Noise coupling mechanisms with pseudo impedance

Pseudo (source) impedance is the ratio of voltage to current of the noise source, Z = V/I. It tells whether the noise energy is mostly electric field (high V, low I) or magnetic field (high I, low V). It is compared with the intrinsic impedance of free space (377 ohm).

Types of coupling

• Capacitive (electric field): high-voltage, low-current source, so Z is high (Z ≫ 377 ohm). The stray capacitance C passes I = C·dV/dt. Examples: high-voltage lines, unshielded fast switching nodes.
• Inductive (magnetic field): high-current, low-voltage source, so Z is low (Z ≪ 377 ohm). Induced voltage V = M·dI/dt. Examples: motors, transformers, power cables.
• Radiated (electromagnetic): in the far field the electric and magnetic fields are in a fixed ratio; wave impedance Z = 377 ohm (E/H = 377). Examples: radio transmitters, long antennas.
• Conductive: noise flows through common wires; here Z is the impedance of the shared conductor, and noise voltage = I × Z_common.

 Z (pseudo impedance)
  high  >>377 ohm : capacitive (E field)
  377 ohm         : radiated (plane wave)
  low   <<377 ohm : inductive (H field)

Near fields change with distance. For a high-impedance (electric) source, the electric field falls off as 1/r³ and the magnetic field as 1/r²; for a low-impedance (magnetic) source it is the other way round. In both cases the wave impedance moves towards 377 ohm as the distance exceeds λ/2π. So by calculating Z of the source, the dominant coupling can be identified and the correct shield chosen (electrostatic for E, magnetic for H).