#aroundcable

Modernization of 100/400G networks and deployment of 5G networks at the proper quality level require testing of optical fiber communication lines (OFC). Choosing the right methods and additional instruments is critical when conducting bulk testing, as mistakes can be very costly.

If we talk any modern optical networks, fiber and coupling quality are critical. Modernization of existing networks involves checking their condition as well as laying new OFCs—testing the quality of connections. The higher the speed of fiber-optic lines, the more stringent the requirements for the quality of their diagnostics. And here the problem of choosing a testing methodology arises: for example, is "testing to the maximum" using bidirectional tests always the best option?

Bidirectional testing of OFCs with a reflectometer: the answer or new problems?

It is important to understand that fiber-optic communication is related to reflectometry. Without a high-quality optical reflectometer (OTDR), it is impossible to create a reliably operating fiber-optic link. The solution to the problem of testing comes down to the correct initial selection of equipment and determining the most suitable methodology.

There are two main methods of reflectometry: one-way (one compensation coil at one end of the line; launch cable box) or two-way (Bi-dir OTDR, with a compensation coil at the near end and the same at the far end). .

The direction of the emitted light can influence the test results. In any fiber, there is a difference in the return loss coefficients, and in one particular direction, the light loss may be greater.

Single-sided OTDR testing can miss many anomalies. Wire junctions with different return loss values can compensate for the signal loss in one orientation and attenuate it in the other. In addition, there are dead zones in which the OTDR does not record events.

The figure below shows an example of the difference in signal loss depending on the measurement direction. One side of the testing even shows a negative loss of -0.3 dB, which is, of course, impossible. In this case, an amplification effect is observed due to the difference in the backscatter coefficient (BSC) at the junction of the two cables.

Therefore, one-way OTDR testing is better suited for simple tests such as finding and locating fiber breaks and bends, assessing total signal attenuation in fibers, testing connectors, etc. In this case, there is no need to waste time on bidirectional testing. The main advantage of unidirectional fiber-optic testing is that it is enough to inspect and clean the fiber just once for any one line. This is important since contamination of the connectors can lead to failure of the fiber-optic line, which was working properly before testing. Sometimes customers insist on two-way "full testing," but this can be excessive and even harmful as it introduces an unnecessary risk of damage while cleaning the optical connectors.

Bidirectional fiber inspection without running around

One-sided tests are not accurate when laying new lines and monitoring the performance of FOCs. In such cases, industry standards require bidirectional testing with an optical time domain reflectometer (Bi-dir OTDR) measuring the signal at both ends of the line. This is necessary to identify anomalies that are not detected by conventional one-way reflectometry. In addition, there are unique cases where a line uses cables with different fiber diameters, where bidirectional effects can be important. Bi-dir OTDR testing makes it possible to average the measurement parameters and give a clear assessment of the quality of the fiber-optic link. Ultimately, it is possible to unambiguously determine whether the tested line supports high-speed data transfer. This saves time and money that the customer may lose if the cable system fails on launch.

Traditional bidirectional testing of fiber-optic links implies first measuring one side:

And then testing the optical line on the other side:

Then obtain the average parameters. This is an important job that requires manual data processing. It can be accelerated by using two similar instruments with two external cable routes simultaneously (the bidirectional reflectometer technique), but this requires additional equipment expenses.

Another effective technique is testing with an optical reflectometer with a closed circuit (loopback). This method involves using a reflectometer at one end of the line and a reference fiber loop at the other. This way, one can test two optical lines in both directions, alternately swapping cables. This is the most effective method in terms of cost and labor savings since it requires only one reflectometer, loop switching, and two compensation coils.

A major drawback of OTDR-loopback testing is the labor-intensive process of recording and merging test data. Often, manual work with data leads to errors, and repeated tests are needed. This problem has been solved in modern optical reflectometers, such as the VIAVI T-BERD/MTS-4000 V2.

When using such advanced devices, the task of specialists is reduced to the high-quality implementation of procedures for cleaning contamination of optical connectors and one-way checking of launch cables before starting bi-directional testing.

