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] 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] >> >> > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society > emc-pstc discussion list. Website: http://www.ieee-pses.org/ > > To post a message to the list, send your e-mail to [email protected] > > Instructions: http://listserv.ieee.org/request/user-guide.html > > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > > Scott Douglas [email protected] > Mike Cantwell [email protected] > > For policy questions, send mail to: > > Jim Bacher: [email protected] > David Heald: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc > > - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
