Hi Ed, (and group)
Great, this is something we can work with. Now prove us that the error caused by interpolation is less then 0.5 dB, that means make a math model (not too complex for linear interpolation) Then we should draw conclusions from that…. First: The calibration graph should well extend the working frequency range Second: I believe it should be mandatory that each max and min point of the graph should be a calibration point (ie dAF/df = 0). (differentiate AF versus frequency: when variation per step is 0, we have a flat area, a top or a bottom in the graph) Third: Each inflexion point should be a calibration point ( d2AF/df= 0) (The second order differentiate = 0) Fourth: Each calibration point AF should not differ more than plus 1 or minus 1 >from the previous and next point. Linear Interpolation is maybe not the best way of interpolation. Are there any math savvy users on this list that can create an error approximation for several interpolation methods ?? Linear Cubic Cubic Spline Cosine Hermite See http://local.wasp.uwa.edu.au/~pbourke/miscellaneous/interpolation/ And http://en.wikipedia.org/wiki/Interpolation Applet showing how it works: http://www.dr-mikes-maths.com/DotPlacer.html (just try it using 4 point only, just as software does) Gert Gremmen Ce-test, qualified testing bv Van: [email protected] [mailto:[email protected]] Namens Price, Edward Verzonden: maandag 8 juni 2009 16:56 Aan: [email protected] Onderwerp: RE: Measurement Accuracy and antenna factors My antenna cal lab gives me gain and antenna factor data at 57 points, from 20 MHz through 300 MHz, using 5 MHz intervals, for my Raven biconical antenna. I normally use this antenna only between 30 MHz and 200 MHz, but it's nice to see how the antenna performs beyond its normal use range. Over the 30 MHz to 200 MHz range, the AF change from one data point to the next rarely varies by more than 1 dB, and often varies only 1/4 dB. I enter all 57 data points into my acquisition system antenna factor file, and the HP-85869PC software does a linear interpolation between data points. I feel that this gives me less than a 1/2 dB uncertainty in all cases, and likely better than 1/4 dB over most of the frequency range. Ed Price [email protected] <blocked::mailto:[email protected]> WB6WSN NARTE Certified EMC Engineer Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 Military & Avionics EMC Is Our Specialty ________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of Cortland Richmond Sent: Sunday, June 07, 2009 11:19 PM To: [email protected] Subject: RE: Measurement Accuracy and antenna factors I favor scanning somewhat past the prescribed scan edge (and either including calibration points, or if not provided, extrapolating straight line in the transducer tables) because often, automated systems fail to display an emission right on the edge of the scan and extending it makes sure these are caught. Many of us may also have noticed that automated systems can produce misleading printouts unless scan segments are kept short enough to insure all data is properly represented on paper. That is properly another discussion. It's already been mentioned that where large excursions are present, more data points should be entered in transducer tables. Data points far enough apart to introduce an additional 1 dB of error are in my opinion too far apart. I suspect that in-band computational errors due to reliance on missing out-of-band data points may be satisfactorily contained by short straight line extrapolations as above. Cortland Richmond, KA5S GE Aviation Opinions my own, not my employers'! ----- Original Message ----- From: ce-test, qualified testing bv - Gert Gremmen <mailto:[email protected]> To: [email protected] Sent: 6/7/2009 12:49:05 PM Subject: Measurement Accuracy and antenna factors A lot of effort has been put into specification of measurement accuracies in radiated emissions. CISPR 16-4-2 has a number of uncertainty budgets listed. One factor that I have not seen in any budget is the error introduced by interpolation between antenna factor calibration points by the measuring receiver. In general the characteristics of a calibrated antenna are entered into the measuring receiver as a number of F/AF pairs, more or less randomly selected from the calibration graph. Then the AF values for frequencies in between those pairs a quadratic spline function is used to interpolate. The function requires 4 calibration pairs to operate correctly of which 2 must be lower and 2 must be higher then the interpolated frequency. Especially near 30 MHz, where modern antennas have steep AF graphs, a calibration point below 30 MHz is not always available and I assume the software duplicates the 30 MHz pair to say 25 MHz to complete the function’s requirements. This must introduce interpolation errors near 30 MHz. I do now know the error that might be introduced by this Type of function. I know that Taylor series have alternating sign In their expansion, and that the values diminish each term, so the error of approximation remain smaller as the last term used to interpolate. But Taylor does not suit itself for approximation of non computable data (such as AF). My questions for the group are: What requirements are to be met for the F/AF pairs to minimize errors? What are the errors introduced by interpolation? How do YOU handle this additional uncertainty…? Gert Gremmen Ce-test qualified testing bv - - 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. 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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]>
