Re: [time-nuts] Opera coordinator has resigned
On 03/31/2012 10:29 PM, Jim Lux wrote:
On 3/31/12 12:24 PM, Achim Vollhardt wrote:
Dear John and all,
I do work in high energy physics and we use LEMO and other standards.
For some years now, I have started to advertise against LEMO (in
particular the LEMO 00 size), as it is VERY sensitive to mechanical
defects and partial connection (yes, you can ..). We have found very
often defective LEMO connections, which could only be detected via time
domain reflectometry. Or LEMO interfaces, which changed impedance
significantly when rotating them in the fully mated position.. this list
could be extended a lot more. And this famous click when mating is
inaudible in typical high energy physics electronics barracks.. too much
fan noise.
In addition, for the very fast signals of modern DAQ systems, LEMO 00 is
just not up to speed (literally) anymore. If size is not of ultimate
importance, we switched to SMA, or SMC if size matters.
On bad connectors.. "K" connectors are horrible.. they can intermate
with SMA, but they don't tolerate the range of SMA mechanical variation,
so it's possible to damage a K jack by mating an SMA plug with it. Not
all SMA plugs, but some percentage of them. And it's not obvious that
the jack has been damaged unless you measure it. You can mate a K or SMA
(properly) with it, it will work, mostly, but now, there's a tiny gap in
the center conductor, sometimes, depending on the connector flex, etc.
We have a dreadful connector at JPL which is a mini twinax (used for
balanced pairs, like MIL-STD-1553). The key is a stamped metal bump so
small that it's easy to mismate in the 180 degree reversed position.
There's a much better version which is hermaphroditic (each side has one
male and one female pin) and it can only mate one way. But that other
connector hangs on, and on, because you get the "device A uses connector
type X", so you build test equipment with mating Connector Type X, then
the next device B has that connector, so we can re-use the test
harnesses. The next batch of test equipment is made to be compatible
with device B, and so it goes. The last people I know (not at JPL) who
bought those connectors called up the mfr and was quoted a spectacularly
long lead time (6 months or a year), so they asked who else had bought
them, maybe they could buy the half dozen they needed. Naturally, JPL
had bought the last batch, something like 15 or 20 years ago, and of
course, we had dozens of them sitting in bonded stores.
I was at an IEEE conference a few years ago on high speed interconnects
(10 Gbps is slow for those guys), and a bunch of the folks were talking
about how too much time is being spent on trying to get impedance
controlled connectors, instead of reliable connectors. Their point was
that in any real high speed (which would, for time nuts be "high timing
precision") system, you already have multiple transitions: die to IC
package pin to PWB trace to connector pin to connector, before you worry
about the connector's potential mismatch. These other ones are even
harder to control the impedance on, and trying to do so makes the boards
hard to route, etc, so why not bite the bullet and just implement a good
adaptive equalizer in your high speed interface. Then you can use a
mechanically robust connector that has a positive mating, can't be
mismated or partially mated, and in fact, the equalizer can tell you if
the cabling is screwed up.
From a cost standpoint, it might actually be cheaper. Implementing the
equalizers in a standard high speed SERDES doesn't cost much more in
silicon (yes, the IP development costs money, so the selling price is
higher), and you save a lot in the rest of the system.
We have equalizers as well as pre/post emphesis to shape the eye of the
signal. It's really required. We adjust the sampling point on our bits.
That's standard business today.
Cheers,
Magnus
On 03/31/12 09:38 PM, Jim Lux wrote:
On 3/31/12 1:15 PM, Brooke Clarke wrote:
Hi Achim:
SMA RF connectors have a very limited life (number of matings) until
they are worn out.
I don't think so. Yes, they're only rated for 500 cycles, but there's a
paper by a guy at Maury Microwave that I ran across when trying to get
statistics on the reflection coefficient variation, and he set up a
automated rig to mate/demate SMAs something like 10,000 times.
I suspect that the jig he built does a better job of aligning them than what
humans do. We put them on not quite square, move them around until the thread
mates etc. I wonder if his jig tried to replicate a human or not?
I suspect that there is a WIDE variation among mfrs in terms of life
performance and manufacturing precision.
Yes, I've worked in places where they use the cheapest they can get, and others
where the more expensive, but much better quality Huber and Shuner connectors
are used. I would expect the more expensive ones, which are machined more
accurately, would last for more
There is also a relationship between the torque you tighten them and their life.
