How "Dynamic Range" is Important Characteristics of an OTDR - technopediasite-Ultimate Resource For Telecom Technical Support

Monday, November 5, 2018

How "Dynamic Range" is Important Characteristics of an OTDR

How "Dynamic Range" is Important Characteristics of an OTDR: "Dynamic Range",this is the specification of OTDR. Today I will explain about the Dynamic Range of an OTDR, how it is important? The dynamic range is one of the most important characteristics of an OTDR, since it determines the maximum observable length of a fiber and therefore the OTDR suitability for analyzing any particular network. The higher the dynamic range, the higher the signal to noise ratio and the better the trace will be, with a better event detection. This dynamic range is relatively difficult to determine since there is no standard computation method used by all the manufacturers.

One method of determining dynamic range is to take the difference between the extrapolated point of the backscatter trace at the near end of the fiber (taken at the interception between the extrapolated trace and the power axis) and the upper level of the noise floor at or after the fiber end.
The upper level of the noise is defined as the upper limit of a range which contains at least 98% of all noise data points.
The level is expressed in decibels (dB).

This measurement is performed with a 3 minute period for the averaging.
This value of the dynamic range was also recommended by Bellcore.

OTDR Dynamic range
Dynamic Range
How "Dynamic Range" is Important Characteristics of an OTDR
Other definitions of the dynamic ranges are given by different manufacturers, which makes the values comparison very difficult:I hope that you can understand very easily with the help of below figure.
dynamic ranges are given by different manufacturers
Dynamic ranges are given by different manufacturers

RMS. The RMS (Root Mean Square) also termed SNR=1 dynamic range is the difference between the extrapolated point of the backscatter trace at the near end of the fiber (taken at the intersection between the extrapolated trace and the power axis) and the RMS noise level. 
N=0.1 dB. This dynamic range definition gives an idea of the limit to which the OTDR can measure when the noise level is 0.1 dB on the trace. The difference between N=0.1 and SNR=1 RMS definition is approximately 6.6 dB. This means that an OTDR which has a dynamic range of 28 dB (SNR=1) can measure a fiber event of 0.1 dB up to 21.5 dB.
End detection: The dynamic range end detection is the one way difference between the top of a 4% Fresnel reflection at the start of the fiber and the RMS noise level. This value is approximately 12 dB higher than the IEC value.
Bellcore measurement range: The Bellcore measurement range is defined as the maximum attenuation that can be placed between the OTDR and an event for which the instrument will still be able to measure the event within acceptable accuracy limits. The event can be reflective or non-reflective, or a fiber break. For example, an event can be a 0.5 dB reflective splice (> 40 dB).
4% Fresnel: This is more an echometric parameter than a reflectometric parameter. It represents the ability of the instrument to perceive the peak of a Fresnel reflection for which the base cannot be perceived. It is defined as the maximum guaranteed range over which the far end of the fiber is detected, sometimes with a minimum of 0.3 dB higher than the highest peak in the noise level.
Peak level plus 0.3 dB: the dynamic range is the difference between the front-end backscattered trace and 0.3 dB more than the peak noise level.

Friends Please remember that "The value of the dynamic range, for each definition can also be given according to different conditions":

typical value: this represents the average or mean value of the dynamic range of the OTDRs which come out of production. An increase of around 2 dB is usually shown in comparison with the specified value.
specified value: this is the minimum dynamic range specified by the manufacturer for its OTDR.
over a temperature range or at room temperature. At low and high temperature, the dynamic range decreases usually by 1 dB.

Summary: An OTDR (Optical Time Domain Reflectometer) is a fiber optic tester characterizing fibers and optical networks. The aim of this instrument is to detect, locate and measure events at any location in the fiber link. One of the main benefits of the OTDR is that it can fully test a fiber from only one end, as it operates as a one dimensional radar system. The OTDR is similar to an accurate radar as its resolution can be between 6 cm and 40 meters.The OTDR technique produces geographic information with regard to localized loss and reflective events thereby providing a pictorial and permanent record which may be used as performance baseline.

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