Time domain reflectometer for cable lines: characteristics and working principle


2018-03-31 16:58:16




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The Need for such devices as time-domain reflectometer for cable lines, originated in a time when the transition from analog to digital. The thing is that with the analog connection it was thought that if one person hears another, then it is good, and the existing noise and crackling on the line is inevitable. With digital communication it was not so simple.

General description of digital communication. The quality of the connection

The Main difference between the digital signal transmission from analog in the fact that it transmits more clear sound. In other words, the noise and crackle must be absent during a conversation. The increasing demands for quality of communication has led to the fact that there is a need to check cables. Any malfunction or defects arising in these segments, negatively affected the transfer of information between communication points. It is in order to find these defects, their causes and eliminate them, they invented instruments such as reflectometers for cable lines.

time domain reflectometer for cable lines

Basic working principle

It is Worth saying immediately that there are several types of this device. Some of the models are designed to work with cables of the old model. Others are used for testing fiber links, which is currently considered one of the best, as the signal is transmitted through them much faster, and the losses are much smaller.

So, the basic principle of the reflectometer is as follows. The device connects directly to the cable and sends it a brief electrical pulse. If the path of this signal will be any defects, inhomogeneities, dislocations, breakage, etc., then an electrical pulse will be reflected in the opposite direction. It is important to note that the characteristics of the reflected signal will depend on why exactly he came back. It is possible not only to establish the existence of the defect, but also to understand its cause. The time-domain reflectometer for cable lines is able to capture the returned signal, measure its parameters, to determine the time after which the pulse returned. In addition, it automatically compares the original signal and the one returned.


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cable reflectometer

Human Involvement in the operation of the device

Characteristics of the device are made of a special program that is able to analyze all the available data and on their basis to make a conclusion at what distance is the defect and what was the root cause. All the information that the device receives during operation, is displayed on the display device. If to speak in General about the human part, all that is required is to connect the device to a line, click the start button, and then retrieve the data to read them. The rest of the work time domain reflectometer for cable line runs in automatic mode. In addition, the use of such devices helps to understand in advance the cause and place of occurrence of the defect, which allows you to respond to them.

the principle of operation of OTDR

What features does the device

In addition to the direct purpose, the OTDR can also be used to diagnose cable and can also communicate with power and signal cables. The range of each device depends on its power. The main limits of their work are in the area from 10 to 50 km. One of the most convenient features of modern models is that they can be connected to the computer. This feature allows you to store obtained data, and then compare them with the results that have been previously defined. With this device it is possible to determine not only the location of cable breaks connection, but an electrical short circuit, reversed pairs, parallel bends, floating defects.

time domain reflectometer for cable lines features


Of the major characteristics that affect the operation of the device, can be identified such as the cross-section area of cable, wire quality and how the device is connected to the communication line. For example, with the increase of the cross section of the cable cores, will decrease the attenuation, which undergoes an electrical impulse, and this means that the distance of its passage will increase.

If to speak about work with the old cables, they often have disadvantages such as low insulation resistance and high damping. Any of these disadvantages have a negative impact on the working distance of the device. Another important point affecting the same characteristic is the connection method of cable time domain reflectometer to the cable. Connect the device needs so that it can transmit the maximum possible power boost. In addition, there is another characteristic, which refers to filtering out the noise. The OTDR has an automatic filtering feature. This device is there an option to automatically get rid of the noises that interfere with work.

measurement reflectometer

Entrance units

Time-domain reflectometer for cable lines are divided into two types depending on how the unit is used in the design. Most oftenused narrowband receiving unit. The use of such devices is justified by the fact that it allows the use of narrow-band amplifier and ADC (the result is reduced power consumption, and lower the cost of the device). In front of the amplifier are such details as the scheme of the sample-and-hold. The use of such schemes has led to the fact that there is an opportunity to apply the pulse length of 2 NS. However, as with any other device and it has its disadvantages. The disadvantage of this scheme is that it displays the OTDR noise. To use devices with such indicators is possible only on short distances.

