us.
    • The total current driving capability of the circuit is almost unchanged since the 20kohm + 4kohm nearly equals the old 25kohm resistance.
  7. Made the Vmon paths for monitoring the HV output of the PA95 differential sending, seems like a good practise to follow for all new designs.
  8. Added on-board bypass capacitors (2 x 10uF WIMA film caps) for cleaning up the HV supply noise.


A series of tests were done. Note that only 1 channel was stuffed (I am out of PA95s), and the HP power supplies borrowed from Rich were used for the HV rails. For the +/-18V, a regular bench-top unit was used.

  1. Low voltage stage tests
    • TF of the differential receiving stage was measured and the overall unity gain and corner at 24kHz were verified, see Attachment #1.
    • With the input of the circuit grounded, I measured the noise of the circuit at various points (see legends on Attachment #2). I didn't bother to do a detailed verification against a SPICE model as the levels seemed roughly what is expected.
  2. Overall noise performance with HV stage enabled
    • For a range of drive currents, generated by applying the appropriate voltage using an Acromag XT1541 DAC module to the J1 connector, I measured the voltage on the circuit side of the 20 kohm resistor (by clipping onto the resistor leg. Note that the path to ground for the current was provided by connecting a 20 ohm resistor between pins 1 and 6 on J3a, and then grounding pin 6.
    • Results are shown in Attachment #3
    • For the drive currents at the edge of the range of operation, there is a small excess relative to lower drive currents. My best hypothesis for why this is happening is that the HV rail is too close to the requested output voltage (the HP units are rated for 320V, which is borderline if we want 300V at the output of the PA95). In any case, the R-C-R passive filter means that above ~60 Hz, there is excellent agreement between model and measurement.
  3. Time domain tests
    • Used a function generator. to drive a 50 mHz, 3Vpp sine wave to the "Bias Input" (=J1), and monitored (i) pickoff of drive signal, (ii) High voltage output at the circuit side of the 20kohm resistor, and (iii) the Vmon output (=pins 1/6 on J4), all on a 100 MHz Tektronix scope.
    • Results shown in Attachment #4. Once again, I see no red flags.
    • While I had the unit hooked up to the scope, I also checked the time domain signal with the scope set to 100 ns/div time base. I saw no evidence of any oscillatory features, either when no input signal was applied, or when a DC signal was provided (in which case the scope was set to AC coupling). 👍 
  4. Checked that the protection diodes at various locations in the circuit work.
  5. Checked the pin-mapping on all 6 D9 connectors is consistent with the top level diagram in the schematic.

PCB remarks:

As I was stuffing the board, I noticed a few improvements that can be made. Just noting these here for documentation purposes - these changes are mostly aesthetic and I personally see no need to order another set of PCBs.

  1. In some places, the silkscreen designators don't have the correct "orientation" relative to the component they are designating. I didn't find any serious ambiguity in terms of being misled to stuff the wrong component onto the wrong pads, but in the spirit of doing a professional job...
  2. The current limiting resistors on the +/-430V LEDs (R37/R38) have footprints for leaded components rather than SMT (which is what the +/-15V LEDs have).
  3. R45 and R46, the current limiting resistors for the rear panel power indicator LEDs, have 0805 footprint rather than 1206.
  4. When I drew up the PCB, R23, the 4kohm resistor in the R-C-R network, was set up as a 1W resistor. Let's say there can be 15 mA flowing through this resistor - the power dissipated is 15e-3 ^2 * 4e3 is 0.9W, which is uncomfortably close to the limit. For all the tests above, I used a 3W resistor, and didn't find any serious noise issues. The drilled holes are a little tight for this higher power rated resistor, but I don't think this is a showstopper.

Communications with Apex:

I've been talking to support at Apex, and pointed out that I couldn't match the SPICE model performance even for a simple non-inverting amplifier with the PA95. The feedback I got from them was that 

  1. They don't optimize the SPICE models for input noise and so it was a nice coincidence that model and measurement are somewhat close (but not exactly).
  2. They recommend the PA194, which is actually advertised as "low-noise". The PA95 is apparently not a "low-noise" part, with its 2uVrms input noise. 

