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The poet's physics twisted-pair cable guide

by:FOT     2020-07-17
The poet's twisted-pair cable guide I have been on a professor of physics class, the professor likes to use two ways to explain various concepts. First of all, he can tap into a concept of mathematical concept, various equations on the blackboard, and explains the relationship between them. 'That he calls' the engineer's point of view. After that, he will explain the concept for a again, but this time, completely does not involve the use of mathematics. 'That he calls' the poet's point of view. Why, then, communication is a twisted pair cable structure, and transmission wire is straight? This is totally out of bandwidth considerations. Power signal frequency is low, there is no need to worry about bandwidth, but it is the communication cable issue to consider. Can produce magnetic field along the wire transmission of high frequency signals, induction signal is caused in the adjacent cables. The induction signal is known as the 'crosstalk', because when using old-fashioned analog phone line, you can often hear the dialogue in the background, which is caused by induction signal. Imagine that your computer's network interface is transmitting a signal. When a signal from the sending end line, the receiver circuit will produce a induction signals at the same time. Came to so much trouble, because according to the rules of the operation of the Ethernet, if other people in the 'remarks' ( Signal) , you have to stop 'speak'. But if every time your computer is trying to 'speak', it can hear his speech, so will stop signal action immediately. In this way, can't even send an email out. Induction signal strength, in fact, many times smaller than the original signal, so the problem is not so serious. But the signal receiver need to be very sensitive. This is because the high frequency signal transmission attenuation happens in the cable. For example, the IEEE 802. Three standards for base - 1000 T twisted-pair signal regulation maximum loss of 24 db, the equivalent of Ethernet interface signals arrive at your computer than distal from the launch of the original intensity attenuation by 6%. So the crosstalk signal must not exceed this level. The farther the distance computer interface, the greater the strength of the received signal, and the less susceptible to the influence of the crosstalk. This means that the place closest to the transmitter crosstalk problem is most serious, therefore the key indicators is called near end crosstalk ( 下一个) 。 The engineers have a lot of ingenious ways to solve the near end crosstalk ( 下一个) The problem. First of all, as to the 'difference' model code and data signal input cable, mean a wire on every positive pulse pairing with the corresponding lead the negative pulse forming on the match. This means that the cable will produce with the same intensity but opposite polarity electromagnetic field, the two will offset each other, so as to eliminate crosstalk. But if just keep a pair of cables between parallel, in which a cable is a magnetic field is closer, so a cable electromagnetic fields will be a little stronger than the other, slightly crosstalk eventually will still exist. The second method is to put the cable twist into a twist. This method can make the distance between the cable change, sometimes closer to the positive conductor, and closer to the cathode wires. This will help offset the induction effect, reduce or even eliminate crosstalk. But if the line on the ground from the same, it could keep the same with the magnetic field in the whole distance spacing, cause greater crosstalk. Need to introduce the third cable at this moment, let the line with different lay, which makes them throughout the transmission distance does not keep the same distance with the same wire. Different lay it is when you're using cable tester to test line pair found that they have different length. If the line to unlock, stretched straight, lay a big cable to be longer. Length difference is usually 5% or more. TIA standards the limitation on the length of the cable to the shortest line shall prevail. Although the wire in the module ( RJ - 45) Connector in only a parallel state in a short distance, but in the installed link, they are usually the main cause near end crosstalk. And when installing a connector, even a little more loose winding could significantly enhance the induction effect, lead to the failure link certification. A new design scheme through other ways to get better crosstalk resistance, including through the cable used in the insulation layer, control of lay more carefully, and the line of together. And some new technologies, such as 10 gbase - T and PoE, the requirement for the link is not only a better crosstalk resistance. But when it comes to high quality wiring, crosstalk is still one of the most important parameters. If you have any questions or problems, you are welcome to contact us at any time
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