Coupled Noise Suppression and Brief Overview of Inductive/Capacitive Coupling

I came across a nice illustration (misspellings "chock" aside, haha) of common and differential mode interference and interference suppression. The document concentrates on AC Power Supplies but would be applicable to any signal line.

As a reminder, mutual-inductance indicators ("dots") demonstrate how the common-mode choke suppresses high frequency common-mode currents by generating an opposing/canceling current for each entering line (imagine two equal current pulses both entering in the same direction, each one generating a corresponding pulse through mutual inductance that suppresses the other line's pulse in a kind of cross-cancellation)

via Fresheneesz and Creative Commons Attribution-ShareAlike 3.0

High frequency differential noise is shunted across lines by the "X"-style capacitors and high frequency common-mode noise is shunted to chassis/ground by the "Y"-style capacitors (Class X and Y are based on safety capacitors types but I'm using them here to describe their connection configuration).

This document illustrates the difference well.

A discussion on Capacitive (Electric-Field) and Inductive (Magnetic Field) coupling (causes of interference) follows the break. 




In Capacitive Coupling a voltage between two conductors separated by dielectric/insulator (e.g. air, FR4 PCB) can create a parasitic capacitance that will pass/couple high frequency signals. 

In Inductive Coupling, changes in current cause changing magnetic fields which in turn can induce currents in nearby conductors:

• Current in a conductor generates a magnetic field oriented around it based on the right-hand rule/Ampere's Law
• Time-varying currents therefore result in time-varying magnetic fields
• Time-varying magnetic fields induce a voltage (emf - Electromotive Force) (and correspondingly a current if the medium is a conductor) in accordance with Faraday's Law
• Based on Lenz's Law, the emf is generated such that the magnetic field created by the induced current OPPOSES the inducing magnetic field. By the right-hand rule this means for parallel conductors the induced current with be travelling opposite to the inducing current (i.e. an increasing clockwise field is opposed by creating a counter-clockwise field - a decreasing clockwise field is opposed by creating a clockwise field)

Here is a link with further illustration.