>Dear All, > >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? > >Would you? > >1. Ignore that frequency. > >2. Move the EUT to 3m and hope the EUT level is above the abient level. >What happened if the EUT is too large to be regarded as "electrical >small". > >2. Use the 3m semi Anechoic Chambers results -10dB, if this means fail >then modify the EUT in the chamber until it passes. > >4. Use different OATS. > >Are they any guideline from the Trade Industry regarding this situation? > >Any comments are very welcome. > >Best Regards > >Vi Van >EMC engineer >Mitsubishi PC.
Vi, (2) would be the preferred method. Failing that a substitutition, or narrowband peak/average (10KHz), measurement of the signal could be performed. This usually helps when the emission is riding on the skirts of, say, a TV signal. The narrowband level should be very close to the QP level provided that the emission is a narrowband signal. If the emission is a broadband signal then..... The test house should be aware of its ambient signals. Whenever selecting which signals are to be measured outside on the OATS they should be ensuring that the test engineer is given plenty of information about the type of emission when they fall close to known ambients. They can also select similar signals at frequencies either side of the bothersome signal and interpolate between indoor and outdoor measurements - CISPR 22/EN55022 section 11.4 (b) refers. Mark
