Hmm... This is a carrot that I need to nibble on. I take a little issue with folks by blaming software, regardless of the supplier. The software does what it's designed to do... In reality there are a number of factors, some of which are: 1) The test environment attempts the reproduce consistent results. It may not correlate with real fields encountered in use. So, there is controversy on how a test should run. 2) These days, most EN testing uses a ( typically 16 point ) cal file. I am unsure that this philosophy is easy to implement with an Analogue sweep. So it is reasonable to assume that most test use a form of stepped test. 3) Assuming that we are stepping between frequencies, what process is followed? and what actually happens? In a good chamber, with a 1% step, the jump in level may not be significant. But, there will be regions where the jump is significant. In a system driving hard, the control loop may not be linear, and the predictive field level may be substantially higher than actually needed: essentially this is overshoot. Manually, overshoot rarely occurs. Step sizes greater than 1% exacerbate this problem. 4) ANY amplifier, subjected to a step input, doesn't respond perfectly. It's not unusual to see amplifiers overshoot several dB. Not a big deal at low field levels, but when leveling at 10 v/m, its a huge deal. 5) Some signal sources switching frequency mute the output during a transition. In reality, the mute momentarily kills the field. Some EUTs can live with RF in their circuits, BUT, respond badly to a CHANGE in RF level. This phenomena doesn't occur with a spot value where the field is typically stepped up ( relatively ) slowly. 6) I think someone mentioned that some signal sources emit transients when switching. This gets worse as Sig gens get older: attenuates in automated setups really do get beaten up. 7) Some software, if incorrectly "tuned", can hunt for the field level, the hunting process invariably results in some overstressing at some frequencies. 8) The assumption is often made that the field in a setup is uniform across it. NOT so. In fact, 10 dB in not undeasonable. Before folks jump on me, remember that ONLY 75% of the points have to be in 6 dB ( which is still a huge variation! ). Modelling of a chamber, or visualisation of the field measured during testing, shows points that a really strong it one frequency, but not the next. Now, if the EXACT frequency for the spot test is used as that in the sweep, then conditions are equal. HOWEVER, I observe that swept testing often uses calculated frequencies ( ie has 13+ significant figures ), and manual spot testing is rounded up to maybe 3 or 4. Such frequency difference has a significant impact on localised fields in a setup, and if your EUT occupies that region..... So what can be done about it... Well, this is where test philosophy is important: Should the field level be reduced before changing frequency? if so, how much, and over what duration? Should the field be killed between frequencies? Some standards address this, some do not... MHO, all standards should have an appendix like MIL STD 461 E which explains why the tests are like they are, and why the limits are what they are. This approach results in significantly better correlation in testing within a lab, and test results between labs, because the Standard is clearer on what is INTENDED. FYI, this document is free, and should be part of any training in a lab, even if the lab doesn't do MIL testing. So, the answer to your question, is "it depends".... Cheers,
Derek Walton L F Research ---------------------------------------------------------------- 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/listserv/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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
