Two EMC issues were discussed how to populate rack for radiated emission
test and correlation of radiated emission tests at different test distances.
For some EUTs there are too many possible mix and match configurations, with
different or same modules, that testing all of them in order to identify the
"worst" would be practically impossible. There is also no guarantee that
full rack will be worse than less populated rack. Switching power supplies
under different loads might have different EMC profiles. Cavity created by
empty slots could resonate and increase certain emission levels. There is no
good way to identify "worst case" configuration. But reasonable effort
should be there and procedure to maintain test consistency and
repeatability. CISPR SC G developed 2 dB procedure to determine
representative ITE configuration for EUTs with multiple ports and/or modules
of the same type. Procedure is in Third Edition CISPR 22:1997 Section 8.1
EUT Configuration. CISPR 11:1997 has procedure from Second Edition CISPR
22:1993 Section 9.1 which requires one module/cable of each type.
Reference test distance is test distance at which limit is specified. Other
test distances could be used, but test at the reference distance would take
precedence. Manufacturer who decide to test product at other than reference
test distance will assume the risk of poor correlation of test results,
resulting in costly over or under EMC design. Potential customer or
enforcement agency could re-test EUT and decide on further actions based on
test data at the reference distance.
My 2 cents...
Mirko Matejic
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