Debugging Automotive Ethernet Transmitter Output Droop

Everyone manufacturing devices used for Automotive Ethernet knows what compliance limits they must meet, based on the electrical test specification, but it is not always obvious where to look for the source of the problem when a compliance test fails. We'll provide more Automotive Ethernet debugging tips in future posts, but here is one that arose from a reader's question: namely, why the 45% limit on output droop for 100Base-T1, and what might cause the problem?

Figure 1: Automotive Ethernet electrical testing is performed at the transmitter connector and is largely governed by channel/connector recommendations.  Between the transmitter and the connector is a Low-Pass Filter and a Common Mode Choke both affecting signal droop.

Automotive Ethernet electrical compliance is determined by the state of the signal at the connector of the transmitter.  Figure 1 shows a typical Automotive Ethernet signal path. The Transmitter Output Droop test is performed at the connector to verify that the transmitted signal arrives with not more than a 45% drop in amplitude (droop) and remains recognizable to the receiver.

Figure 2: Automotive Ethernet Test Mode 1
pattern is a square wave.

The Test Mode 1 pattern used for this test leaves the transmitter as a square wave —a series of +1 and -1 symbols determined by the signal amplitude (Figure 2). Between the transmitter and the connector, the signal goes through a Low-Pass Filter (LPF) and a Common Mode Choke (CMC) (Figure 1). This filtering is often designed to alleviate EMI/EMC issues within the automotive environment.

The droop test is performed to ensure that there is not excessive filtering happening between the transmitter output and the MDI output (connector). If the droop is greater than 45%, the LPF and/or the CMC are excessively impacting the signal and need adjustment so that the effect is not as extreme.

As to why the 45% limit, the obvious answer is that is what has been deemed necessary for the technology to work, given the receiver tolerances and other factors that can impact the signal. The limit has actually varied with different versions of the Automotive Ethernet electrical test specification. For 100Base-T1/BroadR-Reach, output droop was initially specified at 45%, dropping to 26.9% with v.2.0  before returning to 45% with v3.5. For 1000Base-T1 it is specified at 10%, and for 10Base-T1S it is 30%.

Some of this reflects changes in the underlying technology, but almost certainly there is a “political” element, as well, as transmitter and receiver manufacturers negotiate how much margin-of-error each can keep on their side of the link. It may be that 45% was necessary to cover everyone’s contribution to the overall signal degradation.

Watch our on-demand webinars to find out How to Become an Expert in Automotive Ethernet Testing:

Part 1: Fundamentals of Automotive Ethernet Compliance Test, Validation and Debug

Part 2: Mastering MDI Return Loss and Mode Conversion Loss Electrical Compliance Test