Between Ohmmeter and TDR, what are your cable-test options?
Cable assemblies, meaning the electrical conductor and associated connectors at one or both ends, are components with a bipolar life. On one side, they’re simple copper links and easy to check out for basic continuity; all it takes is an ohmmeter function on a multimeter. On the other side, we expect more and more of these interconnects when handling power and especially when conveying data. After all, who would have predicted years ago that a thin, twisted-pair cable would be handling megabit-per-second data rates?
That level of performance is what we routinely get from a properly designed and constructed cable rated for Cat5 or Cat6 standards, for example. Even coaxial cables have their imperatives, with rating to hundreds of megaHertz and tens of gigaHertz. Yet when a system’s performance is not what it should be, we usually don’t think about the cable, since it was marked and marketed to fulfill the requisite specifications.
Reality is that counterfeit, substandard, or poorly installed cables are out there, shocking as it may seem. After all, why bother to produce fake ICs when it is so much easier to make or take an inadequate cable and stamp a certification on it? Even better, the substandard assembly will work somewhat, and link problems will usually be blamed on excessive external noise, inadequate noise-margin, timing skew and jitter, marginal line driver/receivers, and many other possibilities.
That’s why I like to look through the publications such as Cabling Installation & Maintenance, which target the folks in the field (literally, as well as figuratively). These people are the ones who have to deal with the consequences of designs that worked in prototype and even pilot installations, yet somehow are having issues during or after wider deployment; these problems must be often be worked on under adverse circumstances and in challenging locales. By seeing how field personnel handle these problems, you can often find solutions which work more effectively than you would by using the tools and techniques that design engineers normally have available.
It turns out that specialized instruments are available that quickly and easily test cable assemblies in the field. They’re small, lightweight, battery-powered, low cost, and easy to use. For example, among their very practical products from T3 Innovation is their Net Chaser Ethernet Speed Certifier (Figure 1) which provides gigabit Ethernet speed certification to IEEE 802.3 and cable-test qualification to TIA568 A/B requirements. It checks for faults in the wiring, noise in the network, and provides network discovery and organization information. (They also have comparable products for CAT-spec’ed cables, coaxial cabling, and more.)
These instruments fill the gap between the basic continuity test, which is necessary but usually insufficient, and the more sophisticated, full-range TDR (time-domain reflectometry) instrument, which many engineers use. The latter is an extremely powerful tool for fully quantifying cable-assembly specifications, or finding impedance bumps and discontinuities, but it can be too much of a good thing in terms of amount of data it offers, or basics such as weight, convenience, or cost. Testing of cable assemblies may not have the glamour or visibility of other test protocols and the instruments that serve them. Without these links, however, even the best electronic circuits and products are either useless or severely compromised. Worse, their shortcomings when installed can result in lots of time and energy wasted in troubleshooting.
Testing and integrity issues don’t end with copper, either. The increased use of optical interconnects for links of all lengths means that there’s a need to quickly and effectively evaluate a single optical link or a rack with many of them. Obviously, a ohmmeter-based continuity test is useless here, so what’s the next step? Certainly, you can get advanced optical-fiber test sets, which provide information on factors such as bit error rate (BER), but what it you really first want to know is which optical fiber goes where (for both dark and lit fibers), and their overall quality (both fiber and connector). Again, there are handy tools available, such as the VFT-30R visual fiber tester from Click2 (Figure 2).
Have you ever discovered or used an instrument designed for field test rather than engineering design, development, and debug, and been impressed with what it can do, and the ease of use?
Bill Schweber is an electronics engineer and author who has written for EE Times, was analog editor at EDN and prior to that worked in marketing communications for Analog Design and was also editor of its technical journal.
This article first appeared on EE Times Test & Measurement DesignLine
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