终端未知的长电缆

安捷伦制造时域反射仪，如86100/54754系统，可测量PCB，连接器，背板和短电缆，其幅度为200mV，上升时间为35pic。
您所谈论的电缆长度需要不同的设备，分辨率更粗糙，幅度更大/步进刺激更慢。
我们的设备仅适用于高速数字应用的不同应用，因此我们必须建议您找到合适的TDR并考虑“安装和维护”应用。
我知道，帮助不大。
抱歉。



     以下为原文

  Agilent makes Time Domain Reflectometers like the 86100/54754 system that measure PCBs, connectors, backplanes and short cables with a 200mV amplitude 35picosecond risetime step. The length of cables you are talking about require different equipment that is much more coarse in resolution and much higher amplitude/slower step stimulus. Our equipment is just for a different application for high speed digital applications, so we must recommend that you find a suitable TDR with "Installation and Maintenance" applications in mind. Not much help, I know. Sorry.

我不知道原始海报想要衡量的是什么 - 它是否是电缆中的损失？
当损耗为100 dB时，我无法想到任何系统能够从一端测量电缆损耗。
假设理想的完美50欧姆电缆，从VNA，TDR或类似信号注入的任何信号都会以低于入射信号200 dB的幅度反射。
在没有一个200 dB的情况下，我想不出能够测量这种信号的任何仪器。
如果它是想要的损耗，是否可以降低频率以使损耗低得多，使用VNA测量反射的幅度，然后以更高的频率推断损耗？
存在众所周知的同轴电缆损耗的数学模型，其由于电介质和铜损耗而随频率上升。
也许最初的海报可以更多地说明她想知道的这条长11公里，损失100 dB的电缆。
戴夫



     以下为原文

  I don't know what the original poster wanted to measure - was it the loss in the cable? 

I can  not think of any system able to make a measurement of the loss of a cable from one one end, when that loss is 100 dB. Any signal injected from a VNA, TDR or similar would, assuming an ideal perfect 50 Ohm cable, be reflected with an amplitude of 200 dB below the incident signal. I can't think of any instrument that would be able to measure such a signal, in the presence of one 200 dB larger. 

If it is loss that is wanted, would it be possible to reduce the frequency such that the loss is much lower, use a VNA to measure the magnitude of the reflection, then infer the loss at a higher frequency? There are well known mathematical models for the loss of coax, which rises with frequency both due to dielectric and copper loss. 

Perhaps the original poster could say more about what she wants to know about this cable that is 11 km long and has a loss of 100 dB. 

Dave

* NEXT *是近端串扰。
因此，从技术上讲，电缆的长度无关紧要，因为测量是在电缆的开头进行的。
现在，Far-End Crosstalk将是另一回事！
请参阅：http：//en.wikipedia.org/wiki/Crosstalk_%28electronics%29



     以下为原文

  *NEXT* is Near-End Crosstalk.
So technically the length of the cable does not matter since the measurement is performed at the beginning of the cable.  Now, Far-End Crosstalk would be a different matter!

See: http://en.wikipedia.org/wiki/Crosstalk_%28electronics%29

是的，事实上，你可以很好地测量NEXT而不需要任何终止，或者更确切地说，保持电缆终止（我认为它必须连接到远端的东西，否则为什么有i，但无论如何）。
NEXT的关键是在电缆开始处理过渡。
我认为这就像LAN电缆？
当您将分析仪连接到电缆时，用于连接它的固定装置本身就会产生串扰（这种固定装置通常不能很好地平衡。使用自动夹具移除并在输入端使用时间选通可以改善您的结果。
你在考虑使用，你感兴趣的频率范围是多少？



     以下为原文

  Yes, in fact, you can pretty well measure NEXT without any termination or rather, leaving the cable terminated as it is (I presume it must be connected to something on the far end or else why have i, but no matter).

The key thing in NEXT is to deal with the transition at the beginning of the cable.  I presume this is something like a LAN cable?  When you connect your analyzer to the cable, the fixutre used to connect it can have crosstalk in itself (such fixtures are often not well balanced.  Using Automatic Fixture Removal and also using time-gating right at the input can improve your results.

What analyzer are you considering using, and what is the range of frequencies that you are interested in?

你好，谢谢你的回复。
电缆是定制的，内部有几对双绞线和屏蔽。
我们有一台带有LF和S参数测试装置的E5061B网络分析仪。
我还有50欧姆不平衡的100欧姆平衡变压器，我考虑一次连接到一对。
我还需要一次测量一对的特征阻抗。
测量S11是否正确？
感兴趣的频率为100kHz至2MHz。
此致JuliaEdited：Loevinnen于2014年1月29日上午2:21



     以下为原文

  Hi,
thanks for the reply. The cable is a custom-made one, with several twisted pairs and shields inside. We have a E5061B network analyzer with both the LF and S-parameter test sets. I have also 50 Ohm unbalanced to 100 Ohm balanced transformers which I consider using to connect to one pair at a time. 
I need also to measure the characteristic impedance of one pair at a time. Will it be correct to just measure S11?

The frequencies of interest are 100kHz to 2MHz.

Regards,

Julia

Edited by: Loevinnen on Jan 29, 2014 2:21 AM

应该是合理的，但要认识到你无法真正测量特征阻抗。
时域是使用选项10测量它的更好方法。请记住，电缆的阻抗随着频率和电缆的距离而变化（好吧，也许你的电缆是完美的，并且沿着它们的长度没有阻抗偏差，但大多数电缆
有一些阻抗变化）加上变压器引起的误差。
我在本书的第5章中详细讨论了电缆阻抗测量：http：//www.wiley.com/WileyCDA/WileyTitle/productCd-1119979552.html



     以下为原文

  Should be reasonable, but recongnize that you cannot really measure characteristic impedance that way.  Time domain is a better way to measure it using option 10. Remember the cable has an impedance that changes both with frequency and with distance down the cable (well, maybe your cables are perfect and have no impedance deviation along their length, but most cables have some impedance variation) plus error due to the transformer.

I discuss cable impedance measurements in some detail in Chap 5 of my book: http://www.wiley.com/WileyCDA/WileyTitle/productCd-1119979552.html