The Power Clamp method measures the current flowing on a wire into a fixed common mode impedance formed by the added ferrites following the current clamp. They sum up to form a +/- 150-200 Ohms impedance, independent of the cable behind it. This eliminates the effects of cable length , reflections on it and makes a good average reproducible measurement set up. Now this measures ONLY the EMI effects from the attad cables.
Of course the EMI from the enclosure does not pass the clamp and is not taken into account here. Therefore any test engineer should only apply this test when it is sensible to conclude that the EUT does not radiate except by it's cables. This is the case when: - the wavelength of the measured impedance is much greater than the enclosures maximum size - the max size of the enclosure is so small that it is much smaller then the highest frequencies wave length (most often 300 Mhz :: 1 meter) - much smaller is at least 1/20th - The clamp method is useless above 300 Mhz as the included current transformer CANNOT be constructed in reasonable sizes for frequencies above that. - the larger the clamp; the lower the usable frequency. If one uses a current clamp only, the remaining cable does influence the test values. One should vary their lengths or add additional ferrite cores behind the clamp for common mode impedance stabilization. The limit values (40/44 dBpW) most often used with a ferrite clamp correspond to the 30/37 dBuV/m CISP22 etc. limits, when this power is thought to be radiated by a resonant dipole antenna. This can be proved very easily. So if used for pré compliance measurements, limit them to 300 Mhz, use the smallest clamp (or the official luthi clamp) available, add ferrites if necessary, and make sure the enclosure is much smaller then wavelength. However, if your clamp measurements show that no power is emitted at all, you might conclude that no power is radiated either from the enclosure too. Regards, Gert Gremmen Ing. == Ce-test, Qualified testing == Consultants in EMC, Electrical safety and Telecommunication Compliance tests for European standards and ce-marking Member of NEC/IEC voting committee for EMC. Our Web presence: http://www.cetest.nl List of current harmonized standards http://www.cetest.nl/emc-harm.htm 15 great tips for the EMC-designer http://www.cetest.nl/features01.htm -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Rene Charton Sent: zaterdag 13 februari 1999 16:13 To: [email protected]; EMC-PSTC (E-mail) Subject: Re: Pre-Compliance testing for radiated emissions using a current probe 1. For a long time it was stated in CISPR22/EN55022 that some regulatory agencies think that the clamp method is a good substitute for a OATS measurement. But this method never was accepted for CISPR22/EN55022 and can not be found in any other standard but CISPR14/EN55014. Why? 2. The clamp measures the HF power available on the wires going out of the equipment. But is that all? Why do we have to put so much attention on the enclosure seams of personal computers? Would you also put a clamp on the enclosure seam? 3. Putting all wires together into one clamp is certainly not a good idea. The HF-currents can add up, but as well can also cancel each other. 4.Look at a dipol antenna: In the middle a Balun (Power Source) Each side one rod coming out. Look at a personal computer: In the middle the computer, on each side a wire acting as antenna coming out. Do you think a absorber clamp can remodel that scenario? Regards Rene Charton ---------- | 1H%s*L: POWELL, DOUG <[email protected]> | &,%s*L: EMC-PSTC (E-mail) <[email protected]>; 'Treg (E-Mail)' <[email protected]> | %D&.: Pre-Compliance testing for radiated emissions using a current probe | $i4A: 1999&~2$k12$i AM 03:28 | | Hello Group, | | Some time ago (1996) in an email thread on absorbing clamps, Michael A. | Royer stated that CISPR 14 defines a method for using current clamps instead | of open area test sites. There was another thread for an on-site radiated | emissions test when in noisy environments that mentioned using clamp-on | probes. | | Additionally I recall and article in one of the trade magazines (Compliance | Engineering, ITEM, etc.) that described a method where a person could bundle | all of the service connections for a product into an RF current clamp-on | probe. The article also had information on converting the data on a | spectrum analyzer into equivalent open area test site numbers. | | I would like to perform a test like this but have been unable to find a | clamp-on probe that has at least a 100mm inner diameter and a frequency | range of 30 MHz to 1000 MHz, possibly this is limited by physics. | | Is there anyone out there who is able to point me to articles or websites | on: | | 1) RF current probe manufacturers. | 2) The article and/or mathematical algorithm on measuring radiated | emissions with current probe. | 3) Application information. | | Thank you for any help you can provide. | | ================================== | Doug Powell, Compliance Engineer | Advanced Energy Industries, Inc. | Fort Collins, Colorado USA | | [email protected] | www.advanced-energy.com | ==================================
