Coaxial Cable and optical fiber cable Detection

I. coaxial cable Detection
1. After the cable enters the water, we have tested the change of Characteristic Impedance. When the cable enters the water due to moisture and the TV signal passes through the cable, the measured level is lower than the specified loss value of the cable. According to the length of the cable inlet and damp time, the loss attenuation value varies depending on the degree of corrosion and oxidation Rust of the internal and external cables.
Under the face of moisture, water in different degrees of SYDY-75-9.5 bamboo coaxial cable made a variety of test records.
1) instrumentation: MT500 multimeter, MC7 spectrometer, and DL6243 capacitive inductor tester.
2) test project: 100 m cable)
· Cable DC Resistance;
· Cable capacitance and Inductance;
· Level.
3) test results:
The changes in the resistance, capacitance, inductance, and level of cables with different degrees of moisture and inlet are shown in the table.
From the above test data, it is shown that the distribution capacitance and inductance will also change, and the characteristic impedance will also change, because there is an electric field between the two conductors inside and outside the coaxial cable, there is a certain amount of electric capacity. When the conductor passes through the alternating TV signal, a certain amount of electric sensing will be generated. These inductance and capacitance are distributed in the cable, the capacitance combination of the series Inductor and the parallel capacitor is measured by the electric sensing Volume L and the electrical capacity C of the coaxial cable per meter, forming the characteristic impedance Z of the coaxial cable.
According to the characteristic impedance formula, changes in inductance L and capacitance C will lead to changes in the characteristic impedance. Analyze the measurement data of the capacitance and inductance of the cable after water accumulation. The more incoming water, the larger the changes in capacitance and inductance, the characteristic impedance must be changing, resulting in the signal source input impedance and the characteristic impedance of the cable, said mismatch. At last, the power loss increases, and some valid signal power losses are eliminated.
Theoretically, the attenuation of the coaxial cable is mainly caused by the loss of the "Skin Effect" of the inner conductor. The higher the frequency, the stronger the skin effect, the higher the sensitivity of series inductor wL), and the lower the capacitance and wc). The higher the frequency, the higher the signal leakage on the internal conductor, the longer the cable transmission distance, the larger the attenuation.
After the cable suffers moisture and water accumulation, the measured data shows that the inductance and electrical capacity are increasing by a greater percentage than the electric capacity), and the inductance and electrical capacity of the series inductor are also increasing, the parallel capacitor's capacity to resist wc) is decreasing, and the signal on the inner conductor causes the leakage of the bypass on the outer conductor, which increases the signal level attenuation.
When the cable is wet, the levels of each channel are greatly decreased after water accumulation, but the attenuation varies with different frequencies. The levels of the cable after water accumulation change as high and low as the waves, the change amount depends on the degree of wet water of the cable.
2. Coaxial Cable Fault Detection Method
Use the QXZ04 tester to find the fault point. The QXZ04 cable fault tester is a digital instrument used to measure the cable fault. It uses the reflection principle of the transmission line to determine the fault point of the cable in the time domain. Easy to use and quick measurement. Intuitive, accurate, and easy to operate. Measure the fault location and Properties of open circuit, short circuit, and impedance mismatch of cables, and the length of cables and the speed at which signals are transmitted, the failure of open, short and indirect short circuits in the coaxial cable of the transmission TV signal can be quickly determined.
To accurately determine the faulty cable location, the transmission speed of the cable should be measured in advance. The method is to first operate a 50-M cable of the same known length according to the instructions of the instrument, to measure the transmission speed of the cable, according to the tested transmission speed, perform the following inspection on the faulty cable.
1) connect the damaged coaxial cable, connect the power supply of the instrument, and twist the transmission speed to the position where the transmission speed of the cable is known in advance.
2) the whole process, delay switch K4-2 twist on the whole process of the location, according to the length of the tested cable, the test range selection switch K5 in the appropriate position, adjust focus, horizontal displacement, vertical displacement potentiometer W1, W2, W3, W4 to make the image clear, adjust the balance regulation potentiometer W5 and W6 so that the balance circuit impedance in the machine matches the impedance of the tested Cable End, so that the reflection amplitude increases, and then select the time-scale polarity based on the fault nature. After all the above adjustments, adjust the reflection time to the switch K6, K7, K8, so that the moving electronic time scales close to the reflected wave, and fine-tune the reflection time potentiometer W8, so that the time front of the time point is aligned with the front of the reflected wave, the number displayed on the digital monitor is the fault point distance.
3. How to determine the nature of fault points
1) when the line is normal, there is no reflection.
2) when the line is disconnected or the contact is poor, the reflection pulse is the same as the emission pulse.
3) the insulation is reduced when the line is short-circuited or the cable is damp or the inlet, and the reflection pulse is reversed with the emission pulse wave.
