50ohm 问个弱弱的问题

abe_zhang

小弟刚从事手机不久,经常见50ohm这个名词,谁能帮我解释一下,还有,刚接触手机这行业要看一些什么方面的资料啊?

MELE

30歐姆有最大傳輸功率 77歐姆有最小損耗 50歐姆是取平衡的結果 實際應該是在51或52左右

oxt_2000

去看看关于传输线特性阻抗的基础内容

awp666

<DIV class=quote><B>以下是引用<I>MELE</I>在2006-4-11 11:36:00的发言：</B>
30歐姆有最大傳輸功率 77歐姆有最小損耗 50歐姆是取平衡的結果 實際應該是在51或52左右 </DIV>


干微波几年了第一次听到这个。虚心求教一下，这是为什么？
[em13]

rf1234

<b>为什么RF电路的特性阻抗大多选择50欧姆?<p></p></b><P> <p></p></P><P>The History Of 50 W <p></p></P><P>A lot of people ask, so here's the answer to the eternal question, "How did 50 W 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 &amp; 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</P>