[Automotive Ethernet Testing] Series 1: Full-duplex communication brings testing challenges, Tektronix signal segmentation method gives you a unique insight

Only by separating them can we better examine and see the real signal.

Author: Tektronix Technology

As the automotive industry accelerates its shift to automotive Ethernet technology, comprehensive design verification is essential to ensure interoperability and reliable operation between multiple ECUs. The concept of automotive Ethernet was proposed by the OPEN Alliance SIG, also known as IEEE 802.3bw (formerly BroadR-Reach), which is an Ethernet physical layer standard designed for automotive networking applications, such as advanced safety features, comfort and infotainment functions. Through automotive Ethernet, multiple vehicle systems can access information simultaneously through an unshielded single twisted cable. For automakers, this technology reduces networking costs and cable weight while increasing signal bandwidth.

To achieve higher signal bandwidth, automotive Ethernet uses full-duplex communication links on twisted pair cables, supporting simultaneous transmit and receive functions and PAM3 signaling. Using PAM3 to achieve full-duplex communication can make viewing automotive Ethernet services and signal integrity testing very complicated.

The OPEN Alliance has developed automotive Ethernet test specifications for components, channels, and interoperability. The test system integrates electronic control units (ECUs), connectors, and untwisted pair cables. The test requires the system to operate under harsh environmental conditions and noise conditions in the car. To do this, users must be able to characterize and view signal integrity and traffic at the system level in order to perform reliability testing. Application examples where customers need to perform signal integrity testing at the system level include:

• TC8 signal quality test

• ECU component characterization and testing

• Automotive Ethernet cables, connectors, cable length and routing characterization and testing

• Electromagnetic noise or Gaussian noise test

• High current injection test

• Production unit testing

• Impact of automotive systems on automotive Ethernet performance

-DC motor on/off

-Engine on/off

• Automotive Ethernet system debugging

Tektronix recommends performing signal integrity testing during the design phase to identify potential problems before system integration.

Figure 1: Automotive Ethernet full-duplex communication chain.

Full-duplex communications and testing challenges

Full-duplex communication and PAM3 signaling add complexity to verifying ECUs under real-world conditions. Most serial standards operate in simplex mode, with only one device communicating at a time, and some communication standards use a separate link for transmission and reception, while in automotive Ethernet, the master and slave devices can communicate simultaneously over the same link. (See Figure 1)

Therefore, the signal from the master is superimposed on the signal from the slave. The master knows which data it is sending, and it can determine the slave's signal from the superimposed signal, and vice versa. Although transceivers are designed to handle this situation, isolating the signal and performing signal integrity testing or protocol decoding on an oscilloscope is almost impossible.

Automotive Ethernet signaling as seen when the master and slave signals are not separated.

To perform signal integrity analysis on the links, automotive designers must look at each link separately using an oscilloscope to perform protocol decoding in a real system environment. Users must separate the signals before performing analysis.

It should be noted that signal integrity testing is best performed during the automotive integration phase to select cables, check ECU performance under electromagnetic noise conditions, determine optimal cable length and routing, etc. For this type of analysis, eye diagram testing can be a very important tool to view system health, which we will discuss later.

Separating Automotive Ethernet PAM3 Signals

Currently, there are two methods for separating the master signal from the slave signal. The first is the traditional method, which requires the user to disconnect or cut the automotive Ethernet cable and insert a directional coupler to separate and test the signals. This method has its own shortcomings in achieving accurate testing with minimal interference. The second method is the Tektronix signal separation method, which is a new method that uses advanced software and probes to separate the signals non-intrusively, allowing users to see the true signal more clearly. This method overcomes the shortcomings of the traditional directional coupler method. Below we will discuss and compare these two methods.

● Directional coupling method

As mentioned earlier, the directional coupler method requires disconnecting the Automotive Ethernet cable and entering a directional coupler to separate the signals. Cutting the cable at the system level is not an easy task, so this method is not suitable for system-level testing.

