Reliability of atomic clocks
Hi
In the case of the Temex Rb’s in their “reliability sample”, I have hard
evidence in front of me that their MTBF numbers were wildly optimistic.
If their numbers were correct, it would be impossible for me to have as
many broken ones in front of me as I do.
Bob
On Mar 27, 2016, at 7:11 PM, Attila Kinali attila@kinali.ch wrote:
On Sun, 27 Mar 2016 10:13:32 -0400
Bob Camp kb8tq@n1k.org wrote:
The “typical life” numbers on the tubes in the various Cesium standards
are fairly accurate. Most units that are well cared for “die” when the tube
goes out and come back to life when it’s replaced. The tube life dominates
the MTBF in this case.
Lifetime is only one variable I am interested in. I suspect that most
of the standards have a much longer lifetime than their MTBF between
intermittent faults. E.g. Chirstopher Ekstrom reported at FSM8 that their
newly build Rb fountains show a frequency jump once in a while. After a
restart of the fountain it's back to normal.
Attila Kinali
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Titles sound about right. But I don't need them. Someone else was asking.
As I say I stumbled across them and since I don't really have a suite of
Cesiums never saved it. :-)
Thanks
Paul
WB8TSL
On Sun, Mar 27, 2016 at 5:15 PM, Michael Wouters michaeljwouters@gmail.com
wrote:
Dear Paul,
You are probably thinking of one of these:
Chadsey et al “Maintenance of HP5071A frequency standards at
USNO” in Proc. 29th PTTI, p49-60 (1997)
Chadsey “An automated alarm program for HP5071A frequency
standards” in Proc. 31st PTTI, p649-655 (1999)
Brock et al “End-of-life indicators for NIMA’s high-peformance
cesium frequency standards” in Proc. 34th PTTI, p117-125 (2002)
Cheers
Michael
On Mon, Mar 28, 2016 at 1:50 AM, paul swed paulswedb@gmail.com wrote:
I do not have it but I stumbled into it on the internet. There was one
paper it was military, naval observatory or NIST and it did indeed show
failure rates of cesiums of the reference that were owned and it must
have
been 30-50 of them.
I remember it showed failures of units over years.
Since it did not at all addres my need I did not keep it.
Regards
Paul
WB8TSL
On Sun, Mar 27, 2016 at 7:53 AM, Attila Kinali attila@kinali.ch wrote:
Moin,
Maybe someone here can help me.
I am looking for data on the reliability of atomic clocks.
I.e. how often and, if possible, how they fail.
Unfortunately, if I google for reliability then all that pops up
are descriptions of the accuracy and stability of atomic clocks.
If I go for MTBF I only get two papers from the 70s that tackle
the problem in general, without giving any data.
Does someone know where I could find current data about MTBF and
failure modes of atomic clocks? Given the number of 5071's installed
in labs, there must be at least some data on them....
Attila Kinali
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Am Mon, 28 Mar 2016 01:32:03 +0200
schrieb Attila Kinali attila@kinali.ch:
Yes, the MTBF is a very simplicistic measure and there are a couple
of assumptions in its calculation which do not hold generally (or
rather, it's rather seldom that they hold). Yet it gives a number to
something that is otherwise relatively hard to measure and the number,
even though flawed, makes it possible to compare different devices
on their reliability. As this is more a rule of thumb comparison,
you shouldn't read too much into a 10% difference. Yet a 100%
difference is significant, no matter which of the assumptions do not
hold.
Umm, well, even a 100% difference still might mean nothing if the
derivation of MTBF between different devices is based on different
assumptions. That both these derivations might be seriously flawed does
not help at all.
Yet, even MIL-Spec parts documentation does rarely contain sufficient
detail to assess the validity of the numbers in a certain application.
At the very least, one would need to know acceleration factors for the
different failure mechanisms, and shape parameters of the
failure-vs-time plot. This kind of data I wouldn't expect to find
outside the manufacturers premises, and even there it's not likely to
be accessible if it exists at all.
Best regards,
Florian
We make some thick film, plastic molded , plug in attenuators for the cable
TV industry. We have been asked on several occasions to provide MTBF data.
