Test Cables Don’t Last Forever

I frequently hear customers complain that although nothing has changed in their fiber optic cable assembly production process, the measured Insertion Loss (IL) and Return Loss (RL) values of their product aren’t as good as they once were. Is something wrong with the measurement equipment? Well, it’s possible but unlikely.

Obviously something has changed. If it’s not the measurement equipment, then it must be either the product or the measurement setup. Over time the production process can become a familiar, perhaps boring, routine. Operators may become less meticulous in cleaning, use polishing film longer than appropriate, or even take shortcuts. Eventually this takes a toll and yield suffers.

Let’s suppose you’ve eliminated this possibility with a clean sweep of procedures and a reset to rigorous production guidelines. Yet the problem persists. A “gold-standard” product is useful at this stage – a “known-good” example of the best of your previous production, carefully kept, with very good IL and RL values recorded. Using your current rigorous procedures, re-measure this gold-standard product. It should still look good. If it doesn’t, read on …

I have written that IL and RL tests on a product cannot be made in isolation. Remember, we are measuring connector loss, and the device under test (DUT) needs to be connected to a test cable (aka jumper cable, reference cable, or test lead). It’s important to note that the quality and condition of test cables have a direct bearing on the measured product’s IL and RL measurements.
 

Firstly, ensure the test cable’s connectors are clean. Next, take a close look to ensure they are in good condition. Keep in mind that both connectors and coupling sleeves don’t last forever. They will degrade with use, and it’s not just scratches and pits. Sliding surfaces wear out. As the fit becomes loose, alignment will suffer.

How many matings can a test connector undergo before it should be replaced? It would be nice to have a definitive number to work with. However, it’s difficult to give a definite answer as it depends on the type and quality of the connectors and the skill of the operators.

Suppose replacing all the test cables and couplers restores your gold-standard product to the previous good measured values. This is useful information. Take a minute to determine approximately how many matings these particular test cables have undergone. What number did you arrive at? Now you know how many couplings are too many!
 

I propose you set usage limit at, say, half the above number. Weigh the ongoing cost of replacement and time required to replace test cables. How does this compare with the cost if you let products go to the failure stage?

A regime of frequent monitoring of the gold-standard device might help nail down the test cable usage limit. (Don’t do this too frequently, or your gold-standard cable will degrade.) This monitoring regime will help you create a reasonable replacement schedule.

Here’s a helpful tip: If you use the Viavi MAP/PCT system, it can track measurement jumper usage and warn if the cable exceeds a user-defined limit. If you use another test instrument, I encourage you to look into whether it can track usage and provide this type of warning as well.
 

Test cables do have a finite useful working life. And this lifespan will vary. For example, one improper mating or cleaning can potentially damage a test cable’s end-face enough to render it unusable.

Implementing robust process controls will go a long way to extend the test cable’s life and identify when it needs to be replaced. As discussed, I recommend:

Here’s another tip: Telcordia’s GR-326 specification provides excellent guidance on this topic. It is highly recommended that any cable assembly manufacturer procures a copy and, at minimum, adheres to the requirements of Section 8.0.

If you are seeing degraded IL/RL results with your fiber optic cable assemblies, I recommend you first establish rigorous production controls. Next, follow the guidelines in this article to implement procedures to assess the condition of test cables, track usage, and create a reasonable replacement schedule.