EMI Suppression Filters (EMC and Noise Suppression) An Introduction to Noise Suppression in Onboard Ethernet 100Base-T1

1What is Onboard Ethernet?

Onboard Ethernet:"Communication systems used in automobiles"
Various interface standards are used in automobiles, depending on the application.

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Table 1: Communication systems used in automobiles
Communication system Speed Applications, features
CAN/LIN 33k/125k/250k/500kbps Onboard LAN standard adopted for general ECUs
FlexRay 2M/5M/10Mbps Used in the powertrain, chassis, and other control systems
MOST 25M/50M/150Mbps Adopted for use in onboard audio multimedia systems
Ethernet 100M/1Gbps PC LAN standard, also used in diagnostics
100BASE-TX, 100Base-T1
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Table 2: Onboard Ethernet standards
Standard Transmission speed Number of pairs Notes
100Base-T1 100Mbps 1 pair Supports full-duplex communication at 100 Mbps with 1 pair. Standardized by OPEN Alliance.
100Base-TX 100Mbps 2 pairs Based on the existing 100Base-TX and used to improve onboard EMC performance.
1000Mbps 1 or 2 pairs Being reviewed for standardization by IEEE. The standard reduced the amount of wiring in the existing 1000Base-T for vehicle and industrial device applications.

2Necessity of Noise Suppression in Onboard Ethernet

In differential transmissions including Ethernet, noise occurs due to a common mode current caused by various factors.

The cables used for Ethernet are different from USB and HDMI cables.

Onboard devices have a shorter antenna distance and stricter limit values than consumer products.

A common mode choke coil is an effective way to suppress noise in a differential transmission.

  • The reverse phase (differential mode) signal is canceled by the magnetic field → penetrates
  • The magnetic field superimposes the in-phase (common mode) noise → reflects

You can selectively drop just the common mode!

We recommend the DLW43MH series as the optimal common mode choke coil for 100Base-T1 noise suppression.