Being a small company with limited resources, we have never been able to
provide that data. The parts we make will easily outlast the equipment they
are used in, because such equipment is frequently upgraded. We tell our
customers that very few, if any, parts have ever been returned for being
defective. While true, it is somewhat misleading. The plug in attenuator is
an inexpensive part that is carried in a tech's belt bag by the handful. If
he encounters a bad one, he simply removes it, tosses it on the ground, and
installs a new one. In recent years the Chinese have taken most of the
market away from us with lower cost products. A lot of them use FR4 and
chip resistors. I have heard that some OEMs are going away from those
because of reduced reliability. I have been seeing some Chinese products
that are actually thick film. Most are either copies of ours, or they
didn't spend any time to optimize the RF performance. Sometimes, I do see
parts that out perform our own. The Chinese also make plug in, molded
equalizers, but I have yet to see one that works well. I must say to their
credit though, that they have gone from cheap imitations to parts that are
actually well designed and built. AT least some of them are.
On Mon, Mar 28, 2016 at 6:04 AM, Florian Teply usenet@teply.info wrote:
Am Mon, 28 Mar 2016 01:32:03 +0200
schrieb Attila Kinali attila@kinali.ch:
Yes, the MTBF is a very simplicistic measure and there are a couple
of assumptions in its calculation which do not hold generally (or
rather, it's rather seldom that they hold). Yet it gives a number to
something that is otherwise relatively hard to measure and the number,
even though flawed, makes it possible to compare different devices
on their reliability. As this is more a rule of thumb comparison,
you shouldn't read too much into a 10% difference. Yet a 100%
difference is significant, no matter which of the assumptions do not
hold.
Umm, well, even a 100% difference still might mean nothing if the
derivation of MTBF between different devices is based on different
assumptions. That both these derivations might be seriously flawed does
not help at all.
Yet, even MIL-Spec parts documentation does rarely contain sufficient
detail to assess the validity of the numbers in a certain application.
At the very least, one would need to know acceleration factors for the
different failure mechanisms, and shape parameters of the
failure-vs-time plot. This kind of data I wouldn't expect to find
outside the manufacturers premises, and even there it's not likely to
be accessible if it exists at all.
Best regards,
Florian
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It's been a while since I designed aerospace hardware, but seems I remember we had both a calculated AND a demonstrated MTBF.
Back then we called it Mil-Std 781. (I am sure it morphed into more modern tests).
We had both a pre-production qual-test and a production acceptance-test, both required to meet MTBF's, that were run for reliability.
I took stock in them; as did others. They did have merit in predicting weak engineering designs catching weak designs during 'life' production.
It wasn't 'simplistic' at all.
Maybe the military and aerospace world is different from the 'commercial' world.
-Don
---=============================
On Mon, 28 Mar 2016 13:04:23 +0200
Florian Teply usenet@teply.info wrote:
Am Mon, 28 Mar 2016 01:32:03 +0200
schrieb Attila Kinali attila@kinali.ch:
Yes, the MTBF is a very simplicistic measure and there are a couple
of assumptions in its calculation which do not hold generally (or
rather, it's rather seldom that they hold). Yet it gives a number to
something that is otherwise relatively hard to measure and the number,
even though flawed, makes it possible to compare different devices
on their reliability. As this is more a rule of thumb comparison,
you shouldn't read too much into a 10% difference. Yet a 100%
difference is significant, no matter which of the assumptions do not
hold.
Umm, well, even a 100% difference still might mean nothing if the
derivation of MTBF between different devices is based on different
assumptions. That both these derivations might be seriously flawed does
not help at all.
Yet, even MIL-Spec parts documentation does rarely contain sufficient
detail to assess the validity of the numbers in a certain application.
At the very least, one would need to know acceleration factors for the
different failure mechanisms, and shape parameters of the
failure-vs-time plot. This kind of data I wouldn't expect to find
outside the manufacturers premises, and even there it's not likely to
be accessible if it exists at all.
Best regards,
Florian
time-nuts mailing list -- time-nuts@febo.com
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and follow the instructions there.
