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.

The first of the two MDI S-parameter tests is SDD11, which tests that there is an acceptably small amount of power loss due to reflections coming back on port 1. Specifically, it measures the magnitude in dB of the reflections and compares it to a defined mask of the limit. Reflections indicate a loss of power, not good overall for transmissions, so ideally the losses should be as small as possible. If the characteristic impedance of the twisted pair is 100 Ohm, ideally the measured impedance at the MDI should also be close to 100 Ohm.

The second of the two MDI S-parameter tests is SDC11, which tests that there is an acceptably small amount of mode conversion due to reflections coming back on port 1—in this case, common mode signal converting to differential signal. It is essentially a test of noise tolerance. Like SDD11, the SDC11 measurement is compared to a defined mask of the acceptable limit. The desired ratio is 60 dB, or voltage ratio of 1/1000: not more than 10 mV reflected differential signal occurring for 10 V of common mode signal injected. More than that could lead to serious signal disruption and a failure of autonomous systems.

Besides testing the DUT, the 1000Base-T1 standard requires that you also test the test fixture to ensure that it is not contributing excessively to the overall mode conversion. 

As we mentioned in an earlier post, Automotive Ethernet designs may incorporate filters such as a Low-Pass Filter (LPF) and a Common Mode Choke (CMC) to help improve EMC/EMI tolerance. However, too much filtering can cause too much droop on the line, which is addressed by the Transmitter Output Droop test, performed using an oscilloscope.

The S-parameter tests can be performed using a Vector Network Analyzer (VNA) or a Time Domain Reflectometer (TDR). The Teledyne LeCroy solutions use the WavePulser 40iX High Speed Interconnect Analyzer. From the same pulse measurement, the WavePulser calculates the frequency domain S-parameters and the time-domain impedance profile. The advantage of this is that not only are you able to measure the S-parameters as required by the standards, you are also able to measure the characteristic impedance of your Base-T1 pairs and use its TDR capabilities to locate any impairments in the line where discontinuities occur. QualiPHY software options supporting 10Base-T1S, 100Base-T1 and 1000Base-T1 automate the use of the WavePulser to perform the S-parameter tests along with the oscilloscope tests as part of a complete, Automotive Ethernet physical layer test solution.

Watch Giuseppe Leccia explain this further in the on-demand webinar, How to Become an Expert in Automotive Ethernet Testing, Part 2: Mastering Return Loss and Mode Conversion Testing.

See also:

Automotive Ethernet in the Vehicle

Fundamentals of Automotive Ethernet

Mode Conversion

Introduction to Mixed-Mode S-parameters