Types of noise coupling mechanisms

Noise coupling is the transfer of unwanted energy from a noise source to a receiver (victim) circuit through a coupling path. Every noise problem has three parts: source, coupling path and receiver. There are four main coupling mechanisms.

Conductive coupling

Noise travels along a real conductor connecting source and receiver, such as a shared power supply line or common ground. The common impedance (resistance and inductance) of the shared path turns one circuit's current into a noise voltage seen by the other: V = I × Z (and L·di/dt for the inductive part).

Capacitive (electric field) coupling

A stray capacitance exists between any two conductors at different voltages. A changing voltage on the source injects a current into the receiver: I = C·dV/dt. It is strong with high dV/dt, high-impedance receiver, long parallel wires and small spacing.

Inductive (magnetic field) coupling

Current in a loop creates a magnetic flux. If another loop encloses part of it, a voltage is induced: V = M·dI/dt, where M is the mutual inductance. It is strong with high dI/dt, large loop area and close loops.

Radiated (electromagnetic) coupling

At a distance greater than about λ/2π, electric and magnetic fields combine as a radiating wave. Wires and traces act as antennas, so noise from radio transmitters, motors or switching supplies is picked up. It depends on frequency and the antenna length.

Source ---(conductive: wire)-------> Receiver
Source ---(capacitive: Cs)---------> Receiver
Source ---(inductive: M)-----------> Receiver
Source ~~~(radiated: EM wave)~~~~~~> Receiver