Lesson 2/1020%
MODULE 02

CE vs. RE, Noise Paths & Common/Differential Mode

Conducted emissions travel on wires, not through the air — a distinction that changes everything about how they're measured and fixed. This module compares CE to RE, walks through the three physical paths noise takes out of a product, and covers the single most important idea in CE debugging: common-mode vs. differential-mode current.

CE and RE are two views of the same underlying noise problem, separated mainly by frequency and by which physical medium carries the energy to the measurement equipment.

Interactive: Slide Between Conducted and Radiated

CONDUCTED RADIATED
Property Conducted Radiated
Medium Cable / conductor Air (free space)
Typical frequency Lower (150 kHz–30 MHz) Higher (30 MHz–40 GHz)
What's measured Voltage (dBµV) on the cable Electric field strength (dBµV/m) at a distance
Equipment LISN + EMI receiver Antenna + EMI receiver, in a shielded/anechoic environment
Representative standard CISPR 11 / CISPR 32 conducted limits CISPR 32 radiated limits
The two aren't independent: a cable carrying conducted common-mode noise is often also the product's single largest radiating "antenna." Fixing conducted common-mode emissions frequently improves the radiated result for free.

Conducted emissions aren't limited to the AC mains cord — any conductor leaving the enclosure is a potential path. Switch between the three tabs below.

This is arguably the single most important distinction in CE work. Every filter component, every debugging step, and every fix targets one mode or the other — get the mode wrong and the fix won't work no matter how well it's executed.

Interactive: Switch Between Differential and Common Mode

Property Differential Mode (DM) Common Mode (CM)
Return path The opposite power conductor Chassis / earth ground, via parasitic capacitance
Current direction on the two conductors Opposite (out on one, back on the other) Same direction on both conductors together
Typical dominant frequency range Lower conducted frequencies (150 kHz–few MHz) Higher conducted frequencies (few MHz–30 MHz and above)
Primary filter component X-capacitor, differential-mode inductor Y-capacitor, common-mode choke

Interactive: Typical DM vs. CM Contribution Across Frequency

On a typical switching supply, differential-mode noise dominates the lower conducted band while common-mode noise takes over as frequency rises — the crossover point varies by design but is often somewhere in the low-MHz range.

Quick diagnostic: measuring only total noise on the LISN can't tell you which mode dominates. Dedicated DM/CM separation networks (or a current probe used correctly on each conductor) are needed to split the two apart before choosing a filter component.
KNOWLEDGE CHECK

Module 2 Quiz

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