Automotive Ethernet MDI S-parameter Testing

Figure 1: MDI S-parameter tests treat the Base-T1
pair as a balance transmission line and check that
reflections don't cause either excessive power loss
or mode conversion that can disrupt the signal.

As said earlier, the automotive industry has very stringent EMC/EMI requirements, and all Automotive Ethernet standards are designed to ensure good operation even in the presence of high EMI. Not only is there the potential for interference from all the different electronic systems within the vehicle, nothing is stopping you from parking your vehicle below high-voltage transmission wires or in other high EMI fields. 

For this reason, all Automotive Ethernet standards have defined S-parameter tests to be performed at the Medium Dependent Interface (MDI). The assumption is that the single twisted pair that is the basis for all Base-T1 transmissions can be treated as a balanced, differential transmission line with some crosstalk. It is a very real-world application of S-parameters, which can seem so academic.

Two, mixed-mode S-parameters are measured at the MDI reference plane. The tests ensure that there is neither too much loss of power from reflections, nor too much mode conversion into differential signal, that it will disrupt the information of the PAM3 encoded signal.

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.