-------------- Original message -------------- Neven, I do not understand, Why in EMC world people always believe in mesurement. Mesurement can also be wrong especially in precomliance world. With regard to prdiction, look for for IBM, INTEL etc. Do you think they do not use any prediction tool during design?. IBM has a propietory tool, which is very effective in EMC modelling and prediction. Sudhakar
Sudhakar, Measurement is: a) The final confirmation and internationally accepted through standards as a proof of emission levels b) The best method known so far to demonstrate compliance with the standards I agree with you, and it is a generally known that in the EMC world the measurement accuracy can be poor. But, it is not due to the test equipment, i.e. antennas and receivers. It is mostly due to uncontrollable parameters related to the DUT setup and characteristics, as well as the test site characteristics. For every measurement, one must know the accuracy levels, otherwise it is not really a measurement. With a DUT that is not changing, the typical accuracy and repeatability of the chamber measurement can be a couple of dB, assuming the measurement is preformed by competent test engineers. On the bench pre-compliance measurements, if one understands the measurement, the accuracy and repeatability can actually be much better than in a chamber. The question here is not the accuracy but the correlation between the bench-methods and the chamber measurement. The relative correlation, e.g. relative level of improvement obtained on the bench after the product has been characterized in the chamber, measured on bench and in the chamber can be excellent, I'd say within a couple of dB. Prediction levels are of course not so accurate, but they can be useful too. The assumption I am making is that one understands the technical issues with the measurement and is familiar with the product. I believe that by "tool" you mean a numerical modeling tool, and I would yet have to see one that is useful to model a real product. There are numerous tools that can be pretty accurate in modeling a simple situation, say a trace on a PCB, source and a load. Even for a little more complicated situation and if you put it in an enclosure with defined characteristics, there are tools that can give you accurate results for the input that you define. These numerical tools are excellent for what-if analysis and relative A-B comparisons. However, here it ends. One problem each tool would encounter is the same as described above - variations in the DUT setup (including cables). A real product is typically much more complex that any SW tool can handle, given realistically available time and information on the relevant parameters. Many if not most of the parameters relevant for the emission profile are not well-defined. Just to ! mention some: common-mode voltages across the planes due to non-ideal non-zero impedance variations of contact resistances along the seams common-mode voltages due to power and GND bounce on unknown cross-conduction currents through the (often unknown) impedance in the package leads effects of non-ideal current paths ..... and many more. Regards, Neven From: [email protected] mailto:[email protected]]On Behalf Of [email protected] Sent: Tuesday, December 14, 2004 8:37 AM To: [email protected] Subject: RE: Simple EMC Prediction If the definition of prediction ends at wetting the finger and trying to feel it, I agree it is not possible - at least for us mortals. In my opinion, prediction can be based on information ganed from sources other than a large CISPR compliant facility, typically using some pre-measurements obtained on a bench or similar. So, it can still involve measurement. Neven -------------- Original message -------------- Neven, Ravinder, I think that many people here know of current clamps and how to use them ... I feel that you addressed rather special cases than a general possibility like Robert invoked. Speaking of the ASIC which was the source for common mode noise, this was a good root-cause analysis, but I guess nobody _predicted_ it. A good root-cause analysis is the base for later estimations (I will not call it prediction) which modification will have an impact at what frequencies. After that you can use whatever test method to evaluate the differences. Having full control over the design, prediction can be made possible. The problem starts when you use components where the manufacturer tends to implement modification without telling a word (and believe me, many IC vendors tend to). It becomes worse when third-party products are involved and the vendors usually do not report very much on changes unless you encourage them ... Maybe it's just a matter of definition - prediction ends for me when I have to start measuring ... Regards, Michael From: [email protected] [mailto:[email protected]] Sent: Montag, 13. Dezember 2004 20:08 To: [email protected] Subject: RE: Simple EMC Prediction I agree with Neven. EMC prediction after a design change is possible if the sources of EMI have been identified and characterized. As Neven has pointed out, current clamp or near-field probe measurements can provide a good indication. One of our design had problem due to high common-mode noise on the differential lines (caused by