This approach allows the user to view both the master and slave signals, but it introduces insertion and return loss, making it difficult to determine if the error is caused by the system or the added hardware. In addition, while we may be able to remove the effects of the directional coupler, de-embedding may amplify the noise in the system, affecting measurement and characterization accuracy.

Directional coupler method.

The eye diagram of the main signal shows the effect of the directional coupler insertion loss and return loss.

The setup we used included a fixture that converts Automotive Ethernet to SMA connectors, a directional coupler, and a fixture that converts SMA to Automotive Ethernet cables.

The eye diagram shows the effect of insertion loss and return loss on an Automotive Ethernet signal after installing a directional coupler. The maximum amplitude is 100 mVpp, and because the directional coupler operates in a directional principle, the insertion loss and return loss result in the eye being closed. Until recently, the directional coupler method has been the default Automotive Ethernet test method because there has been no Tektronix software-based signal separation test method.

● Tektronix signal separation method

Introduced in July 2019, the Tektronix signal separation method separates full-duplex signals by viewing voltage and current waveforms from both the master and slave test points, and uses a proprietary software algorithm to provide the separated signals. The Tektronix signal separation method is a software-based solution that allows users to see the true signal without cutting the automotive Ethernet cable. One of the advantages of this method is that it can display the master and slave signals without the added insertion loss and return loss and de-embedding effects of the directional coupler method.

Tektronix signal separation method.

The eye diagram below was created using Tektronix Signal Separation software. The signal quality is higher and the eye diagram is "cleaner" compared to a directional coupler eye diagram. Users can accurately represent Automotive Ethernet signals, make signal quality measurements, and identify potential performance issues more quickly.

Eye diagram of the main signal using Tektronix Signal Separation software.

Comparison between signal separation and directional coupling methods

We use the two test methods mentioned above to perform measurement tests and compare the test results.

In the test, we first set up and ran the test using Tektronix signal separation technology, a current probe and voltage probe. For the directional coupler method, we cut the automotive Ethernet cable and inserted the directional coupler with SMA connectors. We then ran the test with the same test conditions as the directional coupler method and called the signal separation method waveform to compare the two test methods.

Comparison of test results using the Tektronix signal separation method and the directional coupling method.

Comparison of the results shows a clear difference in amplitude between the two methods, illustrating the effect of the directional coupler. With the directional coupler method, the amplitude of the master signal is approximately 90 mVpp (peak-to-peak voltage) and the amplitude of the slave signal is approximately 85 mVpp. In contrast, the amplitude of the master signal in the signal separation method is approximately 1.5 Vpp and the amplitude of the slave signal is approximately 1.45 Vpp. In this case, the directional coupler adds 20 dB of loss.

To remove the discontinuity introduced by the directional coupler, de-embedding is necessary to compensate for insertion loss and return loss. As mentioned earlier, while it may be possible to remove the effects of the directional coupler, de-embedding can amplify noise in the system, affecting measurement and characterization accuracy. It should also be noted that de-embedding can be time consuming and challenging. In addition, cutting cables and installing directional couplers can be challenging for system-level testing and maintenance of the vehicle.

In contrast, the signal separation approach shows the true signal without disturbing the system. With this new approach to Automotive Ethernet testing, users can characterize signals with greater accuracy and in less time without the added expense and measurement challenges. With this approach, users can perform signal integrity testing at the system level and perform all the tests available in the application environment.

summary

This article introduces Automotive Ethernet, full-duplex communication, the need to isolate master and slave signals, signal separation test methods, and a comparison of current directional coupler insertion methods and Tektronix's new signal isolation method.

By comparing the two Automotive Ethernet test methods, we can understand the advantages of Tektronix signal separation solutions, such as more accurate viewing of real signals than directional coupler signal methods, simplified component-level and system-level test setups, shortened test time, and meeting the testing needs of the entire life cycle of the vehicle.