Pre-compliance EMC Test
A pre-compliance EMC test is measuring, in one's own environment, the electromagnetic interference a product emits and is exposed to, before going to an accredited laboratory, and correcting the design accordingly. Its results do not replace an official certificate; the aim is to catch and fix obvious problems before entering accredited measurement and thus reduce the number of correction rounds.
EMC has two directions: the interference the device emits into its surroundings (emission) and its immunity to interference coming from the surroundings. Switching power stages, motors driven with PWM and relay contacts are the most typical emission sources.
- What a pre-measurement gives: in which frequency region there is a problem, and whether a change that was made improved the situation or not. The decision on the absolute limit value belongs to the accredited measurement.
- Typical corrections: adding a filter at the supply input, using a ferrite on the cable, slowing down the switching edge, shortening the ground return path, terminating the cable shield from the correct end.
- Effect of cabling: the measurement result is determined by the cable layout as much as by the board itself; the test setup should resemble the real installation.
Corrections made at the pre-test stage are cheap because they are incorporated into the board revision. Making the same corrections after accredited measurement rounds brings both time and repeat-measurement burden. Which certificates and tests will cover the scope of conformity varies according to product type and is clarified at the start of the project.
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