A common mode noise filter for high speed and wide band differential mode signal transmission

This study shows the noise reduction characteristics of a noise filter composed of a common mode choke and a differential mode choke. A common mode choke is usually used for the common mode noise reduction on a differential mode signal transmission line. The common mode choke attenuates the common mode noise and passes the differential mode signal on the signal transmission line. The common mode choke works as a high impedance element for the common mode noise and as a low impedance element for the differential mode signal. However, it is difficult to realize a large reduction effect of noise in wide frequency range by using only a common mode choke. A special feature of the common mode noise filter proposed in this paper is to have a differential mode choke added to a common mode choke. The differential mode choke works as a low impedance element for common mode noise and as a high impedance element for differential mode signal. The noise filter studied in this paper reduces common mode noise 20dB or more and demonstrates the good transmission characteristics of a differential mode signal in the wide frequency range from 1MHz to 1GHz. In addition, this filter generates very small differential mode component converted from a common mode noise.