Not dwelling long enough is a serious problem in emission measurements reproducibility. The prescan in peak mode is used to make a selection of frequenties to be evaluated in QP/AV. A usual dwell time of 100 mS per frequency copes with broadband interferences from 10Hz (repetition) off. Slower EUT emission sequences will result in an inadequate preselection of QP frequencies and may result in emission peaks not evaluated at all.
Example from our test practice where this happens: Many instruments contains a memory (flash or HDD) which is accessed substantially less than once per second. HDD buses in small (non shielded) instruments are a major source of emissions. Small burst < 100 mS) of emissions happen at random in these instruments. One way of detecting these kind of phenomena is switch ON the loudspeaker of your MEASURING RECEIVER and probing 20 - 30 frequencies evenly spread over the band and listen some time. This is best done with a larger bandwidth as usual. If any transient emissions happen, you will hear them most of the time. Alternately, if you have an analog peak indicator, look at it. It will vary in time. Beware, unmodulated interference 'sounds as' silence, no interference as loud noise. Broadband interference sounds like Science fiction movies. A way to be sure not to miss any, is measuring up to infinitely long on every frequency. For a standard scan of over 10K measurement frequencies this will cost a lot of time and money. Conclusion: No emission measurement can be 100% accurate with 100 % certainty. Regards, Ing. Gert Gremmen ce-test, qualified testing bv Van: [email protected] [mailto:[email protected]] Namens Price, Edward Verzonden: Thursday, December 04, 2008 4:24 PM Aan: [email protected] Onderwerp: RE: Radiated emission - time at each frequency > -----Original Message----- > From: [email protected] [mailto:[email protected]] On Behalf > Of John Woodgate > Sent: Thursday, December 04, 2008 7:08 AM > To: [email protected] > Subject: Re: Radiated emission - time at each frequency > > In message > <9d04b979323dcd428297dda95108893e0120c...@bb-corp-ex2.corp.cubic.cub>, > dated Thu, 4 Dec 2008, "Price, Edward" <[email protected]> writes: > > > >The simplistic answer is that you should dwell at a frequency long > >enough for the detector to respond to any emission that occurs. > > It's a base rumour that some regulatory test houses dwell > until they get a 'FAIL'. (;-) > > Seriously, it's a very difficult question to answer. If the > product has some sort of cyclic behaviour within a short > time, waiting some tens of milliseconds may be acceptable, > but what about a washing machine with a 150 minute cycle? > John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK I think the conservative way to approach that would be to see if you could eliminate the time delays in a cycle without affecting the events of the cycle. For instance, you could rig the "wash" cycle to turn on, then off again in a second or two, eliminating 5 minutes of boring motor running. You could rig a water level detector to trip without the tub actually filling, etc. You might also do an analysis to show that certain functions are redundant or duplicative, for instance, the spin cycle may consist of a motor start/stop operation identical to a spin cycle, so you could eliminate one of the whole cycles. Some of my test setups have used multiple strings or cords so that I can yank a cord to fake some mechanical sensor in an EUT into thinking it is ready to move on to the next function. Regards, Ed Price [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 - 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]> 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]>
