Fundamentals of Automotive Ethernet

Figure 1: Automotive Ethernet is designed to support
increasingly complex vehicle electronic systems.

When we speak of “Automotive Ethernet”, we’re referring to a group of Ethernet interfaces intended for in-vehicle use, customized to meet the needs of the automotive industry. The first Automotive Ethernet standard was defined by Broadcom in 2011 with BroadR-Reach. Since then, IEEE has released standards for 100Base-T1, 1000Base-T1, 10Base-T1S and most recently MultiGBase-T1. Together, these standards define the general technology known as Automotive Ethernet.

Probably the first question you ask is, “Why not just use standard Ethernet?” A summary of the fundamental features of Automotive Ethernet will show how much better Automotive Ethernet is than standard Ethernet at meeting the industry’s demand for a higher speed, robust, lightweight and lower cost data interface, one that can ultimately replace the many other protocols currently used throughout the vehicle.

Take a Coffee Break and Learn Five Tips to Improve Dynamic Range

Dynamic range is the ratio of the maximum signal level to the smallest signal level achievable in a measurement.  Tools with good dynamic range are especially helpful for analyzing wide dynamic range signals in which a full-scale signal must be acquired, while at the same time, very small amplitude signal details must be visible. Here are five tips for improving the dynamic range of your measurement instrument.

Mode Conversion

Figure 1: The lower-left and upper-right quadrants of this
matrix show the S-parameters that represent mode conversion
from differential to common signal, and vice versa.

As said earlier, mixed-mode S-parameters describe the general case of combinations of differential and common signals. When we speak of mode conversion in mixed-mode S-parameters, we are referring to the change of a differential signal into a common signal, or a common signal into a differential signal, as it travels the transmission line. If we look at the matrix of mixed-mode S-parameters in Figure 1, we see that those mixed mode S-parameters affected by such a mode conversion—with a different type of signal going out than what went in—are in the lower-left and upper-right quadrants.  

Let’s take the S-parameters SCD11 and SCD21 to see how the combination of single-ended S-parameters they represent reveal the source of mode conversion. If we look at SCD11, the reflected mode conversion, as a function of its single-ended S-parameters, we see:

Converting Single to Mixed-Mode S-Parameters

Figure 1: Model of two transmission lines with crosstalk
showing the transmission and crosstalk related S-parameters.

We have introduced mixed mode S-parameters and developed a formal structure for handling them. It is now time to discuss converting single-ended S-parameters into mixed-mode S-parameters. This is important because every instrument manufacturer obtains mixed mode S-parameters by first measuring single-ended S-parameters, then converting them mathematically to mixed-mode. This assumes that the interconnects being measured are passive, linear and time invariant.  Let’s begin with our model of two transmission lines with crosstalk shown in Figure 1.

Introduction to Mixed-Mode S-parameters

Figure 1: Single-ended vs. differential signal
"world views" of S-parameters

We’ve treated single-ended S-parameters quite extensively in this blog. Links to several entries are listed at the bottom of this post. Now, we’re going to look at how we go from single-ended to mixed-mode S-parameters and what new information we can find in them. This will come in handy when we start looking at some of the MDI S-parameter tests that are performed for Automotive Ethernet compliance a bit down the road.

With single-ended S-parameters, we look at every combination of ‘going in signals’ and ‘coming out signals’. For example, two single-ended transmission lines and their return paths would yield a four-port S-parameter file. We take the complex ratios of each port combination to obtain the S-parameter value in the form of:

S_(OUT,IN) =  V_OUT/V_IN 

The bold typeface indicates complex quantities. 

But what happens if we drive two transmission lines with a differential source? Figure 1 compares the single-ended and differential signal world views.

Take a Coffee Break and Learn How to Use Memory Properly

In a recent post, we addressed setting sample rate for serial data acquisition, but let’s look again at how time per division (time/div), memory length and sample rate all interact, and what you can do to optimize your use of oscilloscope capture memory when setting up your timebase.

Figure 1: Sample rate as a function of time/div for three different memory lengths.
Longer memory extends the range of time/div settings that support the highest sample rate.

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.