EMC Engineering: Ferrite Chokes Block Common-Mode Currents and AC Line Cord Emissions

Ferrite Chokes Block Common-Mode Currents

• Commonly used on keyboard, mouse or video cables
• Extremely useful for troubleshooting
• Manufactures frequently have free evaluation kits
• Best used on low-impedance cables

AC Line Cord Emissions

• If AC line cord is suspected of emitting high-frequency harmonics, insert an external line filter in series. If the emission level (or measured common-mode current) drops, then improved line filtering is required.
• An external filter can be constructed with an extra power line filter module and spare AC line cord.
• Cut the cord down to minimal size and carefully solder the stripped ends to the filter, observing the correct connections. Wrap with electrical tape.

A Current Probe used to Measure a Cable's Common-Mode Currents

Testing a Bare PCB (printed circuit board) for Radiated Emissions and Analyzing of Crystal Oscillators