After 1000Base-T1, the DPI test was also conducted for 100Base-T1. The CMCC for 100Base-T1 satisfied the limit values. However, the CMCC for CAN performed worse than the CMCC for 100Base-T1 at low frequencies of 1 MHz and less, and it also did not meet the limit values at 8 to 60 MHz, and failed the test.
The differences at 2 MHz and less are thought to be due to common mode attenuation (Scc21). Also, the differences at 8 to 60 MHz are thought to be due to the mode conversion characteristics.
One factor why the mode conversion characteristics of the CMCC affected the test results for 100Base-T1 is that the common mode noise that entered from an external source was converted to differential mode noise, which distorted the signal waveform, resulting in a communication error.
In the same way as conducted emissions, in addition to the CMCC, this may also be caused by mode conversion due to imbalances on the board, and so careful attention is needed in board design.
Key Points in Occurrence of Mode Conversion