[time-nuts] Meaning of MTBF (was: Reliability of atomic clocks)

John Green wpxs472 at gmail.com
Mon Mar 28 08:33:25 EDT 2016


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 at teply.info> wrote:

> Am Mon, 28 Mar 2016 01:32:03 +0200
> schrieb Attila Kinali <attila at 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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