Detector Watch S6 V1

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RE: ... the sort of things

Message 93764 in response to message 93763

Quote:

... the sort of things that need maintenance. A stuffed photodetector :

so it's days of lasers are over.


So what are all the black specs? What was the failure?

Quote:
As for underground power lines, the reason may be quite banal. While more expensive to lay they cost relatively little to maintain. Whereas overhead lines are cheaper to erect, but need ongoing care.


Then that suggests that underground is the way to go!

Regards,
Martin

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Mike Hewson
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RE: So what are all the

Message 93765 in response to message 93764

Quote:
So what are all the black specs? What was the failure?


Burnt out elements that have cooked off with the constant heating from the laser. It's a semiconductor lattice. It counts photon fluxes, doing so by catching each one of them and causing movement of an electron(s) - hence a current is generated. Much of the photon energies become lattice vibrations ie. heat. If that occurs in specific areas then spot temperatures will go up and wrecks the local lattice structure. So as time goes on the device as a whole converts a lesser fraction of the incoming photons and it's function degrades.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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Here's a replacement

Here's a replacement photodetector to compare with :

I don't think one can underestimate the importance of this component type. The whole interferometer is attempting to translate some spacetime change to light phase changes to a detectable current/voltage. If these parts are bad, or have poorly understood characteristics then the project slides. I know alot of things have to go right for a detection ( Yeah! ), but these photodetectors are definitely one place where the rubber meets the road.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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This entry from Hanford is

This entry from Hanford is worth copying entirely :

Quote:

54.496 & 108.992 Hz detected from isolated CPU
So far, the only lines reported in both L1 and H1 by the pulsar group, that are not at or near integer values, are two lines at 54.496 and 108.992. We tracked these lines to VME CPUs. The plot and photo show that we detected these frequencies with a magnetometer placed near a 7851 (the CPU used in the lscl0 crates) in a VME crate that we set up. This backup 7851 had no PMC cards, no user code running, no connections to the outside world, and no other cards in the crate. The density of detected lines was not very high, and the resolution of the plot is nearly a millihertz, so this is highly unlikely to be a coincidence. When the crate was on but the CPU board pulled, these frequencies were not detected.

We also detected 54.5 Hz from other VME CPUs and from a Foundry switch, so we suspect that this is not a unique characteristic of the 7851, and we speculate that it is associated with Ethernet firmware.

We did not resolve how it couples into DARM. Because the magnetic field is small compared to fields inside the crates at other frequencies that do not show up in DARM, it seems unlikely that the coupling is through the magnetic field to analog electronics. One possibility is that whatever process is associated with these frequencies, interferes with data collection, producing a slight corruption of the data at these frequencies.


Note that one frequency is twice the other ie. harmonic. VME is a type/brand of mainframe operating system, which I believe is particularly appropriate for realtime use. One wonders what particular behaviour of that board produces such a specific frequency. That is, what is happening some 54.496 times per second? Task switching? Hardware interrupt? Something evidently worth doing over and over .....

Electromagnetism is a long range force or, if you like, is hard/expensive to shield a component completely. There are Faraday cages which are conducting shells around some volume of interest. Any incoming signal, up to some maximum level determined by the geometry of the cage, will be masked. The electrons in the conducting surround dynamically arrange themselves to form an 'mirror image' of the externally applied field ( a perfect conductor has no electric field component tangential to it's surface ). Thus it soaks up the energy of the incoming radiation, in practice almost perfectly. The film 'Enemy of The State' had Gene Hackman's character operating his electronic snooping within one.

For their operators Livingston has a new graphical interface for the Thermal Compensation System ( keeps the mirrors in good shape ) :

I can imagine myself playing with that! Fire up the lasers, use the slider to select power, choose a mask to heat the centre or the periphery, aim it and cross the fingers. :-)

Cheers, Mike.

( edit ) I've had a glance at the specs for the photodiodes for AdLIGO. They're quite elaborate beasts compared to when I first studied such things. Predominantly Gallium-Arsenide but with Indium, Tin, Aluminium and Nitride in the mix. Not with PN junctions but P-I-N junctions, about 8 layers all up. Gold and/or Platinum contacts. I'll have to look that up.

( edit ) Ah, I is for intrinsic ( semiconductor ), lightly doped equal numbers of n's and p's. Makes a PN junction less of a cliff and more like a ramp. The junction width is wider and thus has a lower capacitance on reverse bias, ie. faster. A photon hitting the I region frees up a carrier -> current. Small dark current. Here's a nifty energy band applet.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

tullio
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Good old VAX boards? Sic

Good old VAX boards? Sic transit gloria mundi!
Tullio

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RE: This entry from Hanford

Message 93769 in response to message 93767

Quote:

This entry from Hanford is worth copying entirely :

Quote:

54.496 & 108.992 Hz detected from isolated CPU
[...]

