Electrical noise — what a racket

By William (Bill) L. Mostia, Jr. May 1, 1998

Every electrical circuit has noise in it. Noise becomes undesirable when the signal-to-noise ratio becomes low enough to adversely effect the operation of the electrical circuit.

Electrical or electromechanical devices that cause fast or large changes in voltage or current are common sources of noise. Radio frequency noise can come from walkie-talkies, wireless computer systems, and other radio based systems. Typical noise sources include lightning, power-line switching, switching inductive loads, arcs, fluorescent lights, welding machines, inadequate separation of conductors of different levels, static discharge, harmonics, and ground loops. Noise can appear in both power and signal (control) lines.

Noise is often described by how it is coupled into a circuit. Five basic types of couplings are capacitive, inductive, radio frequency, common impedance, and conducted.

Electromagnetic interference (EMI) or radiated noise is coupled into a circuit depending on how close the source is to the receiver. In general, if the receiver is less than about one-sixth of a wavelength from the radiating source, the noise coupling mechanism will be dominated by either capacitive or inductive effects, but if it is greater than one-sixth wavelength, the radiated noise is a plane wave and the coupling will be by radio frequency effects. This is commonly called radio-frequency interference (RFI).

Capacitive or electrostatic noise is coupled into a circuit via a capacitive effect and is voltage-based. A voltage difference between two conductors separated by air or other insulating materials create a capacitor through which noise can be coupled.

Inductive or magnetic-coupled noise reaches a circuit via an inductive effect and is current-based. Current flowing through one circuit induces noise current in another circuit. The portion of the circuit into which the inductive noise is coupled can be viewed as a single loop or coil being inductively coupled by a noise coil (circuit).

The complex coupling mechanism of RFI is based on reflection, adsorption, and antenna effects. The effectiveness of coupling RFI into a system is a function of the radiating source, its strength, the characteristics of the transmission path, the distance involved, and the receiver’s sensitivity.

Common impedance coupling occurs when different circuits share common wires (impedances). Shared ground wires and long common neutrals or return paths can cause common impedance coupling.

Conducted noise is coupled into a circuit via transmission of noise by wires or other conducting materials. Sooner or later, all other coupled noise becomes conducted noise. Conducted noise is generally defined as two types, normal mode and common mode. Normal mode noise is defined between the conducting wires and the circuit reference and common mode noise is defined between the circuit conductors and ground.

The different types of noise coupling are illustrated. Two good references in the area of electrical noise are “IEEE 518–IEEE Guide for the Installation of Electrical Equipment to Minimize Electrical Noise Inputs to Controllers from External Sources” and “Noise Reduction Techniques in Electronic Systems,” by Henry W. Ott.