The Instrument Review the length of the new cable, the distance between the erected or buried cables. The determination of the fault location is very convenient, accurate and reliable, and the measurement distance is up to 20 kilometers, it is also suitable for public calls, symmetrical, coaxial, power cables, compound cables, copper wires, and iron wires. due to various reasons, there may be a 5-10-meter error.
4. Use a cable detector to find the fault point
The 6405B cable detector produced by the Ministry of Posts and electronics can quickly and accurately locate the short-circuit, open-circuit and indirect short-circuit fault points.
The instrument consists of a host and an auxiliary machine. The host is actually a signal generator with a mA millennial table on the panel, and the impedance is twisted to a power supply with 8 W, 16 W, and 50 W, 150 W, 160 W, five stalls) one electric meter sensitivity control twist, one power test key, one output control and one continuous and intermittent switch. After the host is turned on, consecutive or intermittent beeps are generated. When there is an open or short circuit fault in the cable, connect the core cable and the outer conductor respectively to the terminal of the output terminal of the host, and remove the other end of the cable separately. If the multimeter pointer swing, it means that the cable is open and does not swing, prove short circuit of the cable.
The host has two kinds of probes: Ground probe and underground probe. You can choose the probe based on the difference between air cables and underground cables. The probe is actually a signal receiver that receives intermittent or continuous signals from the host. troubleshooting steps:
1) connect one faulty cable to the output end of the host, and the other end to remove the internal and external conductors from the device.
2) connect to the power supply of the host with a 13.5 V 1 battery) mA table pointer start to swing; if no swing proves that the cable is short circuit fault, the electric meter sensitivity potentiometer is controlled in the appropriate position, the impedance of the W coaxial cable can be set to a 50-W position. You can choose the bee sound output. At this time, you can wear the headset on your head. When the probe is close to the output end of the host, you can hear the continuous or intermittent bee sound, verify that the instrument works properly. If it is an empty wire, you can bundle the probe on a wooden pole About seven meters, lean the probe on the cable, find it along the faulty line, if the cable is open, when the probe exceeds the disconnected cable position, the beep sound received in the headset will be interrupted or the sound will obviously become smaller, proving that the fault point is nearby, and the probe will be searched several times back and forth in the faulty cable location, the fault point will be found very accurately.
3) if the conductor inside and outside the cable is short-circuited, there is almost no sound after the probe exceeds the short-circuited position. The probe will be searched several times back and forth, and the fault point will be found accurately.
4) if the cable is indirectly short-circuited or insulated to reduce water or moisture), the sound will gradually decrease when the probe goes through the cable fault point. When the sound becomes too small, the probe will look back, when the sound of the beeps received by the probe suddenly increases, it is proved that the fault point is here. After checking, you can find out the fault point.
5) use the Qxz04 cable fault tester and cable detector to locate faults at the same time, which is more efficient.
When an open-circuit, short-circuit, and insulation fault of the coaxial cable is found, use the Qxz04 cable fault tester to determine the approximate fault length of the cable, then, the continuous or intermittent beep signal of the 6405B cable detector is sent to the faulty cable, and then the probe is used to find the fault point in the Qxz04 measurement length position. It is also a flexible and clever way to use two types of instruments to check the fault location of the cable, and to quickly and accurately locate the fault point of the cable.
6. Use A multimeter to detect cable faults
1) when the maintenance line is used, if the input level of the amplifier is lower than the level loss value of the specified cable length, use a multimeter to block the test of the cable resistance R × 1 or R × 100 Euro ), the watch needle is slowly rising like a capacitor charging. This phenomenon indicates that the cable is seriously affected by moisture or there is a large amount of water in the cable. Of course, the cable moisture, the amount of water accumulation, the resistance value measured by the multimeter and the swing amplitude of the table needle like charging is different. The R of a normally intact cable should be infinite, the cable is severely affected by moisture, the accumulated water volume is large, and the cable resistance is usually several hundred euros. If the cable is filled with water, its resistance value is basically equal to zero, which is equivalent to a short circuit, and the table needle is slowly swinging up like a capacitor charging. In this way, it can be determined that the fault is caused by the wet water of the cable; at this time, the TV signal received by the receiver is very poor, or even unable to watch.
2) Remove two faulty cables with open or short circuits, and separate both the internal and external conductors of the cables. If there is a short circuit fault in the cables, the table needle will always point to zero position. When the cables are open, the other end of the cable should be shorted and checked with A multimeter. If the table needle is still not swinging, it indicates that the cable is broken.
