Troubleshooting EMI (Part II, Susceptibility Problems)

Some simple hints . for identifying and fixing EMI troubles. This second part of our EMI Troubleshooting guidelines is devoted to investigation and cure of EMI susceptibility problems. Since, according to our former article (Ref.1) conducted tests are faster and easier to perform, we will concentrate on these and avoid more complex and expensive radiated susceptibility test set-ups. The first half of this article will address the situation where the equipment at stake is still in the plant or lab, at the end of its development phase or in an early stage of production. The second half will cover the more difficult cases where the faultly equipment is already in service, installed at some customer site.


1. Susceptibility Problems on Prototype or Pre-Qualification item

Only a limited set of tests can be carried using essentially conducted excitation set-ups, without (or before) resorting to an actual Radiated Susceptibility check in an EMC lab. Preferrably, as explained in Ref. 1, susceptibility checks should be done after emission test have been carriedout, and eventually their non-compliance been investigated and solved. Notice that so-called ”Susceptibility” tests are in fact made to demonstrate EUT Immunity. Therefore, we should always try to reach, within a reasonable span, the actual ”Fail” threshold to get a measure of our Compatibility margin as compared to the Immunity ”No-Fail” requirements.

Fig. 1. Summary of Susceptibility/Immunity tests.

Like for emissions, we try staying away from formal Radiated Susceptibility (R.S) tests, and use substitute conducted tests instead. These conduction (or injection) tests are deemed to recreate on the EUT what its cables (power and signal) and internal circuits would actually receive from a severe, conducted or radiated threat. So, by order of simplicity and efficiency we will limit our tests to:

  • Electrical Fast Transients (EFT Bursts)
  • ESD ( Direct and Indirect)
  • Bulk Current Injection (Continuous Wave )

2. Requirements for informal Suceptibility Test site and EUT set-up

The site requirements for an informal EMC Immunity test are the same as for emissions (Ref.1), except that the quiet RF ambient requirement is exactly the opposite: a quiet ambient is not necessary. Instead, since susceptibility tests will force strong HF signals or fast pulses, it is prudent to forbid, or temporary discontinue the use of sensitive electronic equipments ( computers, office equipment and instruments other than intentional EMI generators) in close proximity – say less than 5m from the test set-up. For the same mirror reasons as above, it is recommended to filter and if possible isolate the AC power branch circuit that feeds the EUT and set-up. This can be done by the same LISN and isolation transformer that were used for emission testing. During the susceptibility test, we will limit the Auxiliary Equipments (AE) to a minimum, using everywhere possible passive loads at the end of I/O cables. A good example is a Tx/Rx link that can be set in self-looping mode. As for emissions tests, the AEs must have an Immunity level equal (or greater) to the one we are testing for. If this is not feasible, an added, temporary filtering on their interfaces, or Coupling/Decoupling Network (CDNs) can be used to isolate the…


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