Whiel the PA194 is compatible with our voltage and current requirements for this application, it is ~3x the cost, and seems like the R-C-R output filter allows us to realize the goal of 1pA/rtHz, so I'm inclined to stick with the PA95.

Production assembly:

I'd prefer to get as much of the board stuffed by Screaming Circuits as possible. It took me ~3 hours to stuff 1 channel + the power supply parts, standoffs etc. So I estimate it'll take me ~6 hours to stuff the entire board. So not the end of the world if we have to do it in-house.

Attachment 1: inputDiffRecTF.pdf
Attachment 2: LVnoises.pdf
Attachment 3: totalNoise.pdf
Attachment 4: timeDomainTests.pdf
  15815   Thu Feb 18 03:20:09 2021 KojiSummaryElectronicsCurrent Rack Map

For your planning:

Attachment 1: rack_plan.pdf
  15820   Thu Feb 18 20:24:48 2021 KojiSummaryElectronicsA bunch of electronics received

Todd provided us a bunch of electronics. I went to Downs to pick them up this afternoon and checked the contents in the box. Basically, the boxes are pretty comprehensive to produce the following chassis

  • 8 HAM-A coil driver chassis
  • 7 16bit Anti-Aliasing chassis
  • 4 18bit Anti-Imaging chassis
  • 5 16bit Anti-Imaging chassis

Some panels are missing (we cannibalized them for the WFS electronics). Otherwise, it seems that we will be able to assemble these chassis listed.
They have placed inside the lab as seen in the attached photo.

HAM-A COIL DRIVER (Req Qty 28+8)

- 8 Chassis
- 8 Front Panels
- 8 Rear Panels
- 8 HAM-A Driver PCBs
- 8 D1000217 DC Power board
- 8 D1000217 DC Power board

16bit AA (Req Qty 7)
- 6 7 Front Panels (1 missing -> [Ed 2/22/2021] Asked Chub to order -> Received on 3/5/2021)
- 7 Rear Panels
- 28 AA/AI board S2001472-486, 499-511
- 7 D070100 ADC AA I/F
- 7 D1000217 DC Power board

18bit AI (Req Qty 4)
- 4 Front Panels
- 4 Rear Panels
- 8 AA/AI board S2001463-67, 90-92
- 4 D1000551 18bit DAC AI I/F
- 4 D1000217 DC Power board
- bunch of excess components

16bit AI (Req Qty 5)
- 4 5 Front Panels (D1101522) (1 missing -> [Ed 2/22/2021] Asked Chub to order -> Received on 3/5/2021)
- 3 5 Rear Panels (D0902784) (2 missing -> [Ed 2/22/2021] Asked Chub to order -> Received on 3/5/2021)
- 10 AA/AI board S2001468-71, 93-98
- 5 D1000217 DC Power board
- 5 D070101 DAC AI I/F

Internal Wiring Kit

[Ed 2/22/2021]
Asked Chub to order:
- Qty 12 1U Hamilton Chassis
- Qty 5 x Front/Rear Panels/Internal PCBs for D1002593 BIO I/F (The parts and connectors to be ordered separately)

  -> Front/Rear Panels received (3/5/2021)
  -> PCBs (unpopulated) received (3/5/2021)
  -> Components ordered by KA (3/7/2021)

Attachment 1: IMG_0416.jpeg
  15828   Sat Feb 20 10:01:48 2021 gautamSummaryElectronicsA bunch of electronics received

Will we also be receiving the additional 34 Satellite Amplifier PCBs?

  15830   Sat Feb 20 16:46:17 2021 KojiSummaryElectronicsA bunch of electronics received

We received currently available sets. We are supposed to receive more coil drivers and sat amps, etc. But they are not ready yet.


  15846   Fri Feb 26 16:31:02 2021 gautamUpdateElectronicsProduction version of the HV coil driver tested with KEPCO HV supplies

Koji asked me to test the production version of the coil driver with the KEPCO HV supplies. See Attachment #1 for the results. For comparison, I've added a single trace from the measurements made with the HP supplies. I continue to see excess noise with the KEPCO supplies. Note that in the production version of the board that was tested, there are a pair of 10uF bypass capacitors on the board for the HV supply lines. It is possible that one or both KEPCO supplies are damaged - one was from the ASY setup and one I found in the little rack next to 1X2. The test conditions were identical to that with the HP supplies (as best as I could make it so).

