Reducing radiated emissions on high speed signal lines using common mode choke coils

As signal frequencies increase, becoming intermixed with regulated radiation frequencies, low pass filters can no longer attenuate unwanted signals without distorting the required signal's waveform. One technique to reduce emissions from high speed signal lines is investigated. First, the possible ill effect of using a low pass filter on a high speed signal line is illustrated. Next, the effect of placing a return line in close proximity to the high speed signal line is examined. Finally, the effect of two common mode choke coils, placed between the high speed signal line and the return line, is examined. Results show common mode choke coils can effectively be used to reduce noise at frequencies above and below that of the high speed signal line without causing undesired distortion to the waveform.