I just signed on, so my apologies if this has all been mentioned:
Assumption is that there is 6dB possible difference between tests.
It is impractical to get 6dB of margin for Class B in many cases, so 3dB
is a more common margin goal.
Products with less than 3dB of margin are checked more frequently.
Product bill of materials have a quality department sign-off.
EMC critical components are identified, and any changes to them require
extra scrutiny (ref. clock oscillators).
Curt
In real life:
Curt McNamara P.E. // senior electrical engineer
Logic Product Development
411 Washington Ave. N. Suite 400
Minneapolis, MN 55401
T // 612.436.5178
F // 612.672.9489
_www.logicpd.com_ <http://www.logicpd.com/>
Conway, Patrick R (Houston) wrote:
> Robert-
>
> Perhaps you have the answer to the original question- that an
> EMC engineer can provide an accurate number for required margin, but
> only up to a certain point in the life of the product. After that point
> the question about margin no longer belongs in the EMC department. We
> can only provide a number that is relative to a point in time.
>
> Doesn't it just stink that no matter how accurate I perform my
> R&D function, eventually someone else can impact the EMI result more
> then I can?!!
>
>
> It also reminds me that I've known many R&D program managers who
> never cared about EMI margin. It was not their problem, because after
> launch to production, they moved on to the next project. Thus leaving
> the problem of continued compliance on the manufacturing team (and the
> EMC team!).
>
> The fact that a company is asking about margin, is a sign of a
> mature organization.
>
>
> Best Regards,
>
> Patrick.
> [email protected]
>
>
> -----Original Message-----
> From: Robert A. Macy [mailto:[email protected]]
> Sent: Monday, July 02, 2007 10:39 PM
> To: Conway, Patrick R (Houston)
> Cc: [email protected]
> Subject: Re: Internal Margins for Emissions Testing
>
> Ouch! if the origin of the potentially offending tone is an upper
> harmonic of a crystal source and all the units you just tested used the
> high quality Bulova IC which had a very well-behaved 50-50 duty cycle
> the power in the offending 6th harmonic is zero, thus the reading is
> also zero, or quite low.
>
> Then, since Engineering did not spec the duty cycle of the Clock Chip,
> Purchasing bought a whole bunch from ?? that cost half as much, but had
> a duty cycle of 35-65 and ZAP!!
> suddenly there is a huge amount of energy at the 6th harmonic which
> unbeknownst to anybody now finds its way out.
>
> So how do you think in terms of statistical variance in such a
> situation? It's a little like predicting where the curb is by measuring
> the slope at the center of the road.
>
> Just very, very difficult to get there from here.
>
> Robert
>
>
> On Tue, 3 Jul 2007 02:46:41 -0000
> "Conway, Patrick R (Houston)" <[email protected]> wrote:
>
>> That is a great answer, the best so far regarding statistical nature
>> of our field (pardon the pun).
>> Although not a fan of the measurement uncertainty cult within
>>
> our
>
>> discipline, I understand the intention and support the result.
>>
>>
>> It seems to me we are still missing 1/2 of the "statistical" picture.
>> Here is what I think -
>> (I could be whacked out here- so please view this with a
>>
> large
>
>> portion of skepticism...)
>> I think the measurement "system" includes two items:
>> a) the measurement facility, and
>> b) the EUT.
>>
>> I think the "measurement uncertainty" does a good job of defining
>> quantity a).
>> But how do we define b), the variance of a product from an
>>
> assembly
>
>> line?
>>
>>
>> The reason this is important to me is that I do not own, operate or
>> work in a test lab.
>> So the only part of the above equation I can influence is the b)
>>
>
>
>> part.
>> (that is of course, assuming that there truly is two parts to
>>
> this
>
>> "system", and I'm not whacked )
>> (- THAT may be too much to assume!)
>>
>> Has anyone on this list performed measurement to understand the
>> statistical RF nature of the EUT?
>> I have not.
>> And I'm not sure how I would start.
>>
>>
>>
>> Best Regards,
>>
>> Patrick.
>> [email protected]
>>
>>
>
> -
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