Ed, take a look at UKAS Lab 34. there is a nice write up in there about MU for many EMI tests.
Cheers, Derek. www.lfresearch.com www.emclabservices.com Brent G DeWitt wrote: > > Well said Ken. In addition I think it is worth explaining that not all > uncertainty contributions have the same weight, as Ed asked about. > This results in a “k” factor that is applied to the contribution > towards the total uncertainty. Check out the numerous on-line > uncertainty calculators and national guidelines for details. > > Brent DeWitt > > Westborough, MA > > *From:* Ken Javor [mailto:[email protected]] > *Sent:* Tuesday, September 08, 2009 7:46 PM > *To:* [email protected] > *Subject:* Re: [PSES] Measurement Uncertainty > > Since dBs are logs of ratios, you cannot do sums of squares on them, > and take square roots. You would have to convert to numerics, do the > rss, and then convert back to dBs. > > Thus, given your example values: > > probe: 0.5 dB is roughly 6% tolerance > cable: 0.1 dB is roughly 1.1% tolerance > analyzer: 1 dB is roughly 12% tolerance > > RSS: = square root [ (0.06)^2 + (0.011)^2 + (0.12)^2 ] = 0.135 > > That corresponds to about 1.3 dB overall tolerance. > > Note that isn’t very far off from your 1.26 dB answer. However, to get > that, you simply added the dB tolerances together (no rss) and then > took a square root of the sum of dBs. When you add the dBs together, > you are multiplying the tolerances. If you multiply the tolerances and > take the cube root, it is as if you are taking a geometric mean. > Geometric and arithmetic means are pretty close as long as the factors > going into the product don’t vary much from each other. > > For instance, if the factors are identical, the geometric and > arithmetic means are identical. Of course, that is a degenerate case > where the concept of mean isn’t terribly useful. If you had two > factors, say one and ten, the geometric mean would be 3.162, whereas > the arithmetic mean would be 5.5. If the two factors are one and one > hundred, then the geometric mean is ten, and the arithmetic mean is > 50.5. You can see where this is going. > > Ken Javor > > Phone: (256) 650-5261 > > ------------------------------------------------------------------------ > > *From: *"Price, Edward" <[email protected]> > *Date: *Tue, 8 Sep 2009 16:16:09 -0700 > *To: *<[email protected]> > *Conversation: *Measurement Uncertainty > *Subject: *Measurement Uncertainty > > *A quick question about calculating measurement uncertainty. > * > *For example, if I am doing a conducted emission test, and I know the > calibration tolerance for each of the "modules" of my measurement > system, how do I calculate an overall uncertainty figure? > * > *I have been under the assumption that, in the example I cite, that I > have a tolerance for my probe (say +/- 0.5 dB), my cable (+/- 0.1 dB) > and my analyzer (+/- 1 dB). I have been using an "rms" calculation, > where I square each of the tolerances, sum them, and take the square > root. Thus, square root of 1.6 dB equals 1.26 dB uncertainty. > * > *I recently had a customer say that this method implied that I was > using "rectangular" shape factors, and thus, I should be taking the > cube root of the sum. > * > *Rectangular? Cube root? > * > > */Ed Price > /**[email protected]* <blocked::mailto:[email protected]> > <mailto:[email protected]%3e> * WB6WSN > **NARTE Certified EMC Engineer > Electromagnetic Compatibility Lab > Cubic Defense Applications > San Diego, CA USA > **858-505-2780 > **/Military & Avionics EMC Is Our Specialty > /* > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society > emc-pstc discussion list. To post a message to the list, send your > e-mail to <[email protected]> > > All emc-pstc postings are archived and searchable on the web at > http://www.ieeecommunities.org/emc-pstc > Graphics (in well-used formats), large files, etc. can be posted to > that URL. > > Website: http://www.ieee-pses.org/ > 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]> > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society > emc-pstc discussion list. To post a message to the list, send your > e-mail to <[email protected] <mailto:[email protected]>> > > All emc-pstc postings are archived and searchable on the web at > http://www.ieeecommunities.org/emc-pstc > Graphics (in well-used formats), large files, etc. can be posted to > that URL. > > Website: http://www.ieee-pses.org/ > 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] <mailto:[email protected]>> > Mike Cantwell <[email protected] <mailto:[email protected]>> > > For policy questions, send mail to: > Jim Bacher <[email protected] <mailto:[email protected]>> > David Heald <[email protected] <mailto:[email protected]>> > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society > emc-pstc discussion list. To post a message to the list, send your > e-mail to <[email protected] <mailto:[email protected]>> > > All emc-pstc postings are archived and searchable on the web at > http://www.ieeecommunities.org/emc-pstc > Graphics (in well-used formats), large files, etc. can be posted to > that URL. > > Website: http://www.ieee-pses.org/ > 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] <mailto:[email protected]>> > Mike Cantwell <[email protected] <mailto:[email protected]>> > > For policy questions, send mail to: > Jim Bacher <[email protected] <mailto:[email protected]>> > David Heald <[email protected] <mailto:[email protected]>> > - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ 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]>
