Putting the 'e' in 10Base-Te MAU

The UNH-IOL is currently developing a test suite and test system for 10Base-Te MAU conformance. Due to the similarities between 10Base-T and 10Base-Te, the lab decided to integrate both tests onto the same test board and use a very similar software package. This allows us to simplify testing, which saves time and also gives us reason to look over our existing 10Base-T test suite.

Integrating the two standards is made easy for two reasons unique to 10Base-T. Firstly, 10Base-T already implements energy efficient idle signaling by transmitting link test pulses spaced 16ms apart, as opposed to a constant link idle signal as seen in Fast or Gigabit Ethernet. Secondly, a 10Base-Te device can link with a 10Base-T device and remain in 10Base-Te mode, providing the minimum cabling requirements, such as category 5, are met. This allows us to use the same equipment and much of the same code and algorithms.

In order to account for the differences, we added a 10Base-Te line model to our test board. A 4 pole dual throw switch allows us to switch the line model depending on which technologies we are testing. Adding this line model was the only change necessary to combine 10Base-T and 10Base-Te MAU testing on the same board.In addition to the necessary changes, we also made some changes for testing convenience. In previous iterations we used a custom cable, nick-named the dragon cable, to split the Tx and Rx pairs so that our traffic generator could obtain a link. This link is needed so that we can send traffic to the device. For the new board, we have designed an LTP generation circuit that gives a link to the traffic generator without the need for a proprietary cable.

Adding this LTP circuit was nearly trivial, thanks to the fact that an LTP is a simple waveform to reproduce using off the shelf components. To create this circuit we used a 555 timer to initiate the pulses with a monostable multivibrator, to make the pulses uniform and finally a differential line driver as a filter to smooth out the pulse edges.

One issue that we recently discovered which is not only relevant to 10BASE-T MAU testing but also to our emerging 10BASE-Te MAU testing was a discrepancy in our impedance balance test methods compared to those defined in IEEE 802.3 Clause 14.3.1.2.4. This test verifies that the common-mode to differential-mode impedance balance of the TD circuit is greater than the specified limit. Previously we had been measuring this using a VNA to both apply a sine wave voltage with a custom test jig and also measure the reflections from the transmitter.

Due to worries about the characteristic impedance of the S11 port on the VNA (used to source a sine wave) loading down the test circuit, we plan to use an AWG and an oscilloscope with a high impedance probe to handle the measurements. In order to perform the test in this way we need to minimize any reflections lost to the AWG, which will be used for sine wave voltage generation from 1.0 MHz to 20 MHz.

To address this issue we will employ an op amp to act as a buffer which will provide 50 ohms to the AWG and a high impedance output connected to the test circuit. These changes result in a more accurate reproduction of the standard defined test circuit without introducing inaccuracies caused by the characteristic impedance of our test equipment. As well as with the majority of our test suites, the testing devices are controlled via GPIB from a Matlab GUI to make the testing more automated.

After designing the test board with Altium we received our newest revision and started the process of populating it. In a future post, we will discuss the process of testing and verifying this new board’s performance before it’s added to our existing suite of tools.

Collin Huston, Research and Development