New 100BASE-T PMD Jig Prototype
UNH-IOL Staff - May 24, 2012
At the IOL, we have been using the same style jig for our 100BASE-T PMD (Clause 25) test system for many years which allows us to interface a 100BASE-T device to our digital oscilloscope. The jig is made up of an RJ45 connector contained in a plastic sleeve, a PCB that has all the components mounted on it, and the TX and RX pairs are routed through the board to SMA connectors. These jigs have great electrical characteristics, but they wear out mechanically as we use them daily. We have been prototyping different jig designs to replace our current PMD jigs so that we do not have to worry about them degrading as quickly. In this post, we will discuss our latest design that is mechanically more reliable and has electrical characteristics that are very similar to our older design.
The main mechanical failure of the old design was that over time the plastic housing that encases the RJ45 connector becomes loose which puts stress on the wires that route to the bottom of the board (shown in the first picture below). Additionally, the plastic tab on the RJ45 also wears out or snaps off, which causes the connector to not properly lock into another RJ45 receptacle. If any of these things happen, a new jig has to be created as repairing an old jig is just as much work as making a new one.
Old Jigs
New Prototype Jig
Inside the New Jig
The new jig that we have created uses flexible or semi-rigid coax cables that are soldered to a small PCB which has the RJ45 pins mounted on it. The metal casing was designed so that RJ45 connector is a part of the assembly, so there are fewer moving parts that can degrade or break. The SMA cables are stabilized by a rubber cap that is installed at the end of the jig. As mentioned above, the electrical characteristics are very similar to that of the old design, as an example the single-ended return loss data for both jigs are shown below:
The return loss plots for both jigs have the same shape, which is dominated by the connector itself. Despite the relatively simple nature of the design, we did go through quite a few different prototypes, but in the end only the proposed jig in this blog has been able to match the performance of our old design. We are currently making more of these jigs and will be using them for testing in the future. If you are interested in some of the steps between the original and final design, please send along your questions to me at mvg2@iol.unh.edu.
Michael DeGaetano, Research and Development