improper ASIC design), which we had to compensate on the PCB. I had the noise characterized on this design with a current clamp measurement, and the measurements on next revision of ASIC showed a 12 dB drop in the common-mode noise. This clearly shows that we! ! should have no problem passing the compliance, although it is still too early to start pre-compliance/compliance testing. Regards, Ravinder Server PCB Development Hitachi Global Storage Technologies Email: [email protected] [email protected] Sent by: [email protected] 12/13/2004 10:24 AM To: [email protected] cc: From: [email protected] Subject: RE: Simple EMC Prediction Prediction is indeed possible for ceratin classes of products. Sometimes they can be based on comparison with previously maesured products, sometimes it can be a straightforward prediction - e.g. based on CM current on voltages measurements at the cable-interface. The more you know your product, the better chances are you'd be able to predict based on quick mea! ! surements. It is not possible to predict always, but certainly there are methods of efficiently using the bench as opposed to going to a chamber every time one wants to know what impact a modification has or to eyeball the emission perfomance of a product. Elaborating more within this media is not possible. Check current clamps, near-field probes, passive contact probes, and if you are specifically interested in predicting emission from differential-pair interfaces, I have done significant amount of work etablishing the correlation between radiated emisison and common-mode voltage on the diff pairs (specificalyy of Ethernet devices, but it is more generally applicable). Published in Seattle EMC Symposium or search for "Common mode voltage probe" Neven -------------- Original message -------------- Robert, Predicition sounds to me like clairvoyance - not very common to EMC :-) My opinion on this is: Rather than trying to predict the quantitative EMC impact of a modification, it is preferrable to understand if a certain modification could have an impact and which. Example: Cha! nging the supp! lier of a resistor is uncritical as long as this type of resistor is used as a pullup on a quasi-static signal (e.g. reset), it may become critical if it is used in the signal path of a RF circuit. When changes are evaluated under a design point of view, it is easier to figure out _what_ ils likely to happen. There is no 100% security, unfortunately. When it comes to IC manufacturers, you may have the situation that components behave differently in function of their date code and the manufacturer did not announce any mask change (we had! this). In a few words: I do not see a simple way of predicton, but by using the knowlegde of the involved designers and EMC engineers with the help of a well implemented process (documentation helps a lot in this case) it becomes easier to concentrate on the real issues. Implementing some sort of "risk rating" can help to priorize the different tasks. If GTEM cells would be an option for you - a test can be done quickly compared to other methods. Best regards, ! Michael Michael Nagel Senior EMC Engineer Motorola Embedded Communications Computing Group Lilienthalstrasse 15 D-85579 Neubiberg/Muenchen - Germany Ph: +49-89-60814-0 Fax: +49-89-60814-376 e-mail: [email protected] Speaking for me. From: [email protected] [mailto:[email protected]] Sent: Montag, 13. Dezember 2004 13:31 To: [email protected] Subject: Simple EMC Prediction Good Morning EMC Gurus, We have the constant issue here in our lab of test time vs. resources. Often we are proposing different design changes for various reasons (cost savings, new supplier because of discontinued component etc.) This deals with existing products that can be updated for various reasons. The problem is we have a hard enough time keeping up with our new business/projects that require full validations. In most cases failures occur and solutions must be implemented to sell the products to the! ! customer. In the case of a current production product often design changes are proposed but of course some sort of validation must be performed before making this design change. The design change can consist of electronic components, material change, and mechanical changes. My question is if anyone else has a similar issue in their lab and what kind of tools can be used as a quick and efficient "filter" to either proceed with a full boat of testing or eliminating the idea and moving on to something else. The most common problem is most design changes require Radiated Emissions testing. With only one Anechoic Chamber and the length of the test this is hard to deal with. Any ideas besides knowledge o! f! a quick and simple test we can use to perform this type of filterin g? Our products are automotive so in some cases we are talking PCB level but in most we! are talking about various dc motors. Of course we have the know ledge but as you all know in the EMC world that is far from the final say all. I would be interested in all responses. Thank you. Rob Kado EMC Engineer VWS - Auburn Hills, USA Tel: 248-340-3828 / Fax: 248-340-3316 Internal Location 3M2 ----------------- ---------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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