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CMRL – Common Mode Return Loss
Common mode return loss (CMRL) is the ratio of an unwanted common mode signal to its part that is reflected to the transmitter. Read on to find out more.
CMRL and RL (return loss) are closely linked to each other. Return loss refers to a differential mode signal like the wanted data signal whereas CMRL refers to a common mode signal like unwanted noise.
CMRL should be as high as possible.
CMRL is the ratio of common mode signal strength to its reflected part on the same wire pair. (CM = common mode)
Trouble-shooting tips if CMRL tests fail
Usually one will experience return loss problems rather than CMRL problems. Causes and trouble-shooting are quite the same.
| Typical cause of the problem | This might help (without engagement) |
| Wrong test program selected | Select the right test program |
| Cable is crushed (cable ties, cable clamps, inside of a backbox, …) | Rework the cable, use a connection module for repair, replace the cable |
| Cable is kinked | Rework the cable, use a connection module for repair, replace the cable |
| Minimum bend radius is not maintained | Change the cable routing so that the minimum bend radius is maintained |
| Cable got repaired | Rework the repaired part, use appropriate connection modules |
| Cable has become wet | Let the cable dray and test again, replace the cable |
| Maximum tensile load of the cable was exceeded when the cable was pulled in | Replace the cable |
| Cable was crushed during installation (someone stepped on the cable, something ran over it or was placed on it) | Replace the cable |
| Cable defective (material defect) | Replace the cable |
| Patch cords defective | Replace the patch cords |
| Category of the patch cords is too low | Replace the patch cords |
| Patch cords are kinked or crushed | Rework the patch cords, replace the patch cords |
| Cabling components are poorly matched | Select matched cabling components |
| Category of the patch panel, outlet or plug is too low | Replace the patch panel/outlet/plug |
| Patch panel, outlet or plug defective | Replace the patch panel/outlet/plug |
| Contacts in patch panel, outlet or plugs are dirty | Clean the contacts |
| Contacts in patch panel, outlet or plugs are worn | Replace patch panel, outlet or plugs |
| Contacts in patch panel, outlet or plugs are corroded | Replace patch panel, outlet or plugs |
| Contacts of the test cord are dirty | Clean the contacts |
| Contacts of the test cord are worn | Replace the plug/test cord |
| Contacts of the test cord are corroded | Replace the plug/test cord |
| Twist of a wire pair the patch panel, outlet or plug is not maintained or there is an air gap between the wires of a pair | Twist the wires up |
| Wires in the patch panel, outlet or plug are poorly terminated or kinked | Re-terminate the wires |
| Wires in the patch panel, outlet or plug have become loose | Re-terminate the wires |
Typical causes of CMRL problems and what might help to solve them.
Contact
Dirk Traeger
