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Event Dead Zone: The Minimum Distance to Distinguish Two Points
The term "dead zone" may sound abstract.
Technicians new to the field might think: as long as the OTDR can measure distance, does the dead zone size really matter?
It matters a great deal. When testing closely spaced splices, a large dead zone causes adjacent splice points to "stick together" on the trace, making them impossible to distinguish. At best, you get inaccurate positioning; at worst, you miss a fault point entirely and have to rework.
Let's clarify this once and for all.
The event dead zone is defined as the minimum distance at which two separate reflective events can be distinguished.
What does that mean? When you inject light into a fiber, encountering a connector or splice point with reflectance causes a spike on the trace. If two reflective points are too close together, the leading edge of the second spike is obscured by the "tail" of the first, and the trace cannot separate them.
The FA8000's event dead zone is 1.2 m. This means that if two reflective events are at least 1.2 m apart, the instrument can resolve them on the trace.
That may not sound like much, but it is critical in practice.
In a building's corridor, the run from an optical splitter box often includes several connectors and splices in close proximity, sometimes spaced less than 1 m apart. Using an ordinary OTDR with a 3 m or 5 m event dead zone, all these points blur into a single lump on the trace, and you cannot determine which connector is problematic.
The FA8000's 1.2 m dead zone, combined with the 5 ns minimum pulse width, handles these dense scenarios effectively.
The 1.2 m specification is not arbitrarily stated. The test conditions are: a reflective event within 10 km with a return loss of 45 dB, measured at the minimum pulse width. These are the industry-standard conditions for specifying dead zones—when comparing OTDRs from different manufacturers, always check the data under these same conditions.
Attenuation Dead Zone: The Distance Required to Accurately Measure Loss After a Reflection
The attenuation dead zone addresses a different issue: how far after a reflective event must you go before you can accurately measure fiber attenuation.
Have you ever measured splice loss? A normal splice loss is a few hundredths of a dB—just a tiny dip on the trace. If the attenuation dead zone is too large, the "spike" from the reflective event has not yet settled, and the instrument begins calculating loss prematurely, yielding inaccurate readings.
The FA8000's attenuation dead zone is 6 m. Six meters after a reflective event, you can accurately measure the fiber attenuation in that segment.
For PON links, this means that after measuring a splice point, you can obtain accurate loss readings within just a few meters—without having to wait for a long distance to stabilize.
Pulse Width Selection: The Trade-Off Between Short and Long
Dead zone is directly linked to pulse width.
Narrower pulse widths produce smaller dead zones but also lower signal energy, limiting measurable distance. Wider pulse widths increase dead zones but provide sufficient energy to reach farther.
The FA8000's pulse width range spans from 5 ns to 20480 ns, with 13 selectable steps.
In the field, a practical selection guide is:
• Short-distance testing—corridors, splitter ports: use narrow pulse widths (5 ns, 10 ns, or 20 ns) for minimal dead zone and clear detail.
• Long-distance trunk fiber testing: use wide pulse widths (5120 ns and above), trading some resolution for greater measurement distance.
For example: at 5–10 ns pulse width, the event dead zone is approximately 1 m; at 20 μs (i.e., 20480 ns), it can exceed 2 km. That is a three-order-of-magnitude difference—so pulse width selection is not something to take lightly.
The sampling resolution is also adjustable from 0.125 m to 8 m. Use fine resolution at close range to capture details; use coarser resolution at long range to reduce data volume and improve speed.
A practical tip: run Auto mode first, letting the instrument select pulse width and parameters automatically. If a specific trace segment looks suspicious, switch to manual mode with a narrow pulse width for a closer look. This workflow handles most scenarios effectively.
Dead Zone Specifications: Irrelevant in Daily Use? Not Necessarily.
Some may ask: in routine PON link testing, splice points aren't that dense—does dead zone size really matter?
Fair point—during routine acceptance testing, you may not notice the difference. But consider this scenario:
The link is up, yet the subscriber complains of intermittent service. You suspect a microbend or stress-induced damage in a certain fiber segment. This type of damage may not be a clean break—it might only cause a slight increase in attenuation, with a subtle change on the trace. You need to locate that anomaly among a cluster of closely spaced splice points.
That's where the 1.2 m event dead zone proves its worth—you can distinguish between splice points separated by just tens of centimeters.
Whether that capability is worth the investment is for you to judge.