In recent decades, the rapid development of optical fiber communication (OFC) lines has required simple, reliable instruments for diagnosing optical communications. An optical time domain reflectometer (OTDR) is one of the most common devices for testing fiber optic links and identifying problem areas in fiber optic communication lines. What criteria should be used to choose a reflectometer so that it performs correctly and does not require excessive financial investments?

Reflectometer: luminous intensity

A reflectometer directs a beam of laser light into an optical fiber. Then, it measures the parameters of the reflected light, thus analyzing the characteristics of the optical fiber. This way, one can not only detect but also determine the location of any damage to the fiber optic line: a lousy receptacle or connector, a cable bend, light loss, poor splicing, etc.

This is a very effective technology, but it has two severe limitations. First, the reflectometer's probe pulse is reflected from all connectors, including the first one, which is why "lighting" creates a dead zone in which measuring is impossible. This problem is solved using an additional piece of optical fiber (launch cable) connected to the line under test. The dead zone is on this fiber, and the entire line can be tested. It is necessary to consider the length of the line that is supposed to be tested and select the correct length of the compensation coil; sometimes, the length can reach several miles.

The second limitation is that different types of optical fiber have the highest light reflectance coefficient at different wavelengths. Of course, the best choice seems to be the most versatile device that can operate in a wide range of wavelengths, for example, from 850 nm to 1650 nm. In particular, the VIAVI MTS-8000 universal measuring platform and a set of modules capable of solving almost any problem of fiber-optic communication analysis.

One must keep in mind that expanding the capabilities dramatically increases the cost of the device. However, these capabilities are not always necessary. More straightforward solutions are often sufficient for checking and even last-mile optical line certification, such as an optical reflectometer with the tester function and damage visualizer Greenlee 930XC-20C-UPC-FC.

The situation is similar to the dynamic range—the strength of the reflectometer signal and its ability to detect even slight attenuation of the optical signal. This can result in a severe deterioration in the efficiency of fiber optic lines on long, critical lines. Therefore, more expensive reflectometers with a wide dynamic range are used to check them. Generally, an OTDR with a dynamic range of 6 dB is more excellent than the loss of the longest optical communication line that the OTDR will ever have to service, which is sufficient for reliable testing.

These are the main aspects to consider first when choosing a reflectometer. However, many reflectometer models are on the market, and selecting them is not always easy. Fortunately, there is a simple set of questions, and answering them will give you a "portrait" of the device best suited for a specific set of tasks.

The right questions to ask when choosing an optical reflectometer

First of all, you need to answer questions about using your new reflectometer:

What networks and types of optical fiber will be tested (for example, multi-mode optical fiber or single-mode optical fiber)?

What is the maximum length of the fiber-optic link to be tested?

What measurements are aimed at (certification, troubleshooting, regular maintenance)?

The answer to these questions will significantly narrow the field of suitable reflectometers. For example, 850 nm and 1300 nm wavelengths are used for multi-mode optical fiber, and 1310 nm and 1550 nm are used for single-mode optical fiber. In the case of PON testing, wavelengths of 1490 nm and 1625 nm may be needed in addition to 1310 nm and 1550 nm.

If the reflectometer's main task is to localize damage, then buying an expensive device may be a waste of money.

However, if detailed diagnostics of a fiber-optic link and its certification are needed, professional reflectometers with a large dynamic range, small dead zones, and good software for processing reflectograms and generating a report are necessary.

It is also necessary to consider the aspects related to the device's operation. In particular, the size and weight of the reflectometer are directly related to the team's mobility. Devices with a larger screen (more than 5") are most often chosen for indoor work or as part of a mobile lab. Specialists use portable devices to work on city networks. Such devices must have waterproof housing and withstand a wide range of operating temperatures.

The minimum operating time on one battery charge is preferably at least 8 hours so that field measurements do not extend over two working days. The ability to upload data to the cloud for subsequent analysis and results processing will significantly save time.

Often, several instruments can be combined in one housing: a reflectometer, a tester, a damage visualizer, an optical spectrum analyzer, a dispersion analyzer, etc.

There are two general measures of video perception: resolution and color. Those notions are intentionally vague for the purpose of explaining their relation to HDMI transmission.

