There are differences between the cables used in Ethernet and the cables for HDMI, USB, and other standards. HDMI, USB, and similar cables consist of a pair made up of a signal line and a separate ground line. As a result, even if a common mode current flows, that current passes through the ground line and returns, and so the magnetic field generated by the common mode current is canceled out, and noise emissions do not tend to occur. Ethernet cables, on the other hand, do not have a ground line, and so the route where the common mode current returns is a ground that passes through stray capacitance, and noise emissions tend to be generated more easily.
Because the ground line flows along the cable,
a portion of the magnetic field generated by the common mode current is also canceled.
This reduces the emissions due to common mode current.
Because there is no ground line in the cable,
the magnetic field generated by the common mode current is not canceled.
Because the emissions due to common mode current become large, noise suppression measures using components must be implemented.
A common mode choke coil (CMCC) is effective in noise suppression measures for automotive Ethernet and other differential transmissions. A common mode choke coil is formed by winding two lines in opposite directions around a common core. The magnetic fluxes generated by both lines for the differential mode current cancel each other out, and so there is no effect on the differential current. The magnetic fluxes generated by both lines for the common mode current reinforce each other, and so it acts as an inductor. Due to this action, common mode noise can be effectively attenuated without affecting the differential signals.
Common mode only can be selectively dropped.