UNH-IOL Test Fixture Overview: Passive Line Tap

Earlier this week, we provided insight into the lab’s MIPI TLIS board.  Today, we’ll discuss the Passive Line Tap. We created this fixture to observe and measure bi-directional traffic being sent over a differential pair for 1000BASE-T and 10GBASE-T environments with minimal interference to the signal. Since Ethernet signaling is differential, we use a balun to transform the differential signal into a single-ended signal. This reduces the complexity of the traces on the tap and requires fewer scope channels to decode the data. Following the balun we use a bi-directional coupler that allows us to passively “tap” off of the signal which will be connected to our test equipment through a SMA cable. This board has gone through many revisions and we have experimented with a variety of unidirectional couplers. Mini-Circuits eventually developed the ADC-ED12199 which has 13dB of coupling up to 1 GHz, and down to 18 dB of directivity between 500 MHz and 1 GHz, which makes it a perfect component for our applications. The performance of the ADC-ED12199, along with good board design, has minimized the crosstalk coupling between Ethernet pairs. We have characterized the Passive Line Tap board so that we can develop a crosstalk removal algorithm for applications that are sensitive to crosstalk noise - although that development is not yet complete. We have adopted this board into our 1000BASE-T Energy Efficient Ethernet PMA (Clause 40) testing and will someday use it in 10GBASE-T PMA (Clause 55) testing.

Passive Line Tap Rev 8

Block diagram of the Passive Line Tap Board

Although we’ve only covered two test fixtures this week, the lab has also created breakout jigs for 100BASE-T, 1000BASE-T, and 10GBASE-T that convert an RJ45 jacks to SMA connectors, an auto-negotiation test board that can capture Ethernet auto-negotiation exchange, a low power idle detector board that can identify Energy Efficient Ethernet (EEE) links between devices, and a MIPI Probing Board that allows interfacing with a scope or logic analyzer for monitoring active D-PHY links. The MIPI family of test fixtures will be getting another addition soon as a new TLIS board will be needed for the newly adopted M-PHY standard.

Michael DeGaetano, Research and Development