Automotive Ethernet in the Vehicle

Figure 1 Block diagram of a typical ADAS system
showing the in-vehicle networks used.

As all Automotive Ethernet technologies utilize a single twisted-pair cable (T1) and a point-to-point network topology, they differ primarily in their data rates and encoding methods. For the most part, the data rate determines the applications for which a particular Automotive Ethernet standard can be used. If we take the case of an automatic driver assistance system (ADAS), we can see where each Automotive Ethernet variant might best be used to replace existing automotive protocols. 

(Click on any figure to enlarge the image.)

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.

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.

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.