In this tutorial we are going to learn about WHAT ARE THE TWO MODE OF INTERFERENCE ?
EM disturbances can appear in the form of Common-Mode & Differential-Mode voltage & current components. Differential mode also called symmetrical noise/interference occurs when noise currents travel between live & neutral. The differential mode voltage components are measured between the phase conductors. Differential mode signals are usually used to convey the desired information & don’t usually cause that much interference as the EMI fields generated by differential currents oppose each other (180° out of phase) causing a cancellation effect.
Common mode signals, on the other hand are usually the major source of EMI from power & transmission (all I/O) cables. They cause the cables to behave as mono pole antenna. These currents flow from the phase & neutral conductors to GND. The circuit for the common mode component is completed by the stray impedance (capacitance) to GND. When solving EMI problems its important to distinguish between differential & common mode interference.
Differential mode noise where is the noise of a signal as it travels down its trace to the receiving device, then back along the return path. There is a differential voltage between the 2 wires. Make sure there is no more noise than needed to get the job done, in terms of both frequency content (rise & fall times) & the magnitude of the current.
Differential mode is desirable & common-mode is an EMI problem. The fields generated by differential currents oppose each other & nearly cancel out; therefore, little interference is caused. In common mode, a voltage travels down both the signal & return lines at the same time. There is no differential between the signal & its return. The voltage is caused by an impedance that is common to both the signal & the return. Common impedance noise where is the most common source of noise in most micro computer-based systems that are not using external memories. The common mode source adds noise to be differential mode signals. This noise is usually coupled from HF currents affecting the PCB and wiring inductance, causing the wiring to operate as a radiating mono pole antenna. These common mode signals are not useful, & are the major cause of radiated EMI from cables, PCB traces & wiring. The wiring inductance is a significant problem & must be tightly controlled. A major source of common mode noise is clocks & repetitive I/O.