We also detected 54.5 Hz from other VME CPUs and from a Foundry switch,...

... That is, what is happening some 54.496 times per second? Task switching? Hardware interrupt? Something evidently worth doing over and over .....


All good interesting stuff!

That comes to about 18.3ms...

Something to do with the ethernet data packet size?

Is the pickup coming from the ethernet network cabling? Or from PSU loading during packet data TX?

When you're trying to clean up noise from that far down into ultra-high sensitivities, you could find almost anything!

Good luck!

Regards,
Martin

See new freedom: Mageia Linux
Take a look for yourself: Linux Format
The Future is what We all make IT (GPLv3)

AMD_is_logical
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Well, 54.496 Hz could come

Well, 54.496 Hz could come from a fan running at 3270 RPM. I once had a fan motor cause jitter on a CRT several feet away.

I wouldn't expect a fan to run at such a precise speed, though, unless it were speed regulated.

Donald A. Tevault
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RE: Note that one

Message 93771 in response to message 93767

Quote:

Note that one frequency is twice the other ie. harmonic. VME is a type/brand of mainframe operating system, which I believe is particularly appropriate for realtime use. One wonders what particular behaviour of that board produces such a specific frequency. That is, what is happening some 54.496 times per second? Task switching? Hardware interrupt? Something evidently worth doing over and over .....

Actually, judging by the context, I would say that the other definition of VME fits better.

That is, VERSAModule Eurocard, which is where you build an electronic component--computer, etc.--by plugging VME cards into a backplane. (I used to work with them when I was in the Navy.)

VERSModule Eurocard

Mike Hewson
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RE: RE: Note that one

Message 93772 in response to message 93771

Quote:
Quote:

Note that one frequency is twice the other ie. harmonic. VME is a type/brand of mainframe operating system, which I believe is particularly appropriate for realtime use. One wonders what particular behaviour of that board produces such a specific frequency. That is, what is happening some 54.496 times per second? Task switching? Hardware interrupt? Something evidently worth doing over and over .....

Actually, judging by the context, I would say that the other definition of VME fits better.

That is, VERSAModule Eurocard, which is where you build an electronic component--computer, etc.--by plugging VME cards into a backplane. (I used to work with them when I was in the Navy.)


Well done! That makes rather better sense. I wasn't actually sure. I didn't pick that one up with Google. :-)

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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Here's another good example

Here's another good example of tracking down noise, from Livingston :

Quote:

TCSX ISS 60Hz peaks reduced
The 60Hz peaks in the TCSX ISS loop has been reduced more than a factor of 20 by replacing a BNC cable in the ISS box between the "To-AOM" BNC and the board.

While poking around the ISS board, the shield crimping of the BNC cable in the ISS box looked little bit loose. As the way of bending changed the peak amplitudes, the cable was replaced. Suddenly the 60Hz and harmonics disappeared.

Then, the cable crimping and the resistance were checked but there was no sign of failure. Actually this presumably bad cable was once restored to the system, however the 60Hz peaks were not reproduced...

Just in case, the cable was replaced to the new one, and the old cable was salvaged to the KA's pocket. (Available upon request)

The output resister at the last stage to the AOM driver was left as it was.
(i.e. left changed to 0 ohm from the original 100 ohm)


BNC is a type of coaxial cable. From your aerial to the TV set, say. ( One can think of it like a loop/circuit of wire but with the return leg wire coming back inside the outward bound one - so it has some characteristics of a Faraday cage. Twisted pair is a cheaper attempt to emulate this ). I call it the fiddly type ( but aren't they all? ) as one needs to be quite careful when fashioning terminations. If the shielding mesh isn't continued carefully to the connector then stuff will leak in - also inadvertent grounding or floating. In this case the 60Hz influence would leak into the circuitry that controls the laser that shapes the end mirror on the X arm.

I recall not having a proper understanding of BNC termination with an office network some years ago. To reduce the performance depending upon the wiring of the building and other factors, then in addition to an impedance load at a T-piece on one end of the line of computers ( to deal with signal reflection ) one has to ground to the case on the other end. Computer power supplies only give you a virtual/AC ground for the network card outlets via the mobo. So a DC offset can arise - you may detect this when you touch around the T-piece! :-)

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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