Pulse Electronics Understanding Common Mode Noise

Understanding the difference between common-mode and differential mode signals is crucial for the correct understanding of how Pulse’s magnetic interface modules work. Transformers, common-mode chokes and autotransformer terminations play a key role in the reduction of common-mode interference in our LAN and Telecom interface circuits. Common-noise plays an important role in the generation of Radio Frequency Interference (RFI) in communications systems using Unshielded Twisted Pair (UTP) cable, so understanding commonmode noise will lead to a better understanding of Electromagnetic Compatibility (EMC) issues concerning magnetic interfaces. It is the purpose of this document to explain the key characteristics of differential mode and common-mode signals, the principle of operation of common-mode chokes and autotransformer terminations, and why common-mode noise on UTP cable leads to noise emissions.