Multimode OTDRs require a pulse suppressor (dead-zone) box.

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    Pulse Suppressor Boxes

    Pulse suppressor boxes, also known as dead-zone boxes or fiber rings, are vital to the success of an OTDR measurement. These boxes are simply long launch cables placed between the OTDR and the near-end patch panel, and serve two key purposes:


    Reflections caused by connector interfaces “blind” OTDRs for a short period of time. The period of time it takes for an OTDR to recover from this “blindness” is commonly referred to as a “dead-zone”. During this dead-zone period, OTDRs are unable to distinguish one anomaly (e.g. breaks, shatters, bends, or even another connector) from another. Without a sufficiently long launch cable, the reflection from the near-end patch panel will be undetectable because it is within the dead-zone caused by the OTDR port.

    Loss Measurement Through Inter-Connections

    To measure the optical loss of any event found on an OTDR trace, there must be sufficient measurable backscatter both before and after the inter-connection. Lack of a dead-zone box means there is no measurable backscatter outside the fiber link under test, preventing the OTDR from measuring the relative loss through both near-end and far-end patch panel connections. As a general rule, pulse suppressor boxes should be longer than the longest pulse width setting of the OTDR used for testing. In most cases, 1-kilometer launch cables are sufficient to account for dead-zones.

    Pulse Suppressor Box Example Usage