User's Manual

Uniwave Performing Diagnostics and Testing

User Manual

14 Performing Diagnostics and Testing

14.1 Diagnosing Optical Links with an OTDR

The Infrastructure Cable between Hub Optical Module and Remote Unit must meet the following

specifications:

• Maximum Optical Loss 5dBo

• Minimum Optical Return Loss 45dB

This can be guaranteed only with Single mode fiber and SC-APC connections throughout the

installation.

14.1.1 Diagnose an Optical Link

In presence of high levels of back reflection due to poor Return loss, the Zinwave system performance

is degraded. The effects of this can be easily seen on the system and diagnosed using appropriate test

equipment. The effects of back reflection can be seen by looking at the level of the noise floor. In cases

where there are high levels of back reflection, the noise floor in either the downlink or the uplink can

vary significantly (10-15dB). It may also show as increased levels of harmonics which will also vary in

level by significant amounts.

The best way to check the return loss of a fiber link is to use an Optical Time Domain Reflectometer

(OTDR) with the appropriate connectors and launch cables. A typical graph result is shown below:

The graph shows the performance of fiber along its length. In this case the first horizontal line

represents the first 250m of fiber in the launch box. Each of the subsequent peaks relates to the

situations where internal reflection has occurred along the fiber length. This could be connectors,

splices or even damaged cables. Generally, the highest peak shows the worst case of back reflection

and hence source of return loss and potential interference, depending on the OTDR used results, can

be displayed in a tabular form giving distances and the relevant reflection or return loss.