--
dlewis6767 dlewis6767@austin.rr.com
On 28 March 2016 at 00:32, Attila Kinali attila@kinali.ch wrote:
Yes, the MTBF is a very simplicistic measure and there are a couple
of assumptions in its calculation which do not hold generally (or
rather, it's rather seldom that they hold).
It get's "interesting" when you look at the MTBF times on hard disks. Some
of the figures quoted in hours related to an MTBF of over 100 years. From
what I read before, this was based on you replacing the drive at the end of
its service life (typically 3 years for consumer drives and 5 years for
enterprise grade disks). So no individual drive was ever expected to last
100 years, but if you kept replacing the drives ever 3~5 years, the average
time of an unexpected failure would be 100 years. I guess its a bit like a
car - the engine might run for 250,000 miles, but if you never change the
oil or the camshaft belt, it is not going to last.
I note Seagate have dropped the use of MTBF:
http://knowledge.seagate.com/articles/en_US/FAQ/174791en?language=en_US
changing to an Annualized Failure Rate (AFR). I don't think Seagate will
ever get a real measure of this, as in many cases people are just going to
throw a hard disk in the bin if it fails, even if under warranty. In many
cases the warranty is with an OEM, so even if you buy a new drive sold
originally to Dell, you can't return it unless you are Dell. Also with hard
drive capacities growing quite fast, if a drive does fail you will probably
chose to replace it with one of higher capacity.
Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
UK.
Registered in England and Wales, company number 08914892.
http://www.kirkbymicrowave.co.uk/
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
It get's "interesting" when you look at the MTBF times on hard disks. Some
of the figures quoted in hours related to an MTBF of over 100 years. From
what I read before, this was based on you replacing the drive at the end of
its service life (typically 3 years for consumer drives and 5 years for
enterprise grade disks). So no individual drive was ever expected to last
100 years, but if you kept replacing the drives ever 3~5 years, the average
time of an unexpected failure would be 100 years. I guess its a bit like a
car - the engine might run for 250,000 miles, but if you never change the
oil or the camshaft belt, it is not going to last.
I note Seagate have dropped the use of MTBF:
http://knowledge.seagate.com/articles/en_US/FAQ/174791en?language=en_US
The article you link here actually explains what MTBF on drives is
measuring -- and it has nothing to do with when you replace your drives.
MTBF is basically expressed as "1 failure per N power-on hours". So if
you have a MTBF of 100,000 hours and you have 100 drives running
continuously, you will (on average) have one failure every ~42 days (1000
hours). If you have 100,000 drives, you'll have (on average) one failure
every hour. MTBF does not address the expected life of any specific
drive in any way.
(It also does not address the bathtub curve that drive failures tend
to follow -- there's a high 'infant mortality' rate for new drives,
then a number of years of service with a low failure rate, followed by an
increase in failure rate after some number of years.)
FWIW, there have been a few interesting things published on drive failure
rates. One of the most interesting is a study[1] Google published in 2007,
which drew some rather unexpected conclusions (e.g. drive temperature is
not associated with failure rate, except at the higher ends of the
temperature range). Backblaze (a cloud backup provider) also publishes
regular reports on drive reliability[2], and have been for a few years now.
-j
On 30 Mar 2016 09:00, "Jay Grizzard" elfchief-timenuts@lupine.org wrote:
It get's "interesting" when you look at the MTBF times on hard disks.
Some
of the figures quoted in hours related to an MTBF of over 100 years.
From
what I read before, this was based on you replacing the drive at the
end of
its service life (typically 3 years for consumer drives and 5 years for
enterprise grade disks).
I note Seagate have dropped the use of MTBF:
http://knowledge.seagate.com/articles/en_US/FAQ/174791en?language=en_US
The article you link here actually explains what MTBF on drives is
measuring -- and it has nothing to do with when you replace your drives.
That article does not. But I have read articles from other manufacturers
where the MTBF was defined in terms of drives being replaced at the end of
their service life. Seagate have obviously dropped the use of the term MTBF
for hard dusks.
Dave.