find broken cable

Broadband device

The Main feature of the reflectometer to the cable lines using the design for such a receiver unit, such as broadband, was that the noise level is reduced to a minimum. This property is ideal for the application of the device over long distances. This advantage is due to the fact that in the Assembly of the device, there are no sampling frames and storage in narrow-band, and the ADC remains. However, the use of this device on sprint will not work at all as it doesn't support pulsing low range. The main principle on which is based the work of the broadband amplifier is a measurement of the speed pulse cable to the collision with the heterogeneity and back. It is worth noting that this characteristic is submitted in the form of coefficient, which is selected from the table. It follows that the accuracy of the OTDR of this type directly depends on how accurately was picked up by factor.

Optical devices

In addition to conventional devices are also available OTDR. They have a lot in common with the first type of device, but there are significant differences.

To Begin you must understand with the design of the device. Like all other devices, it has a pretty good display of the color type with a good resolution, a powerful microprocessor and a battery that will provide long-term Autonomous operation. Naturally, inside is a and the reflectometer. Connectors for connecting top design and are protected by caps.


Working Principle

The Principle of operation of OTDR is somewhat different from the conventional pulse model. His work is similar to a radar. When you connect and run the device into the fiber is sent to a short powerful pulse of light that immediately begins to measure all reflections that occur along the wire. In that moment, when the light pulse comes to any kind of inhomogeneity, for example, welding, damage, etc., he immediately reflected back, where it registers on the sensor unit. Here begins the differences from the search of the breakage of a cable of a conventional device. During the collision is returned, not the whole pulse, but only part of it. This means that the probing signal, albeit weakened, but still continues to move forward along the cable in search of inhomogeneities. Thus, the signal should be either before the fall or before the end of the wire by recording all problems.

Features and details of the device

Characteristics of the same device, as well as its accuracy depend on three main elements available to the OTDR is a laser, splitter and receiver. These parts provide a measure of the OTDR inhomogeneities.

The First element of the design is the laser led. It is responsible for the formation of the short pulses of the probe type. The most common length of these signals is in the range from 5 to 20 NS. It is worth noting that for each length inside the device there is a laser. In other words, if the device works in two waves, and the LEDs will two.

The Second element is the splitter. This device is responsible for ensuring that the signal can freely enter the fiber, but do not let it immediately to the receiver. In addition, this device will pass the signal reflected from inhomogeneities inside the cable.

The Last item is a sensitive photodetector. This device is designed to accurately determine and measure the level as well as delay time of all received reflections. Fixation and measurement will apply to all signals that will be reflected during the passage of the signal. The quality of this personal details will depend such important device characteristics as dynamic range and dead zone. In addition, the quality of this item would also greatly affect the accuracy of the measurements.

Basic configuration of the optical unit

The Range of changes of the signal varies depending on the four baseline settings, which are available with this device.

The First of these is the wavelength. This setting is the most simple and almost always it is tuned to the wavelength of 1310 and 1550 nm. This is speaking of the single-mode. Single-mode almost always works in such settings as 850 and 1300 nm. Exception to these rules may be the only one device that operates on a PON with three wavelengths - 1310, 1490 and 1550 nm. Here are the settings to look for when using a reflectometer.

The Second setting is the measured distance. With this option, you will needbe guided by one important rule - the reflectogram must be visible at the end of the optical line. For example, if the cable has a length of 500 meters, the distance should be set to 1.25 km. If the wire is 4 km away, to put distance to 5 km Is necessary in order to reflectogram could automatically processed by the camera.

The Third is the duration of the probe pulses. The pulse duration is directly associated with parameters such as dynamic range and dead zone. With a decrease in pulse improves the dead zone, and therefore increases the ability of the instrument to distinguish between those variations, which are located at a close distance.

The Fourth setup is the measurement time. The more time will be allocated to work, the better will reflectogram, and will also be considerably a reduced amount of noise.

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