Several EMC antennas have AF step performance of 1 dB per 5 MHz or so at certain frequencies.
If the antenna would have a linear behavior, 2 points would be enough as all other frequencies could be linearly interpolated. But my particular Chase BILOG 6111A has a bump at 14.1 dB at 258 MHz and at both 250 MHz and 266 MHz the antennafactor is 13.0 dB. This is what I read from the calibration graph. The manufacturer however, also has calibration (numeric) data provided as discrete points at comfortable round numbers of 250 (12.9) and 300 MHz. (13.6) These points are in general used for creating a interpolation curve. Linear interpolation using the table provides a antennafactor of 13.1 dB at 258 MHz which is a full -1 dB off from what I read from the graph. The bump has virtually disappeared. Nice result with a 0.2 dB receiver. That is why the output level change and required accuracy should be the driving factor in creating a calibration table. It can easily be automated during a sweep. What complicates this further is that not all manufacturers provide data about their interpolation system. My R&S receiver uses quadratic spline (I suppose Lagrange). My R&S FSP only uses LIN interpolation (with log option) for the transducers. Regards, Ing. Gert Gremmen, BSc [email protected] www.cetest.nl Kiotoweg 363 3047 BG Rotterdam T 31(0)104152426 F 31(0)104154953 Before printing, think about the environment. Van: [email protected] [mailto:[email protected]] Namens Ken Javor Verzonden: Tuesday, December 06, 2011 11:12 PM Aan: [email protected] Onderwerp: Re: [PSES] Calibration practice for EMI test transducers This discussion has gone far a field from the original post. I'm assuming some sort of scan has been run and a continuous sweep is available. The question is, how densely does that sweep need to be digitized? Not as a practical matter, but as a matter of compliance with standards or standard practice. What manner of antenna would have performance as cited below where at 100 MHz the antenna factor is 10 dB and at 120 MHz it is 12 dB, but at 110 MHz it could be 20 dB? Let's generalize the question to any closely spaced frequencies. Let's look at the types of antennas available. 30-200 MHz: A half-wave tuned dipole is nowhere near that sharp. Neither is a biconical. 200 - 1000 MHz: A half-wave tuned dipole is nowhere near that sharp. Neither is a logperiodic, log-spiral, nor a Yagi. 1 GHz+: Logperiodic, log-spiral nor pyramidal horns act the way surmised. My conclusion, and the point here is to invite discussion, not close it out, is that no test-type antenna is a high "Q" device. Antennas can have arbitrarily high gains, depending on construction, but the high gain is a geometrical quality, not a high quality factor in the frequency domain. Ken Javor Phone: (256) 650-5261 ________________________________ From: <[email protected]> Date: Tue, 6 Dec 2011 11:25:43 -0800 To: "ce-test, qualified testing bv - Gert Gremmen" <[email protected]> Cc: <[email protected]> Subject: Re: [PSES] Calibration practice for EMI test transducers Yet how far would you take that argument? If it's say 11.1db at 110MHz and 11.5dB at 112MHz, would you still worry about factor at 111MHz? At some point surly this all becomes rather silly. ________________________________________________________________________ _____________ Ralph McDiarmid | Schneider Electric | Renewable Energies Business | CANADA | Regulatory Compliance Engineering From: "ce-test, qualified testing bv - Gert Gremmen" <[email protected]> To: [email protected] Date: 12/05/2011 10:06 PM Subject: Re: [PSES] Calibration practice for EMI test transducers ________________________________ If at 100 MHz your sensor indicates during calibration 10 dB and at 120 MHz 12 dB, how are you going to know the value at 110 MHz. It maight be 20 as well as 0 dB, you don't not know as you did not measure/ calibrate. This requires some knowledge about the behavior of the sensor, and the 1/BW is and indication of that. There is another aspect in EMI measurements. As the measuring receiver interpolates between the calibrated samples, the max interpolation error (lin interpolation) is 50% of the vertical differences between calibration points. If the measuring receiver is +/- 0.5 dB you should register a calibration each time the difference between previous and current sample is 0.5 dB. Then the total error will be slightly more than 0.5 dB. There are more sophisticated interpolation methods such as cubic spline and polynomial interpolation, and the error item is subject to higher mathematics..... Play with it at: http://terpconnect.umd.edu/~petersd/interp.html <http://terpconnect.umd.edu/~petersd/interp.html> <http://terpconnect.umd.edu/~petersd/interp.html> or at http://www.ibiblio.org/e-notes/Splines/Lagrange.htm <http://www.ibiblio.org/e-notes/Splines/Lagrange.htm> <http://www.ibiblio.org/e-notes/Splines/Lagrange.htm> you can actually move the points on the latter page. Gert Gremmen -----Oorspronkelijk bericht----- Van: [email protected] [mailto:[email protected] <mailto:[email protected]> <mailto:[email protected]> ] Namens Cortland Richmond Verzonden: dinsdag 6 december 2011 3:01 Aan: [email protected] Onderwerp: RE: Calibration practice for EMI test transducers I recently had occasion to verify on an 8591EM generic factors for an EMCO current clamp during pretest, and used its tracking generator to see if with a short wire the result was a straight line on the screen. That simply called for paying attention to 1/BW. Cortland KA5S On 12/5/2011 5:33 PM, Ken Javor wrote: If an antenna, current probe or other transducer is going to be calibrated over its frequency range of operation, what determines the step sizes between measured frequencies, or if swept, what determines the density of test points reported? - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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