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Last Updated: August 21, 2019

QUESTION:   

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?

 

ANSWER: 

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 measure less than it should.

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.

 

READ THE FULL BLOG ARTICLE HERE:  Patch cord gain: Shedding light on this perplexing issue

 

ADDITIONAL CONTENT & RESOURCES:

 

 

  Answered by AskFOC Technical Team  March 22, 2019

 

 

 

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