The need for such devices as reflectometers for cable lines arose at the time when the transition from analogue to digital communication was made. The thing is that with an analog connection, it was believed that if one person hears another, then this is good, and the noise and crackling on the line is inevitable. With digital communications, things were not so simple.
General Description of Digital Communications. Connection quality
The main difference between digital signal transmission from analog is that it transmits a cleaner sound. In other words, noises and crackling should be absent during a conversation. Increasing communication quality requirements has led to the need for cable inspection. Malfunctions or any defects arising on these segments had a dramatic negative effect on the transfer of information between communication points. It was in order to find these defects, the causes of their occurrence and eliminate them, such devices as reflectometers for cable lines were invented.
The basic principle of work
It is worth saying right away that there are several types of this device. Some of the models are designed to work with old-style cables. Others are used to test fiber-optic connections, which are currently considered one of the best, since the signal is transmitted through them much faster, and the losses are much smaller.
So, the basic principle of the OTDR is as follows. The device connects directly to the cable and sends a short electrical pulse through it. If there are any defects, inhomogeneities, malfunctions, breaks, etc. on the path of this signal, then the electric pulse will be reflected in the opposite direction. It is important to note here that the characteristic of the reflected signal will directly depend on the reason for which it returned. That is, you can not only establish the presence of a defect, but also understand its cause. OTDRs for cable lines are able to record the returned signal, measure its parameters, determine the time after which the pulse returned. In addition, it automatically compares the performance of the original signal and the one that returned.
Human participation in the operation of the device
Special programs have been introduced into the characteristics of the device, which are able to analyze all available data and, based on them, draw a conclusion about how far the defect is and what caused it to occur. All information that the device receives during operation is displayed on the display of the device. Speaking generally about human participation, all that is required is to connect the device to the line, press the start button, and after receiving the data, familiarize yourself with them. The reflectometer for cable lines performs all other work 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 quickly respond to them.
What features does the device have?
In addition to their intended use, reflectometers can also be used to diagnose a communication cable, and can also interact with power and signal cable types. The range of each device depends on its power. The main limits of their work are in the region of 10 to 50 km. One of the most convenient features of modern models has become that they can be connected to a computer. This feature allows you to save all the data obtained, and then compare them with the results that were previously determined. With the help of this device, it is possible to determine not only the place of breakage of the communication cable, but also the place of a short circuit, entangled pairs, parallel taps, floating defects.
Device specifications
Of the main characteristics that affect the operation of the device, it is possible to distinguish such as the cross section of the cable cores, the quality of the wire, as well as the method of connecting the device to the communication line. For example, with an increase in the cross-section of the cable cores, the attenuation that undergoes an electrical impulse will decrease, which means that the distance of its passage will increase.
If we talk about working with old cables, then they often have such disadvantages as reduced insulation resistance, as well as increased attenuation. Any of these disadvantages will adversely affect the working distance of the device. Another important point affecting the same characteristic is the way the cable reflectometer is connected to the wire. The device must be connected in such a way that it can transmit the maximum possible pulse in power. In addition, there is another characteristic that relates to noise filtering. The OTDR has an automatic filtering function. Due to this, the device has the ability to automatically get rid of noise that interferes with operation.
Receiving units
OTDRs for cable lines are divided into two types, depending on which receiving unit is used in the design. The most commonly used narrowband receiver unit. The use of just such a device is justified by the fact that it allows the use of a narrow-band amplifier, as well as an ADC (as a result of this, power consumption is reduced, and the cost of the device is also reduced). In front of the amplifier are parts such as a sampling and storage scheme. The use of such schemes has led to the possibility of applying pulses with a length of up to 2 ns. However, like any other device, and this has its drawbacks. The disadvantage of such a circuit was that it displays the interference reflectometer on the display. Because of this, it is possible to use devices with such indicators only at short distances.
