EMC Question of the Week: December 16, 2019
A computer keyboard malfunctions when it is placed on a desktop near a power strip. There is nothing plugged into the power strip, and the keyboard works fine when the power strip is moved further away. What is the most likely EM coupling mechanism responsible for the interference?
- conducted coupling
- electric field coupling
- magnetic field coupling
- radiated coupling
Answer
The best answer is "b". Because there is nothing plugged into the power strip, we don't expect much current to be flowing. Magnetic fields are generated by currents, so it is unlikely that the coupling is magnetic field coupling. Conducted coupling also requires current, and conducted coupling is not likely to be attenuated by moving the power strip further away. Radiated coupling can be eliminated, because the source and victim circuits are in close proximity (relative to a wavelength at any reasonable interference frequency).
That leaves electric field coupling. There is a voltage across the terminals of the power strip at the power distribution frequency (and at frequencies associated with any noise on the power distribution bus). Voltage differences translate to electric fields. Of the four possible EM coupling mechanisms, electric field coupling is the only reasonable option in this scenario.
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