I've covered the resolution part in previous posts, as this metric is simpler to quantify. Every screen has a limited number of pixel dots; the more, the better. The same logic works for refresh rate; the faster we change the image on the screen, the smoother the motion picture. Screens are rectangular matrixes of pixels with a small number of aspect ratios (roughly, from long 21:9 to square-ish 3:2) and resolutions of 1, 2, 4, 5, or 8K. Higher HDMI standards support more Ks. Thanks to Steve Jobs' legacy, we have the upper limit for the density and, consequently, the number of usable pixels, known as 'Retina'.

The second part, color, is more a matter of taste and perception. Much like audio, there's always a place for debate on compressed vs. lossless and the number and range of color or audio frequency levels. So, let's review color-related terms one by one.

Color space

Color space is basically a color wheel from any color picker window in the software of one's choice. Just arranged in a more scientific, yet two-axis planar pattern. This half-parabolic rainbow-colored thing, called the 'cromaticity diagram', was introduced in 1931 and is still referenced today as a space for all the colors, not counting their luminocity.

Then there's 'color models', a system with a multi-letter name for representing a set of colors (color gamut). The ultimate goal for color models is to cover the color space, but in reality, all the RGBs and CMYKs try to cover the most of the color gamut (or technically possible with the technology they are made for).

RGB

One of the most known color models is RGB. I intentionally don't make any distinction between all the variations of RGB (sRGB, AdobeRGB, and xRGB), as this makes no difference for today's topic. RGB, as the name suggests, is copying the human eye's cone cells that make any color a mixture of red, green, and blue.

RGB is the go-to format for PCs, monitors, and most image processing activities. Specifically, sRGB is the standard for web images.

The channels, R, G, and B, are represented as three 8-bit numbers, 0 to 255. Some models can use more raw data to get more chrominance levels—16 bits per channel or 48 bits per pixel. So should we aim to use as many bits per pixel as we can? Well, no, it's not practical. A 24-bit Full HD RGB image takes 1920 × 1080 × 24 = 5.93 MiB of space (or 11.87 MiB for a 48-bit one). It seems manageable, but if we consider a minute of FullHD 30 fps 24-bit video, it will occupy 10.43 GiB of storage, which is obviously too much even for modern networks (that's 938 GiB for a 90-minute movie).

There are two ways to mitigate this: either to use compression algorithms like Youtube's H.264/H.265 or to use a better color space to use less data for better results. In terms of RGB, one could use a limited set known as R'G'B' with values of 16 to 235.

CMYK

Worth a quick mention. It has no use in HDMI video transmission as it is made for printed physical products.

YUV, YPbPr, and YCbCr

Those are used interchangeably, although they differ slightly. But all we care about here is the general idea. YUV works best for photo and video content. Human eyes perceive images as a combination of luminance and color. The luminance is the most important part; black-and-white images are enough for us to have a pretty good idea of what's shown in the image. In the dark, we see just black and white, as there's no light to reflect from colored objects, and it's fine.

So the idea is to devote one color channel to luminance alone and the other two to color: red (or red-difference) and blue (or blue-difference). YCbCr can be converted to and from RGB with a matrix. But the levels of the components Y, Cb, and Cr are gamma-corrected, meaning they are non-linear. When RGB has the same steps for color channels, luminance and, subsequently, red and blue-difference channels don't work like that. Levels of luminance are perceived in a more complex pattern; the white part of the spectrum should have fewer steps, and the dark part needs more.

This attention to luminance levels brings us to HDR.

HDR

HDR stands for high dynamic range, as opposed to SDR, which stands for standard dynamic range. Wider range means more bits for the channels (usually 10 or 12), higher maximum luminance levels, and more metadata for the display.

That means that the number of luminance steps (the same goes for both chromatic channels) increases; the maximum luminance level is an order of magnitude greater than in SDR. In addition to raw data, there's metadata for monitor settings. All the screens are built differently in terms of color calibration and maximum peak brightness. The source communicates luminosity and contrast values to the display for it to adjust. HDR was introduced in HDMI 2.0 as 'static HDR', meaning it sends one set of metadata for the whole video stream. HDMI 2.1 can use 'dynamic HDR', sending different metadata for every scene. HDR is a well-known, widely marketed feature that really elevates the user experience. Dynamic HDR is usually titled 'HDR10+' on the box (compared to static 'HDR10').

