Hi Andy,

I'm going through something like that right now. My intent was
to do it all by myself. This is at a system level. Individual
assemblies such as communication modules and power supply modules
I have calculated using the Bellcore TR-332 Reliability Standard
5th Edition that addresses MTBFs and FITs (Failure In Time).

1 FIT = 1 failure / Billion Hours

MTBF = (10^9)/FITs

For simple assemblies:
It's all spelled out in TR-332 for *assemblies*. I've done it
with an Excel spreadsheet that I copied right out of the
standard. I also bought special software from a company called
Relex that does an excellent job. Bellcore also sells software
for the same thing, but as of 2 years ago, my  experience with
it was less than favorable.  Perhaps they have improved it.
TR-332 even provides tables of industry accepted FITs for all
sorts of parts.

For systems with redundancy:
But, this doesn't cover what happens at a system level. Trying
to calculate this by hand, one must use reliability probablistic
models where the math can get rather hairy. I tried doing this,
but the downfall is that my system has redundancy. Just simply
running the numbers from some given equations doesn't take into
acount things such as "mission time", "availability", and most
important repair. If I ran just dry numbers for my system,
I am assuming that the system gets locked away in a closet and
stays there until everything fails without regard to someone
ever coming by and replacing failed units.

For simple assembly calculations, get a copy of
Bellcore TR-332 (I think it's about $150)
Tel: 201-829-4785

For system level calculations, request a more accurate
MTBF calculation by contacting two labs:

John Healey at Bellcore (the man responsible for TR-332)
Tel: 732-758-3065

Dr. Ronald Giuntini at Wylie Labs (extremely helpful person)
Tel: 205-837-4411

Regards, Doug


> From: Andy Kowalewski <[log in to unmask]>
>
> Hi,
>
> The company I'm currently working with has been asked to provide an
> unusual form of MTBF figure across a complex microwave/baseband system.
>
> Can anyone point us to a source of information we can use to build the
> figure?

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