Attachment 1: totalNoise_KEPCO.pdf
  15847   Fri Feb 26 20:20:43 2021 KojiUpdateElectronicsProduction version of the HV coil driver tested with KEPCO HV supplies

This is very disappointing. Even with KEPCO linear supply with the improved HV driver circuit, the noise level is significantly higher than the 20kOhm R thermal noise.

What is special with the HP supplies? Can you replace KEPCOs with the HP supply, one by one to specify which one is making the noise bad?

  15848   Sat Feb 27 17:25:42 2021 gautamUpdateElectronicsProduction version of the HV coil driver tested with KEPCO HV supplies

I will try the test of switching out KEPCOs one at a time for the HP. Given that the passive RC filter doesn't filter out the excess, I am wondering if the KEPCO is somehow polluting the circuit ground? The measurement was made between the circuit side of R24 (see schematic) and a ground testpoint, so the passive R23/C15 pole should filter the noise above ~15 Hz.


This is very disappointing. Even with KEPCO linear supply with the improved HV driver circuit, the noise level is significantly higher than the 20kOhm R thermal noise.

What is special with the HP supplies? Can you replace KEPCOs with the HP supply, one by one to specify which one is making the noise bad?

  15865   Thu Mar 4 23:57:35 2021 KojiSummaryElectronicsInspection of the new custom dsub cables

I made the inspection of the new custom DSub cables (came from Texas).

The shelled version gives us some chance to inspect/modify the internal connections. (good)
The wires are well insulated. The conductors are wrapped with the foils and then everything is in the braid tube shield. The braid is soldered on one of the connectors. (Attachment  3/4 shows the soldering of the conductor by intentionally removing one of the insulations).

It wasn't clear that if the conductors are twisted or not (probably not).

Attachment 1: 20210304235251_IMG_0527.jpg
Attachment 2: 20210304235302_IMG_0528.jpg
Attachment 3: 20210304235339_IMG_0529.jpg
Attachment 4: 20210305000050_IMG_0530.jpg
Attachment 5: 20210305000610_IMG_0531.jpg
Attachment 6: 20210305000615_IMG_0532.jpg
  15866   Fri Mar 5 00:53:09 2021 KojiSummaryElectronicsA bunch of electronics received

Received additional front/rear panels. Updated the original entry and Wiki [Link]


  15868   Fri Mar 5 15:03:28 2021 gautamSummaryElectronicsA bunch of electronics received

The PCBs for the D1002593 BIO I/F (5pcs ea of D1001050 and D1001266) were received (from JLCPCB) today. idk what the status of the parts (digikey?) is.


Received additional front/rear panels. Updated the original entry and Wiki [Link]

  15870   Fri Mar 5 15:32:53 2021 KojiSummaryElectronicsA bunch of electronics received

The parts will be ordered by Koji The components for the additional BIO I/F have been ordered.

  15885   Tue Mar 9 12:41:29 2021 KojiSummaryElectronicsInvestigation on the invacuum Dsub cables

I believe the aLIGO style invac dsub cables and the conventional 40m ones are incompatible.
While the aLIGO spec is that Pin1 (in-vac) is connected to the shield, Pin13 (in-vac) is the one for the conventional cable. I still have to check if Pin13 is really connected to the shield, but we had trouble before for the IO TTs https://nodus.ligo.caltech.edu:8081/40m/7864.
(At least one of the existing end cables did not show this Pin13-chamber connection. However, the cables OMC/IMC chambers indicated this feature. So the cables are already inhomogenious.)

- Which way do we want to go? Our electronics are updated with aLIGO spec (New Sat amp, OMC electronics, etc), so I think we should start making the shift to the aLIGO spec.

- Attachment Top: The new coil drivers can be used together with the old cables using a custom DB25 cable (in-air).

- Attachment Mid: The combination of the conventional OSEM wiring and the aLIGO in-vac cable cause the conflict. The pin1 which is connected to the shield is used for the PD bias.