I happen to work somewhere where I can't get my boss to buy a torque wrench for
them, despite we use a lot of SMA connectors. I don't know how common that
practice is. Everywhere else I have worked does use a torque wrench.
I one tried a quick experiment tightening them up by hand as tight as possible,
then seeing how many extra turns it required to torque them to whatever the
torque wrench was set to (not all SMA torque wrenches are set to the same
figure). As far as I can tell, there is no way of even roughly estimating how
tight they should be by saying "hand tight + x turns".
Dave
On 4/1/12 3:01 AM, Dr. David Kirkby wrote:
On 03/31/12 09:38 PM, Jim Lux wrote:
On 3/31/12 1:15 PM, Brooke Clarke wrote:
Hi Achim:
SMA RF connectors have a very limited life (number of matings) until
they are worn out.
I don't think so. Yes, they're only rated for 500 cycles, but there's a
paper by a guy at Maury Microwave that I ran across when trying to get
statistics on the reflection coefficient variation, and he set up a
automated rig to mate/demate SMAs something like 10,000 times.
I suspect that the jig he built does a better job of aligning them than
what humans do. We put them on not quite square, move them around until
the thread mates etc. I wonder if his jig tried to replicate a human or
not?
Nope.. back and forth in a straight line. He was measuring repeatability.
I suspect that there is a WIDE variation among mfrs in terms of life
performance and manufacturing precision.
Yes, I've worked in places where they use the cheapest they can get, and
others where the more expensive, but much better quality Huber and
Shuner connectors are used. I would expect the more expensive ones,
which are machined more accurately, would last for more
There is also a relationship between the torque you tighten them and
their life.
I happen to work somewhere where I can't get my boss to buy a torque
wrench for them, despite we use a lot of SMA connectors. I don't know
how common that practice is. Everywhere else I have worked does use a
torque wrench.
I one tried a quick experiment tightening them up by hand as tight as
possible, then seeing how many extra turns it required to torque them to
whatever the torque wrench was set to (not all SMA torque wrenches are
set to the same figure). As far as I can tell, there is no way of even
roughly estimating how tight they should be by saying "hand tight + x
turns".
Yes.. if you're talking about finger tight, there's a lot of
variability. With an inexpensive open end wrench, though you can get
pretty consistent.. perhaps it's the "Calibrated thumb" on the little
wrench?
Not anywhere as good as the "click" on the real torque wrench.
If you think about it, tightening torque only relates very roughly to
axial mating pressure.
Torque is essentially the force requires to push an object up an inclined
plane.
That is the sum of two components, the normal component of the mating
force plus the in plane component of (mating force)*(coefficient of
friction).
The latter is a guesstimate at best.
-John
===============
On 4/1/12 3:01 AM, Dr. David Kirkby wrote:
I one tried a quick experiment tightening them up by hand as tight as
possible, then seeing how many extra turns it required to torque them to
whatever the torque wrench was set to (not all SMA torque wrenches are
set to the same figure). As far as I can tell, there is no way of even
roughly estimating how tight they should be by saying "hand tight + x
turns".
Yes.. if you're talking about finger tight, there's a lot of
variability. With an inexpensive open end wrench, though you can get
pretty consistent.. perhaps it's the "Calibrated thumb" on the little
wrench?
Not anywhere as good as the "click" on the real torque wrench.
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On 04/ 1/12 03:33 PM, Jim Lux wrote:
On 4/1/12 3:01 AM, Dr. David Kirkby wrote:
On 03/31/12 09:38 PM, Jim Lux wrote:
I don't think so. Yes, they're only rated for 500 cycles, but there's a
paper by a guy at Maury Microwave that I ran across when trying to get
statistics on the reflection coefficient variation, and he set up a
automated rig to mate/demate SMAs something like 10,000 times.
I suspect that the jig he built does a better job of aligning them than
what humans do. We put them on not quite square, move them around until
the thread mates etc. I wonder if his jig tried to replicate a human or
not?
Nope.. back and forth in a straight line. He was measuring repeatability.
So it tells us nothing very much about the life of them in normal use, with a
human mating and demating them. - or even the repeatability of the reflection
coefficient with a human in the equation.
dave
On 4/1/12 2:25 PM, Dr. David Kirkby wrote:
On 04/ 1/12 03:33 PM, Jim Lux wrote:
On 4/1/12 3:01 AM, Dr. David Kirkby wrote:
On 03/31/12 09:38 PM, Jim Lux wrote:
I don't think so. Yes, they're only rated for 500 cycles, but there's a
paper by a guy at Maury Microwave that I ran across when trying to get
statistics on the reflection coefficient variation, and he set up a
automated rig to mate/demate SMAs something like 10,000 times.
