Last Week's Question

Circuits illustrating common-impedance coupling

Common-Impedance coupling is likely to be the dominant coupling mechanism when the source circuit is operating at,

  1. low frequencies and low impedances
  2. low frequencies and high impedances
  3. high frequencies and low impedances
  4. high frequencies and high impedances

Answer

The correct answer is "a". Of the four EM coupling mechanisms, common-impedance coupling is the only one that doesn't necessarily diminish in strength as the frequency gets lower. In fact, it's the only EM coupling mechanism that works at DC (0 Hz). Also, since the coupled voltage is generally proportional to the current in the source circuit, low-impedance circuits are more likely to be the source of common-impedance coupling problems.

In most situations of practical interest, electric-field coupling and magnetic-field coupling are proportional to frequency. At higher frequencies (e.g. 10s of kHz and higher), these coupling mechanisms are more likely to dominate. 

Radiation coupling requires conductors in both the source and victim devices to behave like reasonably effective antennas. This can only happen when these conductors have sufficient size relative to a wavelength. At low frequencies (long wavelengths), this is less likely to be an issue.

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