Technical note 1

EMC: applied theory of electromagnetism, not black magic.

EMI is caused by leakage of power from or into circuits via parasitic elements, not shown in any diagram:

1) Components and conductors have undesired loss resistance, stray capacitance and stray inductance.
Resistance = voltage drop per unit of current, only meaningful for current-carrying conductors.
Capacitance = stored charge per unit of voltage, only meaningful for charged conductors.
Inductance = magnetic flux per unit of current, has only meaning when current flows via a wiring loop.

2) Signal transport always occurs via transmission lines, implying time delay and risk of reflections.
Electrical diagrams only show ideal components and conductors without parasitic properties or time delay.
Build an electronic circuit as prescribed in the diagram and there will be no EMC problems!
Direct interaction between electric circuits and the surroundings is usually small. Parasitic behaviour causes coupling between circuits and the wiring to the outside world, resulting in conducted and radiated emission and conducted and radiated susceptibility. Reflected signals interfere with directly received desired signals and so distort these signals.
The fundamental solution to any EMC problem: Eliminate the parasitic properties and the time delay.

Complete application notes are available from our information server, Standards & Documents section.