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[問題求助] 為什么數(shù)據(jù)纜的特性阻抗標(biāo)稱值是100？
P:2008-01-23 09:18:58
7
拙見，有不對的地方請多包涵！
特性阻抗是根據(jù)實際情況確定的一個額定數(shù)值，在確定特性阻抗的時候要考慮多方面的因數(shù)：
1.傳輸功率,距離與傳輸條件
比如網(wǎng)絡(luò)線,水晶頭的插孔就決定了線的絕緣外徑應(yīng)該在0.9-1.05之間(近似),太大太小都對裝配有影響.當(dāng)絕緣外徑確定了后,然后需要驗證的是傳輸?shù)墓β?如果傳輸功率達(dá)不到,當(dāng)然就行把導(dǎo)體截面加大了,到時水晶頭都得改了.想一下,水晶頭要是有拳頭那么大那是什么效果,呵呵.再下一步就是考慮傳輸?shù)木嚯x了,要傳輸遠(yuǎn)點衰減就得小點,截面就得大點.導(dǎo)體電阻就得小點.當(dāng)然,如果局域網(wǎng)100米就夠了,就不用那么大那么好的導(dǎo)體了.還有,這些沒有先后的,你當(dāng)然可以先確定功率再算外徑的了.
2.適配問題
比如同軸電纜,接線端子是75OHM的,如果你用100OHM的同軸線,阻抗就不匹配了,翻翻那年代的物理都有了,傳輸有功功率是在阻抗匹配的情況下達(dá)到最大.
3.結(jié)構(gòu)尺寸,材料及經(jīng)濟(jì)性
假如根據(jù)傳輸功率,傳輸距離確定了導(dǎo)體的直徑是0.5mm的銅,然后就要確定絕緣材料,絕緣材料理論上當(dāng)然是介電常數(shù)越小越好了,但是要太小了你就做不出來了,因為要達(dá)到規(guī)定的特性阻抗值,絕緣厚度就小得一蹋糊涂了.當(dāng)然要是可能的話,可以把特性阻抗規(guī)定得大點.不過設(shè)計電纜是要考慮經(jīng)濟(jì)性的,也就是說在滿足使用的情況下,盡量省錢,這些都做到了,特性阻抗就出來了.
另外后面那個公差值,是根據(jù)傳輸?shù)男Ч驮O(shè)備的加工能力確定的.如果公差太大了,傳輸不穩(wěn)定了,誤碼率高了.掉包多了就麻煩了,公差小了,可能你的導(dǎo)體外徑或者絕緣公差就只能控制在0.000001mm了,那是你不可能做到的.包括進(jìn)口的機(jī)器.
不知道IEC是咋確定的,也不知道說對了沒有,呵呵.
non-metallic central element - 無金屬中心元件 （0） 投訴
P:2008-01-23 19:14:34
11
這個可能有點用：<轉(zhuǎn)>
<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>
D.D.D.S. - Die Diameter Dialling System撥號配模系統(tǒng) （0） 投訴
guodong:39
