<q>
Because ALL results of measurements of physical quantities on many
nominally-identical samples show a statistical distribution of values.
<q>
- where is the data to support this statement for EMI measurements of
all products?
<q>
No, for the reason given above. A small number of samples doesn't explore the
whole distribution, especially since they are probably made form one or a few
batches of each component, so even those don't show a real-world distribution
of values.
<q>
- not true.
For example, HALT/HASS testing.
In that discipline a small sample is used to predict the whole
distribution.
(BTW- why do HALT/HASS engineers get so many samples, and I
have to manage with only one?? ;) )
<q>
But if you are going to make a million a month, you don't do just one
test; you apply the 80-80 rule and test however many the rule requires.
At that rate of production, you CAN afford the USD100k it costs.
<q>
- the 80-80 rule also is unfounded, since the assumption is that EMI
measurements are statistical and are well defined.
I agree that it is highly likely that the measurements are random, but
with what distribution?
Perhaps it is more Chaotic than Gaussian? Or perhaps uniform?
Here is an 80-80 counter-example:
What if a company only manufacture one a month, and it is for the
local military?
And suppose that only one out of 10 (better than 80-80 rule) has
unplanned susceptibility so that it malfunctions.
And further suppose that a soldier gets injured because he/she cannot
see the enemy/fuel gauge/altitude.
The 80-80 rule, the statistics and the amount of margin don’t really
help that customer.
In that case, and many others, having a test margin and test site
measurement uncertainty are useless to the customer.
There are no easy answers to (the other)Brian's question.
I applaud Brian, and others like him, who ask the basic questions in
an attempt to understand the problem.
He is doing the background work to support and communicate the reasons
for compliance (limits + margin).
My contribution (if I have one) to Brian's quest is this:
It is given that electronic products, and companies that manufacture
them, have an absolute need for EM compatibility.
I propose that the starting point of all discussion about margin
should be a self-assessment of the need for compatibility.
(is the need life saving? Is it interoperability? Is it
avoidance of public humiliation? Is it …?)
Once a company understands the drivers of compatibility, then the
quantity of margin needed (or not needed) will follow.
Best Regards,
Patrick.
[email protected]
-----Original Message-----
From: [email protected] [ <mailto:[email protected]> mailto:[email protected]]
On Behalf Of John Woodgate
Sent: Wednesday, June 27, 2007 12:50 AM
To: [email protected]
Subject: Re: Internal Margins for Emissions Testing
In message
<09c2d42ff0bfca4b829cdbe89b8f66ffe86...@g3w0637.americas.hpqcorp.net>,
dated Tue, 26 Jun 2007, "Conway, Patrick R (Houston)" <[email protected]>
writes:
>It seems from Brian's email, and the responses so far, that test margin
>is a presumption.
> Why?
> Why is it presumed that we need any margin at all?
Because ALL results of measurements of physical quantities on many
nominally-identical samples show a statistical distribution of values.
With only a limited number of samples to measure before bulk production
starts, the 'shape' and 'width' of this distribution cannot be accurately
determined.
> If we can answer that question, then the "amount" of margin
>needed should follow easily.
No, for the reason given above. A small number of samples doesn't explore the
whole distribution, especially since they are probably made form one or a few
batches of each component, so even those don't show a real-world distribution
of values.
>
>
>
>Here are some basic questions we compliance engineers should ask of our
>companies.
>
> What problem do we solve by having margin during testing?
> (If there is not a well defined problem, then we should
>stop right here!)
It's an allowance for not knowing the real distribution of values.
>
>
> Does that problem get solved "better" by adding more margin?
> (can you show that doubling your margin improves your
>problem solution by 2x ? )
If the distribution is Gaussian, and the margin is expressed in linear units,
not dB, then doubling the margin usually improves the situation by more than
2x, maybe much more.
>
>
> Are the percentages, probabilities and uncertainties associated
>with that margin truly defensible?
> (defensible in a mathematical and manufacturing sense,
>not a legal sense)
Some of them (tolerances on electronic components, for example); maybe
others can't reasonably be calculated (e.g. screening effectiveness of
an enclosure with large removable parts).
>
>
> If a problem occurs in the field, does the required margin shown
>in the test report gain you any advantage in discussions with the
>customer or auditing body?
> (so, you did one test and passed. Then you manufacture
>10 million a month. What does that margin buy you?)
It shows that you understand the effects of statistical distribution and
took them into account with due diligence.
But if you are going to make a million a month, you don't do just one
test; you apply the 80-80 rule and test however many the rule requires.
At that rate of production, you CAN afford the USD100k it costs.
>
>
--
OOO - Own Opinions Only. Try <file://www.jmwa.demon.co.uk>
www.jmwa.demon.co.uk and <file://www.isce.org.uk> www.isce.org.uk
There are benefits from being irrational - just ask the square root of 2.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
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