Here lies the answer to the question some people ask, "Is 10-bit color ('deep color') HDR?" No, it is not. 10-bit color in RGB means more steps for chromatic channels, not an extra-wide range of luminance. Should YCbCr HDR always take upwards of 10 bits per channel? Yes, 10-bit color could be HDR or SDR, but an HDR signal is 10-bit or more.

Chroma subsampling

The last part is subsampling. It is a third middle option between using compression or implementing a limited color model. Chroma subsampling is used for YCbCr to keep luminance data, as it is the most precious for us, and compress individually colored pixels into bigger blocks.

A standard sample is a two-row block of pixels, usually 4×2. Subsampling is represented as numbers J:a:r.

The first digit 'J' of 4:4:4 or 4:2:0 represents the number of pixels in a row. Basically, it's always 4.

'a' represents the number of different colors (chromatic samples) in the first row. Can be treated as a horizontal resolution; do we reduce the 4 pixel colors of the first row to '1', '2', or do we keep all '4'?

'r' is the number of color changes between the first and second rows. This is a vertical resolution and can be either '0' (no difference; the second row has the same chrominance as the first one) or 'a' (there is a difference in color between two rows).

So, 4:4:4 means no subsampling; 4:2:2 or 4:4:0 reduces 8 different colors to just 4; 4:1:1 or 4:2:0 reduces the number of colors to 2 (vertically or horizontally). Practically useful numbers are 4:4:4 and 4:2:0, as they are the most common.

In conclusion, different color spaces are made for different applications. A PC monitor with SDR works best with RGB for displaying pixel-perfect text on flat surfaces. YCbCr is meant for non-productivity applications like photos, movies, or videogames. YCbCr with no subsampling is great for everything; 4:2:0 is visually indistinguishable from perfect when watching movies and sports.

HDR should always be 10-bit; there's no 8-bit HDR and never was.

All this makes sense for units with HDMI version 2.0 and higher, and there's no point in talking about color spaces in previous versions.

Intermittent faults ('floating' defects) are damages that manifest themselves periodically and are caused by poor-quality core connections or reduced insulation resistance. Customer complaints about short-term connection losses are evidence of defects of this kind. Such defects may appear due to mechanical damage to the cable (for example, in the event of vibration from heavy vehicles, rotary equipment, etc., nearby).

Typically, when a technician encounters this type of damage, he has to wait patiently for it to manifest itself, hoping the effect will last long enough to determine its location. There is no guarantee that the damage will reveal itself while the technician is on duty. The use of reflectometers allows one to automate this process and maximize productivity.

Some reflectometers have a special function for detecting intermittent faults. The device connected to the line accumulates all reflectograms over a certain period and displays them superimposed on each other. Where the reflectogram differs, the intermittent fault is located.

Finding intermittent faults

For example, consider the following situation: a particular pair of cables works fine for the better part of the day, but there is a momentary failure out of the blue.

We get two reflectograms for the same pair (with different gain settings) when checked. In the first one, with a gain of 12 dB, a surge of positive polarity is observed on the reflectogram of a working pair at a distance of 6760 feet, corresponding to the end of the cable. In the second one, when the gain increases by 14 dB, an additional spike appears on the reflectogram, the nature of which indicates the presence of a coupling in the cable at a distance of 3280 feet. By further increasing the vertical gain level, the reflectogram will not reveal the slightest sign of damage along the entire length of the cable being tested.

We will need the 'intermittent fault detection' function mentioned above. By continuously monitoring the pair's condition, the OTDR shows any deviations from the cable's rated impedance, allowing the location of intermittent faults to be pinpointed.

The reflectometer display will show the current reflectograms obtained during testing. Periodic inspections allow one to determine whether signs of malfunction have appeared. Once the non-persistent damage has been captured, the result should look approximately as shown in the figure.

The differences will be evident if one compares it with the previous one. A noticeable drop appears where there was nothing before. The location of the fault can be determined by simply moving the cursor to the front of the pulse reflected from the break and reading the distance from the display.

