4 Electromagnetic Compatibility of the Base-T1 Interface

Several standards help make sure that continuous noise does not cause the system to fail; one example is the International Electrotechnical Commission (IEC) 61000-4 tests for radiated and conducted noise types. The standard’s accepted criterion level shows how well the system can perform when the system is subjected to noise. The IEC 61000-4-4 electrical fast transient (EFT) test, in particular, is a compliance test that simulates the switching phases of a motor.

To pass the EFT test, optimize the termination of the communication interface to provide the best performance. There are two ways to terminate a single-pair Ethernet interface: capacitive coupling or galvanic coupling. Figure 4-1 and Figure 4-2 show simplified versions of the two coupling options.

Given the isolation requirements on the communication line in industrial systems, the rating of the capacitor must be as high as the isolation need. For example, use 1.5kV if building a robot in accordance with the IEC 62368-1 standard.

Comparing the size of the galvanic and the capacitive based coupling for a typical 100Base-T1 implementation, based only on component size is shown in Table 4-1.

This summary does not consider coupling of power over dataline (PoDLSPoE) circuits; however, including PoDL does not change much of the analysis from the communication piece of the coupling. The additional circuit of the PoDL coupling can change the result of the analysis. Table 4-1 shows that the size of the components used is not increased by using galvanic coupling – in fact, in the complete circuit, the size of the components is reduced by using galvanic coupling. This analysis is based on components used from Würth electronics. For more details of Würth components for SPE see Single Pair Ethernet.

To compare galvanic and capacitive coupling, two test systems were built to show the performance of each implementation during EMC testing.

During these tests it became clear that galvanic coupling has better performance over capacitive coupling. This is due to capacitor mismatch which makes this circuit more noise sensitive. Also observed was that when with capacitive coupling the Ethernet link was dropped, with galvanic coupling the link was stable with minimal packets lost.

In summary, Texas Instruments (TI) conducted tests on capacitive and galvanic coupling indicating that:

• Galvanic decoupling is less susceptible to noise.
• Galvanic coupling has a better mode conversion loss and return loss performance response than capacitive decoupling, which means that over frequency galvanic coupling attenuates a signal less, thereby improving signal quality and robustness to noise.
• Galvanic tolerances and parasitic effects are more controllable which improves performance when testing against IEC61000-4-x for industrial environments.

In general, if SPE is used in industrial environments, it is most likely necessary to use galvanic coupling to mitigate the poor performance of capacitive coupling observed during EMC system testing and IEC61000-4-x.

Initial tests were conducted which show that galvanic coupling is able to achieve no bit errors to show performance criterion A, during EMC and EMI testing. For further details on these results please contact your local TI representative.