3) the plastic protection layer of the outer conductor of the coaxial cable is damaged due to construction or the poor production quality. The thickness of the outer protection layer of the cable plastic is different. After the sun and rain, the water is split from the thin area of the outer skin, make the external conductor metal layer or metal mesh) corrosion. When the fault is checked with A multimeter, If the measured loop resistance is much higher than the original cable loop value, it proves that the outer conductor is seriously corroded. If the measured loop value is infinite, it indicates that the outer conductor is broken after corrosion. In short, there are many methods for detecting coaxial cables. As long as we carefully analyze and judge and master accurate detection methods, we can detect any faults.
Ii. Optical Fiber Cable Detection Method
1. Routine maintenance and testing of Optical Fibers
1) routine maintenance of optical fiber is very important. It is the fundamental guarantee for ensuring the safe, stable and reliable operation of optical fiber;
2) The technical data of each optical fiber should be tested and compared with the raw data each year or six months. Analyze and discuss problems as soon as possible, and eliminate problems and faults as soon as possible to avoid unexpected accidents;
3) conduct regular inspections on optical cable lines, and record problems such as cables, sheaths, cable connectors, and line sag during inspections to facilitate early detection and handling of problems, this is an important part of maintenance;
4) regularly test the optical power at the optical receiver inlet and the RF level at the exit. If the difference between the RF level and the original record is large, analyze whether the fault is from the optical cable or the optical receiver, it is caused by the active plug-in part or the optical transmitter itself.
2. Working Principle of Optical Time Domain Reflector
OTDR3000 (Optical Time Domain Reflector) works by the back-to-secondary scattering signal generated in the tested optical fiber. The test items are the length of the optical fiber, attenuation of the optical fiber, failure of the optical fiber, and connection loss of the optical fiber, it is a necessary instrument for detecting the performance and failure of optical fiber. Due to the defects of the optical fiber and the uniformity of the doped components, it will produce scattering under the action of photon, if the optical fiber (or joint) if there is a geometric defect or fracture surface, it will produce a freell reflection. The reflection strength is proportional to the optical power passed through the point. It also reflects the attenuation of each point in the optical fiber, because scattering is emitted in all directions, the reflected light will also form a relatively large angle of reflection. Scattering and reflected light are very few parts. It can also enter the aperture angle of the optical fiber and transmit it back to the input end. If the optical fiber is interrupted, that is, the back-to-scattered light power from this point will be reduced to zero. Determine the breakpoint position and length of the optical fiber based on the scattered light returned by Reverse transmission. This is the basic working principle of the Time Domain Reflector.
3. Measurement of fiber attenuation
The vertical axis is the signal strength, the horizontal axis is the time, and the light is transmitted along the optical fiber axial direction with A certain intensity. Therefore, the incoming point A end face has A freell reflected light first received, and the signal is the strongest, followed by B .C. the transfer distance of the second segment D is different, and the time for returning to the entry point A is also different. Since the attenuation is different, the t amplitude is reflected on the vertical axis, in the unit of optical power. Because each point of the optical fiber along the axial direction has scattered light or reflected light. Therefore, the curve is continuous.
There is a sudden drop in Curve B. It indicates that the optical fiber has a contact or defect at this point, which causes great attenuation of the optical signal. The attenuation between point B and point C is also uniform, and the decrease slows down. It is proved that the attenuation coefficient of the optical fiber is smaller than that of the optical fiber at the front end, point C has a sudden rise of the pulse, prove that there is a broken surface is not completely interrupted) or defects caused by the freell reflection, the C-D segment is not a straight line, indicating that the axial structure of the section of the optical fiber is not uniform. That is to say, the structural parameters related to the Ar scattering coefficient, such as the core diameter and numerical aperture. Refractive Index Distribution Along the axial direction is not uniform.
The sudden disappearance of the D-point signal indicates a disconnection or end point of the optical fiber. If the D-point cross section is flat, the reflection coefficient R is equal to 0. In this case, there will be a reflection pulse. If the D point is a comminuted irregular section, the reflection coefficient R is very small or even zero, and its reflection signal is very weak and there is no obvious reflection pulse.
We can measure the attenuation between any two points along the optical fiber and the whole process using the back-to-back scattering instrument. We can also see whether the fiber structure is even and it is very convenient to use.
4. Optical Cable Testing
Optical cable connectors and test instruments are dedicated. They are completely different from common cable connector tools and test instruments. Optical fiber splitters and Optical Time-Domain splitters for measuring distance and loss are expensive. However, optical fiber splitters with good quality and low loss are very easy, and the detection distance error is small, which is accurate and fast. There is also a kind of handheld optical power, photoelectric flat tester is very lightweight and convenient. In addition, there are several specialized instruments. The following describes how to test the optical fiber cable with the Optical Time Domain Reflector.
During optical fiber construction, the length and transmission loss of the optical fiber are the main measurement indicators. The optical Time-Domain Reflector is convenient to measure the above indicators, and the measurement data is accurate. The TFS3031 micro Optical Time-Domain Reflector is a solid structure, an easy-to-use micro-Optical Time-Domain Reflector OTDR) is ideal for field construction and provides precise measurement of single-mode or multi-mode optical fiber systems.
