Ethernet tackles automotive EMI challenge

Handling radiated emissions
Although the use of low cost unshielded cable is clearly desirable, the perception for some time has been that this was not possible with standard Ethernet 100BASE-TX PHY, and more proprietary means were required. When you observe the typical radiated emission characteristics of an Ethernet PHY, in Figure 1, it is easy to understand where this perception originated. The high energy content at the lower frequency band due to the MLT3 coded 65MHz to 80MHz bandwidth typically 10dB to 15dB in excess of automotive OEM limits.



After continued investigations into the EMI behavior of 100BASE-TX PHY circuitry, simple techniques have been demonstrated that sufficiently reduce the emissions, meeting automotive manufacturers’ needs. By adding a low pass filter to the transmit front-end, emissions can be reduced whilst still providing an interoperable standard Ethernet solution. The result is that no changes are indeed necessary to the standard Ethernet PHY. Figure 2 shows the resulting emissions from 100BASE-TX using Micrel’s latest standard Ethernet PHY technology, where such techniques have been applied.




Radiated emissions performance is not the only EMI challenge for in-vehicle Ethernet. Immunity to other electro-magnetic disturbances, both internal and external to the car, needs to be met if Ethernet is going to be considered by the automotive manufacturers. Standard 100BASE-TX Ethernet PHYs have been shown to be robust enough to meet the demands of automotive immunity limits, whilst operating in a reduced emissions configuration. This is demonstrated in Figure 3, IEC 62132-1/4 Direct Power Injection (DPI) performance, with the same PHY configuration. Here no network errors occur across all frequencies with 39dBm injection limit – equivalent to 106dBuA / 200mA (Level 5) Bulk Current Immunity (ISO 11452-4 stripline) levels.



One of the key differentiators for standards-based solutions is the resulting multiple suppliers. Such competition provides the end user with a greater offering of devices, higher integration, lower power consumption as well as lower costs. Such techniques have already been proven to be successful by multiple suppliers, working independently, both in performance and successful interoperability with each other and any other IEEE 802.3 Ethernet PHY. 

It was recently been announced that two Ethernet PHY vendors have independently demonstrated PHY technology with reduced emissions meeting automotive OEM limits, based on standard IEEE802.3 Ethernet. Micrel and Marvell have successfully demonstrated interoperability both with each other and, since these PHYs are IEEE Standard, other IEEE 802.3 Ethernet PHYs.

Utilizing standard IEEE 100BASE-TX Ethernet, PHY technology can take advantage of the existing IEEE802.3 ecosystem which has been widely adopted for networking applications, eliminating the need to create any new tooling or support infrastructure including standardization groups. Perhaps the most significant consequence is the rapid realization of multi-sourced solution portfolios (PHYs, controllers, switches etc.). Reusing the same silicon in multiple Ethernet applications (in addition to automotive) also leads to economies of scale unlikely to be matched by any proprietary or application specific solutions.