I certainly don't disagree with this, and see a fair amount of the
variation from set-up and not measurement. Yet if I do a test in MN and
then send the equipment to a lab in CA for a client to redo the test the
chance of a substantial variation exists. Technically this is +/-3dB as
I understand it, although I do agree that 3dB overall is more realistic.
Curt
Grasso, Charles wrote:
> Hi Curt - A 3db margin is inside most test labs measurement uncertainty.
> One advantage of using MU as your "guide" is that the margin is no
> longer
> a seeming invention pulled out of the air. Typically labs report in the
>
> general area of 3.5 - 4dB. A very defensible number.
>
> By the way - note I said *strive*. Doesn't there is a graph of margin vs
> schedule that appears to have a very rapid dm/dt - the closer we get to
> schedule release - the margin requirements vanish rapidly in almost an
> xsquared slope!! (OOO as John would say!)
>
> Best Regards
> Charles Grasso
> Compliance Engineer
> Echostar Communications Corp.
> Tel: 303-706-5467
> Fax: 303-799-6222
> Cell: 303-204-2974
> Pager/Short Message: [email protected]
> Email: [email protected]
> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On Behalf Of Curt
> McNamara
> Sent: Tuesday, July 03, 2007 1:02 PM
> To: Conway, Patrick R (Houston)
> Cc: Robert A. Macy; [email protected]
> Subject: Re: Internal Margins for Emissions Testing
>
> 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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