For the position of each connection in the cable, the reflection loss can be quickly and clearly displayed on a large screen of 7 inch. Tekranger is the only micro-Optical Time Domain Reflector. If you press a single key, it will tell you the connectivity between 5 meters and 100 kilometers. The Optical Time Domain Reflector Mini-OTDR) automatically adjusts capture parameters to provide the best possible resolution while maintaining the dynamic range required for accurate measurements. Different pulse widths are used for capturing, which produces extremely precise waveforms. It is very easy to read the curve on the display screen and display an event table, indicating all the conditions of the connection.
1) Test the optical fiber length
The instrument tests, judges, and points the line barrier. Before determining the obstacle, the instrument cursor should be located at the starting point of the rising edge of the freell reflection peak at the end of the line curve split. The accuracy of the test depends on the selected fiber core refractive index n value and the selected pulse width. Due to the formula D = ct/2n in which C is vacuum, C = 3x10 m/s, t is the time for OTDR to receive the optical pulse from a emitting pulse to the end of the line after the freell reflection.) the more accurate the n value, the more authentic the result, therefore, the n value must be set at the production plant during the test. For example, after the construction of the four connectors of the whole line is completed during the construction of the guanziping-majiawan optical cable, when the time-domain reflector is used to check the technical indicators of each optical fiber, it is found that the distance of a fiber core is halved, which proves that the fiber is interrupted. After checking the original data, there is an optical fiber interruption in the optical fiber connector of the second connector 4. 2KM. After the position is determined accurately, open the connector box. It is found that during construction, the optical fiber is in the connector box, the bending radius of the junction is small, and the force is large. Therefore, the connection is closed. After the connection is re-connected, the indicators are normal.
2) optical fiber Line Loss Test
After the optical cable construction is complete, if the loss of a certain Optical Fiber Joint measured by the Optical Time Domain reflex meter is extremely high, you must open the receiving box and re-connect after the distance is determined, this is generally a problem left over during construction.
An error occurs in the optical cable in operation. For example, the attenuation curves of several optical fiber cables are measured in steps. After the distance is determined, the fault point is found based on the raw data. The result is that the optical fiber is not completely broken when the fire gun is shooting and injuring the optical fiber.
3) Test the optical fiber connection loss.
One of the methods to test the joint loss is to use the FSM-30s fusion splices to connect the two optical fiber together, the connector is completed, the display immediately shows the Attenuation Loss of the just joint value, the operator can determine whether the header is qualified based on the displayed data. If the loss is too high, disconnect the re-integration.
The second method is to use the Optical Time Domain to test the joint loss. Generally, the five-point average method is used to set the cursor on the optical fiber contact, the two points on the left of the cursor are placed at the smooth curve of the cable near the test end, so that the straight line and curve of the two points overlap as much as possible, the two points on the right of the cursor are placed in the smooth curve position of the next optical fiber, and the line formed by the two points should overlap with the curve as much as possible. In this way, the optical fiber connection loss is indicated by the "Step" formed by a straight line on both sides of the cursor.
In order to accurately determine the fault point, maintenance and detection technicians should be familiar with the inherent error of the OTDR instrument, master the random variation of the refractive index of the instrument and the deviation of the approximate value of the light speed, and pay attention to the error of improper operation of the instrument. When using the OTDR cable, you must adjust the range according to the actual situation, select the appropriate pulse width pw), set the refractive index n value of the fiber core, in the two wavelengths (1310nm, 1550nm) select the appropriate wavelength value based on the optical wavelength used for future transmission of the line. Only after the preceding parameters are set can the optical characteristics of the line be tested.
The above three errors will affect the accuracy of measurement line faults. The error of the instrument itself is reflected in the distance resolution, which is determined by the sampling frequency and sampling pulse width. The smaller the sampling pulse width, the smaller the error, and the larger the error. The randomness of refractive index and the operation method of maintenance personnel are the main reasons that directly affect the distance error. Different types of optical fiber have different refractive indexes, therefore, we should first understand the refractive index of the optical fiber to minimize the test error when measuring the optical fiber.
4) optical power measurement
After the optical cable construction is completed and put into use, it is necessary to measure the light transmission power, light receiving power, and line loss, and adjust the optimal input power to the design, commonly used optical fiber multimeter has domestic portable PMS-1A type optical power meter, it is an intelligent optical power meter with microcomputer control, dedicated to the Construction of optical fiber cable and other high power measurement field, this meter can measure 40 dBm -- + 20dbm optical signals. This type of optical power meter is placed inside the body with exquisite probes, which are well protected and easy to operate. In addition, AQ2150 imported probes can be used to directly measure the optical power and photovoltaic power to be known.