Understanding impedance in twisted-pair cables help you avoid signal problems.
一对扭曲的对，例如在以太网电缆中发现的，旨在在经济上传输差异信号。尽管只有两个导体，但它是一个差异对互连。我们如何理解这种互连，尤其是当我们想考虑其共同阻抗时？
tw的性质了解的秘密isted pair as a differential interconnect is to identify the third conductor: ground. In this context, I don’t mean electrical ground. I mean ground, as in the floor.
Every differential pair is composed of two single-ended transmission lines — no exceptions. Each of the two single-ended transmission lines in a twisted pair is really composed of a signal line (one of the wires in the pair) and its return path. In unshielded twisted pair cable, like Cat5 cable, the return path is a complicated structure composed of the fringe fields to the chassis, to the adjacent wires in the cable, and to any nearby conducting surface, which is usually the ground, as in the floor. This will vary as the cable is moved and the proximity to the floor or nearby metal changes.
The single-ended properties of each transmission line in a twisted pair are terrible. This is not very controlled impedance. The single-ended impedance of the cable varies down its length, depending on the position of the local conductors.
当我们在两条传输线下启动差分信号时，它确实由两个单端信号组成，该信号在同一方向上传播。一个是正电压，另一个是负电压。这些单端信号的返回电流通过复杂且令人费解的路径传播，但通常它们沿着相同的路径。他们互相取消，因为它们朝相反的方向循环。
单端传输线重叠并取消的返回电流越多，返回路径对差分信号的影响就越小。因此，扭曲的一对提供了非常干净，均匀的差分阻抗。
公共信号是不同的。公共信号在两条线上都是相同的电压。这两条传输线的返回电流重叠，但是随着公共信号的返回电流朝着相同的方向循环，它们只是添加在返回路径中。
The common-impedance profile will show the impedance variation due to that convoluted return path, and it will generally be very high impedance.
The plot below shows an example of the measured differential and common impedance profile for a twisted pair, taken out of a Cat5 cable and spread over a table top.
未屏蔽的扭曲对的典型共同阻抗将从约100ω到200Ω不等。只要在扭曲对上没有通用信号，您就无需关心共同的阻抗。面临的挑战是防止常见的信号进入扭曲对。
提交以下：连接器提示
