What is Common Mode Suppression in a Balun?

Common mode suppression baluns, choke baluns, or autotransformer baluns provide both common mode signal attenuation and balancing, unlike transformer baluns, which are sometimes referred to as ununs. Common mode suppression baluns commonly use multiple conductors with tapped winding around a magnetic core material, often a toroid or bars. Typically, this core material is made of a magnetic lossy conductor that is also temperature stable at high heat.

The basic operation of a common mode suppression balun is for the winding around the magnetic core to induce a current in the magnetic core, which reacts and couples back to the windings. Using a tap referenced to ground can then convert the balanced signal at either end of the winding to an unbalanced ground referenced signal between the tap(s) and one end of the winding. The position of the tap point(s) in the winding to ground determines the ratio of the balun and the relative voltage at the tap points.

Common mode suppression is achieved, as signals with the same voltage and phase travel are combined and travel through the lossy magnetic core material, converting some of the RF energy into heat. Signals with the same voltage and opposite phase cancel out the flux they generate in the magnetic core, leaving an unbalanced signal at the output of the windings. A simple example of a common mode suppression balun is a coaxial cable wrapped around a magnetic core toroid. The currents on the inner conductor and outer conductor that are out of phase pass through, while the common mode currents on the outer conductor generate currents within the magnetic core.