A. Cables and connectors characteristic impedance mismatch
With integrated wiring on-site construction experience of the engineers is clearly a phenomenon, when link NEXT to appear problem, we can through to a line or replacement on both ends of the connection hardware to solve; For a professional team of construction project, once the return loss RL test does not pass, unless the instrument HDTDR (time-domain impedance analysis) found after tracking position at both ends of the link, we can remove this part of the cable after reinstalling connection hardware to rule out, otherwise, except for rewiring we can only accept the results of the test to fail.
Why is it so hard to RL fault is almost through the easy way out? Because of RL is due to the bad causes of horizontal wire itself or product quality problems in the construction of cable caused by injury. Once the cable wiring is complete, the state that is of the whole cable has finalized the design, when we found the problem during the test, there is almost no way by a new lineup or simple replacement module.
Return loss RL is cable link due to impedance mismatch of reflection, RL principle as shown in the figure below. This mismatch is mainly from two aspects: 1, the characteristic impedance of the cable and connector does not match; 2, wire itself characteristic impedance is not uniform. Now we from the two to analyze the influence of RL several reasons and solutions. RL principle is as follows:
Cables and connectors characteristic impedance mismatch
Integrated wiring system characteristic impedance for 100 + 15 Ω, reference to industry standards, the impedance of the cable fall within 85 ~ 115 Ω are qualified products. Connection hardware is used with 100 Ω standard impedance load measured by the size of the return loss value to reflect the status of impedance matching, return loss if selected values in a deviation on the connection of hardware and the characteristic impedance in the lower deviation such as close to 85 of Ω cable link, the link can produce very big reflection due to impedance mismatch, if under the condition of engineering construction and not very attention, may produce RL overall link failure during testing results.
Avoid this impedance mismatch problem in the industry the most basic method is:
1, on the whole level of wiring (including both ends jumper wire) choose the same brand product, different products to ensure that the maximum of characteristic impedance are basically identical. This method is the simplest and most insurance.
2, choose all kinds of product characteristic impedance are close to the median 100 Ω, so also can get satisfactory quality links. This method requires more time and energy, and production is not very easy to control.
2. Cable itself characteristic impedance is not uniform
The causes of uneven wire itself characteristic impedance is very much, embodiment is caused by the inhomogeneity of cable structure. Next we analyze several easy to cause the reason of not uniform of cable structure.
1, the cable manufacturing process
Let's look at before analysis characteristic impedance calculation formula:
Through the analysis of the calculation formula of the above, we may safely draw the factors that affect the characteristic impedance include: center distance, the conductor diameter and the dielectric constant of the insulating material. To ensure that the trace length in each unit is consistent with the characteristic impedance, we can control from the following aspects:
1) improve the concentricity of insulated conductor
Only to ensure that each core has high concentricity of the situation, to ensure that each pair of lines throughout the length of the ground on two conductor center distance "a", general concentricity control at more than 96%. Although some domestic production equipment can fully meet the requirements, but in terms of production efficiency and stability, imported equipment and has a certain advantage.
2) to ensure the tension stability and consistency.
The stability of the series production line tension can ensure high concentricity of the insulated conductor, and can guarantee the consistency of the conductor, the conductor diameter, can achieve the ideal center distance "a", "d" conductor diameter value; On the ground, cabling, equipment to ensure stable and consistent tension conductor on the ground of length balance state, if the rope on the ground or into a tension instability or do not agree with other tension, can produce tensile stress on a conductor, resulting in asymmetric structure and make the "a" value fluctuations, and makes the stress of the core, the phenomenon of the dielectric constant epsilon e instability, make to characteristic impedance is not uniform on the whole length.
As the Cat6A, Cat7 cable standards promulgated, the tension control system requirements will be more and more high. There are factories began trying to wring, into line with active feeding way.
3) choose high quality raw materials
Select high-quality raw materials is the most basic and most important of a kind of method, high quality raw materials with good ductility, plasticization, and can ensure the stability of the dielectric constant. High quality material is the key to guarantee all cable products performance.
4) the pre twist back technology
In the production of the eccentricity of the insulated conductor is inevitable, so as to make the center distance "a" wave causes and line of wave impedance, the "pre twisting back" technology can reduce the influence of insulation core eccentricity of characteristic impedance, which reduce insulated conductor concentricity requirement.
In the process of production, also can use a lot of other ways to ensure the uniformity of wire itself characteristic impedance, such as some companies have established adhesion technology and so on, as long as is a useful method to improve the cable structure can try.
3. The construction process
Construction non-standard could lead to disastrous effects for cable link, here we introduce several kinds of non-standard prone construction:
1) laying density is too large
Density is too large, laying cable inside the pipeline, especially the corner parts mutual extruding, cause cable deformation resulting in cable transmission performance especially RL performance degradation. Both metal and plastic pipe, the general filling rate at about 30%, slot filling rate is around 50%, such as a line of D25 tube wear at most 2 root Cat6 UTP cable.
2) when laying tension is too large
Wear tube tension is too large, easy to cause deformation of cable, will cause the cable transmission performance. The impedance change caused by excessive tension is very serious, almost on the entire length of the wiring test of RL is not ideal, and adopts HDTDR track test, display the worst position is at both ends, but no matter how to replace the module cannot be ruled out all the defects. This is due to make the overall structure of the cable in the process of drawing has changed, this kind of defect occurs when repeated construction.
To avoid damage caused by excessive tension when laying, refer to the conventional cable tension of the maximum permissible:
A cable to the line, 4 to 1 0 0 N;
2 4 on line cable, pull to 1 5 0 N;
Three 4 cable to the line, to 2 0 0 N;
N 4 on line cable tension for N x 5 + 5 0 N;
No matter how many 4 on line cable, maximum tension cannot be more than 4 0 0 N.
3) the bending radius is too small
Small bending radius, are serious extrusion between cable conductor, cable products obvious deformation and even skin damaged, resulting in overall transmission performance decline. For trough turning parts, should avoid 90 ° corner, especially in the vertical position.
General UTP cable installation of the bending radius should not less than 8 times of cable diameter, after installation of the bending radius should not less than 4 times of cable diameter; FTP cable installation of bending radius should not less than 10 times of cable diameter, after installation of the bending radius should not less than five times of cable diameter.
conclusion
RL the technical indicators are defined in the Ethernet transmission application has important practical significance, as is often the case as the physical platform of network wiring system to support at least 2 to 3 generations of the application of network equipment, when network equipment subsequent upgrade full-duplex mode transmission applications, if RL performance is poor, the strong reflection signals will directly affect the normal reception equipment the receiving end, resulting in the error rate rise and influenced the stability of the network. As a result, the performance of RL is should cause the attention of all necessary condition, and implement a good performance of RL wiring system need the quality of the product itself is good at the same time also needs to have the excellent installation construction, the combination of the two are inseparable.