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Twisted-pair cable: why do you want to measure resistance and resistance is poor?

by:FOT     2020-07-17
Twisted-pair cable: why do you want to measure resistance and resistance is poor? Mention data transmission cable, why should emphasize the twisted pair copper resistance measurement? Measure the diameter and weighing method I know a little, but it's too original? Yeah! Starting from the fifties and sixties, the widely used to measure resistance. Because resistance is certain, if material is not full of copper, resistance is big, wire diameter is thinner, the resistance will be bigger, with resistance measurement so it can be easily proved that the material of it whether accord with a standard to ask. You can see, in the test indexes of twisted pair, one of the most basic indicators is the resistance. But you will find that, compared with before, in the past decade, there are some different places - resistance test Besides of the twisted pair to test the loop resistance, also requires two wire resistance measurement of poor ( Option) 。 Because if two different thickness of wire or two wire material have differences, so two wire resistance is not the same. So besides can further test cheating, more major is to test the resistance of the wire whether to keep the 'balance' - Resistance balance is balanced twisted-pair transmission signal when a necessary conditions. Otherwise, at the time of signal transmission, two lines of conduction ability is different, which can cause the signal transmission in two wires appeared difference, so that the 'balance' cable in the ability to resist outside interference. Why balance resistance or resistance is poor in recent years to request test? Here, the main reason is because early, twisted pair work environment in general is given priority to with intelligent buildings, you know, 30 years ago, the building is not serious environmental interference. And in the midst of industrial bus, although also USES the balance cable, but as a result of the data transmission rate, low frequency, so the time for paid little attention to external interference. With transmission rate is higher and higher, the room of interference, such as harmonic interference, ground loop interference, etc. , and wireless base stations, mobile phone Wi - Fi, bluetooth devices such as interference, is more and more big. And is widely used in industrial environments such as Ethernet, the power of interference, radiation interference, ground loop interference, while others, such as from high power frequency conversion motor, electric welding, robots and other interference quantity increased a lot, this time we are going to have to consider to the unbalanced resistance testing. Then what is the requirement to the resistance unbalance in the standard? Resistance unbalance, general requirements is less than 3% or absolute value less than 0. 2 ohm, such ability can achieve certain indicators. In the latest PoE++ ( Ethernet power supply) , due to the PoE power increased to 90 watts ( Later may also further) , if you don't balance, resistance will make the two copper wire current is different, this will lead to the signal transmission problems on the PoE - Is the signal of high frequency transformer core saturation problem - Once the high frequency magnetic core saturation or close to saturation, the signal transmission distortion or complete failure. In order not to let the high frequency magnetic core saturated, on the current requirement is two copper wire to the same, which would require the resistance of the two copper wire. If two resistance difference is bigger, can appear bigger difference of current, lead to core of dc magnetic bias deviation is too large or saturation, this is must be avoided. Why would magnetic core saturated? The high frequency magnetic core is a part of the high frequency signal transformer. But this part has a characteristic, that is the dc current in coil is the best to zero, if not zero, there will be the initial dc magnetization phenomenon exists. A small number of dc magnetization ( Dc magnetic bias) No problem, but the magnetic bias is too big, can let a core into magnetic saturation area, and once in magnetic saturation area, core is completely out of the transformer function, signal transmission will be interrupted. More often the story is: magnetic bias slants big, but has not been into the core saturation area, just entered the nonlinear area, it will make balance signal transmission distortion, leading to the bit error rate and packet loss rate. Let me summarize. Measure the resistance of the copper wire is to measure the material of it is in line with the requirements; Resistance measurement is poor in order to improve its balance, namely anti-interference ability; Also in the midst of POE power supply, it is in order to avoid the high frequency magnetic core transformer differential current is too large, the distortion or saturated failure. B: yes. If you have any questions or problems, you are welcome to contact us at any time
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