Random vibrations or other irregular events cause the connections to loosen and electrical contact to be temporarily lost, resulting in a fault similar to a partial break. Note that at the moment this fault occurs, the pulse reflected from the far open end of the line decreases because, due to a poor connection in the cable coupling, the magnitude of the electrical signal reaching the end of the cable is reduced.

What conclusions can be drawn? Almost every type of cable system is susceptible to intermittent faults. Such damage creates severe problems for users and technicians. The intermittent fault detection mode of reflectometers allows one to continuously monitor the cable over a long period, so the technician does not have to waste working hours waiting for the damage to manifest itself.

Pupin coils

Pupin coils can still be found on an analog telephone line. Pupin coils disrupt the homogeneity of the copper pair, turning it into an ideal low-pass filter with more substantial high-frequency attenuation.

Therefore, a prerequisite for using any xDSL technologies on existing phone lines is the removal of Pupin coils, which have been found to have extensive applications in US telephone networks. Servicing xDSL systems can always result in such a problem. In this case, one will need a reflectometer with a function for searching and counting Pupin coils.

Searching for installation locations of Pupin coils

A reflectometer is the only device that allows one to simply and accurately determine the location of Pupin coils. Since the pulses sent by the reflectometer are high-frequency, they are reflected from the Pupin coil, a low-pass filter. The coil on the reflectogram looks like a significant increase in the cable impedance, i.e., similar to the reflectogram of a line break.

As you can see, the outline of the pulse reflected from the Pupin coil is more rounded than the pulse reflected from the cable brake, and the coil itself is located at a distance of about 5600 feet. In Eastern Europe, there are several Pupinization systems: medium, light, extra light, and broadcasting light. All systems have the same pitch of 1.7 km and differ in the inductance of the coils, the bandwidth of the transmitted frequencies, and the distance between the amplifiers. Unfortunately, the reflectogram shows only the first coil.

Counting the number of Pupin coils

The reflectometer can also be used as an auxiliary device when taking resistive bridge measurements. The presence of Pupin coils introduces inaccuracy into the readings since each of them adds about 4 ohms to the resistance value obtained during the measurement. Using a Pupin coil counter allows one to approximately estimate the number of coils installed on the line and determine the error of measurements made using a resistive bridge.

As you know, a data cabling system consists of different segments. To connect them all and bring the data connection to the end user, it is necessary to make a certain number of crossings. Often, staff forgets to disconnect "old" lines. As a result, over time, parallel branches appear, and their presence can have a detrimental effect on the quality of services.

BRANCHES AS A SOURCE OF PROBLEMS

Parallel branches can make it difficult to serve clients and ensure system functionality. With the introduction of digital systems, the search for parallel branches becomes an increasingly important task since they negatively affect the operation of digital transmission systems and, even if in most cases they are relatively short in length, nevertheless lead to significant problems. The bramch creates a second path for digital signals transmitted on the main line, which travel along the branch and are reflected from its open end. Reflected signals (echoes) enter the main line, where they are mixed with "good" digital signals and negatively affect the quality of the transmitted data. Therefore, to ensure correct operation of the digital line, the branches must be disconnected completely.

When connecting to analog lines, branching also creates problems. For example, if there is a fault on such a branch, it may show itself in the form of a decrease in the quality of the transmitted signal.

Finally, unknown branches can affect the accuracy of diagnostic equipment, for example, when measuring cable capacitance and estimating the distance to a break using a capacitive bridge. An unknown branch increases the combined capacitance of the cable pair and causes a measurement error: for the tested pair, the calculated length will be greater than the actual length.

It is very important to have full information about all the parallel branches available on the line in order, if necessary, to select the correct algorithm for troubleshooting and eliminating the problem.

SEARCHING FOR THE LOCATION OF THE BRANCH CONNECTION

The capacitive bridge is the device most often used to measure the length of a cable that is open at the far end. Unfortunately, it only allows one to estimate the total length of a cable pair, including all parallel branches.

Using multi-function devices (combining a capacitive and resistive bridge), it is possible to calculate the length of the branch cable due to the ability to compare the length values obtained from measuring the cable capacitance and the resistance of the loop.

