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?
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?
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.
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.
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, 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.
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.
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.
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.
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
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.
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.
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).
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?
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
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.
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.
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.
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.
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:
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.
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.
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
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.
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.
For your planning:
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
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)
- 7 CHASSIS
- 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 CHASSIS
- 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)
- 5 CHASSIS
- 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
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)
Will we also be receiving the additional 34 Satellite Amplifier PCBs?
We received currently available sets. We are supposed to receive more coil drivers and sat amps, etc. But they are not ready yet.
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).
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?
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.
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).
Received additional front/rear panels. Updated the original entry and Wiki [Link]
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.
The parts will be ordered by Koji The components for the additional BIO I/F have been ordered.
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.
Active TT Pin Issue
and the thread
Active TT Pin Swapping (December 21, 2012)
TT Wiring Diagram (Wiki)
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.
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.
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.
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.
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.
And some more additional items to fill the emptying stock.
We have received 9x 18bit DAC adapter boards (D1000654)
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).
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%.
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.
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.
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.
HP HV power supply ( HP6209 ) were returned to Downs
DC Power Strip Assemblies delivered and stored behind the Y arm tube (Attachment 1)
I also moved the spare 1U Chassis to the same place.
- 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.
All remaining chasses have been reworked and placed on the floor along the west wall in Room 104.
I was preparing a short write-up / test procedure for the custom HV coil driver, when I thought of something I can't resolve. I'm probably missing some really basic physics here - but why do we not account for the shot noise from DC current flowing through the series resistor? For a 4kohm resistor, the Johnson current noise is ~2pA/rtHz. This is the target we were trying to beat with our custom designed HV bias circuit. But if there is a 1 mA DC current flowing through this resistor, the shot noise of this current is 18pA/rtHz, which is ~9 times larger than the Johnson noise of the same resistor. One could question the applicability of this formula to calculate the shot noise of a DC current through a wire-wound resistor - e.g. maybe the electron transport is not really "ballistic", and so the assumption that the electrons transported through it are independent and non-interacting isn't valid. There are some modified formulae for the shot noise through a metal resistor, which evaluates to 10pA/rtHz for the same 4kohm resistor, which is still ~5x the Johnson noise.
In the case of the HV coil driver circuit, the passive filtering stage I added at the output to filter out the excess PA95 noise unwittingly helps us - the pole at ~0.7 Hz filters the shot noise (but not the Johnson noise) such that at ~10 Hz, the Johnson noise does indeed dominate the total contribution. So, for this circuit, I think we don't have to worry about some un-budgeted noise. However, I am concerned about the fast actuation path - we were all along assuming that this path would be dominated by the Johnson noise of the 4kohm series resistor. But if we need even 1mA of current to null some DC DARM drift, then we'd have the shot noise contribution become comparable, or even dominant?
I looked through the iLIGO literature, where single-stage suspensions were being used, e.g. Rana's manifesto, but I cannot find any mention of shot noise due to DC current, so probably there is a simple explanation why - but it eludes me, at least for the moment. The iLIGO coil drivers did not have a passive filter at the output of the coil driver circuit (at least, not till this work), and there isn't any feedback gain for the DARM loop at >100 Hz (where we hope to measure squeezing) to significantly squash this noise.
Attachment #1 shows schematic topologies of the iLIGO and proposed 40m configs. It may be that I have completely misunderstood the iLIGO config and what I've drawn there is wrong. Since we are mainly interested in the noise from the resistor, I've assumed everything upstream of the final op-amp is noiseless (equivalently, we assume we can sufficiently pre-filter these noises).
Attachment #2 shows the relative magnitudes of shot noise due to a DC current, and thermal noise of the series resistor, as a function of frequency, for a few representative currents, for the slow bias path assuming a 0.7Hz corner from the 4kohm/3uF RC filter at the output of the PA95.
Some lit review suggests that it's actually pretty hard to measure shot noise in a resistor - so I'm guessing that's what it is, the mean free path of electrons is short compared to the length of the resistor such that the assumption that electrons arrive independently and randomly isn't valid. So Ohm's law dictates and that's what sets the current noise. See, for example, pg 432 of Horowitz and Hill.
25 HAM-A coil driver units were fabricated by Todd and I've transported them to the 40m.
2 units we already have received earlier.