I suspect that the jig he built does a better job of aligning them than
what humans do. We put them on not quite square, move them around until
the thread mates etc. I wonder if his jig tried to replicate a human or
not?
Nope.. back and forth in a straight line. He was measuring repeatability.
So it tells us nothing very much about the life of them in normal use,
with a human mating and demating them. - or even the repeatability of
the reflection coefficient with a human in the equation.
Not a whole lot, but the whole paper goes into the various factors
involved. Ultimately, it winds up that the mismatch from SMAs is
a) a whole lot less than the usual "worst case" spec of 1.05:1 or 1.03:1
(which is basically a measurement limit)
and
b) doesn't change much with many mate/demate cycles
He did look at things like coupling nut friction and what not.
On 2 April 2012 00:54, Jim Lux jimlux@earthlink.net wrote:
On 4/1/12 2:25 PM, Dr. David Kirkby wrote:
So it tells us nothing very much about the life of them in normal use,
with a human mating and demating them. - or even the repeatability of
the reflection coefficient with a human in the equation.
Not a whole lot, but the whole paper goes into the various factors involved.
Ultimately, it winds up that the mismatch from SMAs is
a) a whole lot less than the usual "worst case" spec of 1.05:1 or 1.03:1
(which is basically a measurement limit)
and
b) doesn't change much with many mate/demate cycles
He did look at things like coupling nut friction and what not.
Without seeing the paper, it's difficult to comment much more. Clearly
if he had shown the performance to be worst than the spec, then I
think he would have a useful result, as he would have put a limit on
what was achieveable even with perfect use. But I think the human
element is quite critical with SMA connectors - or pretty much any
connector for that matter.
Anyway, SMAs are pretty good connectors overall for most RF things.
It's a long time since I have used any LEMO connectors - the subject
of orginal discussion.
I've not been following the orignal thread in detail, or looked at the
relevant papers, but I do feel it odd and unjust that someone should
resign over what was a genuine mistake. We all make them from time to
time, and sometimes the consequencies are quite serious - like someone
dies.
dave
If the FTL neutrino "discovery" had been kept quiet until fully vetted, he
probably would not have had to resign.
Compare with the discovery of "Cold Fusion". The desire for PR got ahead
of the science.
In my view, somebody who is in responsible charge of any project that size
morally ought to be held accountable as "The buck stops here".
Frankly, I'm entirely fed up with those in charge of projects who walk
away scott free after a giant disaster.
Those in responsible charge should be held liable, both professionally and
personally, IMO.
YMMV,
-John
========================
[snip]
I've not been following the orignal thread in detail, or looked at the
relevant papers, but I do feel it odd and unjust that someone should
resign over what was a genuine mistake. We all make them from time to
time, and sometimes the consequencies are quite serious - like someone
dies.
dave
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El 02/04/2012 16:14, J. Forster escribió:
If the FTL neutrino "discovery" had been kept quiet until fully vetted, he
probably would not have had to resign.
I suppose so... Also, things would have been completely different if the
60ns error would be of the opposite sign, that would lead to the
believing that neutrinos are a bit slower than light and not a bit
faster. This of course would also have been of big impact in the physics
community, but not so big :) and also would not have take even a single
line in the newspapers and other media, except on the specialized ones.
Anyway, I think that it was keep quiet during some time and only made
public when though that it was fully vetted... but as we have seen a
couple of details were overlooked.
In the positive side, as Javier Serrano said, it has been the first time
that neutrino speed has been measured with this precision.
Regards,
Javier
Moin,
On Sun, 01 Apr 2012 16:54:09 -0700
Jim Lux jimlux@earthlink.net wrote:
Not a whole lot, but the whole paper goes into the various factors
involved. Ultimately, it winds up that the mismatch from SMAs is
a) a whole lot less than the usual "worst case" spec of 1.05:1 or 1.03:1
(which is basically a measurement limit)
and
b) doesn't change much with many mate/demate cycles
He did look at things like coupling nut friction and what not.
Do you have the name of the paper? It might be interesting to read.
Attila Kinali
--
Why does it take years to find the answers to
the questions one should have asked long ago?