Pic main_img_p7621_thumb.png

In this case, an OTDR is the most optimal and, moreover, the only device that allows one to find the locations of branching, measure the lengths of the branches, and determine the distance to them.

However, in practice, cable analyzers that combine the functions of a reflectometer and a multi-function instrument are more convenient. The implementation of two measurement methods (reflectometric and bridge) in one device allows for comparison of the results obtained for more accurate fault localization.

The classic branch reflectogram is similar to the one for testing a damaged cable, the only difference being that the reflection of the signal from the branch is a straight line rather than a curve.

As an explanation, let's look closely at the reflectograms for an open-ended cable section without a branch and a cable section with one (it is located at a distance of 3385 ft). The corresponding measurement results using a capacitive bridge were transferred to the reflectometer for direct comparison and accounted for. Note how the presence of a branch affects the measurement results of a capacitive bridge—in particular, how the cable section with a branch distorts the pulse reflected from the open end of the cable at a distance of 6500 feet. This occurs because part of the energy of the reflectometer signal was lost passing through the branch. The ideal way to view these graphs simultaneously is to use a dual-channel OTDR to connect and compare the "good" and "bad" cable pairs back-to-back.

Just as echoes affect digital signal transmission, parallel branches affect cable reflectograms. Interpretation of the reflectogram becomes significantly more difficult if two or more branches are connected to the pair under test.

The following graph is similar to the one shown in the previous figure, but in this case there is an additional branch at a distance of approximately 5400 feet.

The open end of the cable at a distance of 6500 feet is almost invisible on this reflectogram since the energy of the reflectometer test pulse is spent on passing two branches. If there are multiple branches connected to the cable under test, the best strategy is to locate the first one, access its location, and only then locate the next branch. This procedure should be repeated until the locations of all branches have been found.

Let's get to conclusions. Branches obstruct the operation of digital systems. The search for branches and their subsequent removal is extremely important to ensure the high quality of the digital services provided.

In order to make sure that the distance to the nearest branch corresponds to the standard for digital lines (maximum and total length of branches), one can use the following algorithm:

Check the distance to the nearest branch.

Check the length of the cable branch.

Let's see how the cables measure influence the range of the reflectometer. How do the noise filtering mode and methods for determining the unknown propagation coefficient affect the measurement accuracy?

KEY FACTORS FOR CABLES

The reflectometer's detection range depends on the cross-section of the cable cores, the overall quality of the cable, as well as on the way the reflectometer is connected to the cable being tested. The larger the cross-section of the cable cores, the less attenuation the electrical pulse supplied by the reflectometer to this cable undergoes and the longer the distance it covers. Old or defective cables may have reduced insulation resistance or increased attenuation. This significantly reduces the ability of the cable cores to conduct electrical signals and, as a result, reduces the maximum distance. The connection of the reflectometer to the cable must be done so that a pulse with the maximum possible amount of energy is emitted from the reflectometer to the cable.

NOISE FILTERING

I want to eliminate the noise in more detail, as it is on any cable. Many reflectometers have a digital noise filtering mode that obliterates the noise from 50 Hz to 1 GHz. This mode is designed primarily for lineworkers dealing with cables near sources of strong electromagnetic interference (for example, railway contact networks, power lines, or antenna poles). The operator can select a filter type with the required characteristics for each test to ensure the acceptable quality of the resulting reflectogram. Suppose an unexpected random voltage value appears in the line during the measurement. In that case, the noise-filtering mode gets turned on automatically. A multi-level and multifunctional filtering system allows one to check antennas and cellular nodes with some received signal interference.

In some cases, the noise-filtering subsystem may slow down the OTDR operation to such an extent that the display becomes unusable. A good example is filtering the noise the power supply induces (60 Hz). One period of alternating current with a frequency of 60 Hz is 16.7 ms. Therefore, it also takes 16.7 ms to generate one point on the reflectometer display. It will take no less than 5.12 seconds to refresh all the 256 image pixels on the display. One way to compensate for this delay is to store the denoised reflectogram. Writing data to the device memory will take some time, but subsequent operations will be as fast as if the filter were turned off. The "averaging" mode, often provided by the manufacturer to eliminate interference at maximum gain, is no exception. This mode also decreases the refresh rate of the display. Four times less noise means the screen is refreshed sixteen times slower. When the image refresh speed is reduced significantly, it becomes difficult to work with the display, so this mode should be used only when necessary. One more thing to mention: cables for digital data transmission should be tested using short pulses with a duration of 2, 10, or, in extreme cases, 100 ns. They do not affect nearby pairs under load, so the data transmission devices' error detection system will not flag them as such.

