EMI Suppression Filters (EMC and Noise Suppression)Automotive Ethernet for ADAS Noise suppression measures for 1000Base-T1 (2)

5Noise Suppression Measures for Automotive Ethernet

Ethernet Problems

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.

Using HDMI, USB, and Similar Cables

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.

Using Ethernet Cables

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.

6Notes on CMCCs Used in Automotive Ethernet

The balance of the CMCC is important in automotive Ethernet. If there are any differences in the lengths or windings of the two lines comprising the CMCC, the current could become imbalanced, causing mode conversion, and generating common mode noise. For this reason, a CMCC that was designed for maintaining the balance of both lines must be selected.

If there are any differences between the two coils,
such as the line length in the CMCC, this could give rise to a new factor in generating common mode noise due to mode conversion.

A component with low mode conversion must be selected.

The DLW32MH101XT2 was developed as the optimal CMCC product for 1000Base-T1 noise suppression. It has an impedance value based on usage in 1000Base-T1 and has a balanced design for preventing the occurrence of mode conversion.

DLW32MH101XT2

  1. Effective for suppressing noise emitted from signal lines in automobile networks.
  2. Fully compatible with the 1000Base-T1 automotive Ethernet standard
  3. Operating temperature range for automotive applications: -40°C to 125°C

Transmission Characteristics of DLW32MH101XT2

  • Sdd21

    (Differential Mode Transmission Characteristics)

  • Scc21

    (Common Mode Transmission Characteristics)

  • Sdd11

    (Differential Mode Reflection Characteristics)

  • Ssd12

    (Mode Conversion Characteristics)