- Attachment Bottom: This can be solved by shifting the OSEMs by one pin.

o The aLIGO cables have 12 twisted pair wires, but paired signals do not share a twisted pair.
   --- No. This can't be solved by rotating the connectors.
o This modification should be done only for the new suspension.
   --- In principle, we can apply this change to any SOSs. However, this action involves the vent. We probably want to install the new electronics for the existing suspensions before the vent.
o ^- This means that we have to have two types of custom DB25 in-air cables.
   --- Each cable should handle "Shield wire" from the sat amp correctly.

Related Links:

Active TT Pin Issue
and the thread

Hacky solution

Active TT Pin Swapping (December 21, 2012)

TT Wiring Diagram (Wiki)

Attachment 1: SOS_OSEM_cabling.pdf
  15933   Wed Mar 17 15:04:20 2021 gautamUpdateElectronicsRibbon cable for chassis

I had asked Chub to order 100ft ea of 9, 15 and 25 conductor ribbon cable. These arrived today and are stored in the VEA alongside the rest of the electronics/chassis awaiting assembly.

Attachment 1: IMG_9139.jpg
  15941   Thu Mar 18 18:06:36 2021 gautamUpdateElectronicsModified Sat Amp and Coil Driver

I uploaded the annotated schematics (to be more convenient than the noise analysis notes linked from the DCC page) for the HAM-A coil driver and Satellite Amplifier.

  15950   Sun Mar 21 19:31:29 2021 ranaSummaryElectronicsRTL-SDR for monitoring RF noise / interference

When we're debugging our RF system, either due to weird demod phases, or low SNR, or non-stationary noise in the PDH signals, its good to have some baseline measurements of the RF levels in the lab.

I got this cheap USB dongle (RTL-SDR.COM) that seems to be capable of this and also has a bunch of open source code on GitHub to support it. It also comes mith an SMA coax and rabbit ear antenna with a flexi-tripod.

I used CubicSDR, which has free .dmg downloads for MacOS.It would be cool to have a student write some python code (perhaps starting with RTL_Power) for this to let us hop between the diffierent RF frequencies we care about and monitor the power in a small band around them.

  15968   Thu Mar 25 18:05:04 2021 gautamUpdateElectronicsStuffed HV coil drivers received from Screaming Circuits

I think the only part missing for assembly now are 4 2U chassis. The PA95s need to be soldered on as well (they didn't arrive in time to send to SC). The stuffed boards are stored under my desk. I inspected one board, looks fine, but of course we will need to run some actual bench tests to be sure.

  15980   Wed Mar 31 00:40:32 2021 KojiUpdateElectronicsElectronics Packaging for assembly work

I've worked on packing the components for the following chassis
- 5 16bit AI chassis
- 4 18bit AI chassis
- 7 16bit AA chassis
- 8 HAM-A coil driver chassis
They are "almost" ready for shipment. Almost means some small parts are missing. We can ship the boxes to the company while we wait for these small parts.

  • DB9 Female Ribbon Receptacle AFL09B-ND Qty100 (We have 10) -> Received 90 on Apr 1st
  • DB9 Male Ribbon Receptacle CMM09-G Qty100 (We have 10) -> Received 88 on Apr 1st
  • 4-40 Pan Flat Head Screw (round head, Phillips) 1/2" long Qty 50 -> Found 4-40 3/8" Qty50 @WB EE on Apr 1st (Digikey H782-ND)
  • Keystone Chassis Handle 9106 36-9106-ND Qty 50 -> Received 110 on Apr 1st
  • Keystone Chassis Ferrule 9121 NKL PL 36-9121-ND Qty 100 -> Received 55 on Apr 1st
  • Chassis Screws 4-40 3/16" Qty 1100 -> Received 1100 on Apr 1st
  • Chassis Ear Screws 6-32 1/2" 91099A220 Qty 150 -> Received 400 of 3/8" on Apr 1st
  • Chassis Handle Screws 6-32 1/4" 91099A205 Qty 100 -> included in the above
  • Powerboard mounting screw 4-40 Pan Flat Head Screw (round head, Phillips) 1/4" long Qty 125 -> Received 100 on Apr 1st

And some more additional items to fill the emptying stock.

