electromagnetic compatibility (EMC)
/ EMC /
A good citizen neither shouts so loud they disturb the neighbours nor falls apart at the first loud noise from next door. Electromagnetic compatibility, EMC, asks the same of an electronic product: it must not spew out enough electrical noise to upset other equipment, and it must keep working properly when other equipment's noise washes over it. Getting along with the electromagnetic neighbourhood, in both directions, is the whole idea of EMC.
EMC has two halves. EMISSIONS is the noise your product sends out, either radiated through the air or conducted out along its power and signal cables, and your fast clocks and switching supplies are what generate it. SUSCEPTIBILITY, or immunity, is how well your product keeps working when interference comes IN, from a nearby radio transmitter, a mains surge, or an electrostatic-discharge zap. Each half has both a radiated and a conducted path. EMI (electromagnetic interference) is the actual disturbance, while EMC is the goal of coexisting despite it. Most countries require products to pass EMC tests, such as FCC limits in the US or CISPR/CE limits in Europe, before they may be sold.
The uncomfortable real-world truth is that EMC is decided in a test chamber by physical orientation: the lab rotates your product on a turntable and sweeps an antenna up and down looking for the worst-case emission, and a single unshielded cable or one gap in an enclosure can make you fail at a particular frequency and angle. The cures are the whole of this field, decoupling, tight loops, ground planes, shielding, twisted pairs, ferrite beads, slowed edges, and filtered cable entries, all designed in from the start. Honest note: EMC is nearly impossible to bolt on at the end, and products that ignore it until the test lab usually fail and get expensively redesigned.
A product fails radiated-emissions testing at 120 MHz because an unshielded ribbon cable is acting as an antenna for one of its clock harmonics. Clamping a ferrite bead on the cable and adding a ground connection at its entry point drops the emission below the limit line.
One cable can fail the whole product — EMC is designed in, not added on.
EMI is the interference itself; EMC is the ability to coexist with it. Emissions and immunity are separate tests, and passing one does not mean passing the other.