The entire performance of a fiber optic (or copper) network depends on the performance of the patchcords used for interconnects and for connecting communications equipment, but it’s easy to take patchcords for granted. They are inexpensive, often not tested properly, and sometimes treated poorly.
Most users buy patchcords in bulk and store them unpackaged and plugged in. Some users hang them on the sides of equipment racks. That is not how they should be handled.
There are three important issues you must consider with regard to patchcords: quality, cleanliness and condition. You might assume the patchcords you buy are good quality. They come in a plastic bag, usually with a label that identifies the fiber and connector type, length and test results. (Recently, I have seen cheap imported patchcords with connectors with locking mechanisms that would not connect because they were improperly made. These patchcords had losses that tested around 1 decibel (dB), although the label said 0.2 dB. Under closer examination, the label appeared to be from a copy machine, not a printer, so I doubt the cable was even tested—more on that next month.)
To maintain cleanliness, all connectors should have a dust cap that covers the polished ferrule to protect the end of the connector ferrule from damage and dirt. A user recently told me that, after examining many connectors with a microscope, he understood why they were called dust caps; they were all full of dust. The dust cap should be used to protect the connector ferrule, but do not assume it keeps it clean. You must clean connectors before inserting them in mating adapters or active devices to ensure their cleanliness.
Finally, those cables connecting equipment to the cable plant or hanging on the sides of racks are often mistreated, leaving kinks in the cable or scratches and scuffing on the connector ferrules. With the reference cables used for testing, the continual mating to other connectors, even when carefully cleaned before use, will cause scuffing that increases loss over time.
All these factors mean patchcords need testing. It’s a small initial time investment that can save you a lot of troubleshooting time later. Start by checking the connector ferrules with a fiber optic inspection microscope around 100 power. You are not inspecting for polishing quality as you would during termination. You are looking for dirt and scratches. The careful user will inspect connectors before and after cleaning.
For testing the loss of a patchcord, you only need an inexpensive 850--nanometer (nm) light-emitting diode (LED) light source, a fiber optic power meter and some reference patchcords. Just remember that the patchcords used for references in testing must be good for the tests to be valid, so you test them as you would other patchcords, just more often.
Testing patchcords is similar to testing a fiber optic cable. Use one reference patchcord to set a 0-dB reference. Connect a patchcord to test to the reference patchcord with a mating adapter. Connect the power meter to the patchcord’s other end, and measure the loss. Since the length of the fiber is short, the fiber’s loss contribution is ignorable. And since one end of the cable is attached to the power meter, not another cable, you only measure the loss of the one connection between the reference cable and the cable under test, so you can test each connector individually.
To complete the testing of the patchcord, reverse the cable you are testing to check the connector on the other end. Sometimes you will find one bad connector and can replace it to make the patchcord useful again, but often, the cost of replacing the connector may be higher than replacing the patchcord itself.
If your test equipment has different connectors than the patchcords you are testing, you will need hybrid reference cables with connectors compatible with the equipment on one end and the patchcord connectors on the other end. You will also need the correct connector adapters for your power meter.
Obviously, all reference cables used for testing must have high-quality connectors to get reliable test results. Use this same method to test your reference cables against each other, and discard any with high losses, usually those with losses greater than 0.5 dB.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.