Laird Technologies has published an application note titled "Common Mode Choke Filtering Improves CMRR in Ethernet Transformer Applications". The application note describes the reason why common mode chokes are a suitable 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, and have increased the need for the components to be multi-functional.
The chokes 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 CMRR (common mode rejection ratio). Using a common mode choke for Ethernet differential signal lines helps in improving 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. The addition of a common mode choke before the connector filters these common mode currents and allows the desired signals to pass unaffected. This results in efficient communication between devices, good product and system operation and product EMI regulatory compliance. The company's products are designed for power and data line EMI filtering where high current, small size or high frequency performance is required.
