Vi, ---------- Your dilema: >Radiated emission can be performed either on an open area test site or a >semi-anechoic chamber. > >Many test house do the initial scan in the anechoic chamber to find >worst frequencies before performing it on an OATS. > >The initial scan in the 3m Anechoic chamber shows that there are >serveral frequencies may be above the limit. > >So EUT were taken out to test on an OATS. > >What I would like to know is if you are performing the test on an OATS, >unfortunately the abient level for those frequency are very high and it >is above your EUT level. What would you do?
If the emissions in question are narrowband emissions (specifically from a specific clock or respective harmonic frequency), then reducing the RBW (e.g., 100KHz to 30KHz) of the analyzer/receiver should not impact the actual measured amplitude significantly and will likely give these pesky emissions more visibility. However, using a lower RBW to obtain data may be looked on unfavorably by the authorities. Document what you do including rationale for any decisions you make. If they are still not detectable or if the emssions are broadband emissions, then see below. >Would you? > >1. Ignore that frequency. Definitely not. >2. Move the EUT to 3m and hope the EUT level is above the abient level. Sure. At a shorter distance the EUT emissions will be made more significant proportionally to the background signals and should be more measureable. >3. Use the 3m semi Anechoic Chambers results -10dB, if this means fail then > modify the EUT in the chamber until it passes. It should be ok provided that use of this particular chamber is acceptable to the FCC via FCC listing and/or NVLAP or other accreditation body. >4. Use different OATS. Possibly, but remember that one OATS will likely be different from another (i.e., site attenuation differences), so the measured results will vary accordingly. >Are they any guideline from the Trade Industry regarding this situation? None that I can think right away. However, I do recommend that an "off" time (at night) test time might prove beneficial since most EMI activity occurs during the daytime. I've had to do this on more than 1 occasion. Whatever you decide, remember that the choice you make is yours. Also, remember that the above is my opinion only and should not relate to anything or anyone. I hope this helps. Regards, Ron Pickard
