One of the questions we get asked often is about order of EMI/EMC testing. Neither MIL-STD-461 nor RTCA/DO-160 specify the order of test performance. Leaving aside the issue of Safety of Flight tests for aircraft (which typically must be performed prior to any other testing), there are a few different approaches to take in this regard.
The first approach is to perform an analysis of the equipment under test (EUT) before going to the lab to determine what tests are most likely to cause problems, and to start with them.
This approach works best if a customer does not have any idea how their product will stand up to the EMI/EMC compliance requirements. A design analysis tends to vet out significant concerns up front, potential design solutions can be discussed prior to qualification testing. At this point, the Subject Matter Expert (SME) should be able to prioritize the threats, and work with the customer to develop a suitable test order. This approach also provides an opportunity for pre-qualification evaluations to ensure the product will not have any issues during the qualification program.
The second approach is to begin with the most benign tests, usually the emissions. These tests have virtually no chance of harming the EUT, but they sometimes prove to be the most problematic. Emissions testing tends to reveal inerrant design flaws the most, and generally requires some level of redesign. Changes in design could necessitate repeating other tests if emissions is not completed first. However, there may be cases were a customer feels their product does not have any emissions concerns but is likely to be susceptible to a particular immunity test. They may choose to get the immunity evaluation out of the way first, and allow time for potential modifications prior to commencing with the remaining tests. This approach would also avoid costly retests or delays due to changes to the EUT.
The third approach is to begin with the most potentially damaging tests first. The philosophy here is that all is well and good if the EUT endures those tests with no issues. However, if there are susceptibilities that require repair/redesign, those problems can be resolved before continuing with the other tests.
In summary, the EMC / EMI testing sequence used should be an iterative decision between the lab and the customer to determine which approach best suits the product and customer’s needs.