Bob, Can a directional coupler be used between the amp and the clamp to measure the forward power during the calibration test, then with the clamp connected to the EUT, re-level on your calibration points to match the forward power? Would this compensate for the impedance difference in the EUT? This is a similar method used for radiated immunity to adjust for impedance differences between the antenna and the field affected by the EUT. The Other Brian
_____ From: [email protected] [mailto:[email protected]] On Behalf Of Bob Richards Sent: Monday, September 17, 2007 9:27 AM To: [email protected]; ieee Subject: Re: Conducted RF immunity Amund, For the current to be that high, the impedance of the EUT and the AE would both have to be very low. The EM clamp is designed to have some directivity, but not at the lower frequencies, and never more than about 10dB through any of the frequency range. In a perfect 150 ohm system, with the EUT and the AE both representing a 150 ohm impedance, you would see only 33 ma current. The reason it would not be 66ma is due to the test level being specified as an open circuit voltage. The voltage in a terminated system would be half that, or 5v. If the AE is 0 ohms impedance and the EUT 150ohms, then using the formula in the standard to limit the current to 66ma or less would actually overtest the EUT in my opinion. This is one reason why CDNs are preferred. But, in the case where the CDN represents a 150ohm impedance and the EUT is close to 0 ohms, you would also see close to 66ma current for a 10vrms test. The software used to run the test should be able to limit the injected current based on feedback from the measuring probe. Some software will, some won't. I've seen software that will replay the pre-calibrated level, measure the current, then cut back the test level if needed. But, the pre-calibrated level may have already caused the EUT to fail. A better way would be for the software to ramp up until either the pre-calibrated level is reached, or the max current is reached, whichever comes first. If you run the test at the pre-calibrated levels and the EUT passes, no problem. But if you run the test and the EUT fails, and it is determined that the current was over 66ma at the failing frequencies, then you could run the test with the software reducing the test level as needed. I hope this explanation helps. Bob Richards, NCT. [email protected] wrote: When testing with EM clamp, the IEC/EN61000-4-6 gives a requirement to not exceed Imax (e.g Imax = 10V / 150 ohm = 66mA). We use a current clamp to measure on a 2-wire unscreened cable, and it is placed between the EUT and EM clamp. The current is measured to approximate 130mA. The test lab say it is due to the EUT impedance that the current is that high, I mean that the max current should not exceed 66mA whatever the EUT impedance is. How is this Imax requirement to understand? Best regards Amund Westin Oslo, Norway - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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