What is common-mode? Interfering signals can flow in cables in two ways: Differential-mode currents and common-mode currents. Differential-mode is the mode used by the desired signal on a transmission line. The cure for the differential-mode interference as in a spurious noise is a low-pass filter. Common-mode currents exist just like differential- mode currents. Worse yet, common-mode noise causes the same problems as in differential-mode noise. You cannot tell the cause, whether it is the differential or common- mode offense by looking at the symptoms. And therefore, common-mode filtering is an essential measure. The cure for the common-mode noise is a common-mode filter; it also prevents a feed-line transmission and reduces noises generated by nearby devices at receiving. The HI-Q COMMON-MODE Filter Series CF250E and CF5K were introduced in 1988. Tens of thousands of these HI-Q COMMON-MODE Filter Series are in use today worldwide.