Electromagnetic compatibility and interference
Electromagnetic compatibility and interference
This domain tests emissions, immunity, coupling paths, shielding, grounding, and co-site interference. The exam expects you to identify the mechanism before choosing a fix.
Core rules
- EMC has two sides: the device must limit conducted and radiated emissions and must tolerate specified immunity levels.
- Coupling can be common-impedance, capacitive (electric field), inductive (magnetic field), or radiated. Fix the dominant path: return current, twist, shield, filter, or layout.
- Shield effectiveness is reflection plus absorption; seams, pigtails, and cable penetrations often set the real limit.
- Good RF grounding uses short, wide return paths and bonded chassis. A ferrite choke raises common-mode impedance without affecting differential signal.
- Co-site interference includes blocking, desensitization, harmonics, and intermodulation. Two strong tones f1 and f2 create 2f1-f2 and 2f2-f1 products.
Worked example
Transmitters at 150.0 MHz and 150.2 MHz illuminate a receiver tuned to 149.8 MHz. The third-order product 2f1-f2=2(150.0)-150.2=149.8 MHz lands exactly on the receiver. The other product, 2f2-f1=150.4 MHz, does not. A filter at the receiver cannot remove an in-band product generated in its own front end, so improve transmitter filtering, antenna isolation, or linearity.
Traps
Calling a shielded enclosure sufficient when a cable shield is improperly terminated, treating chassis ground as an ideal zero-impedance node, and confusing immunity failure with emission failure. Also do not assume a ferrite bead helps differential-mode noise.