Laird Technologies Publishes Common Mode Choke Application Note

Topic Discusses How Common Mode Choke Filtering Improves CMRR in Ethernet Transformer Applications


St. Louis, Missouri, USA – June 30, 2011 – Laird Technologies, Inc., a global leader in the design and supply of customized performance-critical components and systems for advanced electronics and wireless products, today announced the publication of its newest application note titled “Common Mode Choke Filtering Improves CMRR in Ethernet Transformer Applications”.

The application note describes the reason why common mode chokes are an effective EMI filtering solution for Ethernet transformer applications. Ethernet transformers were originally designed to bundle the cabling of the system. But design specifications have forced board components to become smaller, increasing the need for those components to be multi-functional.

Further details are made of how common mode chokes withstand high DC currents without degradation of filtering performance, and if stable, allow most signals to pass unaffected while filtering noise. They filter common mode EMI currents without causing signal degradation and without derating under high currents, and are applied to pairs of conductors.

The application note then examines the difference between differential mode and common mode, and how a common mode choke actually works. It also explains the Ethernet transformer operational principle and defines the Common Mode Rejection Ratio (CMRR). Using a common mode choke for Ethernet differential signal lines helps to improve the CMRR by reducing EMI noise with the transformer altogether.

Suppression of EMI has become a major concern in the transmission, reception, and processing of electronic signals and data. Modern systems are frequently an interconnection of functional blocks and connections made by cables or wiring harnesses, thus the signal speed becomes very fast. The addition of a common mode choke before the connector filters these common mode currents while allowing the desired signals to pass unaffected. The result is effective communication between devices, reliable product and system operation and product EMI regulatory compliance.

Laird Technologies is a leading global supplier of signal integrity products that meet the demands of any application by providing the very best solution. These products are designed for power and data line EMI filtering where high current, small size or high frequency performance is required.

The application note is available free for download on

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About Laird Technologies, Inc.

Laird Technologies designs and manufactures customized, performance-critical products for wireless and other advanced electronics applications.

The company is a global market leader in the design and supply of electromagnetic interference (EMI) shielding, thermal management products, specialty metal products, signal integrity components, and antenna solutions, as well as radio frequency (RF) modules and wireless remote controls and systems.

Custom products are supplied to all sectors of the electronics industry including the handset, telecommunications, data transfer and information technology, automotive, aerospace, defense, consumer, medical, mining, railroad and industrial markets.

Laird Technologies, a unit of Laird PLC, employs over 12,000 employees in more than 49 facilities located in 16 countries.


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