  • 18AWG wires (we have orange/blue/black 1000ft, I'm sending ~1000ft black/green/white)
  • Already consumed 80% of 100ft 9pin ribbon cable (=only 20ft left in the stock)
Attachment 1: P_20210330_233508.jpg
Attachment 2: P_20210330_233618.jpg
  15981   Wed Mar 31 03:56:37 2021 KojiSummaryElectronicsA bunch of electronics received

We have received 9x 18bit DAC adapter boards (D1000654)

Attachment 1: P_20210331_013257.jpg
Attachment 2: P_20210331_014020.jpg
  15986   Thu Apr 1 18:16:28 2021 KojiUpdateElectronicsElectronics Packaging for assembly work

All small components are packed in the boxes. They are ready to ship.


  15990   Fri Apr 2 01:26:41 2021 gautamUpdateElectronicsREFL55 chain checkout again, seems fine

[koji, gautam]


We could not find problems with any individual piece of the REFL55 electronics chain, from photodiode to ADC.  Nevertheless, the PRMI fringes witnessed by REFL55 is ~x10 higher than ~two weeks ago, when the PRMI could be repeatably and reliably locked using REFL55 signals (ETMs misaligned).


  1. Koji prepared a spare whitening board. However, before he swapped it in, he checked the existing board and found no problems with it.
    • 20mV input signal, 100 Hz was injected into the two REFL55 channels on the whitening board.
    • The flat whitening gain was set to +45 dB.
    • The signal levels seen in CDS was consistent with what is expected given an ADC conversion factor of 3276.8 cts/V.
  2. Tried putting the REFL55 demodulated outputs into the next two channels, 5&6, (currently unused) on the same whitening board.
    • After setting the whitening gains of these two channels also to +18dB, the saturation of the ADCs when the PRMI was fringing persisted.
  3. With the dark noise of the whitening filter, we enabled/disabled the on board frequency dependent whitening, and reasoned that the time domain increase in RMS seemed reasonable. So we decided to investigate parts of the electronics chain upstream of the whitening board, since we couldn't find anything obviously wrong with the whitening board.
  4. Injected -10dBm RF signal (=0.2 Vpp) into the RF input on the REFL55 demod board, and saw ~3500 cts-pp signal in CDS. This is totally consistent with my recent characterization of 16,000 cts/V for this demod board at the "nominal" + 18dB whitening gain setting. So the demodulator seems to function as advertised.
  5. Decided to repeat my test of using the Jenne laser to test the whole chain end-to-end.
    • In summary, we recovered the results (RF transimpedance of the PD, and signal levels in CDS for a known AM determined by the reference NF1611 PD) I reported there.
    • So it would seem that the entire REFL55 electronics chain performs as expected.
    • The only remaining explanation is that the optical gain of the PRMI has increased - but how?? 
    • Similar jumps in the REFL55 signal levels have occurred multiple times in the past, and each time, I was able to recover the "nominal" performance by this procedure (though I have no idea why that should work at all).
    • So I am highly skeptical that this has anything to do with the IFO optical gain, but that is the only difference between our AM laser based test and the "live" operating conditions when the signals are saturated.

Discussion and next steps:

Q: Koji asked me what is the problem with this apparent increased optical gain - can't we just compensate by decreasing the whitening gain?
A: I am unable to transition control of the PRMI (no ETMs) from 3f to 1f, even after reducing the whitening gain on the REFL55 channels to prevent the saturation. So I think we need to get to the bottom of whatever the problem is here.

Q: Why do we need to transfer the control of the vertex to the 1f signals at all?
A: I haven't got a plot in the elog, but from when I had the PRFPMI locked last year, the DARM noise between 100-1kHz had high coherence with the MICH control signal. I tried some feedforward to try and cancel it but never got anywhere. It isn't a quantitative statement but the 1f signals are expected to be cleaner?

Koji pointed out that the MICH signal is visible in the REFL55 channels even when the PRM is misaligned, so I'm gonna look back at the trend data to see if I can identify when this apparent increase in the signal levels occurred and if I can identify some event in the lab that caused it. We also discussed using the ratio of MICH signals in REFL and AS to better estimate the losses in the REFL path - the Faraday losses in particular are a total unknown, but in the AS path, there is less uncertainty since we know the SRM transmission quite precisely, and I guess the 6 output steering mirrors can be assumed to be R=99%. 