PROPAGATION COEFFICIENT AND METHODS FOR ITS DETERMINATION

As already mentioned, the reflectometer determines the distance to the abnormal spot based on the signal propagation speed in the cable and the time it takes to reach the point in question and return. In most cases, speed is expressed as a unitless coefficient, the ratio of the signal propagation speed in a given cable to the speed of light. It is an empirically determined value. Reflectometers from different manufacturers require one to set the wave propagation speed, called the Velocity of Propagation (VOP) or Velocity Factor (VF). Typically, this parameter is expressed as a fraction of the speed of light and can have a value from 0.3 to 1. A cable with a VOP value of 0.66 allows an electrical signal to be transmitted at 66% of the speed of light. Some manufacturers express this parameter in terms of actual speed, and then it can range from 45 to 150 m/ms. The choice of the VOP (VF) parameter significantly impacts the accuracy of any measurement made. Therefore, to obtain the most correct results possible, it is necessary to learn the methods for determining the propagation factor for each specific cable.

VOP DETERMINATION METHODS UNDER DIFFERENT CONDITIONS

Suppose the pulse propagation coefficient for the cable is unknown. In that case, an experiment can either calculate or determine it.

‍1. The insulating material's dielectric constant (ε) is a known value.

VOP=1/√(ε),

where ε is the relative dielectric constant for a given cable. For example, for polyethylene, ε = 2.25. Therefore, VOP = 1/√2.25 = 0.667.‍

2. For the first experiment, one needs a short cable of the same type as the tested cable. Connect the reflectometer to the cable section and adjust it so that the pulse reflected from the end of the section is clearly visible on the display. Move the cursor to the beginning of the pulse and start changing the VOP coefficient until the measured distance to the end of the cable is equal to its actual length (mind the length of the leads). The longer the cable, the more accurate the obtained VOP value will be.‍

3. A similar experiment occurs when the cable length is known, and it has a non-defective pair. Connect the reflectometer to the pair and adjust VOP until you get the cable length you already know on the screen. Again, account for the probing leads. Suppose this technique is applied to thick multi-pair cables. In that case, ensuring that the reference and tested pairs belong to the same cable layer is necessary. The length of a pair of outer layers significantly exceeds the length of a pair of inner ones.‍

4. If the cable length is known, but there is no suitable pair for comparison, then it is necessary to have access to the cable from both sides.

Roughly pick a VOP value and determine the approximate distance (L1) to the fault on side A. Using the same VOP value, determine the approximate distance (L2) to the fault on side B. The exact distance to the damage can be calculated with L1 and L2.

The reflectometer's capabilities in terms of maximum distance and damage detection accuracy are determined by the sensitivity of the amplifier and some other important parameters.

Amplifier sensitivity

The sensitivity of the reflectometer, along with the pulse amplitude, is one of its most essential characteristics. It determines the maximum operating range of the device. It is crucial if you're going to check cables with high signal attenuation. Technical documentation often skips this parameter or describes it very vaguely. In an ideal scenario, sensitivity should be characterized as the input voltage when the waveform on the instrument display is contained between the top and bottom edges of the screen (i.e., “x mV for full-screen deviation”). Vertical sensitivity is sometimes measured in decibels. The decibel value is relative and has no meaning unless given a reference level of 0 dB. With this data, the amplifier's sensitivity can be calculated, as each 6 dB step doubles the gain. Based on the parameters mentioned above—pulse amplitude and amplifier sensitivity—it is possible to calculate the maximum overlapped line attenuation, which also serves as a criterion for assessing the quality of TDR. The maximum overlapped attenuation (amax) is defined as the line attenuation when the deviation of the vertical beam is at least one-eighth of the full screen. In this case, amax is calculated using the formula:

‍ Amax = 20 lg8 + 20 lg(Upuls/ampl), - Amax is the maximum attenuation, - Upuls — pulse amplitude under load Zo, - Vampl — amplifier sensitivity for full-screen deviation.

