Eric, You are correct in your evaluation of EN 55011 (Cispr 11). Keep in mind that the situation of not allowing distance correction is only found in Cispr 11, NOT Cispr 22.
In reference to this specification, I had the opportunity to converse with several Competent Bodies in reference to Cispr 11 Class A test distance requirements. All three of the Competent Bodies I spoke to confirmed that they will accept extrapolated Cispr 11 data measured at 10 meters. They all cited that the specification is being re-vamped, and the 30 meter distance requirement is expected to be changed. Eric brought up a good point, and I would be interested to hear if anyone has run into problems due to this 10m/30m CISPR 11 situation. Sincerely, Randy Flinders EMC Test Engineer Emulex Network Systems * opinions expressed are completely personal, and do not reflect the position of Emulex Corporation. ---------- From: Eric Lifsey To: emc-pstc; treg Subject: Distance of Measurements, 55011 List-Post: [email protected] Date: Monday, June 16, 1997 9:36AM Shall I muddy the issue further...let me address an interesting section of EN 55011:1991. But first; for reference; Clause 5 of EN 55011 lists the limits and measuring distance for Class B at 10 meters, Class A at 30 meters. Section 8.1.3 entitled Radiation Measurements (9 kHz to GHz) speaks to the issue of high ambients and moving to a shorter distance to take measurements. However, the wording (IMO) indicates that the 20* Log(D/d) correction is not allowed. Let me quote: "Note - Due to the unpredictability of the relationship between the measured values of the electromagnetic radiation disturbance and the distance from the equipment under test no correction is allowed for in the limits when measurements are made at a closer distance." "no correction" The key implication (IMO again) is that running Class A measurements at 10 meters must meet the limits specified at 30 meters! Please correct me (pun) if I'm wrong! To my knowledge, many labs are doing Class A measurements at 10 meters. (IMO, 10 meters is better because most sites need an additional RF preamp for 30 meter sensitivity; if not handled carefully, measurement uncertainty is worsened. Of course, EUT size and other factors exist.) CISPR-11 is due for an overhaul (and for other interesting problems too) and I hope they fix this one while they're at it. Regards, Eric Lifsey National Instruments