  16033   Wed Apr 14 23:55:34 2021 gautamUpdateElectronicsHV Coil driver assembly

I've occcupied the southernmost electronics bench for assembling the 4 production version HV coil driver chassis. I estimate it will take me 3 days, and have left a sign indicating as much. Once the chassis assembly is done, I will need to occupy the northernmost bench where bench supplies are to run some functionality tests / noise measurements, and so unless there are objections, I will move the Acromag box which has been sitting there.

  16070   Thu Apr 22 01:42:38 2021 KojiSummaryElectronicsHV Supply Comparison

New HV power supply from Company 'M' has been delivered. So I decided to compare the noise levels of some HV supplies in the lab. There are three models from companies 'H', 'K', and 'M'.

The noise level was measured with SR785 via Gautam's HP filter with protection diodes.

'H' is a fully analog HV supply and the indicator is analog meters.
'K' is a model with a LCD digital display and numerical keypad.
'M' is a model with a knob and digital displays.

All the models showed that the noise levels increased with increased output voltage.

Among these three, H showed the lowest noise. (<~1uV/rtHz@10Hz and <50nV/rtHz@100Hz) (Attachment 1)

K is quite noisy all over the measured freq range and the level was <50uV/rtHz. Also the PSD has lots of 5Hz harmonics. (Attachment 2)

M has a modest noise level (<~30uV/rtHz@10Hz and <1uV/rtHz@100Hz)except for the noticeable line noise (ripple). (Attachment 3)

The comparison of the three models at 300V is Attachment 4. The other day Gautam and I checked the power spectrum of the HV coil driver with KEPCO and the output noise level of the coil driver was acceptable. So I expect that we will be able to use the HV supply from Company M. Next step is to check the HV driver noise with the model by M used as the supply.

Attachment 1: HV_Supply_PSD_H.pdf
Attachment 2: HV_Supply_PSD_K.pdf
Attachment 3: HV_Supply_PSD_M.pdf
Attachment 4: HV_Supply_PSD.pdf
  16140   Fri May 14 03:29:50 2021 KojiUpdateElectronicsHV Driver noise test with the new HV power supply from Matsusada

I believe I did the identical test with the one in [40m ELOG 15786]. The + input of PA95 was shorted to the ground to exclude the noise from the bias input. The voltage noise at TP6 was measured with +/-300V supply by two HP6209 and two Matsusada R4G360.

With R4G360, the floor level was identical and 60Hz line peaks were less. It looks like R4G360 is cheap, easier and precise to handle, and sufficiently low noise.

Attachment 1: HV_Driver_PSD.pdf
  16148   Thu May 20 16:56:21 2021 KojiUpdateElectronicsProduction version of the HV coil driver tested with KEPCO HV supplies

HP HV power supply ( HP6209 ) were returned to Downs

Attachment 1: P_20210520_154523_copy.jpg
  16150   Fri May 21 00:15:33 2021 KojiUpdateElectronicsDC Power Strip delivered / stored

DC Power Strip Assemblies delivered and stored behind the Y arm tube (Attachment 1)

  • 7x 18V Power Strip (Attachment 2)
  • 7x 24V Power Strip (Attachment 2)
  • 7x 18V/24V Sequencer / 14x Mounting Panel (Attachment 3)
  • DC Power Cables 3ft, 6ft, 10ft (Attachments 4/5)
  • DC Power Cables AWG12 Orange / Yellow (Attachments 6/7)

I also moved the spare 1U Chassis to the same place.

  • 5+7+9 = 21x 1U Chassis (Attachments 8/9)


Attachment 1: P_20210520_233112.jpeg
Attachment 2: P_20210520_233123.jpg
Attachment 3: P_20210520_233207.jpg
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  16155   Mon May 24 08:38:26 2021 ChubUpdateElectronics18-bit AI, 16-bit AI and 16-bit AA

- High priority units: 2x 18AI / 1x 16AI / 3x 16AA

All six are reworked and on the electronics workbench. The rest should be ready by the end of the week.


  16160   Tue May 25 17:08:17 2021 ChubUpdateElectronicschassis rework complete!

All remaining chasses have been reworked and placed on the floor along the west wall in Room 104. 

Attachment 1: 40M_chassis_reworked_5-25-21.jpg
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