Interference, Electromagnetic Compatibility and Mitigation
This domain tests the vocabulary of interference and EMC, the mechanisms that create and couple unwanted energy, and the practical mitigation toolkit. Expect questions on emissions versus immunity, on identifying the frequency of a mixing product, and on the least disruptive fix.
Definitions that carry marks
Interference is any unwanted energy; harmful interference is energy that actually degrades, obstructs or repeatedly interrupts a radiocommunication service. EMC has two halves: emissions, meaning what a device throws out by conduction or radiation, and immunity, meaning how much disturbance it tolerates. Licence-exempt apparatus enjoys no protection from interference and may not cause it.
Mechanisms
Co-channel and adjacent-channel energy; harmonics at integer multiples of the fundamental; spurious and out-of-band emissions from imperfect modulation, keying clicks and mixer products; intermodulation, where two or more signals mix in a non-linear stage to create sum and difference products; broadband noise from switching supplies and digital clocks; blocking and desensitization in receivers.
Mitigation
Filter at the source first: low-pass, band-pass and notch filters, shielding and tight enclosure seams, ferrite chokes, twisted pairs, single-point bonding and grounding, physical separation and antenna orientation, power control, duty-cycle limits, and channel planning. Filtering the emitter usually beats filtering the victim.
Worked example
Two transmitters at 100.0 MHz and 100.5 MHz share a combiner. Third-order products appear at 2(100.0) - 100.5 = 99.5 MHz and at 2(100.5) - 100.0 = 101.0 MHz. A receiver listening on 99.5 MHz hears a signal that no transmitter emits. The cure is not a filter on the receiver but improved isolation between the two transmitters, an isolator or circulator, cavity filters, or reduced drive level, since third-order products fall faster than the carriers when drive is backed off.
Traps that cost marks
Blaming the victim instead of finding the emitter; confusing immunity with emission limits; quoting out-of-band rejection while ignoring insertion loss; assuming a licence-exempt device that causes harmful interference may keep operating, when it must cease; and forgetting that harmonic levels scale with fundamental power.