EMI/EMC Testing Best Practices – Before & During Your Lab Visit

Most electrical/electronic designs today are subject to electromagnetic interference/electromagnetic compatibility (EMI/EMC) testing.

Preparation before entering the test laboratory is vital.

Before Testing
The first step is to write a test plan. If you are working in the defense industry, a military test plan is usually a contract requirement. If not, you’ll still want one.  If you don’t know enough to create one, ask your lab to do it, or to recommend an independent consultant who can help.

A good test plan includes:

  • A configuration, mode of operation, and monitoring method, which represents a worst case scenario from an EMC perspective
  • Special software, test fixtures, and supporting equipment may be needed to exercise the equipment under test
  • A description of hardware to be tested, including peripherals & I/O configurations
  • An indication of which external power and data I/O ports need to be tested for each test method
  • Required tests
  • A definition of failure criteria
  • How to monitor, recognize & report failures
  • Special needs: Software, power, cooling, etc.

Then, get pricing and scheduling from a leading 3rd party test lab, like MET Labs.

You’ll have to determine what the lab will supply versus what you will supply.  You will need:

  • Equipment under test (EUT) & spares
  • Cables & connectors
  • Test fixture (for some programs)
  • Tool kit
  • EMI suppression supplies – ferrites, copper tape, etc.
  • The equipment’s design or compliance engineer or someone else familiar with the product to witness testing

In the Lab
Preliminary testing (pre-testing) is always a good idea.  Shortened versions of each test method can be performed to identify failures.  Design modifications can be made before final testing is scheduled.  A radiated emissions pre-test, often referred to as a pre-scan, is the most common pre-test performed.  Even if there is no plan to perform pre-testing for the other test methods, a radiated emissions pre-scan can identify failures and allow for design modifications, which will likely cause the equipment under test to perform better for the other test methods as well.

Then run the full program as specified in the test plan.  If you pass the first time, congratulations!  If not, don’t take it personally – it’s not unusual, especially with early-stage pre-compliance testing.  And practically anything can be overcome. 

If you fail a test, do some quick troubleshooting – you may be able to fix the problem right away.  Do the easy things first:

  • Verify that the EUT is still working properly. This is particularly important with immunity tests that might cause damage.
  • Unplug external cables to see if it improves results.  External cables, although not designed to do so, act as antennas to radiate emissions and receive RF interference.
  • Add ferrites to cables
  • Clean mating, conductive surfaces of paint and other materials.
  • Add an RF filter module at the power input to the equipment under test to limit RF emissions and protect from continuous conducted disturbances.
  • Add a MOVs or other transient limiting device at the power input to protect from transient disturbances.
  • To limit leakage, wrap the EUT in aluminum foil

For immunity, back off the test levels to determine the actual failure levels. If you are close, maybe a ferrite will fix things. If not, that’s good information to have – it will help you narrow the possible failure mechanisms.

Don’t be afraid to ask for suggestions.  Test engineers at an experienced lab like MET will have seen hundreds if not thousands of products, and know many debug and quick-fix solutions.

MET Labs is a full-service EMC testing lab with multiple convenient locations.  Contact us for a free quick-response quotation.

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