Broadband device
The main characteristic of an OTDR for cable lines using a receiver unit such as broadband in the design is that the noise level is reduced to a minimum. This property is ideal for long distance use. This advantage is due to the fact that in the assembly of the device there are no such sampling and storage schemes as in narrowband, but the ADC remains. However, it will not work at all to use this device at short range, since it does not support the supply of short-range pulses. The basic principle of operation on which the operation of a broadband amplifier is based is the measurement of the speed of the pulse passing through the cable before colliding with the heterogeneity and vice versa. It is worth noting that this characteristic is presented in the form of a coefficient, which is selected from the table. It follows that the accuracy of this type of reflectometer directly depends on how accurately the coefficient was selected.
Optical devices
In addition to conventional devices, there are also optical reflectometers. They have much in common with the first type of device, but there are significant differences.
It is necessary to begin to understand with the design of the device. Like all other devices, it has a fairly good color-type display with good resolution, there is a powerful microprocessor, as well as a battery that will provide long battery life. Naturally, the OTDR itself is located inside. Connectors for its connection are located on top of the structure and are protected by covers.
Principle of operation
The principle of operation of an optical reflectometer is somewhat different from a conventional, pulsed sample. His work is similar to the action of the radar. When connecting and starting the device, a short powerful pulse of light is sent to the fiber, which immediately begins to measure all the reflections that occur along the wire. At the moment when the light pulse reaches some kind of heterogeneity, for example, welding, damage, etc., it is immediately reflected and returned back, where it is recorded on the photodetector of the unit. Here the differences begin from the search for a cable break with a conventional device. During the collision, not the whole impulse returns, but only part of it. This means that the probe signal, albeit weakened, but still continues to move forward along the cable in search of heterogeneities. Thus, the signal follows either to the very break, or to the end of the wire, registering all the existing problems.
Characteristics and details of the device
The characteristics of the device itself, as well as its accuracy, depend on the three main elements that an OTDR has - a laser, a splitter, and a receiver. The same parts provide an odometer for measuring inhomogeneities.
The first design element is a laser LED. It is he who is responsible for the formation of short pulses of the probe type. Most often, the length of these signals ranges from 5 to 20 ns. It is worth noting that for each length inside the device has its own laser. In other words, if the device operates on two waves, then there will be two LEDs.
The second element is a splitter. This device is responsible for ensuring that the signal can freely enter the fiber, but does not immediately let it into the receiver. In addition, it is through this device that the signal reflected from the inhomogeneity inside the cable will pass.
The last element is a sensitive photodetector. This device is designed to accurately determine and measure the level, as well as the time delay of all received reflections. Fixation and measurements will apply to all signals that will be reflected along the signal. The most important characteristics of the device, such as: dynamic range and dead zone, will depend on the quality of this part. In addition, the quality of this element will also greatly affect the accuracy of measurements.
Basic settings of the optical unit
The range of signal changes varies depending on the four basic settings that are available for this device.
The first of these is the wavelength. This setting is the simplest and almost always it is tuned to a wavelength of 1310 and 1550 nm. This is if we talk about single-mode. The multimode almost always works on settings such as 850 and 1300 nm. An exception to these rules can only be a device that runs on PON with three wavelengths - 1310, 1490 and 1550 nm. Here you need to focus on such settings when working with an OTDR.
The second setting is the measured distance. When working with this parameter, you must be guided by one important rule - the end of the optical line should always be visible on the trace. For example, if the cable is 500 meters long, then the distance should be set to 1.25 km. If the wire is 4 km, then you need to set the distance to 5 km. This is necessary so that the trace can be automatically processed by the device.
Third is the duration of the probe pulses. The pulse duration is directly related to parameters such as dynamic range and dead zone. With a decrease in momentum, the dead zone improves, which means the ability of the device to distinguish those inhomogeneities that are located at close range is enhanced.
The fourth setting is the measurement time. The more time will be allocated to work, the better the trace will be, and the amount of noise will be significantly reduced.