Noise coupling types and checking predominance

Noise coupling mechanisms

Noise coupling is the transfer of energy from a source to a victim circuit. The four types are:

• Conductive: noise flows through a shared conductor (power line, common ground impedance). Voltage = I × Z of the shared path.
• Capacitive (electric field): a stray capacitance C between conductors passes a noise current I = C·dV/dt into the victim.
• Inductive (magnetic field): a mutual inductance M between loops induces V = M·dI/dt.
• Radiated (electromagnetic): a far-field wave from a transmitter, motor or switching device is picked up by wires acting as antennas.

Checking which one is predominant

Use simple experiments and the field's wave impedance (Z = E/H):

1. Remove the physical connection. If the noise disappears when the shared supply/ground path is separated or filtered, the coupling is conductive.
2. Change the receiver impedance. Lower the victim's input impedance (add a low resistor to ground). If the noise falls strongly, it is capacitive (high-impedance electric field). If it does not change much, it is magnetic.
3. Change the loop area and orientation. If shrinking the loop or twisting the wires reduces the noise, it is inductive. Rotating the receiver loop changes the picked-up voltage.
4. Add an electrostatic shield (grounded foil) between the circuits. If the noise drops, it is capacitive. If a magnetic shield (steel/mu-metal) helps, it is inductive.
5. Move the circuits apart. Near-field (capacitive and inductive) noise drops very quickly with distance, faster than 1/r, while a radiated field falls only as 1/r.
6. Compute the source impedance. Z = V/I of the source: Z much greater than 377 ohm means an electric field (capacitive), much lower means a magnetic field (inductive), and about 377 ohm means radiated.
7. Frequency check. If the noise frequency matches a nearby transmitter or clock and appears even with the cables disconnected, it is radiated.