Electromagnetic Compatibility and Interference
Sources and coupling paths
Interference travels by conduction (mains leads, cables) or radiation (fields). Common sources include switch-mode supplies, arcing contacts, digital electronics, and a transmitter's own harmonics and spurious products. Intermodulation occurs when two or more signals mix in a non-linear stage, producing products such as 2f1 − f2 and f1 + f2 − f3. Receiver front-end overload by a strong nearby transmitter is a common cause of phantom signals.
Suppression techniques
Filtering at the source and at the victim, shielding, bonding and good grounding reduce coupling. Ferrite chokes suppress common-mode current on cables without affecting differential signal current. Twisted pairs, screened leads, bypass capacitors and careful antenna siting all help. Site separation and frequency coordination prevent self-interference.
Traps and rule of thumb
A ground loop is a frequent trap; more earth stakes are not always better. Do not assume a filter will fix an interference problem caused by a strong fundamental - the cause may be overload, not a harmonic. Harmonics are exact multiples; intermodulation products are not.
Worked example: two transmitters on 100.0 and 100.1 MHz produce third-order products at 2 × 100.0 − 100.1 = 99.9 MHz and 100.2 MHz, either of which can fall on a nearby receiver.