Explain insertion loss return loss and crosstalk

What you should be able to do

Describe three different transmission effects and interpret their measurement directions. Distinguish a measured value from its margin to a selected test limit.

Insertion loss

Insertion loss describes reduction in transmitted signal through a link. With the positive loss convention used here, lower dB means less loss. The result depends on frequency, so one number without its frequency and test configuration is incomplete. Testing must cover the required range rather than a single convenient point. The forward arrows in the poster represent a conceptual reduction, not measured amplitudes.

Return loss

An impedance discontinuity can reflect part of the signal back toward its source. Return loss compares incident power with reflected power. With RL = 10 log10(incident power / reflected power), a larger positive value means a smaller reflected fraction. “High return loss” is therefore favorable under this convention. Do not interpret it as a large amount of unwanted returned power. The poster's yellow backward arrow shows the reflected direction; the marked mismatch is a conceptual event, not a diagnosed connector.

Crosstalk

A driven pair can couple unwanted signal into another pair. NEXT is near-end crosstalk, measured at the same end as the disturbing signal source. Higher NEXT loss in dB represents greater separation between the wanted source signal and coupled signal, hence less coupling. A NEXT failure alone does not prove a particular connector is responsible. Preserve designed pair geometry and use diagnostic evidence to investigate. The poster depicts whole-pair signal paths, not individual conductor connections; its dashed coupling arrow is not a physical short.

Read the three pictures together

The first picture follows the desired signal to the other end. The second follows unwanted reflected energy back on the source path. The third follows unwanted energy into a different pair. These are distinct comparisons. Do not add the three displayed dB measurements to produce a “total cable score.” Their reference quantities differ.

Worked through

These invented limits are NOT requirements from TIA, ISO or a cable category. Assume one frequency, the same stated configuration, and no special evaluation exclusions:

1. Insertion loss measured 18 dB; permitted maximum 20 dB. Margin = 20 - 18 = +2 dB.
2. NEXT loss measured 38 dB; required minimum 35 dB. Margin = 38 - 35 = +3 dB.
3. Return loss measured 17 dB; required minimum 19 dB. Margin = 17 - 19 = -2 dB: below this invented minimum.

A lower insertion-loss measurement helps A, while higher NEXT and return-loss measurements help B and C. In each case a larger positive margin is favorable, but only relative to its own applicable limit. Raw measurements and margins are not interchangeable.

Power-ratio exercise

Using the positive return-loss convention: If reflected power is 1/100 of incident power, RL = 10 log10(100) = 20 dB. If reflected power is 1/1000 of incident power, RL = 10 log10(1000) = 30 dB. The 30 dB example has less reflected power. These are normalized teaching examples, not a test fixture specification or acceptance threshold.

Trainer discussion

An instructor supplies two saved reports for the same intended cabling configuration. Before comparing their largest numbers, write down:

• Cable ID and tested boundary.
• Selected standard/category/class and test-limit version.
• Measurement name and units.
• Frequency associated with the result.
• Pair or pair combination and measurement end.
• Raw value, applicable limit, margin and overall outcome.

If these fields are missing, label the comparison incomplete. Do not change the limit to make an existing failure disappear. Investigate whether the setup matches the actual project and retain the original evidence. Actual certification procedures, adapters, calibration and applicable exceptions belong to the tester instructions and specified acceptance method.

Practice scenario

Report X lists NEXT “3 dB margin.” Report Y lists NEXT “38 dB measured.” A student says Y is much better. Explain the problem.

Answer: One number is distance from a limit; the other is the measured ratio. Without the applicable limits, frequency, pairs and configuration, that ranking is unsupported.

Knowledge check

Where beginners go wrong

Sources