RF为什么选择50欧姆？讨论下啊

tangallan1

50欧姆这个要求大家都知道，但是为什么要选择50欧姆（或者75欧姆）呢？
    难道是因为别的东西都选择50欧姆（或者75欧姆）才被迫选择50欧姆吗？虽然有点被迫的意思（这就是在标准下做事你得遵守游戏规则），但这个选择肯定有它自身的好处，这个好处是什么呢？
     我希望大家能够踊跃讨论下哦：）    （会觉得这个问题很白痴很幼稚的那就。。。）
     我把我知道的一点点说下，希望能抛砖引玉啊：
     为了负载能得到最大功率，计算出来的结果是传输线特性阻抗为30欧姆；
     为了最小损耗，特性阻抗大概是77欧姆；
     兼顾两者，取数学平均或者算数平均大概都在50欧姆左右，为了方便就取了50欧姆这个数值；
     但对于其中的推倒小弟还不太清楚，请各位大侠说说啊

yora

推导在THOMAS H LEE的那本书上有的。。

iaman

Why 50 ohm coax ? (taken from http://www.hut.fi/Misc/Electronics/docs/wiring/cable_impedance.html)
Standard coaxial line impedance for r.f. power transmission in the U.S. is almost exclusively 50 ohms. Why
this value was chosen is given in a paper presented by Bird Electronic Corp.
Different impedance values are optimum for different parameters. Maximum power-carrying capability
occurs at a diameter ratio of 1.65 corresponding to 30-ohms impedance. Optimum diameter ratio for
voltage breakdown is 2.7 corresponding to 60-ohms impedance (incidentally, the standard impedance in
many European countries).
Power carrying capacity on breakdown ignores current density which is high at low impedances such as 30
ohms. Attenuation due to conductor losses alone is almost 50% higher at that impedance than at the
minimum attenuation impedance of 77 ohms (diameter ratio 3.6). This ratio, however, is limited to only
one half maximum power of a 30-ohm line.
In the early days, microwave power was hard to come by and lines could not be taxed to capacity.
Therefore low attenuation was the overriding factor leading to the selection of 77 (or 75) ohms as a
standard. This resulted in hardware of certain fixed dimensions. When low-loss dielectric materials made
the flexible line practical, the line dimensions remained unchanged to permit mating with existing
equipment.
The dielectric constant of polyethylene is 2.3. Impedance of a 77-ohm air line is reduced to 51 ohms when
filled with polyethylene. Fifty-one ohms is still in use today though the standard for precision is 50 ohms.
The attenuation is minimum at 77 ohms; the breakdown voltage is maximum at 60 ohms and the powercarrying
capacity is maximum at 30 ohms.
Another thing which might have lead to 50 ohm coax is that if you take a reasonable sized center conductor
and put a insulator around that and then put a shield around that and choose all the dimensions so that they
are convenient and mechanically look good, then the impedance will come out at about 50 ohms. In order
to raise the impedance, the center conductor's diameter needs to be tiny with respect to the overall cable's
size. And in order to lower the impedance, the thickness of the insulation between the inner conductor and
the shield must be made very thin. Since almost any coax that *looks* good for mechanical reasons just
happens to come out at close to 50 ohms anyway, there was a natural tendency for standardization
at exactly 50 ohm.

feihongjing

记号，慢慢看，谢谢了[em01]

iamhome

查阅了一下资料,如下:

50欧姆的同轴线兼顾了耐压、功率容量和衰减的要求

75欧姆的同轴线衰减最小
60欧姆的同轴线耐压最大
30欧姆的同轴线功率容量最大

应该都是适用于同轴线,那么在微带\带状线\等传输线中,情况如何?
是不是也是这个结果?如何推倒相应的结论,有没有人有兴趣推倒一下?

iamhome

以下是引用iamhome在2008-12-2 17:46:15的发言：
查阅了一下资料,如下:

50欧姆的同轴线兼顾了耐压、功率容量和衰减的要求

75欧姆的同轴线衰减最小
60欧姆的同轴线耐压最大
30欧姆的同轴线功率容量最大

应该都是适用于同轴线,那么在微带\带状线\等传输线中,情况如何?
是不是也是这个结果?如何推倒相应的结论,有没有人有兴趣推倒一下?

个人认为推导微带传输线的结论还是很有现实意义的,结果会不会和同轴的结论一样?有什么差异?

begin100

学习学习！

KHK19

学习ING

蓝色水滴

[em08]

lijun3720

同楼上!

amen

面试还被问了这个问题，当场昏

amen

为什么RF电路的特性阻抗大多选择50欧姆?(2007-11-29 01:09:50)   

The History Of 50 ohms

A 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.

edwardflee

以下是引用yora在2008-12-2 9:43:09的发言：
推导在THOMAS H LEE的那本书上有的。。

[em32]《CMOS射频集成电路设计》第六章第5节

wyguo1109

很好[em01]

zaiqianxian

[em01][em01][em01]

ss.sean

学习一下[em01]

lucky850315

不明白，来学习了[em14]

miaoying_121

学习一下[em05]

cryptowings

一直听说是因为功率容量和衰减，原来还有耐压一说~~~学习了

dengxiaobin2040

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