The last (1) unit has been completed, but Luis wants to use it for some A+ testing. So 1 more unit is coming.
Running update of Sat Amp modification work, which involves the following procedure (x8) per unit:
TP4 @ LED1,2 on PCB S2100568 is 13V instead of 5V
TP4 @ LED4 on PCB S2100559 is 13V instead of 5V
Decided to do a quick check of the remaining Sat Amp units before component replacement to identify any unit with defective LED circuits. Managed to examine 5 out of 10 units, so still have 5 units remaining. Also installed the photodiode bias voltage jumper (JP1) on all the units processed so far.
TP4 @ LED3 on chan 1-4 PCB was ~0.7 V instead of 5V
Koji checked the solder connections of the various components, then swapped out the IC OPAMP. Removed DB9 connections to the front panel to get access to the bottom of the board. Upon close inspection, it looked like an issue of a short connection between the Emitter & Base legs of the Q1 transistor.
Solution - Remove the short connection between the Emitter & Base legs of the Q1 transistor legs.
This issue was caused by a short connection between the Emitter & Base legs of the Q1 transistor.
Solution - Remove the short connection between the Emitter & Base legs of the Q1 transistor legs.
Defective unit with updated resistors and capacitors in the previous elog
This issue was caused by a short between the Collector & Base legs of the Q1 transistor.
Solution - Remove the short connection between the Collector & Base legs of the Q1 transistor legs
Complications - During the process of flipping the board to get access to the bottom of the board, a connector holding the two middle black wires, on P1, came loose. I resecured the wires to the connector and checked all TP4s on the board afterwards to make sure things are as expected.
All units now have the correct TP4 voltage reading needed to drive a nominal current of 35 mA through to OSEM LED. The next step is to go ahead and replace the components and test afterward that everything is OK.
This issue was caused by a short between the Emitter & Base legs of the Q1 transistor.
Solution - Remove the short connection between the Emitter & Base legs of the Q1 transistor legs
Complications - I was extra careful this time because of the problem of loose cable from the last flip-over of the right PCB containing chan 5-8. Anyways, after I was done I noticed one of the pink wires (it carries the +14V to the left PCB) had come off on P1. At least this time I could also see that the corresponding front panel green LED turn off as a result. So I resecured the wire to the connector (using solder as my last attempt yesterday to reattach the via crimping didn't work after a long time trying. I hope this is not a problem.) and checked the front panel LED turns on when the unit is powered before closing the unit. These connectors are quite flimsy.
I have finished assembling the 1U adapters from 8 to 5 DB9 conn. for the satellite amp boxes. One thing I had to "hack" was the corners of the front panel end of the PCB. Because the PCB was a bit too wide, it wasn't really flush against the front panel (see Attachment #1), so I just filed the corners by ~ 3 mm and covered with kapton tape to prevent contact between ground planes and the chassis. After this, I made DB9 cables, connected everything in place and attached to the rear panel (Attachment #2). Four units are resting near the CAD machine (next to the bench area), see Attachment #3.
Thanks. You should be able to find the chassis-related hardware on the left side of the benchtop drawers at the middle workbench.
Hardware: The special low profile 4-40 standoff screw / 1U handles / screws and washers for the chassis / flat-top screws for chassis panels and lids
Trying to finish 2 more Sat Amp units so that we have the 7 units needed for the X-arm install.
S2100736 - All good
S2100737 - This unit presented with an issue on the PD1 circuit of channel 1-4 PCB where the voltage reading on TP6, TP7 and TP8 are -15.1V, -14.2V, and +14.7V respectively, instead of ~0V. The unit also has an issue on the PD2 circuit of channel 1-4 PCB because the voltage reading on TP7 and TP8 are -14.2V, and +14.25V respectively, instead of ~0V.
Make sure the inputs for the PD amps are open. This is the current amplifier and we want to leave the input pins open for the test of this circuit.
TP6 is the first stage of the amps (TIA). So this stage has the issue. Usual check if the power is properly supplied / if the pins are properly connected/isolated / If the opamp is alive or not.
For TP8, if TP8 get railed. TP5 and TP7 are going to be railed too. Is that the case, if so, check this whitening stage in the same way as above.
If the problem is only in the TP5 and/or TP7 it is the differential driver issue. Check the final stage as above. Replacing the opamp could help.