--- On Thu, 2/2/12, Kunde, Brian <[email protected]> wrote:
Our calibration software monitors the linearity of the amp while performing the Constant Field Strength method calibration test. A 1db change in the amp’s input will result in a 1db change in the amp’s output (or fairly close to it) if operating within its linear range. If the test software determines the amp is operating outside its linear range or has reached the maximum input power without achieving the target output, a value of “99” is stored for that point and frequency. Oh, pretty neat for the software to do that on the fly, ours does not. :-( However, is it checking the harmonics on the fly as well? That can possibly skew the probe readings. For a test that starts at 80MHz, it probably isn't likely to be a factor, but I have seen it happen with a bilog antenna when we had to test down to 26MHz. When the data is calculated according to the procedure in D.4.1 this point will automatically get thrown out at that frequency. Not necessarily. It is still possible for that point to be considered uniform (within the 6dB window) but the amp may run out of power before it can achieve the desired field strength. In that case, the field may be uniform but you don't have enough power to reach the required level at that frequency. Using the Constant Power Method and selecting a power level that will not saturate your amp and generate data that will not fall below the noise floor of your probe is fine for determining Uniformity, but then you have to calculate the power needed to generate 18V/m at 12 of 16 points and then run the linearity check to make sure you are not in saturation. In your case, using the constant-field method and checking the linearity on the fly, the linearity check does not need to be done, but you still need to check harmonics. We combine the linearity check with the harmonics check, so not having to do the linearity check would not save us much time. We use the indirect method listed in D.3.2. If you don’t have enough power to do the Constant Field Strength method then you won’t have enough power to do the test at the target power level. I disagree. It is possible to have a one or two points that are in a dead null, that would not fall withing the 6dB window and would be tossed out of consideration altogether. I've seen it happen. What can I say, marginal chamber and marginal amp, but I have to use what is available. Both methods will obviously work, but I have personally run into situations where the only way I could end up with usable data was to run the constant-power method. I especially had a hard time getting our compact chamber to perform up to 6GHz (it was previously only rated up to 1GHz). I went through several iterations of moving cones on the walls, ceiling and floor, as well as antenna location, before I ended up with something that worked. I tried to rent some EMC goggles so I could see how the waves were bouncing around, but was unable to locate any. :-) Bob R. From: [email protected] [mailto:[email protected]] On Behalf Of Bob Richards Sent: Wednesday, February 01, 2012 1:46 PM To: [email protected] Subject: RE: IEC 61000-4-3 Field Uniformity Measurement Brian, According to the standard, there is a linearity check that must be performed to insure the amp is not in saturation. This check must be performed regardless of which method is used. Granted, you will know at the end of the constant-field method that you might have a power problem, without having to run the linearity check, but it still needs to be done so you don't really save time. I've tried the constant field method before where one of the outlying points was in a null and the amp did not have enough power to pull it up to 18v/m. The only way I could complete the calibration was to use the constand power method, so at least in this scenario I had wasted time with the constant-field method. I wish I had a better chamber and bigger amp, but management just would not buy them for me. :-) Bob R. - ---------------------------------------------------------------- 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.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. 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]>
