首页 > 数据 高频线缆
[问题求助] 为什么数据缆的特性阻抗标称值是100？
P:2008-01-23 09:18:58
7
拙见，有不对的地方请多包涵！
特性阻抗是根据实际情况确定的一个额定数值，在确定特性阻抗的时候要考虑多方面的因数：
1.传输功率,距离与传输条件
比如网络线,水晶头的插孔就决定了线的绝缘外径应该在0.9-1.05之间(近似),太大太小都对装配有影响.当绝缘外径确定了后,然后需要验证的是传输的功率,如果传输功率达不到,当然就行把导体截面加大了,到时水晶头都得改了.想一下,水晶头要是有拳头那么大那是什么效果,呵呵.再下一步就是考虑传输的距离了,要传输远点衰减就得小点,截面就得大点.导体电阻就得小点.当然,如果局域网100米就够了,就不用那么大那么好的导体了.还有,这些没有先后的,你当然可以先确定功率再算外径的了.
2.适配问题
比如同轴电缆,接线端子是75OHM的,如果你用100OHM的同轴线,阻抗就不匹配了,翻翻那年代的物理都有了,传输有功功率是在阻抗匹配的情况下达到最大.
3.结构尺寸,材料及经济性
假如根据传输功率,传输距离确定了导体的直径是0.5mm的铜,然后就要确定绝缘材料,绝缘材料理论上当然是介电常数越小越好了,但是要太小了你就做不出来了,因为要达到规定的特性阻抗值,绝缘厚度就小得一蹋糊涂了.当然要是可能的话,可以把特性阻抗规定得大点.不过设计电缆是要考虑经济性的,也就是说在满足使用的情况下,尽量省钱,这些都做到了,特性阻抗就出来了.
另外后面那个公差值,是根据传输的效果和设备的加工能力确定的.如果公差太大了,传输不稳定了,误码率高了.掉包多了就麻烦了,公差小了,可能你的导体外径或者绝缘公差就只能控制在0.000001mm了,那是你不可能做到的.包括进口的机器.
不知道IEC是咋确定的,也不知道说对了没有,呵呵.
mlship 奖励 20 金币，.1 声望，于 2008-1-23 10:31:28 原因：
mouse proof cable - 防鼠电缆 （0） 投诉
P:2008-01-23 19:14:34
11
这个可能有点用：<转>
<DIV>The History Of 50 ohmsA lot of people ask, so here's the answer to the eternal question, "How did 50 ohms get to be the standard RF transmission line impedance?" Here are a few stories. Bird Electronics will send you a printed copy of their version if you ask for it. This from Harmon Banning of W.L. Gore & Associates, Inc. cable:There are probably lots of stories about how 50 Ohms came to be. The one I am most familiar goes like this. In the early days of microwaves - around World War II, impedances were chosen depending on the application. For maximum power handling, somewhere between 30 and 44 Ohms was used. On the other hand, lowest attenuation for an air filled line was around 93 Ohms. In those days, there were no flexible cables, at least for higher frequencies, only rigid tubes with air dielectric. Semi-rigid cable came about in the early 50's, while real microwave flex cable was approximately 10 years later.Somewhere along the way it was decided to standardize on a given impedance so that economy and convenience could be brought into the equation. In the US, 50 Ohms was chosen as a compromise. There was a group known as JAN, which stood for Joint Army and Navy who took on these matters. They later became DESC, for Defense Electronic Supply Center, where the MIL specs evolved. Europe chose 60 Ohms. In reality, in the US, since most of the "tubes" were actually existing materials consisting of standard rods and water pipes, 51.5 Ohms was quite common. It was amazing to see and use adapter/converters to go from 50 to 51.5 Ohms. Eventually, 50 won out, and special tubing was created (or maybe the plumbers allowed their pipes to change dimension slightly).Further along, the Europeans were forced to change because of the influence of companies such as Hewlett-Packard which dominated the world scene. 75 Ohms is the telecommunications standard, because in a dielectric filled line, somewhere around 77 Ohms gives the lowest loss. (Cable TV) 93 Ohms is still used for short runs such as the connection between computers and their monitors because of low capacitance per foot which would reduce the loading on circuits and allow longer cable runs.Volume 9 of the MIT Rad Lab Series has some greater details of this for those interested. It has been reprinted by Artech House and is available.</DIV>
SNOM - scanning near-field opticalmicroscopy近场扫描光显微法 （0） 投诉
guodong:39
你需要登录才能发表，点击登录。