‍ It should be mentioned that this method can only be used to compare reflectometers from different manufacturers. It will be impossible to accurately calculate the maximum attenuation overlapped by the reflectometer using this method since the attenuation in metal cables is frequency-dependent. Therefore, different duration pulses will correspond to different Amax values. The covered method for estimating overlapped attenuation better suits optical TDRs. Optical fibers have frequency-independent attenuation, and therefore, in optical time domain reflectometers (OTDR), the pulse amplitude and sensitivity of the photodetector are usually not taken into account; they are "integral" parameters of the device. A parameter called "dynamic range" is introduced instead, i.e., the insertion loss of the line, at which the signal-to-noise ratio SNR = 1 for a certain duration of the probing pulse. To widen the dynamic range of OTDR, one needs an increase in probing pulse power and the receiver's sensitivity, as well as a very specific set of digital processing algorithms developed by the manufacturer.

Resolution and accuracy

[illustration from viavisolutions.com]

Before discussing the issues related to resolution and accuracy, some basic concepts should be defined. Display resolution is the interval between two consecutive dots on the screen. Fault detection resolution is the minimum distance between two consecutive faults to be visible as separate faults on the reflectometer display. Sampling precision is just the precision with which samples are carried out. The term "fault localization precision" also speaks for itself.

Fault localization precision 

The accuracy of finding the fault in the vast majority of real-world cases is limited by the availability of reliable information for the cable being tested and not by the sampling precision or display resolution of the reflectometer. Firstly, the pulse propagation coefficient can only be known with an accuracy of a few percent, and on top of that, it is affected by temperature changes. A 1% error in setting the pulse propagation coefficient leads to a 1% error in determining the distance. Secondly, the limited information on the actual cable route, in turn, limits the user’s ability to find a point along the cable trace marked by the reflectometer as the location of the fault. After all, when tracing a cable to localize the damaged spot, it is nearly impossible to account for all the peculiarities like safety loops, different laying levels, specific terrain, etc. The resolution of fault detection is also influenced by the pulse duration.

Sampling precision and display resolution

Sampling precision and display resolution are not the same thing. The device may have a high level of sampling accuracy, but its display may have poor resolution, and vice versa. For example, consider a device with a display resolution of 100 ns. It can detect damage with an accuracy of +100 ns, regardless of the precision of the sampling generation every 100 ns. Similarly, the device may have a display with a very high resolution (say 0.1 ns). Still, if the sampling precision is insufficient, then in this case, it won't be possible to accurately determine the location of the fault. One should remember that when searching for a fault location with a reflectometer, high display resolution, and sampling precision are not always necessary. Even if the reflectometer allows one to find the damage locations within 1 cm, due to the cable routing environment, this level of accuracy is not enough for fault localization; you simply won't be able to find them on a real cable. Thanks to modern clock generation techniques, many OTDRs have sufficient accuracy and resolution of sampling that they can be used in most real-world applications. A few other important characteristics are tied to the parameters we've covered. We are talking about measurement limits (maximum and minimum distance) and the possibility of scale adjustment.

Maximum distance

Modern technologies make it easy for manufacturers to achieve an almost unlimited range for their devices. However, the sensitivity of the device does not guarantee actual detection of damage at the specified maximum distance. The best way to determine the range of a device is to test it with a cable. The OTDR's distance measurement range and cable length should be gradually increased until a critical point is reached, i.e., a state when the OTDR cannot detect the open end of the cable. When comparing different reflectometers in such a way, it is necessary to use pulses of the same duration.

Minimum distance

Suppose a minimum operating distance of 10 m is specified for one device and 20 m for another. In that case, one shouldn't immediately jump to conclusions. Both devices likely have the same display resolution. Still, the second one has a physically larger display or is less capable of adjusting to the same resolution. Therefore, the key characteristic is the minimum display resolution, not the minimum working distance.

Scaling