Here is a question several customers have asked me. Have you experienced this issue?
“When measured on my properly calibrated IL/RL meter, some of my patch cords show ‘gain.’ How can this be possible? Since a patch cord cannot have gain, is there something wrong with the instrument?”
If your IL/RL meter is properly calibrated, there probably is not anything wrong with your instrument. To understand this puzzling issue, we must look at the measurement method.
Keep in mind that it’s not possible to measure a patch cord (aka jumper) in isolation. It needs to be connected to something – test jumpers are required.
One test method uses a pair of test jumpers: a launch and a receive. These are connected together for referencing, then the device under test (DUT) is inserted between them. This gives the total Insertion Loss of the jumper, including connections at both ends. The IL meter simply reports the difference in power between the reference and measurement connections.
If the test cable connectors were perfect, they would introduce no loss at the referencing stage. Any loss in measurement would be due to defects in the DUT’s connectors or fiber. If the test cables have geometry errors, then an offset from a properly centered condition may produce loss when referencing.
In the worst case, the offsets would be at 180 degrees. If the DUT happens to have offsets (at each end) to match the test cable, to some extent it will cancel out the offset effect. Therefore, the loss in measurement (due to this effect) will be less than in referencing, and the cable’s Insertion loss will be negative, it will appear to have Gain!
In general, test jumper offsets will be at random orientations relative to the device under test, and other loss effects may be greater. The effect will be to randomly add or subtract a small amount from the “true” IL of the DUT. But actual “gainer” DUTs will sometimes occur.
As a side note, I recommend using good-quality test jumpers to help ensure accurate Insertion Loss measurements. Clearly, to qualify the performance of a jumper, other cables must be used, and the quality of those cables will affect the measurement you get for your device under test. You could use reference-quality jumpers, but they could be prohibitively expensive for your production facility. On the other hand, it’s not a good idea to use inexpensive jumpers of unknown origin. I recommend you purchase good-quality jumpers produced by established first-tier manufacturers, which we supply here at Fiber Optic Center.
If you have questions about test jumpers, feel free to contact me or email your question to AskFOC@focenter.com – one of our technical experts will respond as soon as possible. Our goal is to provide information and education, so you can continually improve your manufacturing and testing process.
Speaking of information and education, I’ve authored several articles on related topics. I encourage you to take a few minutes to read these articles. Ideally, this information will help you improve your process, your results, and your fiber optic cable assemblies.
How to accurately measure IL/RL – This article discusses why it’s important to measure Insertion Loss and Return Loss and how to accurately measure IL/RL. Plus, I offer specific advice regarding whether to use a mandrel wrap, index matching gel, or Optical Time Domain Reflectometer (OTDR) when measuring RL.
What causes poor IL/RL? – In this nuts-and-bolts discussion, Dan Rocheleau and I address multiple production issues that can cause poor Insertion Loss and Return Loss, along with specific recommendations to remedy the issues.
Insertion Loss measurement, not a trivial task – If your production facility is new, should you make your own launch jumpers? This might not be a good idea, at least initially. Also, did you know that launch jumpers should use the same fiber type, not just the same size? Here at FOC, we have seen problems mating bend-insensitive fiber to the regular type. You’ll get even more tips in this article.
Encircled flux – a relatively non-technical overview – The IEC 61280-4-1 Standard sets out to define the distribution of light (flux) at the launch into the device under test. “Encircled flux” is a rather grand way of saying the amount of light inside each circle of a given size within the core. This article addresses why the standard regarding flux is important and how to get consistent EF-compliant measurements.
Is Bi-Directional testing the silver bullet to cut all your optical testing time in half? – Many cable assembly houses look to Bi-Directional testing as a time savings. However, some do not accurately use this test method. Hint: This test requires the use of a launch AND a receive cable, so when light is shot in one direction there is a launch, and when it’s shot in the opposite direction there is also a launch. Read on to see if you need to adjust your test method to fully utilize the Bi-Directional feature.
Follow Chris at @TestExpert_FOC